IP over WDM - web.fe.up.pthsalgado/pstfc/Projecto_IP_WDM_Final.pdf · 1.3. Report Organization ......
Transcript of IP over WDM - web.fe.up.pthsalgado/pstfc/Projecto_IP_WDM_Final.pdf · 1.3. Report Organization ......
INESC Porto, July 2002
Licenciatura em Engenharia Electrotécnica e de Computadores Ramo de Telecomunicações, Electrónica e Computadores
Graduation project – DEEC
IP over WDM
Designing an Optical IP Router
Supervisors Students
Henrique Salgado, PhD Manuel Ricardo, PhD
Bruno Leite Fernando Pinto Igor Terroso Joel Carvalho
Acknowledgements
We would like to thank some people,
without which, this work would have been difficult
to accomplish:
Orlando Frazão
Filipe Sousa
Our families
Our friends
Abstract
Expanding Internet-based services are driving the need for evermore bandwidth in the
network backbone. Besides that communications needs are continuously growing.
Communication networks must evolve to sustain this growing need for bandwidth.
Today, the only technology that can effectively meet such a demand is WDM. This technology
allows the increment of the available bandwidth in the fiber by the use of different wavelengths.
At present, this technology is used in a point-to-point manner in the network core using heavy
multi-layered protocol stacks.
In parallel the Internet transport infrastructure is moving towards a model of high-speed
routers interconnected by optical core networks.
Large-scale efforts are being made to develop standards and products that will eliminate
some of those issues: reduce one or more of the intermediate layers of the protocol stack, allow
automatic routing and provisioning inside the transport network, an allow fast and cheap optical
switching.
Development of these networks demands the use of new optical nodes. Nodes based in
new high performance optical technologies that are able to control the communications channels
and route them correctly. The purpose of this project is to build one of such nodes.
This reports covers the development of an Optical Router, implementing a control plane,
similar to the ones in the IP world, as defined in GMPLS research (IETF). The work can be split
into two parts:
Part I – Development of the underlying optical fabrics (Optical cross connect – OXC).
Part II – Definition of the structure of the protocol stack, needed to establish the control
plane at the OXC.
Different groups in different research units develop each of these parts. This report covers
the work done in part II.
PART I
INESC - UOSE Porto, July 2002
Licenciatura em Engenharia Electrotécnica e de Computadores Ramo de Telecomunicações, Electrónica e Computadores
Graduation project – DEEC
IP over WDM
Designing an Optical IP Router
Part I
Supervisors Students
Henrique Salgado, PhD Manuel Ricardo, PhD
Igor Terroso Joel Carvalho
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Table of contents
IP over WDM I - 3 of 90
Table of contents Table of figures................................................................................................................................5 Chapter 1 - Introduction ..................................................................................................................7
1.1. Abstract .................................................................................................................................................................7 1.2. Background and Motivation .................................................................................................................................7 1.3. Report Organization..............................................................................................................................................7 1.4. Contributors...........................................................................................................................................................8
Chapter 2 - Optical Networks..........................................................................................................9 2.1. Introduction ...........................................................................................................................................................9 2.2. The three networks generations ............................................................................................................................9 2.3. OTDM .................................................................................................................................................................10 2.4. OCDM.................................................................................................................................................................11 2.5. WDM...................................................................................................................................................................11
2.5.1. WDM Link ..................................................................................................................................................13 2.5.2. Passive Optical Network (PON).................................................................................................................13 2.5.3. Broadcast and Select Networks ..................................................................................................................14
2.5.3.1. Single-Hop Networks.....................................................................................................................................................................14 2.5.3.2. Multihop Networks ........................................................................................................................................................................15
2.5.4. Wavelength Routing Networks...................................................................................................................15 2.6. DWDM................................................................................................................................................................16 2.7. Summary .............................................................................................................................................................16
Chapter 3 - Optical WDM Components ........................................................................................17 3.1. Introduction .........................................................................................................................................................17 3.2. Optical passive components................................................................................................................................17
3.2.1. Fiber.............................................................................................................................................................17 3.2.2. Couplers.......................................................................................................................................................18 3.2.3. Isolators and Circulators .............................................................................................................................19 3.2.4. Filters...........................................................................................................................................................19
3.2.4.1. Tunable 2 x 2 directional couplers.................................................................................................................................................20 3.2.4.2. Gratings ..........................................................................................................................................................................................21 3.2.4.3. Arrayed waveguide grating (AWG) ..............................................................................................................................................21 3.2.4.4. Fabry-Perot Tunable Filters (FPF).................................................................................................................................................21 3.2.4.5. Mach-Zehnder Tunable Filters (MZF) ..........................................................................................................................................22 3.2.4.6. Liquid-crystal Fabry-Perot filters ..................................................................................................................................................23 3.2.4.7. Acousto-optic tunable filters..........................................................................................................................................................23
3.2.5. Multiplexers ................................................................................................................................................23 3.2.5.1. WDM Multiplexers and Demultiplexers .......................................................................................................................................23 3.2.5.2. Add/Drop Multiplexers (OADMs) ................................................................................................................................................24 3.2.5.3. Optical Cross-Connects (OXCs)....................................................................................................................................................24
3.3. Optical active components..................................................................................................................................24 3.3.1. Amplifiers....................................................................................................................................................24 3.3.2. Erbium-doped fiber amplifiers (EDFAs)....................................................................................................24
3.3.2.1. Semiconductor Optical Amplifiers ................................................................................................................................................25 3.3.2.2. Raman Effect Amplifiers ...............................................................................................................................................................25
3.3.3. Transmitters.................................................................................................................................................26 3.3.3.1. Light-Emitting Diodes ...................................................................................................................................................................26 3.3.3.2. Lasers .............................................................................................................................................................................................26
3.3.4. Receivers .....................................................................................................................................................28 3.3.4.1. PIN diodes......................................................................................................................................................................................28 3.3.4.2. Avalanche photodiodes (APDs).....................................................................................................................................................28
3.3.5. Wavelength Converters...............................................................................................................................29 3.3.6. Optical Gating .............................................................................................................................................29 3.3.7. Wave Mixing...............................................................................................................................................30
3.4. Summary .............................................................................................................................................................30 Chapter 4 - Signal Degradation on Optical Networks...................................................................31
4.1. Introduction .........................................................................................................................................................31 4.2. Attenuation..........................................................................................................................................................31 4.3. Dispersion ...........................................................................................................................................................32 4.4. Nonlinearities ......................................................................................................................................................33 4.5. Kerr effects..........................................................................................................................................................33
4.5.1. Self-phase modulation (SPM).....................................................................................................................33 4.5.2. Crossphase modulation (XPM)...................................................................................................................34
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4.5.3. Four-Wave Mixing (FWM) ........................................................................................................................34 4.6. Scattering effects.................................................................................................................................................35
4.6.1. Stimulated Raman Scattering (SRS)...........................................................................................................35 4.6.2. Stimulated Brillouin Scattering (SBS) .......................................................................................................35 4.6.3. Conclusions on nonlinear effects................................................................................................................36
4.7. Crosstalk..............................................................................................................................................................36 4.7.1. Heterodyne crosstalk...................................................................................................................................36 4.7.2. Homodyne crosstalk....................................................................................................................................37 4.7.3. Conclusions on crosstalk.............................................................................................................................37
4.8. Summary .............................................................................................................................................................38 Chapter 5 - Physical devices used in this project – Fiber Bragg Grating’s ..................................39
5.1. Introduction .........................................................................................................................................................39 5.2. History of photosensitivity and fiber Bragg gratings.........................................................................................39 5.3. Photosensitivity in optical fibers ........................................................................................................................40 5.4. Fiber Bragg grating properties............................................................................................................................41
5.4.1. Physical properties ......................................................................................................................................41 5.4.2. Spectral response of fiber Bragg gratings ..................................................................................................43 5.4.3. Coupled Mode Theory ................................................................................................................................45 5.4.4. Apodization of the spectral response of Bragg gratings ............................................................................47
5.5. Fabrication processes ..........................................................................................................................................47 5.5.1. Interferometric technique............................................................................................................................48 5.5.2. Point-by-point technique.............................................................................................................................49
5.6. Applications of Fiber Bragg Gratings ................................................................................................................51 5.6.1. Laser stabilization .......................................................................................................................................51 5.6.2. Fiber lasers ..................................................................................................................................................51 5.6.3. Reflectors in fiber amplifiers ......................................................................................................................51 5.6.4. Raman-Shifted Lasers and Raman Amplifiers ...........................................................................................52 5.6.5. Sensors.........................................................................................................................................................52 5.6.6. Isolation Filters in Bidirectional Lightwave Transmission........................................................................52 5.6.7. WDM Demultiplexers.................................................................................................................................52 5.6.8. Add/Drop Multiplexers and Optical Cross-Connects ................................................................................52 5.6.9. Dispersion Compensators and Wavelength Converters .............................................................................52
5.7. Summary .............................................................................................................................................................53 Chapter 6 – Wavelength Routers...................................................................................................54
6.1. Introduction .........................................................................................................................................................54 6.2. Optical Add-Drop Multiplexers – OADMs........................................................................................................54
6.2.1. Comparison of common OADM structures ...............................................................................................55 6.3. Optical Cross-Connects - OXCs.........................................................................................................................58
6.3.1. SWITCHING TECHNOLOGIES ..............................................................................................................59 6.3.1.1. OPTOMECHANICAL ..................................................................................................................................................................59 6.3.1.2. MICRO-ELECTRO-MECHANICAL SYSTEM (MEMS) ..........................................................................................................59 6.3.1.3. THERMO-OPTICAL ....................................................................................................................................................................61 6.3.1.4. LIQUID CRYSTAL.......................................................................................................................................................................62 6.3.1.5. GEL/OIL-BASED – “Bubble” Switching.....................................................................................................................................63 6.3.1.6. ELECTRO-OPTICAL ...................................................................................................................................................................64 6.3.1.7. ACOUSTO-OPTIC........................................................................................................................................................................64 6.3.1.8. ELECTROHOLOGRAPHIC.........................................................................................................................................................65 6.3.1.9. BRAGG GRATING BASED ........................................................................................................................................................66
6.3.2. Comparison of OXC technologies..............................................................................................................67 6.4. Summary .............................................................................................................................................................68
Chapter 7 – Work Developed ........................................................................................................69 7.1. Introduction .........................................................................................................................................................69 7.2. Grating Fabrication .............................................................................................................................................69 7.3. Development of an Optical Add-drop Multiplexer ............................................................................................71
7.3.1. Implementation of the first structure – OADM 1.......................................................................................71 7.3.2. Performance Assessment ............................................................................................................................72 7.3.3. Upgraded OADM Structure – OADM 2 ....................................................................................................73 7.3.4. Performance Assessment ............................................................................................................................73
7.4. Optical Crossconnect Architectures ...................................................................................................................74 7.4.1. Description of first structure and its implementation.................................................................................74 7.4.2. Performance Assessment ............................................................................................................................75
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7.4.3. Development of an upgraded OXC ............................................................................................................76 7.4.4. Performance Assessment ............................................................................................................................77
7.5. Summary .............................................................................................................................................................82 Chapter 8 – Concluding remarks ...................................................................................................83 References .....................................................................................................................................85 Appendix A – Peltier Devices Appendix B – Peltier electronic Controller Appendix C – OADM Paper submitted in Física 2002 Appendix D – OXC architecture paper submitted in Física 2002
Table of figures Figure 1 – Three Generations of Networks ...................................................................................10 Figure 2 – Wavelength Division Multiplexing..............................................................................12 Figure 3 - WDM Link....................................................................................................................13 Figure 4 - Passive Optical Network (PON) ...................................................................................14 Figure 5 - Broadcast and Select Network: a) star topology; b) bus topology ...............................14 Figure 6 - Wavelength Routing Network: a) phisical topology; b) virtual topology ....................16 Figure 7 - Optical Fiber .................................................................................................................18 Figure 8 - Optical Couplers: a) equal ; b) non-equal .....................................................................19 Figure 9 - a) Isolator; b) Circulator; c) Logical scheme of a three port circulator ........................19 Figure 10........................................................................................................................................20 Figure 11 – Concept of a tuneable multielectrode asymmetric directional coupler ......................20 Figure 12 - Fiber Bragg Gratings ..................................................................................................21 Figure 13 - Array Waveguide Grating...........................................................................................21 Figure 14 - Operational setup of a Fabry-Perot Tunable Filter .....................................................22 Figure 15 - Mach-Zehnder Interferometer; b) Three Mach-Zehnder Chain .................................22 Figure 16 - Basic acousto-optic tunable filter ...............................................................................23 Figure 17 - Operational Principle of an EDFA with a: a) 1480 nm pump laser; b) 980 nm pump
laser........................................................................................................................................25 Figure 18 – Semiconductor Optical Amplifier (SOA) ..................................................................25 Figure 19 - Raman Amplification..................................................................................................26 Figure 20 – Stimulated Emission...................................................................................................26 Figure 21 - General structure of a laser .........................................................................................26 Figure 22 - a) Fabry Perot Laser b) DFB Laser.............................................................................27 Figure 23 - 3R Regeneration principle ..........................................................................................29 Figure 24 - a) Mach-Zehnder interferometer and b) Michelson Interferometer configurations
using pairs of SOAs for implementing CPM wavelength conversion scheme......................30 Figure 25 – Optical fiber attenuation.............................................................................................31 Figure 26 - Intersymbol Interference (ISI) ....................................................................................32 Figure 27 - Spectral broadening due to SPM ................................................................................34 Figure 28 – FWM: The mixing of f1 and f2 generate sidebands ..................................................34 Figure 29 - SRS transfers optical power from shorter wavelengths to longer wavelengths .........35 Figure 30 - Heterodyne crosstalk in a WDM system ....................................................................37 Figure 31 - Homodyne crosstalk in a WDM system .....................................................................37 Figure 32 -Power penalties from Heterodyne and Homodyne crosstalk for 8 and 16 WDM
channels .................................................................................................................................37
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Figure 33 - Schematic representation of a Fiber Bragg Grating....................................................42 Figure 34 - Spectral Response of a diffraction Grating Filter. ......................................................46 Figure 35 - Refractive index profile of an apodized Fiber Bragg Grating. ...................................47 Figure 36 - Setup for interferometric fabrication of Fiber Bragg Gratings. ..................................49 Figure 37 - Setup for Point-by-point Fabrication Technique. .......................................................50 Figure 38 - Setup for Phase-Mask fabrication of Fiber Bragg Gratings. ......................................51 Figure 39 - Dispersion compensation using an aperiodic grating with a length of 6cm associated
with an optical circulator. ......................................................................................................53 Figure 40 - Common OADM structures: FB-a), FB-b) and FB-c) – Interferometric structures
based in Fiber Bragg gratings; FB-d), FB-e) and FB-f) – Fiber Bragg grating and circulator based structures; FA-a), FA-b) and FA-c) – Array Waveguide Grating Mux structures......56
Figure 41 - OADMs based in Multiport Optical Circulators (MOC’s) Configurations. ...............58 Figure 42 - Toshiyoshi and Fujita’s 2×2 MEMS optical switch . .................................................60 Figure 43 - 8x8 2-D Optical switch ...............................................................................................60 Figure 44 - Schematic of 3-D MEMS switching...........................................................................60 Figure 45 - 3-D micromachined mirrors........................................................................................61 Figure 46 - Scratch Drive actuator switching 1x2. ........................................................................61 Figure 47 - Thermooptical switching - a)Schematic diagram of a Mach Zehnder switch ; b)
Light path in one of the switching. ........................................................................................62 Figure 48 - Total internal reflection switching – Agilent’s Champagne Bubble Switch. .............64 Figure 49 - Acoustooptic Tunable filter ........................................................................................65 Figure 50 - Electroholographic Matrix with ferro-electric crystals...............................................66 Figure 51 - A 4x4 rearrangeable nonblocking OXC using: a) 12 three-port Optical Circulators
and b) Four five-port Multiport Optical Circulators (MOC’s). .............................................67 Figure 52 - Transmission and reflection grating spectra. ..............................................................70 Figure 53 – First Optical Add-drop structure implemented - OADM 1.......................................71 Figure 54 – Optical WDM source obtained through slicing of a Broadband optical source’s
(LED) spectra. .......................................................................................................................71 Figure 55 – A – Input signal composed of three WDM channels (l1, l2,l3); B– Output Signal
composed of signals l1 and l3 ; C – Dropped Channel l2 . ...................................................72 Figure 56 – Heterodyne Crosstalk caused by imperfect FBG filtering. ........................................72 Figure 57 - Second Optical Add-drop structure implemented - OADM 2....................................73 Figure 58 - C – Dropped Channel λ2 in OADM 1; D – Dropped Channel λ2 in OADM 2. ........74 Figure 59 – Optical Crossconnect 1 ..............................................................................................74 Figure 60 – Optical Crossconnect 1 performance test...................................................................75 Figure 61 - Power Spectral Response of the OXC in a detuned state. ..........................................76 Figure 62 - OXC 2. ........................................................................................................................77 Figure 63 - Multiwavelength Fiber Ring Laser Source using Fiber Bragg Gratings. ...................77 Figure 64 - Input Signals ...............................................................................................................78 Figure 65 - Output port signals with optical filters detuned..........................................................78 Figure 66 - Outputs when Channel 1 is switched from Input 1 to Output 2. ................................79 Figure 67 - Tuning and detuning speed. ........................................................................................80 Figure 68 - Tuning and detuning of the optical filter (FBG).........................................................81 Figure 69 - Wavelength tuning and detuning of the optical filter (FBG)......................................81
Chapter 1- Introduction
IP over WDM I - 7 of 90
Chapter 1 - Introduction
1.1. Abstract
Optical networks have evolved to sustain the growing need for bandwidth in communication
networks. Wavelength switched networks using Dense Wavelength Division Multiplexing –
DWDM, are a solution to this problem. Development of these networks demands the use of
nodes capable of controlling the communication channels and route them correctly. These nodes
are sometimes referred to as Wavelength Routers.
As explained earlier in this report, the purpose of this project is the development of an
Optical Router and the implementation of a control plane similar to the ones in the IP world, as
defined in GMPLS research (IETF). The work can be split into two parts:
Part 1 – Development of the underlying optical fabrics.
Part 2 – Definition of the structure of the protocol stack, needed to establish the control
plane at the OXC.
Each of these parts was developed by different groups in different research units of INESC
Porto.
1.2. Background and Motivation
This project is currently a hot topic of research. The pursuit of an All-Optical network is
considered the “Quest for the Holy Graal” of Optical Network Research. Several technologies
have tried to reach the same goal and some of them are getting very close to their objective. The
optical technology used is this project is Fiber Bragg Grating based. These devices have proved
to be an essential building block in optical networks nowadays and have many other
applications. Work carried out by some research units like the Optoelectronics unit at INESC and
the Institute of Telecommunications (IT) in Aveiro is used as example1 and architectural
advances are introduced.
1.3. Report Organization
This report is divided in chapters which give readers a possibility to choose the topics of
interest. Following this introductory chapter, where the main purpose and motivation of the
Chapter 1- Introduction
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project is presented, the state of the art of the technology at the present time is described. Chapter
(2) gives an overview of Multiwavelength Optical Technology and Networks where fundamental
notions about optical networks and their current state are developed. Since this is a project where
an optical device is actually developed chapter 3describes the state-of-the-art in optical
components for WDM networks. Components ranging from the fiber itself, to active components
and complex amplifiers based in nonlinear effects are described briefly.
Performance tests are an important part of this project and this means that optical networks
and device impairments should be detailed. This is done in Chapter 4.
The key elements in the fabrics of this device are Optical Filters Based in Fiber Bragg
Gratings, their characteristics being fully described in Chapter 5.
The final Chapter, before the analysis of the physical implementation, (Chapter 6) describes
the state-of-the-art in Wavelength Routers, a category in which the device developed in this
project can be classified.
The physical implementation of this work is detailed in Chapter 7 where all the structures
developed are explained and their performance tests are presented.
Concluding remarks are given in Chapter 8. Finally, Appendix A describes the physical
characteristics behind a Peltier Device and the electronic control circuitry is described in
Appendix B.
1.4. Contributors
As a result of this work two articles and a device patent have been submitted for admission
by proper authorities. The following articles where written:
- I. Terroso, J. P. Carvalho, O. Frazão, M. Ricardo, H. M. Salgado, “Avaliação de duas Arquitecturas de OADM Baseadas em Circuladores Ópticos e Redes de Bragg em Fibra Óptica”, Física 2002. - J. P. Carvalho, I. Terroso, O. Frazão, V. Barbosa, M. Ricardo, H. M. Salgado, “Comutador Óptico (OXC) Baseado em Circuladores Ópticos e numa Rede de Bragg em Fibra Óptica”, Física 2002. The device developed was object of a patent described as: - Comutador Óptico (OXC) de 2 × 2 Portas para Sistemas de Multiplexagem em comprimento de Onda e Escalável a N × N Portas.
Chapter 1- Introduction
IP over WDM I - 9 of 90
This invention refers to the area of Optical Fiber Communications and can be defined as an
2x2 Optical Cross-connect for wavelength switched networks (WDM) that can be scaled to NxN
ports.
It should be noted that the main objective of this project was, initially, to develop an Optical
Add-drop Multiplexer but initial encouraging results made us proceed to build an Optical Cross-
connect based on the same technology. Insight into various areas in terms of optical networking
and technology was gained. The “hands-on” approach of this work when dealing with the optical
components and electronic circuits provided us with a thorough practical experience and know-
how that could not be gained through only the theoretical study of these matters.
Chapter 2 - Optical Networks
2.1. Introduction
In the last years we have seen a growing interest in optical networks in order to increase the
capacity of communication networks. The purpose of this chapter is to provide a level set in
about purely optical networks, or All optical networks (AONs)2,3,4 as they are usually called:
What they are good for, how far we have gotten with them, and how far we have yet to go.
2.2. The three networks generations
All optical networks are those that in which the path between the using nodes at the ends
remains entirely optical from end to end. Such paths are termed lightpaths. Each lightpath may
de optically amplified or have its wavelength altered along the way, but it’s a purely optical path.
As the optoelectronics technology to build optical networks is gotten closer to functional and
economic feasibility, more and more groups worldwide are studying them as a possible base
upon which to build the networks of the future, both within the wide-area backbone and for
metropolitan and local area distribution facilities. In light of the potential and recent advances,
all optical networks are very often considered to be the main candidate for constituting the
backbone that will carry global data traffic whose volume has been growing at impressive rates
that are not expected to slow down in the near future.
According to the physical technology employed, one can identify three generations of
networks (Figure 1). Networks built before the emergence of optical fiber technology are the first
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generation networks (i.e. networks based on copper wire or radio). The second generation
networks employ fibers in traditional architectures. The choice of fiber is due to its large
bandwidth, low error rate, reliability, availability, and maintainability. Although some
performance improvements can be achieved by employing fibers, the performance for this
generation is limited by the maximum speed of electronics (a few Gbps) employed in switches
and end-nodes. This phenomenon is called the electronics bottleneck. In order to satisfy the
increasing bandwidth requirements of emerging applications, totally new approaches are
employed to exploit vast bandwidth (approximately 30THz in the low loss region of single mode
fiber in the neighborhood of 1500nm) available in fibers. Therefore, the third generation
networks are designed as all-optical to avoid the electronics bottleneck. That is, information is
conveyed in the optical domain (without facing any electro-optical conversions) through the
network until it reaches its final destination. The emergence of single mode fibers, all-optical
wide-band amplifiers, optical couplers, tunable lasers (transmitters)/filters (receivers), optical
add-drop multiplexers and all-optical crossconnects5,6 make third generation networks a reality.
Figure 1 – Three Generations of Networks
In order to make use of the vast bandwidth available without experiencing electronics
bottleneck, concurrency among multiple user transmissions can be introduced. In all-optical
networks, concurrency can be supplied through time slots (OTDM - Optical time division
multiplexing), wave shape (OCDM - Optical code division multiplexing) or wavelength (WDM -
Wavelength division multiplexing)7.
2.3. OTDM
In optical time division multiplexing (OTDM)8, many low-speed channels, each transmitted
in the form of ultra-short pulses, are time interleaved to form a single high-speed channel. By
this method, the information carrying capacity of the network can be improved to 100 Gbps or
higher without experiencing electronics bottleneck. In order to avoid interference between
Chapter 2- Optical Networks
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channels, transmitters should be capable of generating ultra-short pulses, which are perfectly
synchronized to the desired channel (time slot), and receivers should have a perfect
synchronization to desired channel (time slot).
OTDM is a promising multiplexing technique, but it is still on research level. It is a very
similar to electronic TDM, the only difference is that OTDM is faster, and the devices used in
OTDM networks are optical. The main advantages of this multiplexing technique are the facts
that only one source is required and the node equipment is simpler in the single channel
architecture. The problems concerning OTDM are associated with the immaturity of all-optical
devices. Very fast lasers are needed and timing synchronization and alignment are still problems.
Additionally, although systems were technically feasible, it would not be economically viable.
Another weakness is that the OTDM is not a transparent multiplexing technique.
2.4. OCDM
In Optical code division multiplexing (OCDM)9, each channel is assigned a unique code
sequence (very short pulse sequence), which is used to encode low-speed data. The channels are
combined and transmitted in a single fiber without interfering with each other. This is possible
since the code sequence of each channel is chosen such that its cross-correlation between the
other channels' code sequences is small, and the spectrum of the code sequence is much larger
than the signal bandwidth. Therefore, it is possible to have an aggregate network capacity
beyond the speed limits of electronics. Like OTDM, CDM requires short pulse technology, and
synchronization to one chip time for detection.
Summarily, in OCDM each optical code (channel) is assigned to its own, independent path
(OCP). Cells with different OCPs can be transmitted in the same fiber at the same time. OCDM
is transparent and no synchronization between different channels is needed. Using OCDM in
addition to WDM can remarkably improve the communication capacity of the network. OCDM
can also be used to improve the communication security.
2.5. WDM
In WDM10, the optical spectrum (low loss region of fibers) is carved up into a number of
smaller capacity channels (Figure 2). Users can transmit and receive from these channels at peak
electronic rates, and many users can use the different channels simultaneously. In this way, the
aggregate network capacity can reach the number of channels times the rate of each channel. In
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order to develop an effective WDM network, each user may be able to transmit and receive from
multiple channels. That is why, the tunable transmitter (laser) / tunable receivers (filter) and/or
multitude of fixed transmitters/receivers are employed at end-nodes.
Figure 2 – Wavelength Division Multiplexing
WDM is an optical version of FDM. As we see, the idea is that several signals are
transmitted at the same time in the same fiber at different wavelengths. This way WDM provides
many virtual fibers on a single physical fiber. Today WDM is the most popular alternative to
multiplex signals in the optical domain. WDM is also the most mature all-optical multiplexing
technique. Its main advantages are the signal transparency, scalability and flexibility; the existing
fiber lines can be upgraded by implementing WDM. The main problems are the need for flat
gain amplifiers, increasing noise when the number of channels increases, and the limits in
channel spacing and in the number of channels caused by wave mixing and cross-phase
modulation (see Chapter 4 for further information).
The key parameters of any multiplexing system are the total capacity of the system, number
of channels, the spectral efficiency and the transmission distance.
WDM is the favorite choice over OTDM, and CDM. This is due to the complex hardware
requirements, and synchronization requirements of OTDM and CDM (synchronization within
one time slot time and one chip time respectively). OTDM and CDM are viewed as a long-term
network solution, since they rely on different and immature technology. Whereas it is possible to
implement WDM systems using components that are already (or will be, in a short time),
available commercially. Moreover, WDM has the inherent property of transparency. Since there
is no electronic processing involved in the network, channels act like independent fibers
(transparent pipes) between the end nodes provided that channel bandwidths are not exceeded.
Once a connection is established between the end-nodes on a WDM channel, the communicating
parties have the freedom to choose the bit rate, signaling and framing conventions, etc. (even
analog communication is possible). This transparency property makes it possible to support
various data formats and services simultaneously on the same network. In addition to this great
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flexibility, transparency protects the investments against future developments. Once deployed,
WDM networks will support a variety of future protocols and bit rates without making any
changes to the network. The most commonly used architectural forms for WDM networks are
WDM Link, Passive Optical Network (PON), Broadcast and Select Networks, and Wavelength
Routing Networks.
2.5.1. WDM Link
To increase information carrying capacity, second generation networks employ parallel fibers for
individual channels. In the WDM Link approach, parallel fibers are replaced by wavelength
channels on a single fiber (Figure 3). In long haul WDM links, all channels are amplified
together by a single wideband optical amplifier (no separate amplifier for each channel), and
existing fibers are used efficiently by integrating more than one channel in a single fiber.
Therefore, WDM link offers a very cost-effective system. The other factors that make WDM
links very popular are the maturity of this technology and its simplicity of integration with
legacy equipment.7
Figure 3 - WDM Link
2.5.2. Passive Optical Network (PON)
The main feature of a PON is to share fiber between the Central Office and Optical Network
Units (ONU) (Figure 4). The PON establishes a tree structure that enables bi-directional
communication between a server (central office) and multiple customers (ONUs) with
centralized control and routing at the central office.
This architecture is a good network choice for regional communication providers. The main
technological problem for the PONs is to design cheap, simple, and durable equipment for the
ONUs. 11
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Figure 4 - Passive Optical Network (PON)
2.5.3. Broadcast and Select Networks
Broadcast and Select Networks offer an optical equivalent to radio systems. In these
networks, each transmitter broadcasts its signal on a different channel, and receivers can tune to
receive the desired signal. In other words, the data is broadcasted at a special wavelength to all
nodes and the receivers accept only certain wavelengths, i.e., data channels, therefore the data is
rejected in those nodes that it does not belong to. Generally, broadcast and select networks are
based on a passive star coupler (Figure 5a)). This device is connected to the nodes by fibers in a
star topology. The signals received at the input ports are evenly distributed to the output ports.
The main networking problem for these networks is the coordination of pairs of stations in order
to agree and tune their systems to transmit and receive on the same channel. The most important
disadvantages of these networks are splitting loss and lack of wavelength reuse. Therefore,
broadcast and select networks are suitable for local area networks, but are not scalable to wide
area networks. 12,13,14
Figure 5 - Broadcast and Select Network: a) star topology; b) bus topology
2.5.3.1. Single-Hop Networks
In single-hop networks data is converted to electrical form only at the end of the path. These
networks can be either circuit or packet switched, and packet switched networks furthermore
connectionless or connection oriented. Single-hop networks can function as packet networks if
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fast tunable receivers and/or transmitters are used. Actually, the first implemented optical packet
networks were single-hop broadcast-and-select networks. 15
2.5.3.2. Multihop Networks
In multihop networks every node has only a few fixed transmitters and receivers. Therefore,
a signal cannot always be directly transmitted from source node to destination node. Instead
signals have to be received at some intermediate nodes along the way, converted to electronic
form and retransmitted. 12,16,17
2.5.4. Wavelength Routing Networks
Wavelength-routed networks currently14 represent the most promising technology for optical
backbone networks. They can be either single-hop or multihop. Wavelength Routing Networks
are composed of one or more wavelength selective nodes called wavelength routers and fibers
interconnecting these nodes. Each wavelength router has a number of input and output ports.
These ports are connected to either end-nodes or other wavelength routers. Each wavelength
router makes its routing decision according to the port and wavelength of the input signal.
Signals routed to the same output port should be on different wavelengths. As long as any two
channels do not share the same fiber link anywhere on the network, they can use the same
wavelength in wavelength routing networks. This wavelength reuse feature results in a
tremendous reduction in the number of wavelengths required for building wide networks.
Depending on design and components in use, a wavelength router may have a variety of
capabilities. For example, its routing matrix may be static or re-configurable, and it may provide
wavelength conversion or not. These features have a direct influence on the operation and
scalability of the network. Therefore, wavelength routing networks are the primary choice for
wide-area all-optical networks.
The network topology (Figure 6) is typically a mesh. In these networks the routing and
switching functions are done on a lower layer called the optical layer. The data transfer is done
using lightpaths. The network is transparent and protocol insensitive and component expenses
are saved because less high layer logic is needed. This network is ideal for circuit switching but
not suitable for packet switching. However, hybrid networks, i.e., networks consisting of both
circuit and packet switching can be implemented.12,18,19,20,21,22
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Figure 6 - Wavelength Routing Network: a) phisical topology; b) virtual topology
2.6. DWDM
The literature often uses the term DWDM (Dense Wavelength Division Multiplexing) in
contrast to regular WDM. This term doesn’t denote a precise operation region or implementation
condition, but, instead, is a historically derived designation. The original use of the WDM was to
upgrade the capacity of installed point-to point-transmission links. Typically, this was achieved
by adding wavelengths that were separated by several tens, or even hundreds, of nanometers, so
that strict requirements would not be imposed on the different laser sources and the receiving
optical wavelength splitters. In the late 1980’s, with the advent of tunable lasers that have
extremely narrow linewidths, one could then have closely spaced signal bands. This is the basis
of DWDM.23
2.7. Summary
The emergence of new optical technologies enables the realization of third generation
networks. That type of computer networks completely avoids the electronics bottleneck
appealing to all optical technology. Between the three techniques of concurrency among multiple
user transmissions introduced, WDM was presented as the favorite choice over OTDM, and
OCDM and the reasons for that have been presented. The most commonly used architectural
forms for WDM networks where also mentioned with particular evidence given to Wavelength
Routing Networks. Finally, a small definition of the new standard – DWDM - is made.
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Chapter 3 - Optical WDM Components
3.1. Introduction
Over the past two decades, the telecommunications industry has witnessed an unprecedented
growth in data traffic and the need for networking. The exploitation of WDM as a networking
mechanism where signals are routed, switched, and selected based on wavelength marks the
dawn of a new era in optical communications.
To enable the exploitation of the new, vast, and versatile wavelength dimension, new
technologies had to be developed and demonstrated. Optical sources and receivers operating at a
fixed wavelength were no longer adequate, and tunable as well as switched sources together with
tunable filters were thus developed. New components that can exercise selection, switching, and
routing based on wavelength were also needed, and as a result, components such as Fiber Bragg
Gratings, arrayed waveguide gratings (AWGs), tunable filters, and wavelength multiplexers and
demultiplexers.
3.2. Optical passive components
3.2.1. Fiber
As a transmission medium fiber has several benefits over copper. The attenuation is lower,
the transfer rates can be higher and there is no electromagnetic interference. Additionally, the
fiber is lighter and stronger than the copper. Naturally the fiber also has some weaknesses, such
as dispersion, nonlinear refraction or attenuation. Yet, compared to other transmission media the
fiber is an attractive alternative. In fact, as early as in the 1980's it was clear that the optical fiber
would be necessary to support high capacity systems.
An optical fiber consists of a core and a cladding surrounding it (Figure 7). Both are made of
pure silica glass, but their refractive indices are different. The use of the optical fiber as a
transmission medium is based on this refractive index difference. Depending on the difference
between refraction indexes and the angle at which the light strikes the interface of two different
transmission media, it either reflects or refracts. If the refraction indexes differ sufficiently, a part
of light confronting the interface is reflected. By controlling the angle at which the light waves
are transmitted and encounter the interface of core and cladding, the proportion of the reflected
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light can be increased and the signal can be contained within the core and guided to the other end
of the fiber.23, 31
Figure 7 - Optical Fiber
The signal traveling in the fiber has an infinite number of possible paths. These paths are
called modes. The optical fiber can be either multimode or single-mode. Multimode fibers have a
core approximately five times larger. Because the propagation time along different paths is
different, multimode fibers suffer from Intermodal Dispersion, i.e. differences in the propagation
times of waves in the fiber. The multimode fiber is simpler to manufacture making it cheaper,
but the single-mode fiber has several advantages. Today multimode fibers are used if a cheap
alternative is needed, whereas the single-mode fibers are used over the longer distances. The
most remarkable problem in using optical fiber is dispersion. The problems resulting from
dispersion increase as the bit rates increase, which make this problem significant. There are three
kinds of dispersion. Chromatic dispersion follows from the fact the color of the light has an
effect on its propagation speed. If the core of the single mode fiber is asymmetric, and the light
beams traveling along different sides of it have therefore different speeds, the polarization mode
dispersion occurs. Slope mismatch dispersion occurs in single-mode fibers, because dispersion
varies with wavelength.
3.2.2. Couplers
Couplers are simple passive optical components which are used to split or combine signals.
A coupler consists of n input and m output ports. A 1 x n coupler is called a splitter and an n x 1
coupler is called a combiner. Figure 8 a) describes a 2 x 2 coupler. In 2 x 2 coupler a part of
input signal 1 is directed to output port 1 and the rest to the output port 2. In a similar way a part
of input signal 2 is guided to both output ports. The fractions directed to output ports can be
either equal or non-equal (Figure 8b)). Couplers function as building blocks of other
components. A coupler can also be used for measurements by separating a small fraction of
signal for this purpose.
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Figure 8 - Optical Couplers: a) equal ; b) non-equal
3.2.3. Isolators and Circulators
Isolators (Figure 9a)) are devices that allow transmission only in one direction and block the
transmissions in the opposite (reverse) direction. This way reflections from, e.g., amplifiers or
lasers can be prevented. Typically the insertion loss, i.e. the loss in the forward direction is
around 1 dB and the isolation, i.e. the loss in the reverse direction, is approximately 40 to 50 dB.
A circulator is a device similar to an isolator, but with multiple ports. Figure 9 b) shows a
circulator with 3 input and output ports. A signal from each port is directed to the next adjoining
port and blocked in all the other ports, as in Figure 9 c). Circulators can be used as a component
in optical add/drop multiplexers and optical cross-connects.34
Figure 9 - a) Isolator; b) Circulator; c) Logical scheme of a three port circulator
3.2.4. Filters
To filter or multiplex channels which are based on wavelengths, one has to separate different
wavelengths from the signal. There are many different ways to do this, but in principle they are
all based on the same idea: some wavelengths are delayed in phase compared to other
wavelengths. This is done by directing them through a longer path.
The key parameters of filters are insertion loss and passband flatness. Insertion losses should be
low and independent of polarization and temperature. Passband should be flat and passband
skirts should be as sharp as possible. Figure 10 shows a signal that illustrates this situation. The
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flatter the passband is and sharper the passband skirts are, the smaller crosstalk energy passing
through the adjacent channels is.33
Figure 10
3.2.4.1. Tunable 2 x 2 directional couplers
Tunable 2 x 2 directional couplers have multiple control electrodes placed on the coupling
waveguides. Figure 11 illustrates a multielectrode asymmetric directional coupler fabricated on a
LiNbO crystal, where one arm is thinner than the other. For a wavelength-dropping application
in this device, M wavelengths enter input port 1. Applying a specific voltage to the electrodes
changes the refractive index of the waveguides, thereby selecting one of the wavelengths, say λi,
to be coupled to the second waveguide, so that it exits through port 4. The remaining M - 1
wavelengths pass along the device and leave through port 3. To insert a wavelength and combine
it with an input stream entering port 1, one inserts λi into port 2, so that it couples across to the
top waveguide. Thus, it exits port 3 along with the other wavelengths λ1, … , λi-1, λi, λi+1, λM that
entered port 1.24
Figure 11 – Concept of a tuneable multielectrode asymmetric directional coupler
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3.2.4.2. Gratings
Gratings work based on the principle of diffraction where a signal that is guided to a small
hole spreads into many directions. Because one path the wavelength travels is longer than the
other path traveled, it is added in phase. Fiber Bragg gratings (Figure 12) are modified pieces
of fiber that reflect one wavelength and pass through all the others. They are simple to fabricate
and use and they have low insertion losses.34 (See Chapter 5 for further information about Fiber
Bragg Gratings)
Figure 12 - Fiber Bragg Gratings
3.2.4.3. Arrayed waveguide grating (AWG)
In arrayed waveguide grating (AWG) (Figure 13) the idea is that signals with many
wavelengths is copied to several fibers with different lengths and on each fiber all but one
wavelength are rejected. In principle the idea is the same as in gratings: different wavelengths
get different phases, by being delayed differently. AWGs are promising devices. They can be
used in integrated optical circuits, while they can be easily combined with other functions.
Additionally, they can be used as both a multiplexer and a demultiplexer. The disadvantage is
their high temperature coefficient.25,34
Figure 13 - Array Waveguide Grating
3.2.4.4. Fabry-Perot Tunable Filters (FPF)
Fabry-Perot interferometer Filters (Figure 14) are micro-optic cavities in which two mirrors
are placed parallel to each other. When a signal meets a mirror, a part of it continues through and
the other part is reflected. The reflected signal travels straight to the other mirror and is guided
back. It then travels through the same mirror wavelength 1 has already gone and will be added in
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phase. The advantage of Fabry-Perot filters compared to many other filter types is their ability to
be tuned to filter different wavelengths.25,34
Figure 14 - Operational setup of a Fabry-Perot Tunable Filter
3.2.4.5. Mach-Zehnder Tunable Filters (MZF)
Whereas the Fabry-Perot interferometer Filters involves light interference by many repeated
reflections, a single Mach-Zehnder Interferometer (MZF) (
Figure 15 a)) involves interference by only two versions of the same light transversing paths
of slightly different length. The multichannel input signal is split into two equal parts by a 3 dB
coupler. The two versions of the same signal traverse paths of slightly different lengths and
merge together in another 3 dB coupler at the output.
Figure 15 - Mach-Zehnder Interferometer; b) Three Mach-Zehnder Chain
The real advantage of the MZF, which usually are used in MZF chains (
Figure 15b)), is that the filter can be realized using lithographic technology, leading to
potentially low fabrication costs. Also, by designing a square waveguide cross-section, these
filters can be made polarization insensitive. The main disadvantages are the slow tunning speed
due to thermal inertia (a few miliseconds) and the complexity of the multistage tunning
control.,25,34
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3.2.4.6. Liquid-crystal Fabry-Perot filters
Liquid-crystal Fabry-Perot filters are based on the use of high-speed electro clinic liquid
crystals inside a Fabry-Perot cavity. In this case, the liquid crystal is positioned between the two
fiber end faces, and thus becomes part of the Fabry Perot cavity. These filters can be widely
tuned by appiying a voltage across the crystal, which changes the refractive index, and hence the
optical path length, in the cavity material.26, 27
3.2.4.7. Acousto-optic tunable filters
Acousto-optic tunable filters (AOTFs) operate through the interaction of photons and
acoustic waves in a solid such as lithium niobate. Figure 16 shows the basic operation. Here, an
acoustic transducer, which is modulated by radio-frequency (RF) signal, produces a surface
acoustic wave in the LiNbO crystal. This wave sets up au artificial grating in the solid, the
grating period being determined by the frequency of the driving RF signal. More than one
grating can be produced simultaneously by using a number of different driving frequencies.
Input wavelengths that match the Bragg condition of the gratings are coupled to the second
branch of the AOTF, while the other wave lengths continue on through.28,29,30
Figure 16 - Basic acousto-optic tunable filter
3.2.5. Multiplexers
3.2.5.1. WDM Multiplexers and Demultiplexers
Multiplexer is a device which combines several signals with different wavelengths to one
fiber. Respectively a demultiplexer gets one signal as an input and by separating the different
wavelengths from the fiber, assorts each wavelength to its own output fiber. The purpose of these
devices is to increase the capacity of a fiber by increasing the number of channels per fiber from
one to hundreds.
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3.2.5.2. Add/Drop Multiplexers (OADMs)
Add/drop multiplexers are devices that are used for adding or removing single wavelengths,
i.e. channels, from a fiber without disturbing the transmission of other signals. Optical add/drop
multiplexers are widely used in WDM networks. 25 (See Chapter 6 for further information)
3.2.5.3. Optical Cross-Connects (OXCs)
An Optical Cross-connect is a device used in optical switching. An optical channel at one of
the input pots of the OXC could be sent to any of the output ports according to the network
switching requirements. As the OADMs, OXCs are a key component in the AONs.25 (see
Chapter 6 for further information)
3.3. Optical active components
3.3.1. Amplifiers
A signal attenuates while it travels through an optical fiber or through optical components. In
order to travel over long distances the signal has to be amplified. Earlier the signals were
converted to electrical form and regenerated. Today there are optical amplifiers and the signal
can be transmitted over longer distances without conversion to electric form. Compared to
regenerators, optical amplifiers are more flexible to changes in the bit rate. Additionally, they
can be used to amplify several wavelengths at the same time.31
3.3.2. Erbium-doped fiber amplifiers (EDFAs)
Erbium-doped fiber amplifiers (EDFAs) consist of a strand of fiber doped with Erbium
atoms, pumping devices and simple optical components. They are all optical devices. The
operation of doped amplifiers is based on the stimulated emission (Figure 17). The signal gets
more energy if there are more transitions from a higher energy level to a lower than from the
lower to the higher. The energy can be pumped to fiber by giving Erbium atoms more energy
and lifting electrons to higher energy state. When electrons fall to lower level, energy is released
and the signal is amplified.
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Figure 17 - Operational Principle of an EDFA with a: a) 1480 nm pump laser; b) 980 nm pump laser
For a long time the major problem with amplifying was the fact that different wavelengths
were amplified with different gains. With EDFAs this is not a remarkable problem as the gain is
relatively flat. Erbium-doped amplifiers were the first doped amplifiers constructed.31,34
3.3.2.1. Semiconductor Optical Amplifiers
Semiconductor optical amplifiers (SOAs) (Figure 18) are based on stimulated emission
similarly as EDFAs. The difference is that instead of energy levels of dopant atoms, the process
is based on electrons and electron holes in the semiconductor. The amplifier consists of two
semiconductors separated with a band gap.
Figure 18 – Semiconductor Optical Amplifier (SOA)
Actually, SOAs are not as good amplifiers as EDFAs. SOAs have wider bandwidth, i.e. they
can be used to amplify more wavelengths. However, their output power is weaker, there is more
polarization and coupling losses and they suffer from crosstalk. Still, there is a lot of interest for
SOAs at the moment. SOAs are small compared to EDFAs which makes them suitable for
building blocks for other devices, e.g. switches and wavelength converters.34
3.3.2.2. Raman Effect Amplifiers
A Raman amplifier is not a compact device in the same way as the other amplifiers described
previously. The amplification process (Figure 19) happens slowly in the transmission fiber over
several kilometers. The main idea is that a light beam with lower wavelength and thus higher
energy is guided to the same fiber with the signal to be amplified. The light beam with higher
energy then delivers energy to the signal and signal is therefore amplified. Gain flatness is a
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critical issue with Raman Effect Amplifiers and several pumping bandwidths are needed to get
the desired result.32,34
Figure 19 - Raman Amplification
3.3.3. Transmitters
3.3.3.1. Light-Emitting Diodes
If the distance is short and the data transmission rate is low a cheap device called a light-
emitting diode (LED) can be used as a light source instead of laser. LEDs have broad passband
and low output power. A typical output power of LED is -20 dBm, which is low compared to 0 -
10 dBm output power of lasers.33
3.3.3.2. Lasers
Figure 20 illustrates the stimulated emission. A photon interacts with the atom that is in the
higher energy state. Then the electron drops down to the lower energy state and an equal amount
of energy is released. Because of the stimulating photon the new photon produced has exactly
the same direction, phase and wavelength as the first one. The stimulated emission is a necessity
for the operation of a laser.
Figure 20 – Stimulated Emission
Figure 21 - General structure of a laser
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Additionally, a source of energy, a cavity filled with suitable material for the emission and
two mirrors are needed (Figure 21). The material can be solid, liquid or gas. It is suitable for a
particular laser if the energy difference between the lower and the higher state of the atom is
correct such that a photon with desired wavelength could be produced by stimulated emission.
To produce a coherent light beam with the laser, energy is guided to cavity filled with the
particular material. The electrons are then elevated to the higher energy state and photons are
emitted spontaneously. Most photons emitted exit the cavity through the walls of the cavity.
Some of them, however, confront the partial mirror with certain angle and are reflected back to
the cavity. When this kind of reflected photon encounters an electron in the higher energy state
stimulated emission happens and a new photon with exactly the same direction, phase and
wavelength is produced. These photons continue toward the other mirror, stimulate new photons
and are reflected back. When the photons meet the partial mirror, a part of them continue
through and a part is reflected and stimulates new photons. Soon there are plenty of photons
traveling in the cavity, and a great amount of light is also directed through the partial mirror.
In addition to the mentioned conditions, the length of the cavity has to be a multiple of half
the wavelength of the output beam. Therefore the output light consists of only a limited number
of wavelengths. However, the output beam of the lasers should be as narrow as possible and to
improve the laser all these wavelengths but one should be rejected. This can be done by filtering
or by using external cavity. Another alternative is to use a very short cavity. Today simple
Fabry-Perot lasers (Figure 22 a)), as the one discussed previously, are often replaced with
distributed feedback (DFB) lasers (Figure 22b)). In these lasers the width of the cavity has
periodic changes and the beam is reflected several times back and forth over short distances.
This device can work as designed to be a narrow band single mode laser.
Figure 22 - a) Fabry Perot Laser b) DFB Laser
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Another remarkable improvement to Fabry-Perot laser is the tunability. External cavity lasers
can function as slow tunable lasers, if the length of the other cavity can be changed. Usually this
is done by moving the mirror or grating which forms the external cavity. A faster way is to
chance the bias current, which chances the refractive index. The disadvantage of this approach is
variation in the output power. Another possibility is to use two independent currents, one for
controlling the output power and the other for controlling the wavelength of the laser.34, 35
3.3.4. Receivers
There are many ways of converting incident light into a current or a voltage, and the general
class of devices that do so are photodetectors. In all different types of photodetectors, only the
semiconductor photodiode subclass has proved to have the right combination of sensitivity, low
noise, small size, low cost, and high speed of response to serve satisfactorily in a fiber optic
communication system. The internal processes by which they convert a photon flux into a
current are exactly the reverse of the processes of a light semiconductor LED or a Laser diode.
There are three types of semiconductor photodiodes: PN diodes, PIN diodes and avalanche
photodiodes (APDs).33,3
3.3.4.1. PIN diodes
At one time PN photodiodes were the most prevalent detection device used in lightwave
systems, but PIN devices have now superseded these. These photodiodes are called pin
photodiodes because the material is made up of p-type, intrinsic (lightly doped intrinsic
semiconductor material), and n-type material. In such a device, for every single photon incident,
a single electron will rise to its excited state. This will be satisfactory for most short-range and
low bit-rate systems. However, if a signal has been weakened significantly, then a more
advanced type of detector may be required to detect it.34,36
3.3.4.2. Avalanche photodiodes (APDs)
One option would be to use an “avalanche photodiode” (APD), which is a discrete
semiconductor device like the PIN photodiode. It differs, however, in that for every incident
photon, it can generate several excited electrons – as many as 100. Therefore the signal is
boosted many times over and so lower optical powers can be detected successfully. An APD
operates at a much higher voltage than a PIN and is designed so that a photon moves an electron
to its excited state with enough energy to cause further electrons to be excited also. These extra
electrons can themselves cause further electrons to rise to their excited states, and so there is a
chain reaction process – or an “avalanche.”
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Unfortunately, APDs are not without their problems. First, the amount of amplification is
limited. If too much amplification is provided, the device will “run away” and simply provide an
essentially continuous (large) current. Second, APDs are noisy because thermal action can
randomly promote an electron to the conduction band. The current resulting from this thermal
action is noise. And finally, the avalanche action takes time to occur, meaning that the device has
a certain response time. APDs have been used a great deal in the network but at the highest
speeds, the equipment providers tend to use pin diodes with optical preamplifiers.34,36
3.3.5. Wavelength Converters
Wavelength converters are used for converting the wavelength of an incoming signal to a
different wavelength. They can be used for instance as a part of a switch or cross-connect or in
3R regenerators. 3R regenerators are devices that regenerate signals amplitude and regenerate the
signal also in time and frequency domains. The 3R regeneration principle is shown in the Figure
23.
Figure 23 - 3R Regeneration principle
Wavelength converters can be optoelectronic, or based on optical gating or wave mixing.
Optoelectronic converters are devices which convert signal to electronic form, regenerate and
retransmit it. The problem with optoelectronic converters is that they require fixed data rate and
format. The two other possibilities are discussed here in more detail.25
3.3.6. Optical Gating
There are two possibilities to use optical gating: cross-gain modulation and cross-phase
modulation (CPM). The first is based on the fact that the gain of a SOA dependents on input
power. A low power probe signal with desired wavelength is sent into the SOA. Because the
probe signal has a low gain compared to the input signal, it will experience high gain when the
gain of the input signal is high (state 1) and low gain when gain to the input signal is zero (state
0).
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In cross-phase modulation (Figure 7) the phase of the probe signal is changed on basis of the
input signal. The phase modulation is then converted into intensity modulations by using
interferometer. Cross phase modulation is the more attractive alternative of these two. It requires
less power and it has a better extinction ratio. Additionally, pulse distortion, from which cross-
gain modulation suffers, is not a problem but this feature is the operational principle of the
device.37,38
Figure 24 - a) Mach-Zehnder interferometer and b) Michelson Interferometer configurations using pairs of SOAs for implementing CPM wavelength conversion scheme
3.3.7. Wave Mixing
In wave mixing the idea is to construct a desired signal by using probe signals with such
wavelengths that together with the input signal they form another signal with desired
wavelength. The advantage of this approach is the transparency, and the disadvantage is the fact
that there are additional signals in the output and these signals have to be filtered out. [39,40,41] (for
further information in four wave mixing see Chapter 4)
3.4. Summary
In this chapter an overview in optical WDM components has been made. The main
characteristics and functioning principles of the active and passive components had been
described briefly. From all, the special evidence went for optical fiber, gratings, circulators,
isolators, OADMs, OXCs, EDFAs, Leds and Lasers because those have been used in this project
as will be described in Chapter 7.
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Chapter 4 - Signal Degradation on Optical Networks
4.1. Introduction
The attractiveness of lightwave communications is the ability of silica-optical fibers to carry
large amounts of information over long repeaterless spans. To utilize the available bandwidth,
numerous channels at different wavelengths can be multiplexed on the same fiber. To increase
system margins, higher transmitter powers or lower fiber losses are required. All these attempts
to fully utilize the capabilities of silica fibers will ultimately be limited by attenuation, dispersion
and nonlinear interactions between the information-bearing lightwaves and the transmission
medium. The optical nonlinearities can lead to interference, distortion, and excess attenuation of
the optical signals, resulting in system degradations. At the system level, and with the dense
packing of channels, attention was given to another WDM impairments such as channel
crosstalk.
4.2. Attenuation
Attenuation, also known as fiber loss or signal loss, is one of the most important properties of
an optical fiber, because it leads to a reduction of the signal power as the signal propagates over
some distance. When determining the maximum distance that a signal can propagate for a given
transmitter power and receiver sensitivity, one must consider attenuation. So, attenuation largely
determines the maximum unamplified or repeaterless separation between a transmitter or a
receiver. Since amplifiers and repeaters are expensive to fabricate, install and maintain, the
degree of attenuation in a fiber has large influence on the system cost.23,42,43
Figure 25 – Optical fiber attenuation
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4.3. Dispersion
Dispersion is the widening of a pulse’s duration as it travels through a fiber. As a pulse
widens, it can broaden enough to interfere with neighboring pulses (bits) on the fiber, leading to
intersymbol interference (ISI), thereby create errors in the receiver output. Dispersion thus limits
the bit spacing and the maximum transmission rate on a fiber-optic channel.42,43
Figure 26 - Intersymbol Interference (ISI)
One form of dispersion is intermodal dispersion. This is caused when multiple modes of the
same signal propagate at different velocities along the fiber. Intermodal dispersion does not
occur in a single-mode fiber.
Another form of dispersion is material or chromatic dispersion. In a dispersive medium, the
index of refraction is a function of the wavelength. Thus, if the transmitted signal consists of
more than one wavelength, certain wavelengths will propagate faster than other wavelengths.
Since no laser can create a signal consisting of an exact single wavelength, material dispersion
will occur in most systems.
A third type of dispersion is waveguide dispersion. Waveguide dispersion is caused because
the propagation of different wavelengths depends on waveguide characteristics such as the
indexes and shape of the fiber core and cladding.
At 1300 nm, material dispersion in a conventional single mode fiber is near zero. Luckily,
this is also a low attenuation window (although loss is lower at 1550 nm). Through advanced
techniques such as dispersion shifting, fibers with zero dispersion at a wavelength between
1300–1700 nm can be manufactured. In a dispersion shifted fiber, the core and cladding are
designed such that the waveguide dispersion is negative with respect to the material dispersion,
thus canceling the total dispersion. The dispersion will only be zero, however, for a single
wavelength.44
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4.4. Nonlinearities
Nonlinear effects in fiber may potentially have a significant impact on the performance of
WDM optical communications systems. Nonlinearities in fiber may lead to attenuation,
distortion, and cross-channel interference. In a WDM system, these effects place constraints on
the spacing between adjacent wavelength channels, limit the maximum power on any channel,
and may also limit the maximum bit rate.
There are two categories of nonlinear effects: Kerr effects and scattering effects. The first
consists of three phenomena. In an optical fiber the core in which the optical signals travel has a
specific refractive index that determines how light travels through it. However, depending upon
the intensity of light traveling in the core, this refractive index can change. This intensity-
dependence of refractive index is called the Kerr effect. It can cause Self-phase modulation
(SPM) of a signal, whereby a wavelength can spread out onto adjacent wavelengths by itself. It
can also cause cross-phase modulation (XPM) whereby several different wavelengths in a WDM
system can cause each other to spread out. Finally, it can result in Four-wave mixing (FWM) in
which two or more signal wavelengths can interact to create a new wavelength.
There are two nonlinear scattering effects. “stimulated Raman scattering” involves light
losing energy to molecules in the fiber and being re-emitted at a longer wavelength (due to the
loss of energy). In “stimulated Brillouin scattering” light in the fiber can create acoustic waves,
which then scatter light to different wavelengths.42,43,45,46
4.5. Kerr effects
4.5.1. Self-phase modulation (SPM)
SPM is caused by variations in the power of an optical signal and results in variations in the
phase of the signal. In phase-shift-keying (PSK) systems, SPM may lead to a degradation of the
system performance since the receiver relies on the phase information. SPM also leads to the
spectral broadening of pulses, as explained below. Instantaneous variations in a signal’s phase
caused by changes in the signal’s intensity will result in instantaneous variations of frequency
around the signal’s central frequency. For very short pulses, the additional frequency
components generated by SPM combined with the effects of material dispersion will also lead to
spreading or compression of the pulse in the time domain, affecting the maximum bit rate and
the BER.42,43,45
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Figure 27 - Spectral broadening due to SPM
4.5.2. Crossphase modulation (XPM)
XPM is a shift in the phase of a signal caused by the change in intensity of a signal
propagating at a different wavelength. XPM can lead to asymmetric spectral broadening, and
combined with SPM and dispersion may also affect the pulse shape in the time domain.
Although XPM may limit the performance of fiber-optic systems, it may also have
advantageous applications. XPM can be used to modulate a pump signal at one wavelength from
a modulated signal on a different wavelength. 47,48
4.5.3. Four-Wave Mixing (FWM)
FWM occurs when two wavelengths operating at frequencies f1 and f2, respectively, mix to
cause signals at 2f1-f2 and 2f2-f1. These extra signals, called sidebands, can cause interference if
they overlap with frequencies used for data transmission. Likewise, mixing can occur between
combinations of three or more wavelengths. Using unequally spaced channels can reduce the
effect of FWM in WDM systems. FWM can be used to provide wavelength conversion. 49,50,51
Figure 28 – FWM: The mixing of f1 and f2 generate sidebands
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4.6. Scattering effects
4.6.1. Stimulated Raman Scattering (SRS)
SRS is caused by the interaction of light with molecular vibrations. Light incident on the
molecules creates scattered light at a longer wavelength than that of the incident light. A portion
of the light traveling at each frequency in a Raman-active fiber is downshifted across a region of
lower frequencies. The light generated at the lower frequencies is called the Stokes wave. The
range of frequencies occupied by the Stokes wave is determined by the Raman gain spectrum,
which covers a range of around 40 THz below the frequency of the input light. In silica fiber, the
Stokes wave has a maximum gain at a frequency of around 13.2 THz less than the input signal.
The fraction of power transferred to the Stokes wave grows rapidly as the power of the input
signal is increased. Under very high input power, SRS will cause almost all of the power in the
input signal to be transferred to the Stokes wave.
In multiwavelength systems, the channels of shorter wavelength will lose some power to
each of the higher wavelength channels within the Raman gain spectrum. To reduce the amount
of loss, the power on each channel needs to be below a certain level.42,43,45,46
Figure 29 - SRS transfers optical power from shorter wavelengths to longer wavelengths
4.6.2. Stimulated Brillouin Scattering (SBS)
SBS is similar to SRS except that the frequency shift is cause by sound waves rather than
molecular vibrations. Other characteristics of SBS are that the Stokes wave propagates in the
opposite direction of the input light, and SBS occurs at relatively low input powers for wide
pulses (greater than 1µs) but has negligible effect for short pulses (less than 10 ns). The intensity
of the scattered light is much greater in SBS than in SRS but the frequency range of SBS, on the
order of 10 GHz, is much lower than that of SRS. Also, the gain bandwidth of SBS is only on the
order of 100 MHz.
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To counter the effects of SBS, one must ensure that the input power is below a certain
threshold. Also, in multiwavelength systems, SBS may induce cross talk between channels.
Cross talk will occur when two counterpropagating channels differ in frequency by the Brillouin
shift, which is around 11 GHz for wavelengths at 1550 nm. The narrow gain bandwidth of SBS,
however, makes SBS cross talk fairly easy to avoid.42,43,45,46
4.6.3. Conclusions on nonlinear effects
Nonlinear effects in optical fibers may potentially limit the performance of WDM optical
networks. Such nonlinearities may limit the optical power on each channel, limit the maximum
number of channels, limit the maximum transmission rate, and constrain the spacing between
different channels.
The details of optical nonlinearities are very complex and beyond the scope of this report.
They are a major limiting factor in the available number channels in a WDM system, however,
especially those operating over distances greater than a few dozen kilometers. The existence of
these nonlinearities suggests that WDM protocols that limit the number of nodes to the number
of channels do not scale well.
4.7. Crosstalk
The narrow channel spacing in dense WDM links give rise to crosstalk, which is defined as
the feedthrough of one channel’s signal into another channel. Crosstalk can be introduced by
almost any WDM component, including optical filters, wavelength multiplexers and
demultiplexers, optical switches, optical amplifiers and by the fiber itself. Crosstalk from
neighbouring inputs is a fundamental difficulty of wavelength routing which cause severe
degradation in system performance. Nonlinear crosstalk is that induced by fiber nonlinearities
refereed in previous section. Linear crosstalk can be classified into two categories, depending on
its origin.42,43,52,53,54,55
4.7.1. Heterodyne crosstalk
Optical filters and demultiplexers often leak a fraction of the signal power from neighbour
channel operating at a different wavelength that interferes with the detection process, resulting in
noise addiction on the detector. Such incoherent crosstalk is called heterodyne crosstalk or
interchannel crosstalk. Figure X shows an example of the origin of heterodyne crosstalk in a
WDM component, case of a demultiplexer.56
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Figure 30 - Heterodyne crosstalk in a WDM system
4.7.2. Homodyne crosstalk
Another case of crosstalk is the Homodyne crosstalk or intrachannel crosstalk, due to is
coherent nature is far more penalizing that the Heterodyne crosstalk. This kind of crosstalk
usually occurs on wavelength routing where already exists a leakage signal at the same
wavelength due to incomplete filtering. Figure X shows an example of the origin of homodyne
crosstalk in a WDM component, case of an optical switch.56
Figure 31 - Homodyne crosstalk in a WDM system
4.7.3. Conclusions on crosstalk
The power penalties from Heterodyne and Homodyne crosstalk for WDM channels are
function or the individual crosstalk level. Figure 32 illustrates that fact for 8 and 16 WDM
channels. Here each channel contributes an equal amount of crosstalk power. This shows that the
Homodyne crosstalk effect is more severe, since it falls completely within the receiver passband.
Figure 32 -Power penalties from Heterodyne and Homodyne crosstalk for 8 and 16 WDM channels
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4.8. Summary
Impairments in optical networks such as attenuation, dispersion, non-linearities due to Kerr
and scattering effects, and crosstalk have been discussed. The attenuation, is the most important
property in a optical fiber, in fact it is in the low attenuation region (near from 1550 nm) that the
WDM techniques are actually been used. Three dispersion types were also analyzed ant it was
said that it is the widening cause of an optical pulse when it travels through a fiber. Nonlinear
effects in fibers, such as the three Kerr effects and the two scattering effects also potentially limit
the performance of WDM networks. Finally a discussion in the two crosstalk types evidences the
Homodyne crosstalk effect as the worst of the impairments in WDM system devices.
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Chapter 5 - Physical devices used in this project – Fiber Bragg
Grating’s
5.1. Introduction
The fiber optic filter based on Bragg gratings is the main component in this project and it
would be appropriate to introduce some of the fundamental concepts of these devices. The
following is a description of the photosensitivity effect responsible for the formation in the fiber
of permanent diffraction structures. These structures are what make this device such an effective
wavelength filter.
5.2. History of photosensitivity and fiber Bragg gratings
The first observations of refractive index changes were observed in germanosilica fibers and
were reported by Hill and co-workers at the CRC – “Communication Research Center – Canada”
in 197857. These observations were made during a test to study the non-linearities of a special
type of germanosilica fiber. While injecting visible light from a 488 nm Argon laser into the core
of the fiber the attenuation kept increasing and as a consequence of this the intensity of reflected
light from the fiber also increased. This increase in the reflectivity was a consequence of an
optical diffraction pattern that had been formed in the fiber’s core due to the photosensitivity
characteristics of the fiber.
Discovery of this characteristic of optical fibers brought forth a whole new field in optical
fiber investigation. A new era in terms of fiber optics based devices had begun. In combination
with other types of diffraction gratings, Bragg gratings permit the design of complex in-fiber
devices like resonant cavities, pass-band filters and wavelength multiplexing devices. New and
exciting structures based in fiber Bragg gratings such as optical sensors, Fabry-Perot type Bragg
gratings used in pass-band filters, non-uniform diffraction pattern gratings used in dispersion
compensation have been studied and developed58. Many more structures based on the same
principle are always appearing as a consequence of the intense investigation being developed in
this field.
Although this discovery was extremely important, after the initial discovery, the advances
were slow for almost 10 years. Diffraction gratings were initially all self-induced. This means
that they were formed without human intervention or control. This led to a lack of applicability
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of these components because the only wavelength that could be reflected was that of the laser
that created the diffraction pattern initially. Lasers used at that time were Argon ion lasers at 488
nm. Some adjustments can be made through mechanical stress during the fabrication process but
this adjustment does not bring enough flexibility to the device because adjustments up to the
infra-red band (used in telecommunications applications) were not possible. Moreover, in the
blue-green wavelength regions, these gratings are intrinsically unstable owing to the continuing
photosensitivity of the fiber. This causes the grating to continually evolve during its use as a
Bragg reflector. The grating can even disappear completely if it is exposed to light of a different
blue-green wavelength. The instabilities reported and other problems with self-induced gratings
made the use of these devices impossible in optical communications or sensing applications.
5.3. Photosensitivity in optical fibers
Photosensitivity in optical fibers can be defined as the maximum refraction index change that
can be induced in an optical fiber through exposure to UV (ultra-violet) light59. A well known
characteristic of optical fibers is the high absorption verified for wavelengths in the UV band
(<300 nm). In fact, at these wavelengths the photon energy is considerable and electronic links
resonance is seen. UV exposure is a single photon process and as such, the induced index
alteration is ~6 orders of magnitude higher than the one verified in exposure with visible light.
Fibers with high germanium doping proved to be highly photosensitive. Theoretical models (still
not proven) that explain this phenomenon state that the defects introduced by the germanium in
the crystalline structure of the fiber are responsible for the achieved photosensitivity. Recently,
new techniques were developed that increase the photosensitivity of any type of fiber. This is
important because a standard fiber with germanium doping does not have a sufficient doping
profile as to obtain enough photosensitivity to inscribe the diffraction pattern correctly. Among
these techniques we have hydrogenation or hydrogen loading, flame brushing, and boron
codoping. The first technique (hydrogenation) is the most widely implemented. Hydrogen
molecules are diffused into the fiber core and penetrate the crystalline structure at high pressure
and temperatures. The presence of these molecules makes the fiber much more sensitive to
refractive index changes due to the exposure to UV light. It should be noted that the increased
fiber/waveguide photosensitivity as a result of hydrogen loading is not a permanent effect, and as
the hydrogen diffuses out, the photosensitivity decreases.
The other processes are also valid as means of increasing the photosensitivity of optical
fibers and have their advantages but hydrogenation is considered the best in terms of final results
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for telecommunications applications. There are several advantages of enhancing fiber
photosensitivity though hydrogenation. The first and foremost is that it allows strong Bragg
gratings to be fabricated in any type of germanosilica fiber, including the standard
telecommunications fibers that typically have low germanium concentration, and hence, low
intrinsic photosensitivity. Second, permanent changes occur only in regions that are UV
irradiated. Finally, unreacted hydrogen in other sections of the fiber slowly diffuses out. Thus
leaving negligible absorption losses at the optical communication windows.
The physical mechanisms through which fibers get their refractive index changed are not
completely explained yet. Three models have been reported in the literature that try to describe
this phenomenon but it would become too cumbersome to describe them here in light of this
project’s objectives, but suitable references are provided for those that wish to analyze this
subject further60,61.
5.4. Fiber Bragg grating properties
5.4.1. Physical properties
In its simplest form, a fiber Bragg grating consists of a periodic modulation of the refractive
index in the core of a single-mode optical fiber (See Figure 33). These are uniform fiber gratings,
where the phase fronts are perpendicular to the fiber longitudinal axis and the grating planes are
of a constant period. They considered the fundamental building blocks for most for most Bragg
grating structures. Light guided along the core of an optical fiber will be scattered by each
grating plane; if the Bragg condition is not satisfied, the reflected light from each of the
subsequent planes becomes progressively out of phase and will eventually cancel out. Where the
Bragg condition is satisfied, the contributions of reflected light from each grating plane add
constructively in the backward direction to form a back-reflected peak with a center wavelength
defined by the grating parameters.
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Figure 33 - Schematic representation of a Fiber Bragg Grating.
The reflected wavelength is given by the following equation:
Λ= 02nBλ Equation .1
The Bragg relationship in its differential form is given by:
Λ
∆Λ+∆
=∆0
0
nn
BB λλ ; Equation .2
These equations state that any measurable quantity applied to the grating that causes a
refractive index change or period change, induces a deviation in the resonant wavelength. This is
one of the key features of these devices that will be further explained later on in this report. The
sensitivity of the gratings with temperature is a consequence of thermal expansion of the silica
matrix and thermal dependence of the refractive index. Thus, for a temperature deviation T∆ ,
the correspondent deviation in wavelength is given by:
( ) TTTn
nT BBB ∆+=∆
∂∂+
∂Λ∂
Λ=∆ ξαλλλ 11 Equation .3
Where α and ξ are respectively, the thermal expansion coefficient and the thermo-optical
coefficient. In the case of silica, the thermal expansion coefficient has an absolute value of 161055.0 −−× K and the thermo-optical coefficient a value of 16100.8 −−× K . This means that, the
change in the reflected wavelength as a result of temperature variations is dominated by the
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change in the refractive index. On the other hand, the mechanical stress sensitivity comes
simultaneously from deformation of the silica matrix and alteration of the refractive index due to
the photo-elastic effect. The resulting change in the resonant wavelength through mechanical
strain, for a longitudinal deformation ε∆ , is given by:
( ) ελεεε
λλ ∆−=∆
∂∂+
∂Λ∂
Λ=∆ eBBB pn
n111 Equation .4
where pe represents the photo-elastic constant of the fiber’s material. In the case of silica, this
constant has an approximate value of 0.22.
5.4.2. Spectral response of fiber Bragg gratings
As scientists strive to find a suitable model to describe the physical phenomenon behind the
formation of diffraction gratings, the flexibility of a fiber Bragg grating in terms of filtering
dictated the need to model their spectral characteristics.
Theoretical models have been established that relate the spectral dependence of a fiber
grating and the corresponding grating structure. The need to establish a model for the physical
process that relates the variation of refractive index in the fiber’s core as it is exposed to UV
radiation is associated to the mathematical description of the spatial distribution of that same
variation. The theory presently accepted describes the refractive index spatial variation in terms
of modulation of the refractive index, ),,( zyxn∆ in the fiber’s core. This theory is incomplete
and makes use of spatial coordinates. Besides this theoretical inability, the experimental
verification of the fringes contrast over the interference pattern is very difficult, particularly
when using a pulsed source. So, obtaining ),,( zyxn∆ from the spectral response of a diffraction
grating can’t be done without imposing some hypothesis and additional approximations that
introduce some limitations in the model. As such, it is supposed that the interference pattern is
perfectly sinusoidal and the fiber’s reaction to UV exposure is linear in behavior. Additionally,
the germanium concentration is considered uniform and the fringes are equally spaced. The
intensity profile of these fringes and their visibility are constant over the grating’s length.
Considering all of these restrictions, the refractive index profile is given by:
)cos()( 0 zKnnzn ⋅∆+= Equation .5
Where n0 is the mean value of the refractive index and K is the grating’s associated vector,
which is orthogonal to the index modulation planes. The amplitude is given by Λ/2π , where
Λ is the distance between consecutive planes.
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Incoming light with a propagation vector Ki is deviated towards the diffraction wave vector
Kd=Ki-K. If the diffracted propagation vector matches the one of the incoming wave (forward
propagating wave), a strong Bragg diffraction occurs following Kd; otherwise, the efficiency of
the diffraction is reduced.
The wavelength for guided light within the core of a single mode fiber, that verifies this
resonance condition is given by the first order Bragg condition.
Λ= 02nBλ . Equation .6
The reflectivity of the diffraction grating is given by:
∆=B
VnLRλ
ηπ )(tanh 2 . Equation .7
In the previous equation Bλ is the resonant wavelength of the grating, L is the grating’s
length and )(Vη is the overlapping coefficient between the LP01 guided mode and the index
modulation. This equation is obtained through simplification of the equations explained later on
when the Coupled Mode Theory is explained. The reflectivity depends on two important factors:
the number of modulation planes Λ= /LN , and the modulation magnitude of the n∆ index. In
practice, the magnitude and modulation period of the refractive index are not rigorously constant
over the grating’s length. This means that the obtained value for n∆ is, in fact, the mean value of
)(zn∆ over the grating’s total length. Incoming light is partially reflected in each of the planes of
the diffraction grating. If the Bragg condition is not satisfied, the fractions of light reflected
become more and more out of phase. Balance between the total length and the exact length that
verifies the phase condition determines the width of the grating’s spectral response. The
expression that is used to calculate the Full Half Width Maximum (FHWM) of the spectral
response with these values is a first order approximation, given by:
22
0
12
+
∆=∆Nn
nsBλλ . Equation .8
The parameter s tends to unity in case of reflectivity values near 100% and tends to 5.0≈ for
low reflectivity gratings.
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5.4.3. Coupled Mode Theory
The spectral response of a diffraction grating can be described using the coupled mode
theory62. It is an accurate model for obtaining quantitative information about the diffraction
efficiency and spectral dependence of fiber Bragg gratings. This is not the only theory available
but it is the most widely used and most effective.
Analysis with this theory involves the calculation of the eigen-modes of the fiber and
consequently it is used to represent a disturbance induced in the field by the refractive index
modulation. This study demands the characterization of optical fibers with small index difference
between core and cladding. As such, it is possible to use linearly polarized fields. Besides this,
two approximations are made: the absorption losses are neglected and the propagation modes are
not significantly coupled by the radiative modes. In these conditions, it can be demonstrated that
the solution to the wave equation is a pair of coupled differential equations:
ziv
v eCidz
dC β∆−+ Ω= 2 Equation .9
ziv
v eCidz
dC β∆−+− Ω= 2 Equation .10
Where the symbols + and – signal the forward-going propagation path and counter
propagation path, and
Λ−=∆ πββ Equation .11
Where β is the propagation constant and Ω is the transverse coupling coefficient, given by:
B
nλ
ηπ∆=Ω . Equation .12
Solving the set of coupled differential equations, using the normalized boundary conditions
1)0( =+C and 0)( =− LC , the final equation for the reflectivity is obtained63:
∆<ΩΩ−∆
Ω
∆>Ω+∆
Ω
=,
)(cos)(sin
,)(cosh)(sinh
)(sinh
),(22
222
22
222222
22
ββ
ββ
λpara
QLSL
paraSLSSL
SL
LR Equation .13
Where
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=),( LR λ Reflectance or reflectivity as a function of Wavelength and filter length;
=λ Wavelength;
L = Filter Length;
=Ω Coupling coefficient;
;
Λ−=∆ πββ
=β Eigen propagation constant;
=Λ Perturbation period;
22 β∆−Ω=S and
iSQ =Ω−∆= 22β ;
This equation is an accurate definition of the spectral response of an uniform Fiber Bragg
Grating (FBG), but in practice, the equation used most frequently is Equation .7.
Figure 34 shows the spectral response of a diffraction grating with the following
characteristics: L = 10 mm, mµ078.1=Λ , 4102.1 −×=∆n .
Figure 34 - Spectral Response of a diffraction Grating Filter.
Reflectivity has its maximum value R= 97% for 1563 nm. This is the wavelength that obeys
the Bragg resonance condition. Multiple reflections between opposite sides of the grating lead to
the formation of sidelobes in the spectral response outside the resonance condition. These
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sidelobes compromise the performance of the grating when acting as a filter and a process
known as apodization is used to eliminate these sidelobes.
5.4.4. Apodization of the spectral response of Bragg gratings
The reflection spectrum of a finite-length Bragg grating with uniform modulation of the
index of refraction is accompanied by a series of sidelobes at adjacent wavelengths as referred in
the previous section. It is very important to minimize and, if possible, eliminate the reflectivity
of these sidelobes (or apodize the reflection spectrum of the grating) in devices where high
rejection of the nonresonant light is required, as is the case with the filters used for this project.
In practice, apodization is accomplished by varying the amplitude of the coupling coefficient
along the length of the grating64. This can be seen in Figure 35. A method used to apodize the
response consists in exposing the optical fiber with the interference pattern formed by two non-
uniform UV light beams. In the phase mask technique, apodization can also be achieved by
varying the exposure time along the length of the grating, either from a double exposure or by
scanning a small writing beam or using a diffraction efficiency phase mask.
Figure 35 - Refractive index profile of an apodized Fiber Bragg Grating.
5.5. Fabrication processes
In the following, the fabrication processes through which fiber Bragg gratings are inscribed
in optical fibers will be described. Special interest will be given to the process used to inscribe
the gratings used in this project.
Inscribing diffraction patterns in a fiber is a complicated and high precision process. As
previously stated, inscription of these patterns requires a high power UV light source, centered at
244 nm, in the Germanium-Oxygen (GeO) defects or wrong bonds. Energy densities by unit area
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used to efficiently write diffraction gratings have a threshold of approximately 2/150 cmmJ≈ .
These values demand the use of a UV laser source. Although there are several possibilities to
obtain laser emission at 244 nm, like the Árgon or Nd-YAG laser with, respectively, doubled and
quadrupled frequency, the choice fell in the use of a KrF excimer laser. This laser’s wavelength
is perfectly centered in the 244 nm absorption peak, meaning that the writing of gratings can be
performed directly without the need for frequency multiplication crystals. This laser emits
optical pulses with a duration of 20 ns with an energy of 100 mJ to 1.4 J per impulse. The energy
density over the transversal section of the beam is 213 cm×≈ and it is non-uniform having a
high divergence profile. This means that a set of slits and lenses must be used to obtain a
uniformly distributed energy area. In this process 70% of the initial energy is lost and the beam
must be focused over a 22.010 mm× area, corresponding to a maximum fluency of 15 J/cm2
corresponds. Writing of diffraction patterns with fluency levels of 500 mJ/cm2 demands
exposure for min5≈ . This corresponds to a total radiation dosis of 7.5 kJ/cm2.
Up to date there are only a few externally written fabrication techniques, namely, the
interferometric technique, the point-by-point technique and the phase mask technique. The latter
was the technique used in this project. Next, a brief description of these fabrication processes
will be made with particular emphasis to the phase mask technique.
5.5.1. Interferometric technique
The interferometric technique was the first to be developed. It was demonstrated by Meltz65.
It uses an interferometer that splits UV light from a source into two beams and then recombines
them in order to form an interference pattern. This pattern is used to expose a photosensitive
optical fiber thus creating a refractive index change in the fiber’s core. Several types of
interferometers are used with this technique each one having its specific characteristic giving the
process different capabilities. This interferometric process is very flexible because it allows the
creation of gratings with any central resonant wavelength without the need for different sources.
It also allows making Bragg gratings with any desired length.
The main disadvantage of the amplitude-splitting interferometric technique is it’s
susceptibility to mechanical vibrations. Displacements as small as submicrons in the position of
mirrors, beam splitter, or mounts in the interferometer can cause the fringe pattern to drift,
washing out the grating. In addition to the above short-coming, quality gratings can only be
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produced with a laser source that has good spatial and temporal coherence with excellent output
power stability.
Figure 36 - Setup for interferometric fabrication of Fiber Bragg Gratings.
5.5.2. Point-by-point technique
Another technique used is the point-by-point technique that is accomplished by inducing a
change in the refractive index a step at a time along the core of the fiber66. Each grating plane is
produced separately by a focused single pulse from an excimer laser. A single pulse of UV light
from the laser passes through a mask containing a slit. A focusing lens images the slit onto the
core of the optical fiber from the side, as shown in Figure 37, and the refractive index of the core
in the irradiated fiber section increases locally.
The fiber is then translated though a distance Λ corresponding to the grating pitch in a
direction parallel to the fiber axis and the process is repeated to form the grating structure in the
fiber core. Essential to the point-by-point fabrication process is a very stable and precise
submicron translational system. The main advantage of this technique lies in it’s flexibility to
alter the Bragg grating parameters. Because the grating structure is built up a point at a time,
variations in grating length, grating pitch and spectral response can easily be incorporated. One
disadvantage of this technique is that it is a tedious process due to its step by step nature. Error in
the grating spacing due to thermal effects and/or small variations in the fiber strain can occur.
This limits the gratings to a very short length making the fabrication of high reflectivity gratings
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difficult. As such, this is not an adequate process to fabricate the grating to be used in this
project.
SubMicron translation device
Pulsed UV beam
Point-by-point fabricated grating
SubMicron translation device
Pulsed UV beam
Point-by-point fabricated grating
Figure 37 - Setup for Point-by-point Fabrication Technique.
5.5.3 – Phase mask technique The last technique to be addressed is, most certainly, the most used and one of the most
effective methods for inscribing gratings in photosensitive fiber. This technique employs a
diffractive optical element (phase mask) to spatially modulate the UV writing beam (See Figure
38)67. Phase masks may be formed holographically or by electron-beam lithography. The mask
has an interference pattern written on it and the UV light passing through the mask and getting to
the fiber core will photoimprint a refractive index modulation with fringe period one-half that of
the mask.
Use of this process greatly reduces the complexity of the fiber grating fabrication system.
The simplicity of using only one optical element provides a robust and an inherently stable
method for reproducing Fiber Bragg Gratings. Since the fiber is usually placed directly behind
the phase mask in the near field of the diffracting UV beams, sensitivity to mechanical vibrations
and, therefore, stability problems are minimized. Low temporal coherence does not affect the
writing capability (as opposed to the interferometric technique) due to the geometry of the
problem.
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Figure 38 - Setup for Phase-Mask fabrication of Fiber Bragg Gratings.
5.6. Applications of Fiber Bragg Gratings
A brief reference should be made to the other applications where Bragg gratings are used
5.6.1. Laser stabilization
The wavelength of laser diodes is sensitive to temperature fluctuations. Fiber Bragg gratings
can be applied to stabilize the diode wavelength with respect to temperature. By inserting a
length of fiber with a fiber Bragg grating, the length of the cavity can be made to dwarf the scale
of the temperature fluctuations, rendering the diode immune to such changes68.
5.6.2. Fiber lasers
Historically it has been very difficult to produce high quality high power lasers at 1550 nm.
The design of grating based fiber lasers falls into two basic categories, end-pumped and ring
lasers. The first uses a 980nm laser coupled to a length of fiber doped with erbium. The pump
laser excites the erbium ions in a manor similar to an EDFA. A cavity is defined in the doped
fiber with Fiber Bragg Gratings. The ring laser schematically consists of a loop of fiber
connected by a pump laser and containing a length of erbium doped fiber. The cavity is defined
though the use of a coupler and a single grating. This second structure will be used in our work
and will be described in Chapter 769.
5.6.3. Reflectors in fiber amplifiers
Bragg gratings can be used to flatten the gain profile of the EDFA. By using special gratings
called Side Tap Gratings, the excess gain can be pushed down. Side Tap Gratings do not reflect
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light back down the core of the fiber, like normal gratings, but instead couple light into the
cladding of the fiber, where it is dispersed and lost.
5.6.4. Raman-Shifted Lasers and Raman Amplifiers
Raman-shifted lasers and Raman amplifiers enable efficient conversion of short-wavelength
light into longer wavelengths suitable for long-distance fiber transmission. Raman gain is
obtained through energy transfer from pump light to the laser output or amplified signal as
mediated by molecular vibrations in the silica fiber70.
5.6.5. Sensors
In a non-communications related example, Fiber Bragg Gratings can be used in sensors. The
area of embedded sensors in composite materials to detect strain in static structures is
extensively studied and has a lot of commercial applications.71,72
5.6.6. Isolation Filters in Bidirectional Lightwave Transmission
With a very effective reflection property, fiber grating filters can be used as isolation filters.
They block adjacent channels and far-end crosstalk.
5.6.7. WDM Demultiplexers
The grating’s property of deflecting light incident upon it with an angle that depends on the
wavelength of that light can be used to demultiplex WDM signal into their different channels.
5.6.8. Add/Drop Multiplexers and Optical Cross-Connects
These structures are fundamental in communications systems using WDM and are fully
described in Chapter 673.
5.6.9. Dispersion Compensators and Wavelength Converters
Using chirped fiber Bragg gratings with a special refractive index profile, the chromatic
dispersion that induces significant distortion on optical pulses can be reduced74.
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Figure 39 - Dispersion compensation using an aperiodic grating with a length of 6cm associated with an
optical circulator.
5.7. Summary
In this chapter, a main component of WDM networks has been presented – Fiber Bragg
Gratings. Their physical characteristics and spectral response have been presented.
Fiber Bragg Gratings are vastly used in various types of applications. The main applications,
fundamentally telecommunications applications where presented. There where many more to
present but we kept to the most important in the context of our work.
Gratings are a fundamental component in wavelength selective networks; they are, in some
technologies integrated in Wavelength Routers. These devices are the core of Optical
Wavelength Selective Networks and will be described in the next chapter.
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Chapter 6 – Wavelength Routers
State of the art in Optical Switching and Routing
6.1. Introduction
A Wavelength Router is a device used in WDM networks. It has one or more input and
output ports through which it is connected to other wavelength routers and/or end-nodes by using
one or more fiber links for each neighbor. Wavelength routers should be able to route signals on
different wavelengths at different input ports to (possibly) different output ports independent of
the signals on other input ports and on other wavelengths. According to the routing matrix
present there are four major types of wavelength router architectures: Add-Drop Multiplexers,
Optical Cross-Connects, Static Wavelength Routers and Reconfigurable Wavelength Routers.
In this project the first two architectures are the ones implemented and as such, a description
of the state of the art in terms of these devices is suitable at this point. These devices, in the
context of optical networks in general, have been briefly described above in Chapter 3. Here a
description of some of the technologies used in terms of optical switching and routing is made.
6.2. Optical Add-Drop Multiplexers – OADMs
These devices perform a simple but very important task in terms of optical network operation
as described in Chapter 2. Optical add–drop multiplexers (OADMs) will be required in future
wavelength-division multiplexed (WDM) ring and bus networks to link the network with local
transmitters and receivers. OADMs can also provide interconnection between network
structures75.
These devices are generally evaluated in terms of performance through crosstalk
measurements. Although there are other problems related with these devices like losses, non-
linear effects, etc… Crosstalk is the most limiting factor in terms of performance. It arises in
OADMs through component imperfections. Optical crosstalk at the same wavelength as the
transmitted signal is generally referred to as homodyne or in-band crosstalk. It is particularly
serious because it cannot be removed by filtering76,77, and has been shown to severely limit
network performance. Within homodyne crosstalk, incoherent crosstalk causes rapid power
fluctuations, while coherent crosstalk changes the optical power of the signal78. Incoherent
crosstalk occurs in ring and bus networks when the signal and interferer are from different
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optical sources. It causes power fluctuations at the receiver, resulting in bit error rate (BER)
degradation and a power penalty. Coherent crosstalk in these networks is due to multiple paths
between ports within the OADMs. It causes variable attenuation levels between OADM ports. So
long as the level variation is slower than the decision threshold at the receiver, the effect of
coherent crosstalk is generally to reduce the size of the eye, without increasing eye closure.
There is no accompanying power penalty, because the BER is measured against the optical
power at the receiver. Incoherent and coherent crosstalk together give a range of possible power
penalties, because coherent crosstalk can cause variation in both signal and incoherent crosstalk
powers at the receiver79. Combination of coherent and incoherent crosstalk leads to a range of
possible BERs and power penalties for OADMs deployed in a network link.
6.2.1. Comparison of common OADM structures
A brief description of common OADM structures will be made here. In Figure 40 some
structures are presented. Only the most common OADM structures are referred, Bragg gratings
and arrayed waveguide grating multiplexers (AWGMs). These two technologies have been
selected because they are widely published, cover a range of proposed OADM structures, and are
comparatively mature. Among the other technologies, we have ring resonators80, acousto-optic
tunable filters81, and micromirror arrays82,83 and finally, the technology used in this project:
tunable gratings84,85. This last technology will not be analyzed here because it will be described
thoroughly in Chapter 1.
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Figure 40 - Common OADM structures: FB-a), FB-b) and FB-c) – Interferometric structures based in Fiber Bragg gratings; FB-d), FB-e) and FB-f) – Fiber Bragg grating and circulator based structures; FA-a), FA-b) and FA-c) – Array Waveguide Grating Mux structures
The structures in the figure where initially developed as Fixed-Wavelength structures, this
means that they deal only with wavelengths for which they are initially tuned; there is no
possibility of tunability during functioning.
The first set of structures (from a) to c)), uses interferometric processes as Bragg gratings to
select the path that the optical signal must follow. Although having good crosstalk
characteristics, these structures use couplers which is a big problem because it causes a lot of
attenuation especially to dropped channels. Besides this, interferometric systems incur in a
specific problem called back-reflection that can severely harm the light sources if they are placed
directly at the input of these devices. Slow tunability (as referred earlier in this report about
Mach-Zehnder interferometers)
In the second set of structures (d), e) and f)) we have approaches using gratings, circulators
and couplers. Structure d) is very simple but very poor in performance too. Is exhibits high
attenuation due to the use of the coupler, and high levels of crosstalk. Structure e) is a refinement
of the previous structure where attenuation problems are solved by introducing a new circulator.
Crosstalk levels are still relatively high but are almost completely solved using structure f) in
which an isolator is used to prevent crosstalk problems inside the diffraction gratings. Our
OADM structure, described later in this report is very similar to this one but adds tunability
though temperature control of the gratings. The remaining three structures, FA-a to FA-c, are
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based on AWGMs. These OADMs operate quite differently to the Bragg grating-based
structures. Rather than separating out the channel to be dropped, the entire WDM signal is
demultiplexed by the AWGM(s). Each channel traverses a separate path through the OADM
before being remultiplexed. The path for the channel be added and dropped is broken to provide
ADD and DROP ports. Structures FA-b and FA-c are low-crosstalk variations of structure FA-
a86.
Comparing the fixed-wavelength OADMs it can be said that structures using Bragg gratings
generally have lower insertion losses than those based on AWGMs. They also generally have
fewer coherent crosstalk terms, although the coherent crosstalk levels are low for all structures.
The incoherent crosstalk levels show no consistent differences by technology.
The four low-crosstalk design variations do all offer significantly better incoherent crosstalk
performance as expected, at the cost of a higher component count, or more complex components.
Combining multiplexing and demultiplexing functions in single AWGMs leads to more
leakage paths and greater coherent crosstalk.
There are two general trends in the crosstalk performance of the OADMs briefly described
here. The first is that low crosstalk levels (incoherent and coherent) are generally associated with
either a higher component count, or more complex components.
It should be referred that introducing tunability in these structures will increase the crosstalk
levels but overall performance in terms of flexibility is definitely increased.
Bragg-grating and AWGM-based structures can offer excellent homodyne crosstalk
performance. However, the Bragg grating based structures are superior overall, because of their
reduced insertion loss and better filtering characteristics.
The AWGM-based structures optically filter all channels at each OADM, whereas those
using Bragg gratings only add filtering at the OADMs in the network where the channel is added
and dropped. Filtering from AWGM cascades has been shown to cause signal distortion and eye
closure87.
An upcoming technology that uses gratings is the Multiport approach. In these structures
(Figure 41)88, optical circulators with multiple ports are used in order to reduce the insertion loss
due to the use of several circulators. The reduction of component count helps normalize the
attenuations imposed on different channels. But there are also setbacks. The multiport circulators
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are yet to be proven effective when many ports become necessary, (i.e.), they do not support
scalability.
Figure 41 - OADMs based in Multiport Optical Circulators (MOC’s) Configurations.
As previously denoted, there are other solutions to implement these devices. It would be
impossible to describe them all in this report and maintain an introductory structure, but suitable
references are provided and further study can be conducted through those articles.
6.3. Optical Cross-Connects - OXCs
An optical cross-connect (OXC), operates directly in the optical domain, just like the
described OADMs, but, instead of just adding or dropping channels, it switches the input
channels into different output fibers.
Just as electrical switching replaced mechanical relays of the past, optical switching is on the
verge of replacing some of today’s electrical switching functions in telecommunications
networks. Although new approaches to optical switching are constantly being developed, optical
switch designs can be roughly classified into seven categories: optomechanical, thermo-optical,
liquid crystal, micro-electrical mechanical, gel/oil based, electro-optical, and others such as
acoustooptic, semiconductor optical amplifier (SOA) and ferro-magnetic.
In evaluating the performance of the different optical switches, the following individual
technology appraisals include assessments of reliability, energy usage, port configurations and
scalability, optical insertion loss, cross-talk, temperature resistance, and polarization dependent
loss characteristics.
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6.3.1. SWITCHING TECHNOLOGIES
6.3.1.1. OPTOMECHANICAL
Optomechanical switches employ electromechanical actuators to redirect a light beam. One
type of optomechanical switch inserts and retracts a reflective surface into a light stream to
redirect it to another port. Another architecture redirects the light stream by bending a grating-
written fiber.
In terms of optical insertion loss and switching speed, performance characteristics of
optomechanical switches vary according to architecture; performance can range from low to high
loss and slow to fast speed. However, the universal drawback for optomechanical switches is the
durability and cycle limitation of the mechanical actuators. This type of structure found its
success in MEMS that are described bellow.
6.3.1.2. MICRO-ELECTRO-MECHANICAL SYSTEM (MEMS)
MEMS can be considered a subcategory of optomechanical switches; however, because of
the fabrication processes and miniature natures, they have different characteristics, performance
and reliability concerns. MEMS use tiny reflective surfaces to redirect the light beams to a
desired port by either ricocheting the light off of neighboring surfaces to a port, or by steering
the light beam directly to a port89. Analog-type, or 3-D, MEMS mirror arrays have reflecting
surfaces that pivot about axes to guide the light. Digital type, or 2-D, MEMS have reflective
surfaces that “pop up” and “lay down” to redirect the light beam propagating parallel to the
surface of the substrate. The reflective surfaces’ actuators may be electrostatically-driven or
electromagnetically-driven with hinges or torsion bars that bend and straighten the miniature
mirrors.
MEMS devices easily scale to large port counts because of miniature sizes and
semiconductor fabrication processes, but due to the density and microscopic size of the light
paths entering the substrate, MEMS can be a challenge to package.
Although highly accepted by the industry, these MEMs also have some problems. In order to
switch the different wavelengths, an effective spatial division of all the wavelength channels
must be made and each wavelength must have its own switching plane (a set of mirrors that
make it possible for it to be routed across the device).
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Figure 42 - Toshiyoshi and Fujita’s 2×2 MEMS optical switch .
Figure 43 - 8x8 2-D Optical switch
Figure 44 - Schematic of 3-D MEMS switching
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Figure 45 - 3-D micromachined mirrors.
Figure 46 - Scratch Drive actuator switching 1x2.
6.3.1.3. THERMO-OPTICAL
Planar lightwave circuit thermo-optical switches are usually polymer-based or silica on
silicon substrates. Thermo-optical switches use temperature control to change index of refraction
properties of Mach-Zehnder interferometer-based waveguide arms on the substrate. The light is
processed by waveguide interaction and is guided through the appropriate path to the desired
port90.
Thermo-optical switches are small in size but have a drawback of having high driving-power
characteristics and issues in optical performance.
A typical 2X2 thermal-optic switch has an insertion loss of <2.5 dB, an extinction ratio of
>35 dB (for two cascaded switches), and a switching speed of 1 – 3 ms. The switch can be
constructed using a Mach-Zehnder configuration (Fig. 1), which consists of two, 3 dB couplers
connected by two waveguides serving as phase shifters. Thin film resistors are deposited on the
Mach-Zehnder arms, so that one of the arms can be heated to change its refractive index, and the
accumulated phase difference of light propagating through the two arms can be modulated.
When light is launched into one of the input ports, it is split into two MZ arms by the 3 dB
coupler with equal optical power and p/2 phase difference. As light travels through the MZ arms,
the phase difference can be altered due to the temperature difference between the two
waveguides. After passing through the second 3 dB coupler, the two beams recombine either
constructively or destructively at either of the two output ports, depending upon the exact phase
difference between the two Mach-Zehnder arms controlled by the heater. This modulation of
temperature achieves the purpose of switching the light between the two output ports. The
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electrical power needed to switch each path is on the order of a few hundred milliwatts. Switches
can also be cascaded for added extinction ratio without sacrificing much on insertion loss.
Figure 47 - Thermooptical switching - a)Schematic diagram of a Mach Zehnder switch ; b) Light path in one of the switching.
6.3.1.4. LIQUID CRYSTAL
Liquid crystal switches work by processing polarization states of the light. Apply a voltage
and the liquid crystal element allows one polarization state to pass through. Apply no voltage
and the liquid crystal element passes though the orthogonal polarization state. These polarization
states are steered to the desired port, are processed, and are recombined to recover the original’s
signal properties. With no moving parts, liquid crystal is highly reliable and has a good optical
performance, but can be affected by extreme temperatures if not properly designed.
Its drawbacks are essentially three. It is fairly slow (especially at low temperatures, where
switching times can be hundreds of milliseconds); is difficult to integrate with other optical
components; and has relatively high light losses from the liquid crystal itself, the polarization
splitters, and imperfections in the fairly complex optical path.
One of the most challenging aspects of applying liquid crystals to optical switching directly
relates to their use of polarization. The optical polarization of any input signal is completely
uncontrolled. Therefore, the signal must be split into two known orthogonal polarizations using
polarization splitters and switching done separately on each. The results are then recombined to
form the output. This approach is troublesome and costly to implement and could cause
unacceptable polarization mode dispersion (PMD), in which short pulses are spread out in time
because different components of the pulses propagate at different speeds, depending on their
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polarization. In addition, compensating for the liquid crystal’s temperature dependence renders it
too costly for all-optical switching needs in the metro and access networks91,92.
6.3.1.5. GEL/OIL-BASED – “Bubble” Switching
Index-matching gel and oil-based optical switches can be classified as a subset of thermo-
optical technology, as the switch substrate needs to heat and cool to operate. However, their
exists an added dimension in that heating a portion of the switch causes an index of refraction
changed atmosphere to form at the waveguide junctions. This index of refraction changed
“bubble” or liquid redirects the light stream through the appropriate waveguide path to the
desired port.
Total internal reflection—known as TIR, the phenomenon that makes light propagate down
an optical fiber—can, with an added twist, also serve as the basis of a switch. The way the
principle works, if light attempts to cross from a medium of higher refractive index (Dielectric 1)
to one of lower refractive index (Dielectric 2) at too shallow an angle, all of the light is reflected
from the interface back into the high-index medium [see top left part of figure below]. The trick
to exploiting the phenomenon in a switch is to turn the effect off (or on) by replacing (or not
replacing) the second medium with one whose index of refraction matches that of the first. The
best-known product based on this phenomenon is the Agilent Champagne switch, in which
sections of waveguide intersect with fluid-filled channels [see bottom part of figure below].
There are inherent losses in this type of structure, and even worse, these losses cause
crosstalk. To minimize these detrimental effects, the intersection should be kept as small as
possible.
Designers of TIR switches are therefore faced with a pair of conflicting requirements: low
loss must be traded off against high isolation. An additional problem in TIR switches of this type
is that the reflected wave undergoes a wavelength-dependent phase shift because of energy
storage in the bubble. This causes amplitude variations and dispersion in the switch’s output,
lowering its usefulness for some applications.
Note that because this is a matrix switch, the number of intersections equals the product of
the number of inputs and the number of outputs. As mentioned above, each intersection traversed
by the light contributes to the loss and crosstalk, limiting the scaling of the matrix to less than
100 ports because the number of intersections to be crossed by a light beam in the worst case
may equal the total number of ports.
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This technology has been compared to proven inkjet printer technology and can achieve good
modular scalability. However, for telecom environments, uncertainty exists about gel/oil-based
long-term reliability, thermal management and optical insertion losses93.
Figure 48 - Total internal reflection switching – Agilent’s Champagne Bubble Switch.
6.3.1.6. ELECTRO-OPTICAL
Electro-optical switches use highly birefringent substrate material and electrical fields to
redirect light from one port to another. A popular material to use in an electro-optical switch is
Lithium Niobate. An electrical signal is fed as the control into the substrate of the device. This
electrical field changes non-isotropically the substrate’s index of refraction. The index of
refraction change manipulates the light through the appropriate waveguide path to the desired
port. Opto-electrical switches are extremely fast and are reliable, but they pay the price of high
insertion loss and possible polarization dependence.
6.3.1.7. ACOUSTO-OPTIC
Acousto-optic optical switches receive acoustic-wave-induced pressure from a RF-fed
piezoelectric transducer to generate fine gratings in optical waveguides. The gratings diffract
lights to the desired port.
The Acoustooptic Tunable Filter (AOTF)94,95 (Figure 49), utilizes TM and TE polarization
modes in a birefringent optical waveguide in LiNbO3. In AOTF, after passing the first polarizing
beam splitter, all signals have the same polarization. For the signals at selected wavelength(s),
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corresponding RF drive signal(s) are injected in to a polarization converter device. Finally,
selected signals are dropped by the second polarizing beam splitter device. The AOTF can be
used by a passive combiner device (to add new signals) to construct a multi-wavelength
configurable add-drop multiplexer.
Figure 49 - Acoustooptic Tunable filter
6.3.1.8. ELECTROHOLOGRAPHIC
Electroholography is the newest all-optical switching technology. This method features a
solid-state switch matrix created from rows and columns of ferroelectric crystals such as lithium
niobate or potassium lithium tantalite niobate [Figure 50]. Rows correspond to individual fibers,
and each column is for a different wavelength. Each crystal is laser etched with a Bragg grating
(which causes a quasi-periodic modulation in its dielectric properties) to create a hologram in
which the crystal’s optical properties are changed when it is energized, for example, by the
application of an electric field. In current implementations, such as those by Trellis Photonics,
individual crystals are manually assembled, and thus must be greater than 1 mm on a side. As the
technology evolves, the holographic elements may be able to be written more densely into a
single crystal; then patterning will be required only for the electrodes through which the
energizing electric fields are applied to each crystal or holographic element. When a crystal is
not energized, light goes through it. Energized crystals, on the other hand, deflect a controllable
portion of the incident light to the appropriate fiber. Holographic switches are quite fast and
claim instant signal restoration. They, along with other switches made from electro-optic
materials, will be fast enough for the long-term application of optical packet switching. Because
it is an emerging technology, no data about its long-term reliability is available, but past
holographic applications like high-density storage have shown lifetime issues with the holograms
themselves. On the plus side, electroholographic switches may be easily integrated with other
network functions like equalization and monitoring. Being electrostatically controlled, they
consume negligible power. The technique allows a single crystal to be used for switching and for
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variable attenuation, since the fraction of light reflected is controllable by an applied signal. Yet,
from an application viewpoint, the technology is not the ideal solution it is sometimes
represented to be. The approach is that of a wavelength-selective matrix switch. The hologram
blocks are analogous to the mirrors in a 2-D MEMS switch. The number of matrix elements in
an electroholographic switch, therefore, increases as the product of the number of input and
output ports, and will not scale well. As the switch matrix size is increased to the sizes needed
for core network switching, the required optical beam size will expand and optics for collimating
and focusing the beams will be required. Non-energized blocks in the optical path will contribute
to the loss and crosstalk of the switch. Also, holograms are diffractive elements that are
inherently polarization and wavelength dependent, leading to dispersion and polarization-
dependent loss (PDL) issues.
Figure 50 - Electroholographic Matrix with ferro-electric crystals
6.3.1.9. BRAGG GRATING BASED
Although many times not mentioned in literature, this technology has great potential in the
optical switching domain. The main idea is to use optical filters based in fiber Bragg gratings in
conjunction with optical circulators to appropriately direct the light between input and output
fibers. The work done in this report is based in this technology but there are some
implementation studied mainly in academic environments because the industry is more inclined
to the MEMs approach (inspite of its complexity and cost). Bragg gratings, as described in
Chapter 5, are especially suited to filter wavelengths and as such, present good crosstalk
capabilities. As seen with OADM structures, the overall performance of this type of devices is
very good.
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It is unfortunately true that commercial versions of OXCs using this technology are not
available and that high port count are only proven to work in theoretical studies, but research is
being made, this project being an excellent example, in order to make this technology proliferate
and have better commercial acceptance.
In Figure 51, an example of a scalable OXC can be seen. An even better solution is presented
in the patent submitted as a result of this project.
Figure 51 - A 4x4 rearrangeable nonblocking OXC using: a) 12 three-port Optical Circulators and b) Four five-port Multiport Optical Circulators (MOC’s).
Some of the problems associated with this specific configuration are stated in the patent and
how our solution can improve overall performance.
Fiber Bragg grating technology is very mature and has very good temperature stability
besides having easy coupling to fiber and low loss96.
6.3.2. Comparison of OXC technologies
Switching Technology CROSSTALK INSERTION LOSS
POWER CONSUMPTION SPEED WAVELENGTH
DEPENDENCEPOLARIZATION DEPENDENCE COST SCALABILITY
MEMS SMALL LOW LOW ms SMALL SMALL HIGH LARGEThermo-Optical LARGE MODERATE HIGH ms SMALL SMALL MEDIUM SMALLLiquid Crystal LARGE HIGH MODERATE < ms LARGE SMALL HIGH MODERATE
Bubble (Gel/Oil) LARGE HIGH MODERATE ms HIGH MODERATE HIGH SMALLElectro-Optical LARGE MODERATE MODERATE ns - ms SMALL or LARGE LARGE HIGH SMALLAcousto-Optic MODERATE HIGH MODERATE micros LARGE SMALL MEDIUM SMALL
ElectroHolographic LARGE HIGH LOW ms LARGE LARGE HIGH SMALLBragg Grating SMALL LOW MODERATE s SMALL NONE SMALL MODERATE
The only conclusion that can be taken from this analysis is that switching technologies are
numerous and suitable for different types of applications. In the long run there won’t be any sole
winners. Each technology will have its own market niche.
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6.4. Summary
The most important technologies used in the fabrication of Optical Add-drop Multiplexers
(OADMS) and Optical Cross-connects (OXCS) have been showed and a performance evaluation
has been made. As stated above, none of these technologies is vastly superior to another. They
all have their applicability in different types of networks with different requirements. Our work
focused in one of these technologies, namely, Fiber Bragg Grating Based. In the following
Chapter, the practical work developed in our project will be described and the performance of
some architectures will be examined.
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Chapter 7 – Work Developed
7.1. Introduction
The practical implementation of this project could only be accomplished after careful
planning and study of the possible architectures. In order to make the results systematic, a step-
by-step approach was used. The steps followed where:
Grating fabrication – Fabrication of high reflectivity and low insertion loss gratings
Development of an Add-drop Multiplexer (OADM – 1)
Development of a temperature controller to tune the grating
Structure assembly – OADM 1
Performance evaluation and testing
Upgrade of the OADM developed – OADM 2
Structure assembly – OADM 2
Performance evaluation and testing
Development of an Optical Crossconnect (OXC)
Development of additional WDM Laser Sources to place at the input ports
Structure assembly – OXC 1
Performance test
Development of an upgraded OXC
Performance evaluation and testing
7.2. Grating Fabrication
A key component to the success of this work was the fabrication of gratings with good
characteristics such as: high reflectivity, low insertion loss, and other important characteristics.
The gratings used where fabricated using the Phase Mask technique. This allowed the use of
a low spatial and temporal coherence KrF excimer laser at 248 nm. A piece of standard single
mode fiber (SMF) is illuminated with UV light coming from the laser and passing through the
Chapter 7– Work Developed
70 of 90 - IP over WDM I
phase mask creating a diffraction pattern in the fiber (This was previously explained in Chapter
5).
The fiber had been previously kept under high-pressure hydrogen atmosphere in order to
enhance its photosensitivity due to hydrogen diffusion into the glass matrix. This process
described in Chapter 5, helps to reduce the writing time necessary to obtain high reflectivity
gratings. The phase mask used had a spacing period of Λ= 1072 nm. The laser was fired with a
power level of 300mW during approximately 15 minutes to fabricate this grating.
The spectral response of the resulting grating is presented in Figure 52.
1548 1549 1550 1551 1552 1553 15540,0
0,2
0,4
0,6
0,8
1,0
Tran
smiss
ion
(dB
)
Ref
lect
ivity
λ (nm)
-16
-14
-12
-10
-8
-6
-4
-2
0
Figure 52 - Transmission and reflection grating spectra.
The resulting spectral response shows approximately 100% reflectivity at the wavelength of
1550.9 nm. The Full Width Half Maximum (FWHM) is 0.2 nm which is very good for a grating
with this reflectivity. This grating is a little wide in terms of bandwidth occupied but it would be
possible to obtain better results if apodized masks where used. There is a performance trade-off
between reflectivity and spectral width. However, considering that the WDM channel spacing is
0.8nm (100GHz) this is not a severe problem in terms of crosstalk.
A further problem is excessive loss in the shorter wavelength side of the main peak in the
transmission spectrum. This is due to coupling from the waveguide mode to the backward-
propagating cladding mode. This may be overcome by suppressing the coupling itself and by
shifting the region in which the excessive loss occurs outside the waveband being used. In either
case it is achieved by modifying the profile of the fiber [97].
Chapter 7– Work Developed
IP over WDM I - 71 of 90
7.3. Development of an Optical Add-drop Multiplexer
7.3.1. Implementation of the first structure – OADM 1
The idea behind this part of the project was to develop a fixed OADM with good
performance characteristics. A schematic of the first structure that was developed is presented in
Figure 53.
(λ1,λ2,λ3)
Input Output
λ2
Drop Add
Signal A
Signal C
Signal B (λ1,λ3)
(λ2)
Figure 53 – First Optical Add-drop structure implemented - OADM 1.
This structure selectively filters a channel with wavelength λ2 from the input signal composed
of three channels of wavelengths: λ1=1549.9 nm, λ2=1550.7 nm and λ3=1551.5 nm, separated by
0.8 nm (100 GHz). This input signal was obtained through optical spectral slicing of a LED’s
emission spectra. This source is briefly explained in Figure 54.
PCGPIB
BroadbandOptical Source
Coupler
Optical Spectrum Analyser
λ1 λ2 λ3
50:50
Figure 54 – Optical WDM source obtained through slicing of a Broadband optical source’s (LED) spectra.
It should be noted that the power injected in the fiber by this source is low and reflection in
the gratings followed by the coupling loss, will reduce even further the power levels of the
resulting signal. This is not detrimental in the analysis of our OADM but better results could be
obtained using sources with more significant power levels.
Chapter 7– Work Developed
72 of 90 - IP over WDM I
7.3.2. Performance Assessment
The results obtained are shown in Figure 55 where the input signal composed of three WDM
channels is represented together with the output and the dropped channel. We can see that the
Insertion Losses in this first structure are small, in the order of 1dB.
1548 1549 1550 1551 1552 1553 1554-25
-20
-15
-10
-5
0
A B C
Tran
smis
sion
(dB
)
λ (nm)
Figure 55 – A – Input signal composed of three WDM channels (l1, l2,l3); B– Output Signal composed of signals l1 and l3 ; C – Dropped Channel l2 .
Crosstalk is the main performance evaluation parameter for these devices and can be of two
types: Heterodyne or Interchannel Crosstalk (Figure 56) – Derives from interferences of small
power levels that appear outside the channel’s bandwidth, causing an increase in the bit error rate
when detecting the other channels; Homodyne or Intrachannel Crosstalk – Results from
interferences inside the channel’s bandwidth[98].
Figure 56 – Heterodyne Crosstalk caused by imperfect FBG filtering.
In the performance tests on the OADM structures, only Heterodyne Crosstalk has been
tested.
Chapter 7– Work Developed
IP over WDM I - 73 of 90
The Crosstalk isolation level, i.e., the difference between the power level of the channel and
the interference from adjacent channels (Crosstalk), is 14.5 dB. This is not a spectacular result
but having in consideration that this is a simple configuration where one does not intend to
reduce the crosstalk this is actually an acceptable value.
In order to increase the isolation level, a new configuration was developed – OADM 2. This
new structure is shown if Figure 57.
7.3.3. Upgraded OADM Structure – OADM 2
(λ1,λ2,λ3)
Input Output
λ1
(λ1,λ3)
λ3
Drop Add
Signal A Signal B λ2 λ2
Optical Isolator
Signal C (λ2)
Figure 57 - Second Optical Add-drop structure implemented - OADM 2
In this structure, new gratings where added in the drop output in order to remove any
crosstalk residues still present in the output signal. The isolator in the central arm between the
circulator prevents any signal residues coming from signals being added from interfering with
the signal being dropped or signal just following the direct path though the OADM.
7.3.4. Performance Assessment
The results of this new and more complex structure are shown in Figure 58. In this figure we
can see the dropped channel λ2 using the first OADM and that same dropped channel using the
second structure. The Insertion loss has increased by 2 dB but the Crosstalk isolation has
increased 5 dB. The noise seen in the Figure is the noise floor of the equipment and it is not
entirely present in the signal. The fact is that the Optical Spectrum Analyzer (OSA) does not
have enough resolution to read the effective crosstalk level present in the signal which is very
low.
Chapter 7– Work Developed
74 of 90 - IP over WDM I
1548 1549 1550 1551 1552 1553 1554-25
-20
-15
-10
-5
0
C D
Tran
smis
sion
(dB
)
λ (nm)
Figure 58 - C – Dropped Channel λ2 in OADM 1; D – Dropped Channel λ2 in OADM 2.
7.4. Optical Crossconnect Architectures
7.4.1. Description of first structure and its implementation
The next step in the project’s objective was to assemble an Optical Crossconnect. The first
structure to be presented is the one shown in Figure 59. This structure is patented [99] and referred
in our own patent has one of the recent developments in this area. It has relatively good
performance and it is a good starting point to study this type of OXCs.
λ1, λ2 , λ3
λM
Output 1
Output 2
Input 1
Input 2
Figure 59 – Optical Crossconnect 1
As described above in Chapter 5, Bragg gratings can be tuned to reflect different
wavelengths strain or temperature change of their physical characteristics. Now, instead of only
stabilizing the grating so that the central wavelength is fixed, the approach was to heat the
grating with a Peltier Device in order to tune it to the desired wavelength thereby selecting
which channel is dropped and which channel is added. This means that the grating is used as a
Chapter 7– Work Developed
IP over WDM I - 75 of 90
Tunable Optical Filter. Description of the developed control electronic circuit is given in
Appendix B.
Another Fiber Bragg Grating was fabricated with a central wavelength of 1550.7 nm,
FWHM of 0.2 nm and near 100% reflectivity. The phase mask used had a spacing period of Λ=
1070 nm. The laser was fired yet again with a power level of 300 mW during approximately 15
minutes to fabricate this grating. The WDM source was the same used in the OADM testing.
7.4.2. Performance Assessment
When the tunable optical filter is stabilized at room temperature, the central wavelength is
λM=λ2, therefore switching channel 2 to Output 1. The other two channels (1 and 3) are switched
to Output 2.
In Figure 60 it can be seen that the crosstalk isolation level of channels λ1 and λ3 is -16.23 dB
(Output 2). The small residual components centered in λ1 and λ3, are due mostly to residual
reflections in the grating and circulators that give birth to Heterodyne Crosstalk.
1548 1549 1550 1551 1552 1553 1554-35
-30
-25
-20
-15
-10
-5
0
Output 1 Output 2
Opt
ical
Pow
er (d
B)
λ (nm)
Figure 60 – Optical Crossconnect 1 performance test.
As a second step in the analysis of this structure the optical filter (FBG) is detuned by
temperature variation and the central wavelength is placed at an intermediate wavelength
between λ2 and λ3 (λM=1551.1 nm). In Figure 61 we can see the power level of the signals in
both exits of the device. The three channels are switched to Output 2. In Output 1 the signal
measured has a power level 10.53 dB bellow the one at Output 2. This signal is reflected from
the grating and causes Homodyne Crosstalk. It should be noted once more that the use of
gratings with an apodized refractive index profile will reduce significantly the measured
crosstalk levels.
Chapter 7– Work Developed
76 of 90 - IP over WDM I
1548 1549 1550 1551 1552 1553 1554-35
-30
-25
-20
-15
-10
-5
0
Output 1 Output 2
Opt
ical
Pow
er (d
B)
λ(nm)
Figure 61 - Power Spectral Response of the OXC in a detuned state.
It is important to note that there are inherent losses due to the use of circulators. Each time a
signal enters a circulator and exits in the next port it suffers from a 1 dB loss to which are added
0.2 dB of losses due to fiber splices. Each signal incurs twice in these losses, which explains why
the maximum power levels at the Outputs are bellow the reference level by 2.4 dB.
One of the disadvantages of this device is the physical impossibility of placing signals with
the same wavelength in contiguous ports and effectively switching them. The grating would have
to be detuned and the conflicting signals would mix destroying all information in them.
This structure is nevertheless fully scalable (with the mentioned limitation), i.e., it is possible
to build NxN port Crossconnects using basic 2x2 OXC blocks although the described setback is
still a problem.
Even though in this structure only one FBG is used, there is a possibility of controlling more
input channels by placing other FBGs next to the one present.
7.4.3. Development of an upgraded OXC
The most significant achievement of this project was a novel Optical Crossconnect
architecture that will now be described. This structure is under proper procedures in order to be
patented. In Figure 62 a diagram of the OXC – OXC 2 is presented. Unlike the previous structure
presented (OXC 1) this novel structure has no limitations in terms of channel insertion. We can
insert any type of wavelength in any of the input ports and there is always a way to successfully
switch these wavelengths through the Crossconnect. The optical filters (FBGs) are still
controlled by the previously referred temperature controller based on a Peltier Device.
Chapter 7– Work Developed
IP over WDM I - 77 of 90
Input 1 Output 1
Input 2
Output 2
λΜ
λΜ
Signal A
Signal B
(λ2,λ3)
(λ1)
Figure 62 - OXC 2.
7.4.4. Performance Assessment
In the next set of Figures, we will present the signals obtained in the performance assessment
of this configuration. In order to evaluate the performance more effectively new optical sources
where used. A tunable laser with good power output was applied to Input 2 with central
wavelength with center at λ1 = 1548.8 nm. At Input 1 a Fiber Laser was used to create a signal
with two wavelengths λ2 =1549.6 nm and λ3 = 1550.4 nm. This fiber laser is built as depicted in
Figure 63.
EDFA
APC
FBGs
80:20
Circulator
PC
Losses
λ2
λ3
Figure 63 - Multiwavelength Fiber Ring Laser Source using Fiber Bragg Gratings.
The optical signals from these sources are depicted in Figure 64.
Chapter 7– Work Developed
78 of 90 - IP over WDM I
1546 1548 1550 1552-60
-50
-40
-30
-20
-10
0
Input Port 1 Input Port 2
Tran
smis
sion
(dB
m)
λ(nm)
Figure 64 - Input Signals
As a first step in this test, the optical filters (FBGs) where detuned from any of the input
wavelengths and the signals at the corresponding Outputs where measured. The results of this
test are shown in Figure 65.
1546 1548 1550 1552
-60
-50
-40
-30
-20
-10
0
Output 1 Output 2
Tran
smis
sion
(dB
m)
λ(nm)
Figure 65 - Output port signals with optical filters detuned.
As seen in this Figure, the wavelength channels are properly sent directly to their
correspondent outputs, i.e., Output 1 for channel 2 and 3; Output 2 for channel 1. The small peak
at 1548.8 nm that appears in the signal form Output port 1 is Heterodyne Crosstalk and has a
power level of -45 dBm which is fairly negligible. Insertion Losses are calculated with respect
to the Input signals and are in this case 1.31 dB. These losses are originated in the circulators and
fiber splices and grating (optical filter) imperfections. The Interchannel Crosstalk Isolation Level
Chapter 7– Work Developed
IP over WDM I - 79 of 90
is 30.72dBm. The Homodyne Crosstalk Isolation Level is 30.89dBm. These are very good
isolation levels even when comparing this device with commercial ones.
In the next step, the optical filters (FBGs) where tuned to λ1 = 1548.8 nm in order to switch
channel 1 from Input port 2 to Output port 1. The results are in Figure 66.
1546 1548 1550 1552-80
-70
-60
-50
-40
-30
-20
-10
0
Output 1 Output 2
Tran
smis
sion
(dB
m)
λ(nm)
Figure 66 - Outputs when Channel 1 is switched from Input 1 to Output 2.
As seen in this result, Channel 1 is effectively switched to Output port 1. Due to the increase
in the reflections in optical filters and also an increase in the number of times a signal has to
enter an optical circulator (thereby suffering from insertion losses) the total Insertion Losses in
this case have increased to 4 dB. In Output port 2 a -38.2 dBm Crosstalk level is seen. The
Homodyne and Heterodyne Crosstalk Isolation Levels are, respectively 20.4 dB and 21.4 dB.
These are very good results and point to the good performance of this device.
Another performance issue to be addressed is the tuning speed. The temperature control
gives stable results but only slow tunability is achieved. This can be seen in the Figure 67 where
the tuning and detuning speed of the optical filter (FBG) is seen.
Chapter 7– Work Developed
80 of 90 - IP over WDM I
0 5 10 15 20 25 30 35 40
-20
-15
-10
-5
0
FBG tuning FBG detuning
Tran
smis
sion
(dB
)
Time (sec)
Figure 67 - Tuning and detuning speed.
When tuning, i.e., reaching a specified wavelength in order to reflect it, the achieved speed is
around 10 seconds. But it should be noted that detection of the desired channels begins before
the stable wavelength is reached. As the temperature that reaches the grating increases, the
central wavelength reflected by the grating changes until it reaches the desired wavelength. To
show the stability of this temperature control two figures are presented next. In Figure 68 we can
see the power level of the Crossconnect’s Output signal as the optical filter is being tuned.
According to the wavelength that appears in the Output port, the power level increases or
decreases. The important detail to notice is the fact that, once in a specific wavelength, the
device maintains stability. This can be noticed much easily in Figure 69.
Chapter 7– Work Developed
IP over WDM I - 81 of 90
0 20 40 60 80 100 120 140 160 180 200 220 240-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
Tran
smis
sion
(dB
)
Time (sec.)
Figure 68 - Tuning and detuning of the optical filter (FBG).
0 20 40 60 80 100 120 140 160 180 200 220 2401550,4
1550,5
1550,6
1550,7
1550,8
1550,9
1551,0
1551,1
1551,2
λ (n
m)
Time (sec.)
Figure 69 - Wavelength tuning and detuning of the optical filter (FBG).
The optical filter (FBG) seems to be a limiting component in all the configurations presented.
The filter used, although having near 100% reflectivity, does not entirely block incoming signals.
It blocks signal reducing their level by a maximum of 15 dB. Better filters could effectively be
obtained but would come to the expense of wider spectral bandwidth that would cause
interference with adjacent channels. This setback could be avoided if the apodized masks for
fabrication where available.
Chapter 7– Work Developed
82 of 90 - IP over WDM I
7.5. Summary
In this chapter, a description of the practical implementation of our project was made. An
introductory section about the grating fabrication process was presented and subsequently, the
structures implemented where described and their performance tests where shown.
In the next Chapter final conclusions about our work are given and justified.
Chapter 8– Concluding remarks
IP over WDM I - 83 of 90
Chapter 8 – Concluding remarks
In this report we reviewed the fundamental aspects of this project and a theoretical
introduction about its area of interest – Optical Networks – is made. This enabled us to be aware
of the environment that surrounds this area of research and its importance in the
telecommunications scenario.
We reviewed the most important advances in optical networks and their evolution through
successive generations up to the present time. The paths taken to overcome the main problems of
the first two generation are described, in particular the electronics bottleneck problem.
After that, a review of the Multiplexing solutions possible is presented, namely, Optical Time
Division Multiplexing – OTDM, Optical Code Division Multiplexing – OCDM and finally,
Wavelength Division Multiplexing – WDM, which is the most pervasive optical technology
nowadays. Optical Components and network impairments are also described (Chapters 3 and 4).
Fiber Bragg Grating structures are reviewed in Chapter 5 and the state-of-the-art in Optical
Switching is presented.
Finally, attention is given to the practical implementation of the devices developed in the
project. The designed structures and performance tests made are described. As a final conclusion
we can say that the OXC has very good performance although different approaches to its control
could be made.
The Optical Crossconnect configurations, specially the last one described has very good
spectral characteristics which results in low homodyne and heterodyne crosstalk levels which
subsequently permits high cascading levels bringing this 2x2 structure to generalized NxN port
configurations.
In order to improve the spectral characteristics of the filters used, thereby reducing the
crosstalk levels and improving the performance of our solution, apodized grating filters should
be used. Unfortunately there were no facilities to fabricate apodized grating filters but reference
to this fact will be made later on in this report.
The long run goal of this project was to develop good insight on the performance
characteristics and specific impairments of this type of technology and integrate it with routing
protocols, namely, GMPLS – Generalized Multiprotocol Label Switching, that enables the
Chapter 8– Concluding remarks
84 of 90 - IP over WDM I
upgrade of this device to a fully functional Optical Router with completely optical interfaces,
removing the O-E-O conversion which gives birth to the Electronics Bottleneck. This is an area
of intense research at this moment and several solutions to the underlying physical architecture
are in development. Nonetheless, we think that this technology has very good perspectives in
terms of market implementation because of its very good price/quality ratio. This means that it
has visible cost benefits considering the performance levels reached.
Considering other technologies this is the only one where wavelength and spatial switching is
performed at the same time. The wavelength selection, that is, splitting of the WDM channels in
a multiwavelength signal is done in a manner that is inherent to the device itself, because
gratings are, by nature, wavelength selective filters and select the channels in the composite
signal. Other technologies demand the use of prisms or other type of wavelength splitting
technologies to be used and this comes at the expense of excessive losses and switching
complexity. For example in MEMs technologies, much discussed at this time, each wavelength
has to use its specific wavelength switching plane. This increases the complexity and cost of this
solution and severely affects its flexibility.
There is a possibility of controlling the tuning of the optical filters (FBGs) through strain
using piezoelectric control and obtain higher tuning speeds but this also has its setbacks due to
the still necessary temperature control of the gratings which will complicate the control circuitry
and stability requirements.
The mentioned integration with routing protocols is one of the objectives for the other part of
this project, developed in another research unit (UTM – Telecommunications and Multimedia
Unit), which we believe was carried out also with great success.
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IP over WDM I - 85 of 90
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Appendix A - Peltier Devices 1
Appendix A - Peltier Devices
A. Introduction
Peltier devices, also known as thermoelectric (TE) devices, are small solid-state
devices that work as heat pumps. A "typical" unit is a few millimeters thick by a few
millimeters to a few square centimeters. It is a sandwich formed by two ceramic
plates with an array of small Bismuth Telluride cubes ("couples") in between. When a
DC current is applied heat is moved from one side of the device to the other - where it
must be removed with a heat sink. The "cold" side is commonly used to cool an
electronic device such as a microprocessor or a photodetector. If the current is
reversed the device makes an excellent heater.
B. Peltier & Seebeck Effects
The cooling property of these devices is due to the Peltier Effect, while the electrical
power generating property is due to the Seebeck Effect. A thermoelectric device can
be used as either a cooler or a power generator, but not with the best efficiency.
Peltier Effect devices are almost always constructed with Bismuth Telluride (Bi2Te3)
and used around room temperature and below. Seebeck Effect power generators are
often constructed of PbTe or, SiGe as well as Bi2Te3 and are used at much higher
temperatures.
In theory, the Peltier effect is explained the following way: electrons speed up or slow
down under the influence of contact potential difference. In the first case the kinetic
energy of the electrons increases, and then, turns into heat. In the second case the
kinetic energy decreases and the joint temperature falls down. In case of usage of
semiconductors of p and n types the effect becomes more vivid. On Figure 1 you can
see how it works.
Figure 1 - Usage of semiconductors of p and n-type in thermoelectric coolers
Appendix A - Peltier Devices 2
Combination of many pairs of p and n semiconductors allows the creation of cooling
units - Peltier devices of relatively high power (Figure 2). Has we have previously
seen, thermoelectric cooling couples (Figure 1) are made from two elements of
semiconductor, primarily Bismuth Telluride, heavily doped to create either an excess
(n-type) or deficiency (p-type) of electrons. Heat absorbed at the cold junction is
pumped to the hot junction at a rate proportional to current passing through the circuit
and the number of couples.
Figure 2 - Structure of a Peltier device
In resume, a Peltier device (Figure 3a)) consists of semiconductors mounted
successively, which form p-n and n-p junctions. Each junction has a thermal contact
with radiators. When switching on the current of the definite polarity, there forms a
temperature difference between the radiators: one of them warms up and works as a
heat sink, the other works as a refrigerator.
Figure 3 – a) Peltier device; b) Peltier cascade
A typical device provides a temperature difference of several tens degrees Celsius.
With forced cooling of one of the hot radiators, the other one can reach temperatures
Appendix A - Peltier Devices 3
below 0 ºC. For more significant temperature differences the cascade connection is
used. (Figure 3 b)) The cooling devices based on Peltier devices are often called
active Peltier refrigerators or Peltier coolers. Peltier device's power depends on its
size. Low power devices might not be efficient enough. But the usage of the devices
of too high power might cause moisture condensation, what is dangerous for
electronic circuits.
Thermoelectric devices are not the solution for every cooling problem. However, you
should consider them when your system design criteria include such factors as high
reliability, small size or capacity, low cost, low weight, intrinsic safety for hazardous
electrical environments, and precise temperature control. i,ii,iii,iv,v
C. Competitive Advantages
It is possible to build thermoelectric systems in a space of less than 1 cubic inch.
More typically, thermoelectric systems occupy about 20 to 30 im3 of space. These
systems are energized by a DC power input. In addition to the space and weight
saving advantages, thermoelectrics offer the utmost in reliability due to its solid-state
construction. Another feature of importance is the ease with which a thermoelectric
can be precisely temperature controlled, which is an important advantage for
scientific, military and aerospace applications.
D. Technical aspects
Peltier device cooling & heating speed - they can change temperature extremely
quickly, but to avoid damage from thermal expansion control the rate of change to
about 1ºC per second.
These devices can heat as well as cool - they make great heaters. Just reverse the
polarity of the power supply. But, be sure not to exceed the temperature rating of the
devices, usually 80ºC for standard models to 200ºC for high temperature models.
The power supply requirement is a simple DC supply, if the AC ripple is not more
than about 10% or 15%. The devices specified Vmax should not be exceeded.
E. Peltier in Optical Communications
Peltier Devices have encountered widespread use in the temperature stabilization of
optoelectronic components (lasers, switches, detectors, etc.) in high speed and
wavelength division multiplexed (WDM) fiber optic communication systems. This is
Appendix A - Peltier Devices 4
even more so in dense WDM systems where the spacing between adjacent
wavelengths can be from 0.8nm (100GHz) to as small as 0.2nm (25GHz) vi. Since
typical InGaAsP-based DFB lasers operating around 1.55 µm have a wavelength drift
of approximately 0.1 nm/°C, the temperature must be controlled to less than a degree
of variance to prevent excessive loss in multiplexers/demultiplexers or crosstalk
interference. While Peltier devices have successfully met this requirement, they have
added greatly to the total cost of components since they are not easily integrated with
devices vii.
Another disadvantage to the use of Peltier devices is the large mismatch in thermal
mass between that of the cooler and the device. The smallest Peltier devices are a
couple of millimeters squared, whereas a typical optoelectronic device is an order of
magnitude smaller. Much work is currently underway in thin film thermoelectric
refrigeration for other applications, however the same problems of integration with
optoelectronics still exist. The InGaAsP/InP family of materials has poor
thermoelectric properties due to the inherently small Seebeck coefficient viii. However,
the use of thermionic emission in heterostructures was recently proposed and has been
demonstrated in the InGaAsP system to increase the cooling power ix,x
F. Peltier Device in IP Over WDM
The usage of the Peltier Device in this project has the unique purpose of tuning or
detuning the fiber Bragg grating, since the grating period of a FBG depends on
temperature, to reflect or not the wavelength channels according on the switching
requirements. The FBG was placed over the thermoelectric Peltier device, so that its
temperature could be easily set and controlled. The electronic circuit designed to
control the Peltier Device is briefly described in Appendix B.
i http://www.peltier-info.com/ ii http://www.peltierelement.com/english/peltierelement/index.peltierelement.html iii http://www.overclockers.com/topiclist/index21.asp#PELTIERS iv http://www.toomeycomputers.com/Peltiers.htm v http://www.melcor.com/handbook.htm
Appendix A - Peltier Devices 5
vi Y. Yamada, S.I. Nakagawa, K. Takashina, T. Kawazawa, H. Taga, K. Goto, “25GHz spacing ultra-
dense WDM transmission experiment of 1 Tbit/s (100WDM x 10Gbit/s) over 7300km using non pre-
chirped RZ format”, IEEE Elec. Lett., 35, pp. 2212, 1999. vii L. Rushing, A. Shakouri, P. Abraham, J.E. Bowers, “Micro theroelectric coolers for integrated
applications”, Proceedings of 16th International Conference on Thermoelectrics, Dresden, Germany,
August 1997. viii A. Shakouri, C. LaBounty, “Material Optimization for Heterostructure Integrated Thermionic
Coolers”, Proceedings of 18th International Conference on Thermoelectrics, Baltimore, MD, USA
August (1999). ix A. Shakouri, E.Y. Lee, D.L. Smith, V. Narayanamurti, J.E. Bowers, “Thermoelectric effects in
submicron heterostructure barriers”, Microscale Thermophysical Engineering 2, 37 (1998). x C. LaBounty, A. Shakouri, P. Abraham, J.E. Bowers, “Integrated cooling for optoelectronic devices,
Proceedings of SPIE Photonics West Conference”, San Jose, CA, USA, Jan. 2000
Appendix B – Peltier Electronic Controller 1
Appendix B – Peltier Electronic Controller
A. Introduction
To control Bragg wavelengths by temperature alteration it was necessary make use of
the Peltier device described in the previous appendix. To control this device a precise
electronic control circuit is required. Basically the Electronics’ Peltier controller is
composed of three different stages: Digital Controller, Analogical Controller and a
Power Circuit, to which the Peltier Device is connected. The block diagram of the
Peltier Controller is shown in Figure 1.
Figure 1 - Peltier Controller Stages
B. Digital Controller
As described bellow the first of the three stages of the Peltier Controller is the Digital
Controller. In reality it wasn’t necessary to design this part of the circuit, since a
circuit with similar functionalities had already been designed for a previous project in
Lasers Modulators at UOSEi. That digital controller has the capability of generating
digital voltage steps (Vdac), which will be used to control the Peltier device current
(IP). The voltage Vdac is controlled with a microcontroller of the 80C51 family and the
selection of the multiple voltage steps could be done with a personal computer (PC)
(since that stage is connected to the PC through the serial port) or with a simple
keyboard connected directly to the digital controller.
C. Analogical controller ii
The purpose of the second stage, the Analogical Controller, is to convert the digital
signal VDAC into analogical voltage signals (VA1(1), VB1(1), VA2(1), VB2(1)) that will be
sent to the Power Circuit. The electronics behind the Analogical Controller is quite
simple and based in several operational amplifiers µA741, and some passive
PeltierDevice
V dac V A1
V B1 VA2
V B2
1
2
Analogical Controller Power Circuit Digital Controller
Appendix B – Peltier Electronic Controller 2
components such as resistors, potentiometers, diodes, and capacitors. The schematic
representation of this circuit is shown in Figure 2.
Figure 2 - Peltier Device Analogical Controller
The controller’s configuration is described next through a series of schematics.
a. Input stage
In Figure 3 the configuration of the controller Input stage is shown.
Figure 3 - Input stage
As seen in the previous figure the input stage is a non-inverting configuration with a
voltage reference at the non-inverting input terminal. Equation 1 describes this circuit
configuration.
12
2
2
22 1 REF
up
downdac
up
downoutU V
RVRVV
RVRVV •
−•
+=
Equation 1
Appendix B – Peltier Electronic Controller 3
The Vdac voltage variation is from 0V to 5V. VREF1 has a constant value as will be
calculated next. The VU2out signal varies between +15V and –15V and his value is 0V
when Vdac=2.5V.
b. VREF1 Generator
In the Figure 4 is shown the configuration of the VREF1 Generator.
Figure 4 - VREF1 Generator
The operational amplifier U1 works as buffer. The Equation 2 that describes a simple
voltage divider gives his output voltage.
downup
upREF RVRV
RVV
11
11 30
+•=
Equation 2
In this project the voltage VREF1 used is equal to 3V. It’s interesting to detach that this
value of VREF1 imposes VU2out=0V when Vdac is equal to 2.5V.
c. VREF2(1) and VREF2(2) Generators
In Figure 5 the configurations of the VREF2(1) and VREF2(2) Generators are shown.
Figure 5 - VREF2(1) and VREF2(2) Generators
Appendix B – Peltier Electronic Controller 4
The op. amps. U3 e U4 work as comparators with reference voltages in the inverting
terminal. The Equation 3 gives these reference voltages.
updown
downREF
updown
downREF RVRV
RVVRVRV
RVV44
4)2(2
33
3)1(2 15;15
+•=
+•=
Equation 3
Both reference voltages VREF2(1) and VREF2(2) are equal to 0.1V. The purpose of these
low voltage levels is to provide a faster response to this configuration when the VU2out
starts increasing (case of U3) or decreasing (case of U5).
d. Inverting configuration with unitary gain
In Figure 6 the configuration of the inverting configuration for a amplifier with a
unity gain is shown.
Figure 6 - Inverting configuration with unitary gain
In the Equation 4 R5 must equal R4 allowing the inverting amplifier to have unity
gain.
outUB VRRV 2
4
52 •−=
Equation 4
e. The Diodes functions
D1 allows positive voltage excursions to reach operational amplifier U3 and the
output B1, while D2 allows negative voltage signal excursions to the operational
amplifiers U4.
D3 and D4 are respectively connected to the op. amps. U3 e U5 with the purpose of
providing only 2 steps, high or null to the output signals of A1 and A2.
Appendix B – Peltier Electronic Controller 5
f. The resistors R3 and R6 functions
The insertion of these resistors in the digital controller circuit has the unique purpose
of avoiding voltage floating points which could impair the circuit’s performance.
g. The Analogical controller functional description
The functional description of this circuit is done in a very simple way appealing to the
following Table 1.
Vdac (V) VU2out (V) VA1(1) (V) VB1(1) (V) VA2(1) (V) VB2(1) (V) 0 -15 0 0 0 13.6 ⇓ -15 ⇒ 0 0 0 0 ⇒14.3 13.6
2.5 0 0 0 0 0 ⇑ 0 ⇒ 15 0 ⇒14.3 13.6 0 0 5 15 0 13.6 0 0
Table 1
D. Power Circuit ii
Since the analogical control doesn’t supply large currents and the Peltier device drives
lots of power, i.e., it needs large currents to operate, it was necessary to design a
Power circuit that works as an interface between the control and the Peltier device,
producing enough current (IP) to drive the Peltier device. This Power circuit is shown
in Figure 7.
Figure 7- Power circuit
Appendix B – Peltier Electronic Controller 6
The previous circuit is based in two sets of transistors, one composed by four bipolar
transistors (Q2, Q4, Q6 and Q8) and another composed by four bipolar power
transistors (Q1, Q3, Q5, and Q7). The pairs Q1 and Q2, Q3 and Q4, Q5 and Q6, Q7
and Q8, are Darlington pairs. The use of the Darlington configuration is very
important in this case because the output current of the Analogical controller is very
small. Using Darlington configuration reduces the base current requirements for a
power stage and provides a very high current gain. This happens because the current
gains of both transistors are multiplied by each other.
The entry’s 1 and 2 are respectively connected to the outputs 1 and 2 of the analogical
controller. When Vdac spans between 2.5V and 5V the Peltier device acts as a heat
source. The Darlington pair composed by Q1 and Q2 acts like an interrupter while the
other one composed by Q7 and Q8 provides the increase or decrease of the Peltier
current by changing the Darlington base voltage. When the Vdac spans between 0V
and 2.5V the Peltier device act as a cooling source because the other entries (3 and 4)
provide voltage levels that make this possible.
The Peltier device is connected to the output ports (1 and 2 – interface P6) of the
Power circuit stage.
E. Peltier device controller pictures
Figure 8 – Analogical controller Stage
Appendix B – Peltier Electronic Controller 7
Figure 9 - Board with four analogical controller stages
Figure 10 - Power Circuit
i V. Barbosa, “Transmissor Óptico Analógico”, INESC Porto - UOSE, 2001 ii A. S. Sedra, K. C. Smith, “Microelectronic Circuits”, Oxford University Press, 1998
Appendix C – OADM paper submitted in Física 2002 1
Appendix C – OADM paper submitted in Física 2002
AVALIAÇÃO DE DUAS ARQUITECTURAS DE OADM BASEADAS EM CIRCULADORES ÓPTICOS E REDES DE BRAGG EM FIBRA ÓPTICA
I. Terrosoa#, J. P. Carvalhoa#, O. Frazãoa, M. Ricardoa,b , H. M. Salgadoa,c a INESC Porto - UOSE , Rua do Campo Alegre 687, 4169-007 Porto – Portugal
b INESC Porto - UTM , Praça da República 93, 4050-497 Porto, Portugal c FEUP - DEEC, Rua Dr. Roberto Frias, 4200-465 Porto - Portugal
A tecnologia de multiplexagem em comprimento de onda DWDM – Dense Wavelength Division Multiplexing tem evoluído rapidamente pois parece ser a única capaz de satisfazer a crescente necessidade de largura de banda em redes de comunicações ópticas. O aumento do tráfego nas redes de comunicação por fibra óptica, resultante da procura de serviços de Internet e múltimedia, impõe o desenvolvimento de redes baseadas na tecnologia DWDM, para aumento da capacidade, que sejam simultaneamente capazes de efectuar o encaminhamento e comutação de comprimentos de onda, por oposição a sistemas de transmissão ponto-a-ponto. A implementação dessas técnicas de remoção de canais requerem o desenvolvimento de nós ópticos de remoção e adição de canais – OADM Optical Add-Drop Multiplexers. Estes dispositivos são utilizados para remover e adicionar selectivamente um ou vários canais numa rede óptica DWDM, aumentando desta forma a sua flexibilidade. Neste artigo apresenta-se um OADM baseado em circuladores ópticos e em redes de Bragg em fibra óptica (FBG – Fiber Bragg Gratings) [1]. A diafonia (Crosstalk) é um dos problemas de OADM’s deste tipo. Neste trabalho apresenta-se uma avaliação do desempenho de duas arquitecturas OADM (ver figura 1) em termos de diafonia. O OADM apresentado na figura 1, utiliza uma FBG que está estabilizada através de um elemento de Peltier a uma temperatura constante. As FBGs reflectem um determinado comprimento de onda de acordo com a condição em que se encontra a rede de difracção de Bragg segundo a seguinte equação, λB = 2 neff Λ em que neff é o índice de refracção efectivo do modo de propagação guiado, e Λ é o período de modulação do índice de refracção na fibra.
(λ1,λ2,λ3)
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Figura 1) Representação das duas arquitecturas em estudo
# Telf: 22.608.2601 Fax: 22.608.2299 e-mail: [email protected] , [email protected]
Appendix C – OADM paper submitted in Física 2002 2
Na Figura 1 temos representadas as duas arquitecturas em estudo. A Figura 1 a) representa o OADM convencional [2] onde existe diafonia a prejudicar o desempenho, e na Figura 1 b) já temos uma arquitectura capaz de reduzir significativamente a diafonia.
Na Figura 2 estão representados os espectros (reflexão/transmissão) do FBG usado no OADM. O filtro óptico é uma rede de Bragg em fibra óptica centrada em 1550,9 nm, com uma reflectividade próxima de 100% e FWHM igual a 0,2 nm. Foram introduzidos 3 canais WDM espaçados a 100 GHz, nas duas arquitecturas. Na Figura 3 estão representados os três canais introduzidos nos OADM’s na porta de entrada (Sinal a tracejado – A). O sinal B é o sinal à saída do OADM após a remoção do canal 2 e está livre de diafonia pois o filtro óptico apresenta caracte-rísticas excelentes de filtragem. O sinal C, que é obtido na porta de remoção do OADM (referente à primeira arquitectura) já apresenta picos de diafonia referentes aos canais 1 e 3. Estes picos são eliminados graças à arquitectura do OADM (segunda arquitectura) que apresenta FBGs na saída de remoção para eliminar os picos de diafonia presentes na primeira arquitectura, como se pode observar na figura 4, em que se compara o sinal C, obtido com a primeira arqui-tectura e o sinal D obtido com a segunda arquitectura. Foram apresentadas duas arquitecturas de OADM’s baseadas em dois circuladores ópticos e FBG’s. A segunda arquitectura mostrou um melhor desempenho em relação à primeira em termos de diafonia. Referências [1] C.R. Giles, Journal of Lightwave Technology, Vol. 15, Nº. 8, August 1997 [2] P.S. André e J.L. Pinto, I. Abe, H.J. Kalinowski, O. Frazão, F.M. Araújo, Journal of Microwaves and
Optoelectronics, Vol. 2, N.º 3, July 2001.
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Appendix D – OXC architecture paper submitted in Física 2002 1
λ1, λ2 , λ3
Entrada 1
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Saída 2
Fig.1 – Arquitectura do comutador
óptico (OXC)
Appendix D – OXC architecture paper submitted in Física 2002
COMUTADOR ÓPTICO (OXC) BASEADO EM CIRCULADORES ÓPTICOS E NUMA REDE DE BRAGG EM FIBRA ÓPTICA
J. P. Carvalhoa#, I. Terrosoa#, O. Frazãoa, V. Barbosaa, M. Ricardob,c, H. M. Salgadoa,c a INESC Porto - UOSE , Rua do Campo Alegre 687, 4169-007 Porto – Portugal
b INESC Porto - UTM , Praça da República 93, 4050-497 Porto, Portugal c FEUP - DEEC, Rua Dr. Roberto Frias, 4200-465 Porto - Portugal
O incremento nas taxas de transmissão de dados devido ao uso cada vez mais acentuado da Internet e de outras aplicações multimédia tornaram as redes de telecomunicações totalmente ópticas, baseadas na tecnologia de multiplexagem densa de comprimento de onda (DWDM-Dense Wavelenght Division Multiplexing), no candidato maioritário à constituição do backbone que suportará o tráfego global de dados num futuro próximo. O DWDM e o consequente recurso às técnicas de encaminhamento por comprimento de onda, tornam os comutadores ópticos (OXC-Optical Cross Connect) com selecção de comprimento de onda, dispositivos chave neste tipo de redes, dado que permitem aos pontos terminais de rede a possibilidade de comunicarem de forma transparente, flexível e reconfigurável. As propostas para arquitecturas de OXC’s têm sido várias, nomeadamente: as tecnologias baseadas em micro-espelhos reguláveis sobre bases de silício (MEMS-Micro Electro Mechanical System), guias de onda baseados em bolhas, e ainda soluções baseadas em redes de Bragg em fibra óptica (FBG-Fiber Bragg Gratings) [1]. Um grave problema destes sistemas de encaminhamento de comprimento de onda é a diafonia (crosstalk), que causa uma degradação acentuada na performance do sistema. Esta pode ser de dois tipos: heterodina (heterodyne crosstalk) – resultante da interferência de pequenas fracções de potencia fora da banda do canal, ou homodina (homodyne crosstalk) – resultante de interferências dentro da banda do canal. Neste artigo apresenta-se uma arquitectura de um OXC de 2 × 2 portas baseada numa FBG e circuladores ópticos [3]. Apresenta-se uma avaliação do comportamento deste dispositivo (OXC) face a variações controladas da FBG para o reencaminhamento dos comprimentos de onda de entrada. A configuração utilizada é representada na Figura 1, e consiste em dois circuladores (JDS Uniphase) e uma FBG sintonizável controlada por um elemento de Peltier. O comprimento de onda de Bragg λB ocorre quando a constante de propagação do modo guiado no núcleo se encontra em ressonância com a modulação espacial do índice estabelecendo a condição de Bragg: λB = 2 neff Λ, em que neff é o índice de refracção efectivo, Λ é o período de modulação do índice de refracção na fibra [2]. Foi fabricado uma FBG do tipo uniforme com
comprimento de onda 1550.7 nm, FWHM de 0.2 nm e uma reflectividade de próxima de 100%. O desempenho do OXC apresentado foi testado com três canais WDM na entrada 1, de comprimentos de onda λ1=1549.9 nm, λ2=1550.7 nm e λ3=1551.5 nm, espaçados de 0.8 nm, o que corresponde a um intervalo em frequência de 100 GHz. Quando o filtro óptico FBG se encontra estabilizado à temperatura ambiente, a FBG está sintonizada em λM=λ2, encaminhando dessa forma o canal 2 para a saída 1 do OXC, enquanto que os outros
# Telf: 22.608.2601 Fax: 22.608.2299 e-mail: [email protected] , [email protected]
Appendix D – OXC architecture paper submitted in Física 2002 2
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Fig.2 – Potência Espectral no comutador óptico: a) Canal 2 sintonizado e encaminhado para a porta 1; b) Dessintonia da rede de Bragg e encaminhamento dos três canais para a saída 2.
dois canais são direccionados para a saída 2. Na Figura 2 a) verificamos que a rejeição feita pelo FBG dos canais λ1 e λ3 é de -16.23 dB (saída 2). As pequenas componentes espectrais centradas em λ1 e λ3, devem-se sobretudo a reflexões residuais no FBG e nos circuladores e originam diafonia heterodina. Numa segunda fase a rede de Bragg é dessintonizada, por variação de temperatura e o comprimento de onda de Bragg alterado para um intermédio entre λ2 e λ3 (λM=1551.1 nm). Na Figura 2 b) podemos ver o traçado da potência dos sinais em ambas as saídas deste dispositivo, sendo que os três canais de entrada foram encaminhados para a saída 2. Na saída 1 é verificado um nível de sinal com menos 10.53 dB que na saída 2, resultante da reflexão do FBG que provocará diafonia homodina. Note-se que a redução acentuada dos níveis de diafonia referidos anteriormente pode ser
conseguida recorrendo ao uso de FBG’s apodizados. É importante notar que existem perdas associadas ao trânsito de potência duma porta dum circulador para a porta adjacente.
Neste caso essas perdas são de 1.2 dB e dado que cada canal tem de sofrer obrigatoriamente duas perdas deste tipo, assim se explicam os valores máximos de potência registados nas saídas que rondam os -2.4 dB. Este dispositivo apresenta como limitação a impossibilidade de introdução em portas adjacentes do mesmo canal dado que esta situação provocaria níveis de diafonia elevadíssimos no caso da FBG estar sintonizada e colisões do sinal óptico no caso da dessintonia. A configuração estudada é ainda totalmente escalável sendo possível através de blocos básicos do OXC de 2 × 2 portas elaborar comutadores de N × N portas em que cada canal pode ser encaminhado para qualquer uma das portas de saída desde que seja respeitada a limitação acima referida. Embora nesta configuração apenas seja utilizado um filtro óptico FBG, existe ainda a possibilidade de se usarem M filtros ópticos que no limite seriam tantos quanto o número de diferentes comprimentos de onda introduzido nas portas deste OXC. O comutador óptico aqui descrito e estudado apresentou um desempenho razoável. Actualmente estão a ser estudados e implementadas novas configurações de OXC’s com reduzida diafonia baseadas nesta mesma arquitectura. Referências [1] Y. W. Song, Z. Pan, D. Starodubov, V. Grubsky, E.Salik, S. A. Havstad, Y. Xie, A. E.
Willner, J. Feinberg, “All-Fiber WDM Optical Crossconnect Using Ultrastrong Widly Tunable FBGs", IEEE Photonics Technologhy Letters, VOL. 13, NO. 10, Outober 2001.
[2] Andreas Othonos, Kiriacos Kalli, "Fibre Bragg Gratings – Fundamentals and Applications in Telecommunications and Sensing”,Artech House, London, 1999.
[3] Xiangnong Wu, Xau Lu, Z. Ghassemlooy, Yixin Wang, "Evaluation of Intraband Crosstalk in an FBG-OC-Based Optical Cross Connect”, IEEE Photonics Technologhy Letters, VOL. 14, NO. 2, February 2002.
PART II
INESC - UTM Porto, July 2002
Licenciatura em Engenharia Electrotécnica e de Computadores Ramo de Telecomunicações, Electrónica e Computadores
Graduation project – DEEC
IP over WDM
Designing an Optical IP Router
Part II
Supervisors Students Henrique Salgado, PhD Manuel Ricardo, PhD
Bruno Leite Fernando Pinto
Table of contents
i
Table of contents
Table of figures............................................................................................................................... ii Chapter 1 – Introduction..................................................................................................................1
Abstract ........................................................................................................................................................................1 Document Structure .....................................................................................................................................................1 Background ..................................................................................................................................................................2
Telecommunications Networks – the optical era ...................................................................................................2 Data traffic emergence – the Internet revolution....................................................................................................2 Moving to a distributed standardized control plane ...............................................................................................3
The Objectives..............................................................................................................................................................4 Our work.......................................................................................................................................................................5
Chapter 2 - Optical technology networks ........................................................................................6 Background ..................................................................................................................................................................6 Network architectural considerations ..........................................................................................................................6
The usage of WDM technology..............................................................................................................................6 Mesh topology vs ring topology .............................................................................................................................6 A new control plane is required. .............................................................................................................................6 Network integration models – IP over WDM protocol stack. ...............................................................................7
Layer 2 technologies in use..........................................................................................................................................8 Synchronous Digital Hierarchy...............................................................................................................................9 Optical Transport Network G.709 ........................................................................................................................10 Optical Internetworking Forum - UNI 1.0............................................................................................................11 GigaBit Ethernet....................................................................................................................................................12
Migrating from management based solutions to automatic routing and provisioning.............................................15 Moving to IP protocol arena ......................................................................................................................................16 Using MPLS framework ............................................................................................................................................16 Evolving to GMPLS...................................................................................................................................................17
The need for different control plane models ........................................................................................................18 Conclusions ................................................................................................................................................................18
Chapter 3 - GMPLS.......................................................................................................................19 Architecture................................................................................................................................................................19
Switching domains ................................................................................................................................................19 1. Fiber-Switch Capable (FSC)...................................................................................................................................................................19 2. Wavelenght Switch Capable (λSC) ........................................................................................................................................................20 3. Waveband Switch Capable (WSC).........................................................................................................................................................20 4. Time Division Multiplexing Capable (TDMC)......................................................................................................................................20 5. Packet Switch Capable (PSC).................................................................................................................................................................20
OXC control plane functions ................................................................................................................................20 Control plane..............................................................................................................................................................21
1. Link management....................................................................................................................................................................................21 2. Intra domain routing protocols ..............................................................................................................................................................22 3. Inter domain routing protocols ..............................................................................................................................................................22 4. Signaling protocols .................................................................................................................................................................................22
Inband/Outband control channel................................................................................................................................23 Traffic Engineering ....................................................................................................................................................24 Conclusions ................................................................................................................................................................24
Chapter 4 - Our approach ..............................................................................................................25 Core architecture. .......................................................................................................................................................25 How different components interact ...........................................................................................................................26
Forward Information Base (FIB) ..........................................................................................................................26 Traffic Engineering Topology DB........................................................................................................................26 LMP.......................................................................................................................................................................26 OXC Controller .....................................................................................................................................................26 RSVP-TE and OSPF-TE.......................................................................................................................................27 Traffic Engineering control...................................................................................................................................27
Signaling (RSVP and CR-LDP) ...............................................................................................................................27 How to request an LSP..........................................................................................................................................28 Generalized Label Request ...................................................................................................................................29 Label Suggestion by the Upstream .......................................................................................................................30 Label Restriction by the Upstream .......................................................................................................................30
Table of figures
Routing (OSPF and BGP)..........................................................................................................................................31 The roots of GMPLS routing ................................................................................................................................31
Some challenges to consider …..................................................................................................................................................................32 …and some possible solutions....................................................................................................................................................................32
Link management (LMP)...........................................................................................................................................34 Managing links ......................................................................................................................................................35 LMP for DWDM Multiplexer...............................................................................................................................35
Traffic Engineering ....................................................................................................................................................36 Conclusions ................................................................................................................................................................37
Chapter 5 – The proposed architecture..........................................................................................38 OXC Controler ...........................................................................................................................................................38 RSVP ..........................................................................................................................................................................43
OXC controller communication functions. ..........................................................................................................43 RSVP signaling messages.....................................................................................................................................44
1. Generalized label request object .............................................................................................................................................................44 2. Generalized label object..........................................................................................................................................................................45 3. Suggested label object.............................................................................................................................................................................45 4. Label Set object.......................................................................................................................................................................................46
Communication interface...........................................................................................................................................47 Using a PCI board. ................................................................................................................................................47
Conclusions ................................................................................................................................................................48 Chapter 6 – Conclusions................................................................................................................49 References .....................................................................................................................................51
Papers .........................................................................................................................................................................51
Table of figures Figure 1 – Optical Network Model..................................................................................................5 Figure 3 - Protocol Stack evolution for IP-over-WDM solutions. ..................................................7 Figure 4 – 10 Gigabit Ethernet Protocol Stack..............................................................................13 Figure 5 - Optical Media for 10Gigabit Ethernet . ........................................................................14 Figure 6 - 10 Gigabit Ethernet in metropolitans network.............................................................14 Figure 7 – Switching domains. ......................................................................................................19 Figure 8 - Control plane architecture.............................................................................................25 Figure 9 - LSP hierarchical structure traditional in GMPLS networks. ........................................34 Figure 10 - Control structures........................................................................................................39 Figure 11 - OXC Controller Data Structures, full picture. ............................................................40 Figure 12 - FIB data structures. .....................................................................................................40 Figure 13 - Forwarding Information Base Hash Table..................................................................41 Figure 14 - OXC controller and FIB joint operation. ....................................................................41 Figure 15 - Overall program diagram............................................................................................42 Figure 16 - Generalized Label Request object format...................................................................44 Figure 17 - Generalized Label Object format................................................................................45 Figure 18 - Label Set object format ..............................................................................................46
Chapter 1 – Introduction
IP over WDM II - 1
Chapter 1 – Introduction
Abstract
The main objective of this work consists in defining the architecture of an OXC control
plane. This architecture must cope with two main issues.
The first is the diversity of deployed equipments and technologies. That requires a multi-
protocol solution support and some backward compatibility. The second is the movement of IP
related technologies from the edge to the core of the network.
The proposed approach combines the recent GMPLS advances with OXC technology to
provide a framework for real-time routing and provisioning of optical channels and allow the use
of uniform semantics for hybrid network management and operations control.
The development of GMPLS requires modifications to signalling and routing protocols.
The protocols being considered are originated from the IP arena, thus the movement of IP
technologies to the network core is a fact.
Document Structure
This document is divided in chapters, each one covering a given subject.
Chapter 1 - Introduction covers the project presentation and its key elements.
Chapter 2 - Optical Technology Networks and Chapter 3 - GMPLS present actual
technology state and define the main guidelines of IP over WDM control plane within the
definitions of GMPLS.
Chapter 4 - Our approach presents a detailed description of the OXC control plane
components and which are the key elements in this technology. This chapter starts the control
plane definition and references the protocols to be used.
Chapter 5 – The proposed architecture reinforces previous chapter subjects and covers the
work done and the guidelines to follow to complete the implementation.
Chapter 6 – Conclusions, the final chapter, presents the main conclusions and a small
description of the complete OXC usage and final implementation.
Chapter 1 – Introduction
2 - IP over WDM II
Background
Telecommunications Networks – the optical era
Nowadays optical transport networks assume the central role in the telecommunications
networks stage. Synchronous Digital Hierarchy established itself as the absolute must have for
any telecommunications corporation, worthy of bearing that name. However, despite its
widespread use and proven capabilities to explore the benefits of the optical medium, SDH
networks can no longer take their future for granted as data traffic emerges to become the
dominant. The design of SDH networks in the 1980s did not foresee the emergence of data
networks in the late 1990s, thus SDH was developed as an high performance voice traffic carrier,
rigidly structured to carry voice signals and unable to take advantage of statistical multiplexing -
the key word in data networks.
Though, as many said, ‘rumours of the impending death of SDH might have been
exaggerated’, and SDH will keep on being the dominant technology for years to come. The
investments made in building an SDH infrastructure, the tremendous experience and known-how
accumulated in long years of operation and management are a too precious asset to throw away
overnight. Wavelength Division Multiplexing was a significant breakthrough in the physical
layer, providing SDH with an n-fold increase in available bandwidth in a single fibre, where n is
expected to grow from two to four channels today to hundreds in the near future. Though
providing SDH with an absolute surplus value, WDM being a new multiplexing technology
brought an extra burden in an already over burdened network management department. As
carriers strive to keep up with Internet providers ever changing demands and pressure to drop
circuit prices, while having to deal with incompatible network management systems, manually
hop-by-hop configuration of WDM equipment. Whilst keeping up with strong competition from
new comers, in the context of the telecommunication market liberalization.
Data traffic emergence – the Internet revolution
The exponential growth of the Internet in the late 1990s brought great challenges to
backbone data networking technologies. Data networking technologies were aimed always at
small geographic areas, because data networks were usually small and most of the traffic was
internal to the network. Longer links were easily provided in early days through modems using
the public voice network, as demands grew larger using protocol overlay solutions and rented
links (E1 lines, ATM VCs or even STM links). Overlay networks where no more than a short-
Chapter 1 – Introduction
IP over WDM II - 3
lived approach to scale IP networking technology. Stability problems (difficult convergence of
routing protocols in large meshed networks with many adjacencies, specially in failure events),
ineffective use of layer two capabilities, excessive overhead due to large protocol stacks,
expensive framing conversion and the impossibility to do traffic engineering, soon forced the
development of new solutions.
Moving to a distributed standardized control plane
Multi-Protocol Label Switching came as a radical revolution in networking concepts.
Applying the maxim - route once, switch many - and adopting the virtual circuit concept of ATM
(now Label Switched Path), MPLS is taking over Internet Service Provider backbones by
allowing easy and effective integration with layer two technologies (specially ATM but also
emerging Gigabit Ethernet) and proving to be an effective tool for traffic engineering. MPLS has
leveraged IP networking from a collection of small islands interconnected by the public
telephone network to a metropolitan and even national integrated network.
Notwithstanding, this a far from optimal solution, transporting packet traffic, multiplexed
statistically, even using MPLS in an underlying circuit switching technology has been a cause of
major headaches for network management departments all over the world. Today, more than
extra bandwidth, provided by WDM for years to come, fast and flexible provisioning, low cost
and rapid network deployment are the commons goals to the industry, from equipment
manufacturers, standards committees to telecommunications corporations.
International Telecommunications Union - Telecom is devoting considerable effort in the
development of a large number of standards considering distributed network management. Its
main initiatives are the G.ASTN (Automatic Switched Telecommunications Networks) and the
G.ASON (Automatic Switched Optical Networks), both of them are due to produce standards by
early 2002 to ratified by mid that year. The Optical Internetworking Forum has also developed
valuable work in this field, by standardizing the User to Network Interface for signalling, already
approved and tested – UNI-1.0. The Internet Engineering Task Force is producing considerable
work in developing a full control network specification starting from the standardizations
introduced by ITU-T and specially OIF but evolving to an IP like architecture - GMPLS.
As data traffic becomes even more dominant, data networks concepts and protocols will
certainly shape transport networks. This can be already seen in a progressive change in voice
applications towards the usage of IP and the new generation of mobile phone networks - UMTS.
However, IP type networks have more to offer than a flexible network protocol. They were
Chapter 1 – Introduction
4 - IP over WDM II
conceived to allow different multi-vendor equipment to interoperate, resilient to withstand
failure and unreliable media, flexible and auto-managed to simplify its deployment and
survivability. IETF best impersonates the Internet attitude, where an industry community
develops open advisories, which are indeed the strongest standards in the networking world.
Common Control and Management Plane and IP over Optical workgroups have produced a large
number of drafts, proposing the adoption of routing and signalling protocols from the IP world to
provide transport networks with a distributed control plane architecture. This new specifications
were produced in conjunction with ITU and mainly OIF standardization efforts. This will
provide optical networks with an end to end signalling and automatic routing infrastructure.
The Objectives
Some intermediate steps are needed in order to test and define the final architecture.
The goals of this project are:
- Build an Optical Add/Drop multiplexer.
- Build an Optical Cross Connect.
- Define Optical IP Router architecture.
- Specify and develop the Optical IP Router control plane.
The project is divided between two groups each one with a different part of the project.
One part of the project deals with physical optical components, fabrics and the electronic
control. The objective is to design an OXC to be controlled by the control plane defined in the
other part of the project.
Our part of the project discusses and defines, based in the latest developments and
standards in this area, the structure of the protocol stack, needed to establish the control plane
that will control the OXC.
This Optical IP Router is only fiber and wavelength switching capable. Due to this
limitation it is equivalent to a common OXC in the data plane, but in control plane it is capable
of routing and signalling in a GMPLS context.
At physical level the breakthroughs of our OCX are the usage of fully passive optical
technology and the capability of wavelength and spatial (fiber) switching simultaneously.
Chapter 1 – Introduction
IP over WDM II - 5
Our work
Our work focuses on providing the state of the art WDM OXC, designed by our fellow
colleagues and presented in the first part of this document, with a control plane as specified by
GMPLS drafts for wavelength capable equipment. We strongly believe our OXC can have an
important role to play in large data backbones, which we consider will be governed by GMPLS
routing and signalling protocols. Switching transparently high bandwidth WDM channels, of
more and more IP traffic, each time transported closely to the optical medium.
AON
Client Netw ork
λSR
λSR
λSR
λSR
λSR
Client Netw ork
Client Netw ork OXC
OXC
OXC OXC
OXC
AON
Client Netw orkClient Netw ork
λSRλSR
λSRλSR
λSRλSR
λSRλSR
λSRλSR
Client Netw orkClient Netw ork
Client Netw orkClient Netw ork OXCOXC
OXCOXC
OXCOXC OXCOXC
OXCOXC
Figure 1 – Optical Network Model.
The network model represented in Error! Reference source not found. states our view
of the future for optical networking. Our WDM OXC, now a GMPLS lambda-router fits in the
core of this network, future developments (providing it with packet our TDM switching
capabilities) can take it to the border of the WDM cloud acting as an edge lambda router.
Chapter 2 - Optical technology networks
6 - IP over WDM II
Chapter 2 - Optical technology networks
Background
Today, in the Internet era, traffic patterns are unpredictable, what stills remains
predictable is the traffic exponential growth. Operators need a solution that scales well and
allows them to reconfigure the network to cope with rapidly changing traffic patterns. Add to
this the fact that many different types of traffic are entering the network now at optical speeds
(including ATM, gigabit Ethernet, high-speed TDM circuits, and IP). So being, a carrier needs a
way to manage all these protocols and wavelengths through simplified control structures. Today
a carrier may have to enlist three or more different management systems to provision a single IP
connection across the country, and many occasions with manual interventions required along the
way, thus the goal is to have a simplified control structure that selects conduit, fiber, wavelength,
optical switch ports, router ports, and a variety of restoration paths in a single step, according to
traffic engineering constraints.
Network architectural considerations
The usage of WDM technology
It is important to identify the latest changes in optical transport network architecture.
One is the use of Wavelength multiplexing (WDM) in order to explore the available
bandwidth in the fiber, and now at an higher rate with the use of Dense WDM (DWDM).
Mesh topology vs ring topology
Another change is that the early implementations of optical networks were based on ring
topologies. The goal at that time was to have point-to-point connection in the core network. Now
the trend is migrating this technology closer to the end user, so the mesh topology becomes
necessary. Mesh architectures are easy to deploy, allowing a more powerful implementation.
A new control plane is required.
A new optical control plane and a new unifying networking architecture are required that
are equally adapted to manage connections all the way from the heavy-duty fiber in the core to
individual packet flows within a single link.
Chapter 2 - Optical technology networks
IP over WDM II - 7
Carriers can continue to survive, for now, without optical signalling or any other
integrated control schema, but it is a sure thing that they could benefit from such a solution.
Early runners in implementing integrated signalling systems will have the advantage in the near
future.
Network integration models – IP over WDM protocol stack.
Is important at this point to look at the protocol stack. Current implementations of optical
/ WDM technology are usually based on SONET/SDH and ATM, which acts as an interface to
higher layers.
The emergence of Internet and its related applications based on Internet Protocol (IP) are
making IP the dominant protocol, to which all communication network technology converge.
The solution for communication networks will become IP over WDM. IP will become the
convergence layer; WDM is and shall be the high-bandwidth carrier.
OpticalLayer
SDHATM
IPIP
Traditional SDH approaches. - IP/ATM/SDH - POS ( IP/PPP/HDLC/SDH)
Traditional SDH based approaches, with WDM layer.
IP over WDM overlay approach
Direct MPλS approach
WDM Adaptation Layer
HDLCIP
GbE
IP
SDHATM
IP
WDM / OTNOptical LayerGigabit Ethernetover SDH Aproach
PPPIP
HDLCPPP IP
GbEGFP
WDM
PPP EthIP
GFPPPP Eth
ATMIP
OpticalLayer
SDHATM
IPIP
Traditional SDH approaches. - IP/ATM/SDH - POS ( IP/PPP/HDLC/SDH)
Traditional SDH based approaches, with WDM layer.
IP over WDM overlay approach
Direct MPλS approach
WDM Adaptation Layer
HDLCIP
GbE
IP
SDHATM
IP
WDM / OTNOptical LayerGigabit Ethernetover SDH Aproach
PPPIP
HDLCPPP IP
GbEGFP
WDM
PPP EthIP
GFPPPP Eth
ATMIP
Figure 2 - Protocol Stack evolution for IP-over-WDM solutions.
As seen in Figure 2, the first step towards faster networks was the introduction of a WDM
adaptation layer to the traditional SDH approach. This new layer would manage WDM channel
setup/takedown and provide some level of protection recovery. This IP/ATM/SDH based
solution configures an overlay model network (CLIP, MPOA), and carries a significant overhead
due to the four framing layers and extra management burden. Using Packet over SONET (POS),
based in IP/PPP/HDLC into SDH framing some overhead is eliminated. The use of Generic
Framing Procedure (GFP) reduces even more this overhead since GFP uses a more efficient
framing technique than HDLC, and less error prone. In this solution IP runs over Ethernet or
Chapter 2 - Optical technology networks
8 - IP over WDM II
PPP. Traffic adaptation mechanisms such as ATM, HDLC or GFP shape the traffic to the SDH
layer.
GFP and Ethernet (i.e. Gigabit Ethernet) can also run over fiber directly. So it is possible
to eliminate SDH framing, and reduce even more the overhead. This solution is not yet widely
implemented in existing communications infrastructures based in Sonet/SDH, but with the
standardization of the both protocols it has the potential to become dominant, for its simplicity
and low cost.
Once eliminating SDH framing it is possible to maintain GFP and Ethernet to assure the
framing. Functions such as protection/recovery, if needed can be implemented at WDM optical
layer. But, as Ethernet supports fiber directly it is possible to use direct GMPLS. This is the
lighter and more flexible approach since a direct framing without monitoring is used, expensive
synchronizations. Provisioning and survivability actions can be taken by GMPLS. For carrier-
class reliability an optional framing/monitoring sub-layer can be used.
Layer 2 technologies in use
This project focused mainly in building a physical layer device and designing a control
plane according to GMPLS forthcoming standards. Being a fully transparent switch designed for
the core transport networks it was of little concern in our design which would be the protocols
used to carry the data itself, alias its transparency is one of its key advantages.
However we find from the utmost significance to briefly discuss here layer 2 technologies
current available and how they fit in a GMPLS world. Sometimes, however, some confusion
might arise over which technology refers to which layer, we will consider everything that
concerns physical interfacing as layer 1 and all the rest that fits beneath IP as layer 2, even
though sometimes there will be more than one layer 2 protocol, as in Packet Over SDH where
beneath IP lies PPP, HDLC and SDH, in this case SDH is a layer one protocol in what concerns
to physical media specifications and framing.
We shall not discuss ATM networks in this chapter, not that we find them outdated but
we do think they were extensively covered in MPLS standards and the road towards bringing
closer IP and the optical layer will certainly take ATM as its first victim. By this, with do not
mean ATM is unworthy of our attention, just that it didn't succeed and was clearly defeated by
the combination of IP and Ethernet. ATM's only battle victories were in large networks’
backbones, though its deployment was always tampered by too complex protocol stacks and
Chapter 2 - Optical technology networks
IP over WDM II - 9
large overheads, or in an initial stage as a WAN interconnection technology. But now the war
seems definitively lost, as we shall see bellow and only MPLS will keep ATM going in a near
future.
However ATM's effort wasn't in vain and despite defeat it enjoys a sweet vengeance as IP
world sees itself obliged to accept the superiority of most of ATM's concepts, embraced by
MPLS and latterly GMPLS protocols. Route once, switch many left a bitter taste in the mouth of
IP winners.
Synchronous Digital Hierarchy
Long from its introduction in the 80s, SDH has become the widely adopted technology in
carrier telecommunication networks. Its capability to take advantage of the high bandwidths
provided by the optical medium, recently upgraded using DWDM physical layer techniques,
hierarchic structure, tools for medium and equipment monitoring, fast protection and restoration
took it to the status of standard in telecommunications networks all over the world.
SDH is best known for the wideband and ultra fast switching capabilities, however SDH
was conceived not to carry data but voice signals. This has a dramatic impact on its structure,
optimised for multiplexing large amounts of voice traffic coming from lower hierarchies and
switching them in a static fashion - circuit mode. This inability to effectively carry datagram
traffic from the IP world forced the development of several adaptation protocols that ran on top
of SDH structure in an overlay model, most of them inducing large overheads and proving to be
ineffective in using SDH resources.
In an early stage ATM was used as an adaptation layer, due to its cell switching
technology, midway from packet switching and circuit switching. Although ATM was designed
to run over SDH networks and simplified adapting IP nodes to use SDH links, having an ATM
network underneath the IP layer meant a significant traffic overhead, overwhelming effort to
manage three distinct networks (IP, ATM and SDH) and the well known shortcomings of
overlay model networks, namely ineffective usage of network capabilities and instability in
failure events. MPLS solved the overlay problem and raised the billion-dollar question - why use
ATM if SDH can handle the job?
Eager bandwidth network operators were soon taken over SDH high capacity and the
trend to evolve to smaller protocol stacks led to widespread use of SDH in backbone router
interconnection as bandwidth requirements increased dramatically – though still in an overlay
manner. IETF developed a recommendation to meet these requirements, Packet over
Chapter 2 - Optical technology networks
10 - IP over WDM II
SDH/SONET (PoS). IP packets are encapsulated in PPP, which in turn is mapped into an HDLC
like frame. This frame is then mapped together with other frames into the payload of a STM-1
frame, if no data is to be transported in one frame the entire payload is filled with HDLC idle
frames. SDH allows multiplexing the STM-1 frame upon climbing higher in the hierarchy,
nowadays reaching 10Gb/s at framing layer with STM-64.
The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-
protocol datagrams (e.g. IP packets) over point-to-point links. Initially, PPP was used over Plain
Old Telephone Services (POTS). However, since the SDH technology is by definition a point-to-
point circuit, PPP/HDLC is well suited for use over these lines. PPP is designed to transport
packets between two peers across a simple link.
PoS and the use of DWDM in the physical layer have allowed carriers to cope with the
ever growing demand for bandwidth, DWDM makes it possible to aggregate in one link up to
1Tb/s and possibly even more. The solid growth in the number of channels in DWDM systems
and higher switching speeds is changing the network bottleneck from link bandwidth to
provisioning times, as will be discussed in the next section.
Optical Transport Network G.709
SONET/SDH matured to become the standard in transport technology, established in
almost every country in the world. It was conceived at a time when traffic voice was utterly
predominant, thus switching circuit concepts was the sensible choice. With the ever-growing
demand for large bandwidths, especially in data communications driven by the large growth
Internet has experienced, it became clear that even using WDM technology and faster TDM
circuits (STM-64) the limits of the transport medium (the optical fibre) and more critically the
switching capability of electronic circuits were reaching its limit.
The latest recommendation in this field is G.709 "Interface for the Optical Transport
Network" builds on the experience gained from SDH to provide a route to the next generation
optical network. The OTN takes the concepts and structure from SDH and broadens them
essentially to achieve larger bandwidths. OTN shares with traditional SDH a layered structure, in
band service performance monitoring, protection and several other management functions.
However, some key elements were added to improve performance and reduce operational costs,
namely more flexible management of optical channels in optical domain and the introduction of
forward error correction techniques to improve error performance in longer and faster optical
spans.
Chapter 2 - Optical technology networks
IP over WDM II - 11
OTN also standardises the method for managing optical wavelengths (channels), avoiding
the need for expensive opto-electric-opto (O-E-O) conversion at each node in the link, which has
been demonstrated to be the main limiting factor to larger bandwidths in current networks. This
clearly opens the door to integrate new-coming all optical switching technology in current
networks, with extensive cost saving, simplicity and scalability enhancements. In band
monitoring, protection and fast restoration techniques derived from SDH will account for highly
resilient networks and the several overheads allow transporting GMPLS control plane
information in band.
In a nutshell, OTN paves the way for GMPLS deployment in carrier networks by
standardising the management of optical nodes, allowing more transparent switching (fibre,
waveband or wavelength level) and providing error correction to cope with larger speeds and
spans of optical links without compromising data integrity.
Optical Internetworking Forum - UNI 1.0
Dense Wavelength Division Multiplexing has become quite a standard in wide band
telecommunications systems as the most cost-effective technology for increasing capacity of
optical fibre networks. This new optical network layer promises intelligent transport services to
clients such as backbone IP routers interconnection, initially using SDH, but soon evolving to
other optical interfaces.
The bandwidth requirement growth has increased by several others of magnitude
recently, overwhelming carriers management systems, most of them manually operated, to
control large and intricate optical networks. Currently, optical networks are provisioned through
supplier-specific element management systems. Provisioning end-to-end connections across
multi-vendor equipment has involved the use of incompatible EMSs, manual operations and
even hop by hop reconfiguration, leading to long provisioning times in an era when costumers
demand faster responses from their carriers. The long awaited solution for this time-consuming
matter is to bring MPLS like control intelligence to switching networks. To accomplish this OIF
defined a new standard signalling interface - UNI - between client and optical networks to enable
dynamic provisioning requests.
OIF addressed this critical issue by defining an interface that is compatible with the latest
GMPLS signalling specifications. In addition to signalling, the UNI specifications also addresses
two other subjects fundamental in simplifying management of large networks. The first one is a
Chapter 2 - Optical technology networks
12 - IP over WDM II
neighbour discovery mechanism, which will allow adjacent nodes to identify each other.
Neighbour discovery will allow management systems to build inter-connection maps
automatically. The second important aspect of UNI is a service discovery mechanism enabling
clients to determine the services provided by the optical network. Service discovery will allow
clients to automatically discover and take advantage of new services provided by the network as
they are introduced over time. Most of UNI services have been integrated in GMPLS
architecture, specially the mechanism for neighbour and service discovery, which are the basis of
the Link Management Protocol. Though GMPLS will use the services provided by the UNI, a
substantial difference exists between OIF’s point of view and the GMPLS peer to peer network
concept. Whilst GMPLS makes no distinction between client and provider equipment (only the
use of BGP might introduce a concept of border in this otherwise flat architecture), OIF defines
different interfaces, one for inner networks nodes (UNI-N) and one other to external nodes (UNI-
C). The UNI-C is more limited, preventing outer networks equipment to be aware of the inner
network topology in an overlay model, so to speak.
GigaBit Ethernet
Ethernet has come a long way since its introduction as a small and simple network for office
buildings by Xerox in early 1970s. From 10Mb/s over coaxial copper cabling to 1Gb/s over
single mode fibre the protocol evolved to become the dominant technology in local networking
and even menacing to take over WAN networking with its 10Gb/s, STM-64 framing on WDM
support standard. Its low cost technology, easy deployment and ubiquity from desktop
connectivity to campus backbones seized the Internet with over 250 million Ethernet ports
installed.
Moving towards gigabit speeds meant quite a revolution in lower stack layers. In order to
accelerate from speeds 100 Mbps Fast Ethernet up to 1 Gb/s, several changes needed to be made
to the physical interface. It has been decided that Gigabit Ethernet will look identical to Ethernet
from the data link layer upward. The challenges involved in accelerating to 1 Gb/s have been
resolved by merging two technologies together: IEEE 802.3 Ethernet and ANSI X3T11
FiberChannel. Leveraging these two technologies means that the standard can take advantage of
the existing high-speed physical interface technology of FibreChannel while maintaining the
IEEE 802.3 Ethernet frame format (MAC sublayer remains untouched), backward compatibility
for installed media, and use of full or half duplex carrier sense multiple access collision detection
(CSMA/CD). This scenario helps minimize the technology complexity, resulting in a stable
Chapter 2 - Optical technology networks
IP over WDM II - 13
technology that can be quickly developed. The Gigabit interface converter (GBIC) allows
network managers to configure each gigabit port on a port-by-port basis for short wave (SX),
long wave (LX), long haul (LH), and copper physical interfaces (CX). LH GBICs extends the
operations using SM fibre to span up to 40km, making it perfect for metro networks
interconnection and perhaps even WAN.
PMD
LAN PCS WAN PCS
MAC
PhysicalLayer
LLCData LinkLayer
64b/66b encodingSTM64 line rate
SDH framing
8b/10b encoding10.3GBaud
Same as in 802.3
10 Gigabit EthernetProtocol StackIEEE 802.3ae
Figure 3 – 10 Gigabit Ethernet Protocol Stack.
10Gigabit Ethernet is much more then a ten fold increase in link bandwidth, being able to
aggregate several Gigabit Ethernet in one link it presents itself as the optimal solution to scale
enterprise and service providers Ethernet backbones. Being compatible with traditional Ethernet
frame format, preserving minimum and maximum frame size (except Jumbo payload from
Gigabit Ethernet), supporting only full duplex operation and compatible Gigabit Media
Independent Interface makes it the best solution to leverage network equipment with Ethernet
interfaces.
10Gigabit Ethernet standardizes two new and different physical interfaces, one targeted
to the traditional LAN market and the new one to the metropolitan links using SDH networks. In
one hand, LAN PHY operating at 10Gb/s up to 300m spans on MM fibre and 2 to 40km spans
on SM fibre for office backbones and interconnection in small distances. WAN PHY, one the
other hand, made compatible with STM-64 framing - ethernet frames are carried in the payload
of a VC4 without the need to do any protocol conversion. Using DWDM physical layer
techniques it is possible to span almost to 1000km. WAN PHY aims high by threatening to take
on ATM and PoS, since Internet end points are ethernet powered, it makes all sense to avoid
expensive framing conversions in WAN links. WAN PHY accomplishes just that by being
compatible with the fastest up to day SDH hierarchy. The two physical interfaces are broken in
two sub-layers, the upper one – Physical Coding Sublayer – allows different coding,
Chapter 2 - Optical technology networks
14 - IP over WDM II
multiplexing and serialization. The bottom one is the Physical Media Dependent, which specifies
the physical characteristics of transmitters, optical medium and receivers as described in Figure 4
- Optical Media for 10Gigabit Ethernet . The two physical layers only differ in the PCS and fully
support all the interfaces defined in the PMD sub-layer.
Figure 4 - Optical Media for 10Gigabit Ethernet .
10 Gigabit Ethernet is perhaps the most promising technology at the moment and the one
that can most benefit from GMPLS. The possibility to provision a link dynamically, on client
request granted by a transparent network is the sensible choice when even the framing is
preserved from end to end. Using the WAN PHY it will be easy to integrate existing SDH
networks with new Ethernet links for data networks.
10Gb LAN PHY MM fibre <40km
10Gb LAN PHY MM fibre <300m
10Gb WAN PHY MM fibre <300m
TDMTDM
TDM TDM TDMTDM
WDM MetroNetwork
10Gb WAN PHY SM fibre WDM
ISP PoP
Carrier PoP
Figure 5 - 10 Gigabit Ethernet in metropolitans network.
Chapter 2 - Optical technology networks
IP over WDM II - 15
Migrating from management based solutions to automatic routing and
provisioning.
The transition to optical signaling allows the use of automatic routing and provisioning. It
doesn’t have to take place in one profound leap, but can progress in stages, according to a
carrier’s needs.
The first step is already taking place in many carrier networks today. It consists of
improving operational support systems (OSSs) to allow for real point-and-click provisioning of
optical bandwidth.
The second step a carrier may take moves them beyond the management of optical
bandwidth and begins to collapse the boundary between the data services layer of the network
and the optical layer. This step involves implementing a user-to-network-interface, or UNI, that
allows network equipment to “ask” for connectivity across the optical network by signaling for
it. This does require specifications for signaling and provisioning, and it's on the way from the
Optical Internetworking Forum (OIF). In this step, inside the optical "cloud" signaling remains
proprietary, whereas outside the cloud a standardized interface allows client devices attached to
optical systems to talk to each other as though they were neighbours, speeding service creation
and improving resource management.
The third step is the complete standardization of signaling and control planes. With this in
place, carriers can begin to collapse layers of the network, reaching two (a packet layer IP and a
transport layer WDM) or even one, adopting a unified control plane that touches all the
equipment in a carrier network, allowing them all to communicate in real time, dynamically,
asking for bandwidth, connections, grades of restoration, and just about anything they could
want from any layer of the network.
One important question to make is how far to push this technology. Should routers,
dynamically provisioning themselves wavelengths, be making million-dollar decisions on their
own? Can data equipment be expected to manage restoration in the transport layer? Is optical
signaling really the last step to allow a migration to mesh networking in the optical core?
Chapter 2 - Optical technology networks
16 - IP over WDM II
The answer to those questions is not the goal of this document, but the idea is to keep in
mind is that optical signalling and routing is only part of a larger movement toward unifying
control planes for multiple network layers
Moving to IP protocol arena
It is important to note here that the emerging Optical Internet will require both signalling
and routing. The signals provide the communication between formerly disparate network
domains; and the routing, in the form of extensions to existing IP routing protocols, is there to
provide enhanced capabilities in managing optical network resources, such as WDM
wavelengths.
A method was required that would ease the burden put on core routers, where traffic
quantity was highest, by relieving these routers of the task of examining each packet header in its
entirety and instead just reading a “label” and passing the packet along according to label
switching rules. Optical signalling and routing are starting their first steps but the fact that it is
drawing upon recent advances in MPLS control plane technology has accelerated its momentum
considerably.
Using MPLS framework
The foundation of MPLS was constraint-based routing, which provides IP devices the
ability to establish and maintain paths through the network that are optimal with respect to a pre-
determined set of metrics and constraints. These constraints can be either resource-related, such
as bandwidth, or administrative related, such as restricting paths to particular links.
In short, MPLS was created as a combination of a forwarding mechanism (label
switching), connection establishment protocols, and defined mappings onto Layer 2
technologies. Thus, MPLS could behave as a next-gen ATM, improving routing in the core of
the network by putting traffic where the bandwidth is, and enabling a range of new services,
including network-based VPNs, circuits over MPLS, and differentiated data services.
The Internet has demonstrated that the non-centralized control plane of MPLS is required
to reduce provisioning and planning costs, while being extremely robust. The essential feature of
MPLS is to apply virtual circuit notions (such as those used in frame relay or ATM) to IP
networks to support quality of service (QOS) and traffic engineering.
Chapter 2 - Optical technology networks
IP over WDM II - 17
MPLS bases in Link state routing (LSR) protocols, which are used to obtain network
topology information, and signaling/label distribution protocols, which are used to set up virtual
circuits (or LSPs) across the MPLS network. These labels only have local significance to the
switch, called a label switching router (LSR), and do not require the router to perform any time-
consuming route table lookup.
Fundamentally, this means that in MPLS forwarding information is separate from the
content of the IP header; and through this separation of the forwarding plane and data plane, any
kind of data can be mapped into LSPs, which is already making MPLS as attractive to
edge/aggregation systems as it is to core routers.
Evolving to GMPLS
MPLS evolved the fundamental architecture of routed networks by separating the
forwarding plane (looking up packets) from the control plane (deciding where they go). With this
separation in place, a “best-effort” IP network can now support a variety of protection and
restoration functions, as well as providing some measurable level of QOS, reducing or
eliminating the need for an ATM layer in the network, and improving the IP network’s long-term
scaleability.
With all the work underway developing Multiprotocol Label Switching (MPLS) and its
control plane, it became obvious that the same control plane could be abstracted to the lower
layers of the network, namely Sonet/SDH and the DWDM layer.
If core routers are being simplified with a new switching scheme that relies on a separate
control plane, and optical transmission networks are being simplified with the addition of a
switch, then this new control plane for routers could be applied equally well to both routers and
optical switches.
So the Internet Engineering Task Force (IETF) has been off and running, developing its
Generalized MPLS (GMPLS) for just this purpose, applying MPLS control plane techniques to
optical switches and IP routing algorithms to manage lightpaths in an optical network.
Some IETF groups are working in this area. Groups such CCAMP (Common Control
and Measurement Plane) group, IPO (IP over Optical) group and MPLS (Multiprotocol Label
Switching) group are making the most important contributions to GMPLS standardization.
Chapter 2 - Optical technology networks
18 - IP over WDM II
The need for different control plane models
The key distinction between MPLS and GMPLS is that, whereas the control plane for
MPLS was separate from the data plane, in GMPLS it can also be physically separate from the
signal. The GMPLS control plane allows for a wide variety of control plane operation models
and architectures that meet different network operation scenarios. This allows a GMPLS control
plane to manage connectivity and resource management among multiple layers of the network,
from fibers to wavelengths from IP networks do SDH circuits.
Conclusions
The evolution of optical networks towards a simplification and standardization of the
control structures and the trend to carry IP with the least overhead possible in the optical
medium, especially in 10Gigabit Ethernet, opens the door to the introduction of IP protocols in
the management of the AON. As switched networks become more complex, centralized control
systems strive to keep up with the growing pressure for fast network deployment and
reconfiguration. GMPLS with its peer model decentralized control using traditional IP routing,
with traffic engineering extensions, promises to ease network management in a more efficient
and cost-effective way.
Classical MPLS (meaning, derived from IP network elements) and optical MPLS (those
extensions to classical MPLS that control optical network elements) are only subsets of GMPLS.
From now all efforts in MPLS and optical layer management will now be developed under the
rubric of GMPLS.
Chapter 3 - GMPLS
IP over WDM II - 19
Chapter 3 - GMPLS
Architecture
GMPLS assumes a unique control plane, derived from MPLS, that is extended to include
a group of network elements that do not make forwarding decisions based on the information
carried in packet or cell headers, but rather based on time slots, wavelengths, or physical ports.
It is important to understand how these kinds of information channels are related and how
they must be treated.
Switching domains
Those different information channels in the network and the different manners how they
could be switched could be considered as different switching domains. In current drafts of
GMPLS signalling, five types of interface or switching domains are described.
TDMC
FSC/λSC
OXCOXCOXCλSR λSR
PSC
SONETSONET
TDMC
FSC/λSC
OXCOXCOXCOXCOXCOXCλSR λSRλSR
PSC
SONETSONETSONETSONET
Figure 6 – Switching domains.
1. Fiber-Switch Capable (FSC)
These interfaces do not need to recognize bits or frames and do not necessarily have
visibility of individual wavelengths or wavebands. This data forwarding based on the position of
the data in physical space, such as the interfaces on an optical cross-connect that can operate at
the level of single or multiple fibers. These would be found on automated fiber patch panels,
fiber protection switches, or photonic crossconnects that operate at the level of a fiber.
Chapter 3 - GMPLS
20 - IP over WDM II
2. Wavelenght Switch Capable (λSC)
These interfaces do not need to recognize bits or frames. They forward data based on the
wavelength and port on which the data is received, such as an optical cross-connect or
wavelength switch. These are not assumed to be capable of receiving and processing control
plane information on an in-band channel. Examples are interfaces on an all-optical add/drop mux
(OADM) or all-optical crossconnect (OXC).
3. Waveband Switch Capable (WSC)
If adjacent wavelengths are grouped together and the switch has the capability to switch
all of them as a group, this capability is defined as waveband switching. This functionality could
be implemented at the fabrics level, but is easier to implement at control level, as a group of λSC
interfaces.
4. Time Division Multiplexing Capable (TDMC)
These interfaces also recognize bits, though focus on the repeating, synchronous frame
structure of Sonet/SDH. These interfaces forward data on the basis of a time slot within this
structure, and are capable of receiving and processing control plane information sent in-band
with the synchronous frames. Examples are interfaces on Sonet/SDH add/drop multiplexers,
digital cross-connects, and OEO switching systems.
5. Packet Switch Capable (PSC)
Interfaces that make forwarding decisions based on information in the packet or cell
header, such as routers and ATM switches. These interfaces recognize bit, packet, or cell
boundaries and can make forwarding decisions based on the content of the appropriate MPLS
header. Notice, these are also capable of receiving and processing routing and signalling
messages on in-band channels. Examples include interfaces on routers, ATM switches, and
Frame Relay switches that have been enabled with an MPLS control plane.
OXC control plane functions
In order to discuss and develop OXC technologies, it’s important to look at the OXC
place in LSP’s. As presented in Figure 6, OXCs are placed inside FSC cloud. It’s capabilities are
to switch fibers and individual wavelengths from fiber to fiber (FSC and λSC). The possibility to
make wavelength conversion or switch wavebands is an option inside switch architecture. The
option at the OXC is only path’s decision (Fiber and Wavelength switching). Packets decision is
Chapter 3 - GMPLS
IP over WDM II - 21
not possible. So, the goal here is the Wavelength Switch and Fiber Switch Capability and the
related control plane functions.
Control plane
Creating a standardized control plane for optical networks gives carriers a comfort level
that they won’t be tied into a single vendor solution (as they were in the ATM days) and
provides them with a toolkit to support a variety of protection and restoration schemes, traffic
engineering in the optical layer, and provision of optical channels across their networks.
How does a control plane accomplish all this? Which protocols should be used and which
functions should they perform?
1. Link management
The use of technologies like Dense Wavelength Division Multiplexing (DWDM) implies
that we can now have a very large number of parallel links between two directly adjacent nodes
(hundreds of wavelengths, or even thousands of wavelengths if multiple fibers are used). Such a
large number of links was not originally considered for an IP or MPLS control plane, although it
could be done. Some slight adaptations of that control plane are thus required if we want to
better reuse it in the GMPLS context.
For instance, the traditional IP routing model assumes the establishment of a routing
adjacency over each link connecting two adjacent nodes. Having such a large number of
adjacencies does not scale well. Each node needs to maintain each of its adjacencies one by one,
and link state routing information must be flooded throughout the network.
To solve this issue the concept of link bundling was introduced. Moreover, the manual
configuration and control of these links, even if they are unnumbered, becomes impractical. The
Link Management Protocol (LMP) executes automated provisioning of an optical network by
discovering neighbour nodes and their capabilities. LMP allows neighbouring nodes to exchange
identities, link information, and negotiate the functions to be supported between the nodes.
LMP is also used to allow adjacent OXCs to determine IP addresses of each other and
port-level local connectivity information, such as, which port on one optical switch is connected
to which port on a neighbour. In GMPLS, the optical control plane will include capabilities of
establishing, maintaining, and tearing down optical channels in much the same way MPLS-
enabled routers establish label switched paths (LSPs).
Chapter 3 - GMPLS
22 - IP over WDM II
2. Intra domain routing protocols
Resource discovery is a good start, but it does not provide enough information to route
connections across a network.
Basically, there are two different ways for performing distributed routing:
• Link-state protocols involve reliably flooding all changes in network topology to each
network node, after which the node uses this to calculate its routing tables;
• Distance-vector protocols involve the network nodes participating in a joint
calculation of what the least-distance path is to a destination.
When allowing optical switches and network elements to disseminate information about
the network topology and resource availability, topology information can be exchanged only
using a link-state protocol. Reachability information (what end stations or nodes can be reached)
can be distributed via either link state or vector distance.
Link-state protocols have superior speed of convergence and freedom from routing loops,
while distance-vector protocols are somewhat less complex.
Routing link-state protocols such as OSPF step in at this point to distribute current
information about the topology of the network to each node. In GMPLS, extensions are being
defined to allow OSPF to be used for disseminating routing information for optical networks.
OSPF must be considered as an “intra domain” routing protocol.
3. Inter domain routing protocols
Considering OSPF as a “intra domain” routing protocol, an “inter domain” routing
protocol is necessary. This function is to be performed by BGP. New extensions are being
developed in order to adapt BGP to GMPLS arena.
4. Signaling protocols
Path setup and control protocol allows the connection to be created through switch-to-
switch signaling, without the need for network management intervention at intermediate nodes.
There are many protocols in use today that provide an analogous function in packet and
circuit networks. In circuit networks, this function is provided by SS7 or QSIG protocols, in
ATM networks by the PNNI or INNI protocols. In IP networks, a similar function is provided by
RSVP, which uses signaling to reserve resources across the IP network to support a new
Chapter 3 - GMPLS
IP over WDM II - 23
information flow. RSVP has not been extensively deployed, due to scalability concerns, but
extensions have been defined to improve its scalability.
Participants in the IETF group dealing with this issue were unable to reach a unified
approach towards setup signalling, and wound up with multiple signaling standards, leaving it to
the market to decide. This situation has unfortunately extended to GMPLS, where equivalent
modifications have been defined for both RSVP and LDP.
The OIF has similarly been unable to resolve the issue and has incorporated both RSVP
and LDP into its UNI specification. The extensions required for both RSVP and LDP in order to
support optical network signaling are significant. New parameters or formats have been defined
to take into account the need to specify time slots, wavelengths, and wavebands, instead of
packet header labels. New parameters have also been defined to allow connection requirements
such as protection and diversity to be specified.
Inband/Outband control channel
In Generalized MPLS, a control channel can be separated physically from the data
channel. Such a separation brings issues to use of RSVP and RSVP-TE for signaling.
In original RSVP, signaling is assumed to be in-band. Each RSVP message is sent hop-
by-hop between RSVP-capable routers as an IP datagram. The IP addresses of RSVP
downstream messages (Path, PathTear and ResvConf) must be set to DestAddress for the
session. Also these messages must be sent with Router Alert IP option in their IP headers. Thus,
all routers along a route examine received IP packets carrying RSVP downstream messages, but
only RSVP-aware routers recognize and process these messages. The delivery of RSVP
downstream messages to the session destination is based on IP routing scheme and unaffected by
non-RSVP routers on the path.
The assumption of in-band signaling is unchanged in RSVP-TE. Therefore the
convention referred above is inherited. However, the use of some functions is limited to cases
when all routers along the explicit route support RSVP those functions.
On the contrary, Generalized MPLS handles non-packet-switch-capable interfaces. Thus
an in-band signaling cannot be assured any longer. An RSVP messages may travel out-of-band
with respect to an LSP data channel. A Path or PathTear message should be addressed directly to
an address associated with the control plane of the node, which is known to be adjacent at the
data plane, without Router Alert option.
Chapter 3 - GMPLS
24 - IP over WDM II
Traffic Engineering
Traffic engineering (TE) must be considered as an optimization to the network
technology. Its goal is to reduce the overall cost of operations by more efficient use of bandwidth
resources, preventing a situation where some parts of a service provider network are overloaded
(congested), while other parts are idle.
Due to its applicability to transport networks where TE issues are unsurpassable, GMPLS
extends the existing signalling protocols defined for MPLS-TE (i.e. RSVP-TE or CR-LDP) in
order to support TDMC, λSC and FSC traffic engineering.
Conclusions
Communication networks must support different switching domains. Devices that
perform packet switching are needed, but also those that perform switching in time, wavelength
and space domains.
Inside the transport network core, the needs are fast switching and traffic aggregation. On
the contrary, in the edge the needs are packet switching in order to route traffic to its destination.
In communication networks lots of different devices perform those different functionalities. Such
different devices must use a unified control plane that supports and manage all those differences
and deal with available functionalities at each network equipment.
GMPLS assumes itself as a solution to implement a complete and integrated distributed
control plane supporting many different control plane operation models.
Chapter 4 - Our approach
IP over WDM II - 25
Chapter 4 - Our approach
Core architecture.
Based in the study being done in this project, our approach to the GMPLS control plane
to implement in OXC is the one presented in Figure 7. This represents one of many possible
solutions though it is the most common approach done by the market.
It is important to note that the positioning of the OXC inside the network core define
some constraints to the architectural model of the control plane. I.e., inter domain routing
protocols (BGP) are not needed when considering a single carrier scenario.
Traffic Enginnering Control
FIB
OSPF - TETE Topology DB
OXC controller
LMP
RSVP - TE
Path and Wavelength selectionTraffic Enginnering Control
FIB
OSPF - TEOSPF - TETE Topology DB
OXC controllerOXC controller
LMPLMP
RSVP - TERSVP - TE
Path and Wavelength selection
Figure 7 - Control plane architecture.
It is also important considering the need of traffic engineering support in all the protocols.
The use of GMPLS and its related protocols implements distributed and automatic routing and
provisioning. But the functions used inside network core imply the usage of management-based
solutions. Centralized management solutions regarding traffic engineering are still important for
carriers.
GMPLS control plane architecture implements automatic management based in OSPF,
LMP and RSVP, but traffic engineering control is always possible.
Chapter 4 - Our approach
26 - IP over WDM II
Adapted versions of OSPF-TE, RSVP-TE and LMP are needed in order to support the
GMPLS specifications. Out of band signaling, different interfaces and functionalities in network
nodes, forwarding adjacency, path bundling and unnumbered links are some of the changes to
implement.
The OXC controller includes all the functions to configure and control the OXC in order
to identify the network its capabilities and to receive and execute network configuration
instructions.
FIB will include the new data structures needed to exchange information between OSPF,
LMP, RSVP and OCX controller and to represent switch configuration and capabilities.
How different components interact
Forward Information Base (FIB)
The FIB stores the current status of the data flows traversing the switch. Specifying for
each the origin IP address, input port and wavelength set, and the respective destination IP
address, output port and wavelength set.
In OXC controller software description, in Chapter 5 – The , a more detailed description
of FIB information could be found.
Traffic Engineering Topology DB
TE Topology DB is the core component of the control plane, here is stored topology, link
forwarding adjacencies and respective control channel attributes, link TE characterization (i.e.,
bandwidth, protection, priority, node switching types).
LMP
This protocol populates the TE topology DB, with information gathered from its
neighbour and service discovery mechanisms and control channel negotiations.
LMP is the only protocol that directly accesses OXC controller to be able to perform
monitoring of the physical layer according to the switch capabilities.
OXC Controller
The OXC controller uses FIB information to setup and configure the OXC fabrics.
Chapter 4 - Our approach
IP over WDM II - 27
RSVP-TE and OSPF-TE
These protocols act as common signaling and routing protocols but using the GMPLS
and traffic engineering extensions to interact with FIB and TE topology DB.
OSPF-TE will fill and update the information stored in the TE topology DB and use this
same information to distribute to its peers. OSPF not only depends on the information stored in
the TE topology DB but also from the TE configurations from TE Control. Wavelength routing
subject to other TE constraints (protection, traffic balancing, bandwidth limitations) – constraint
shortest path algorithms must be used instead of standard shortest path calculation.
RSVP-TE uses TE topology DB to be able to propagate Path and Resv messages
according to routing info. It will use the information contained in the FIB together with the
information in the TE topology DB to select appropriate switch configuration (port and
wavelength) to fulfill each signaling request (Path and Resv) messages. After a successful
signaling request (Path message) it commands the OXC controller via setting a new FIB entry. A
similar procedure is executed in when a Tear message is received, removing an entry in the FIB.
Every FIB change triggers the OXC to update the fabrics state.
Traffic Engineering control
Traffic Engineering control is not mandatory for this implementation. Its functions
concern providing network operators manual control over certain TE parameters such as load
balancing, protection requirements, recovery rules, etc. TE control has proved to be a major
advantage by allowing operators to interfere with automatic routing calculation, mapping traffic
flows to the network physical configuration.
Path and wavelength selections are the most complex subject in new generation optical
networks. The mathematical aspects of CSPF algorithms with wavelength constraints have been
a matter of extensive research, though we consider it out of our scope. Diverse solutions, most of
them combining offline and online calculations are available and could easily be integrated in
our control architecture.
Signaling (RSVP and CR-LDP)
GMPLS does not specify any profile for RSVP-TE and CR-LDP implementations in
order to support GMPLS - except for what is directly related to GMPLS procedures. It is to the
manufacturer to decide which are the optional elements and procedures of RSVP-TE and CR-
Chapter 4 - Our approach
28 - IP over WDM II
LDP that need to be implemented. Some optional MPLS-TE elements can be useful for TDM,
λSC and FSC layers, for instance the setup and holding priorities that are inherited from MPLS-
TE.
Nevertheless Generalized MPLS formalizes possible separation between control and data
channels, it is unlikely that these control channels are realized via completely different service
provider networks. It is rather reasonable to consider that: there should be direct connectivity for
communication in the control plane, between immediate neighbors, which are connected
physically in the date plane. Even if there is no actual wire, there can be a logical connection by
means of IP tunnels.
Assuming such an existence of one-to-one relationship between a communication
channel in the control plane and a physical connection in the data plane, the conventional manner
of RSVP messages can be considered still effective. Particularly, it is useful within the condition
that a network topology is subject to change. Specifically, setups and teardowns of FA-LSPs in
GMPLS make a network topology transitional itself. For the support of Explicit Route (ERO), it
is even presumable that all the associated nodes in a network support RSVP. Every node on a
path examines received IP packets according to the Router Alert option. If a packet has protocol
number 46, the router recognizes and processes it as an RSVP packet. The router looks the ERO
in the packet, and then determines an appropriate next hop based on its up-to-date understanding
of the network topology.
In signaling of hierarchical LSPs, an ingress node is supposed to build an ERO that
consist of subobjects including LSP region nodes for a Path message. Using conventional RSVP
manner, an LSR can receive this message, even if ERO subsequence, which includes the LSR is
extracted by an upstream node. It gives the LSR a chance to examine the message. Thus, if the
LSR can provide a more optimal route, it may pick up and modify the message. Otherwise, if the
LSR determines to be a transit for the FA-LSP, it acts like a non-RSVP node.
In addition, when a RSVP message is delivered to terminator node directly jumping
intended transit nodes, it is even desirable to provide mechanism that can save the signaling
session from an error.
How to request an LSP
A TDMC, λSC or FSC LSP is established by sending a PATH/Label Request message
downstream to the destination. This message contains a Generalized Label Request with the type
Chapter 4 - Our approach
IP over WDM II - 29
of LSP (i.e. the layer concerned), and its payload type. An ERO is also normally added to the
message, but this can be added and/or completed by the first/default LSR.
The requested bandwidth is encoded in the RSVP-TE SENDER_TSPEC object.. Specific
parameters for a given technology are given in these traffic parameters, such as the type of
signal, concatenation and/or transparency for a SDH/SONET LSP. For some other technology
there be could just one bandwidth parameter indicating the bandwidth as a floating-point value.
The requested local protection per link may be requested using the Protection Information
Object. The end-to-end LSP protection is for further study and is introduced LSP
protection/restoration section.
Additionally, a Suggested Label, a Label Set and a Waveband Label can also be included
in the message. Other operations are defined in TE. The downstream node will send back a
Resv/Label Mapping message including one Generalized Label object that can contain several
Generalized Labels. For instance, if a concatenated SDH/SONET signal is requested, several
labels can be returned.
Generalized Label Request
The Generalized Label Request is a new object to be added in an RSVP-TE Path message
instead of the regular Label Request. Only one label request can be used per message, so a single
LSP can be requested at a time per signaling message.
The Generalized Label Request gives three major characteristics (parameters) required to
support the LSP being requested: the LSP Encoding Type, the Switching Type that must be used
and the LSP payload type called Generalized PID (G-PID).
The LSP Encoding Type indicates the encoding type that will be used with the data
associated with the LSP. For instance, it can be SDH, SONET, Ethernet, ANSI PDH, etc. It
represents the nature of the LSP, and not the nature of the links that the LSP traverses. This is
used hop-by-hop by each node. A link may support a set of encoding formats, where support
means that a link is able to carry and switch a signal of one or more of these encoding formats.
The Switching Type indicates then the type of switching that should be performed on a particular
link for that LSP. This information is needed for links that advertise more than one type of
switching capability. Nodes must verify that the type indicated in the Switching Type is
supported on the corresponding incoming interface; otherwise the node must generate a
notification message with a "Routing problem/Switching Type" indication. The LSP payload
Chapter 4 - Our approach
30 - IP over WDM II
type (G-PID) identifies the payload carried by the LSP, i.e. an identifier of the client layer of that
LSP. For some technologies it also indicates the mapping used by the client layer, e.g. byte
synchronous mapping of E1. This must be interpreted according to the LSP encoding type of the
LSP and is used by the nodes at the endpoints of the LSP to know to which client layer a request
is destined, and in some cases by the penultimate hop.
Other technology specific parameters are not transported in the Generalized Label
Request but in technology specific traffic. Currently, two set of traffic parameters are defined,
one for SONET/SDH and one for G.709.
Label Suggestion by the Upstream
GMPLS allows for a label to be optionally suggested by an upstream node. This
suggestion may be overridden by a downstream node but in some cases, at the cost of higher
LSP setup time. The suggested label is valuable when establishing LSPs through certain kinds of
optical equipment where there may be a lengthy (in electrical terms) delay in configuring the
switching fabric. For example physical motion inside the OXC is needed and subsequent it takes
time. If the labels and hence switching fabric are configured in the reverse direction as usual, the
MAPPING/Resv message may need to be delayed by hundreds of milliseconds per hop in order
to establish a usable forwarding path. It can be important for restoration purposes where alternate
LSPs may need to be rapidly established as a result of network failures.
Note that the use of of Suggested Label is only an optimisation.
Label Restriction by the Upstream
An upstream node can optionally restrict the choice of label of a downstream node to a
set of acceptable labels. Giving lists and/or ranges of acceptable or unacceptable labels in a
Label Set provides this restriction. If not applied, all labels from the valid label range may be
used. There are at least four cases where a label restriction is useful in the "optical" domain.
1. The first case is where the end equipment is only capable of transmitting and receiving
on a small specific set of wavelengths/bands.
2. The second case is where there is a sequence of interfaces, which cannot support
wavelength conversion and require the same wavelength be used end-to-end over a sequence of
hops, or even an entire path.
Chapter 4 - Our approach
IP over WDM II - 31
3. The third case is where it is desirable to limit the amount of wavelength conversion
being performed to reduce the distortion on the optical signals.
4. The last case is where two ends of a link support different sets of wavelengths.
The receiver of a Label Set must restrict its choice of labels to one that is in the Label Set.
A Label Set may be present across multiple hops. In this case each node generates it's own
outgoing Label Set, possibly based on the incoming Label Set and the node's hardware
capabilities. This case is expected to be the norm for nodes with conversion incapable interfaces.
Routing (OSPF and BGP)
GMPLS standards committee (namely the IETF standards) has defined several extensions
to routing protocols towards their integration in GMPLS architecture. Up to now the Interior
Gateway Protocols were the main focuses with several drafts being released concerning
extensions to both ISIS-TE and OSPF-TE (please refer to reference section for draft references).
The work bases were the traffic engineering extensions introduced to OSPF and ISIS in MPLS
standardization. GMPLS requires IGP protocols to deal with a much more heterogeneous
network (optical and circuit switching networks like SDH and OTN) and the possibility of using
an out of band control channel. Following on we will briefly discuss some of this new demands.
Border Gateway Protocol was not yet addressed, but it is expected to receive the
CCAMP group attention in the near future due to its great importance in the inter-connection
between different carriers’ networks (Autonomous Systems). While IGPs will be extensively
used inside each network, BGP’s strong policies and flexibility will be essential to operators who
which to protect their routing information but keeping connection and routing updates from
neighbour networks.
The roots of GMPLS routing
GMPLS is indeed based on the Traffic Engineering (TE) extensions to MPLS, MPLS-TE.
As most of the technologies that can be used below the Packet Switch Control (PSC) level
require some traffic engineering. The placement of LSPs at these levels needs in general to take
several constraints into consideration (such as framing, bandwidth, protection capability, etc)
and to bypass the legacy Shortest-Path First (SPF) algorithm when required.
In order to facilitate constraint-based SPF routing of LSPs, the nodes performing LSP
establishment need more information about the links in the network than standard intra-domain
Chapter 4 - Our approach
32 - IP over WDM II
routing protocols provide. The TE attributes are distributed using the transport mechanisms
already available in IGPs (IP encapsulation and flooding) and taken into consideration by the
LSP routing algorithm. Optimization of the routes may also require and benefit from some
external offline simulation (e.g. some wavelength routing algorithms are quite heavy to perform
online) using suitable heuristics that act as input for the actual path calculation and LSP
establishment process. Extensions to traditional routing protocols and algorithms are required to
uniformly encode and carry TE link information.
Some challenges to consider …
Some of the issues addressed by the CCAMP group were:
The MPLS label space is comparatively large, whereas there are a limited number of
wavelengths and TDM channels.
MPLS LSPs can be allocated any bandwidth value within bounds, whereas in optical
networks bandwidth can only be allocated statically and in a small number of values with little
granularity.
In MPLS there is usually only one link between two adjacent nodes while in TDM and
DWDM networks there might be thousands of labels if multiple fibres are used to interconnect
them.
Assigning IP addresses to each link in an MPLS network is common practice, assigning
IP address to every port, wavelength our TDM slot is a serious concern – even with IPv6.
Identifying which port in a network element is connected to is a complex matter in all
optical networks and one subject to high management burden and error prone.
Fast fault detection, isolation, reroute and protection typical of circuit switched networks
must be preserved.
In transparent domains, such as all optical networks, LSAs cannot be distributed in band
with current technology, this small change wreaks havoc in current LSA flooding techniques
because from now on adjacencies in control channel no longer match data channel adjacencies.
…and some possible solutions
To tackle the first two problems LSP hierarchy can be used. By aggregating traffic that
enters and exits part of the network in common nodes (tunnelling, so to speak) it is possible to
define multiple hierarchies throughout the network. Fine-grained LSPs can be used in outskirts,
while for scalability concerns bundling traffic in coarse-grained LSPs can be performed in the
Chapter 4 - Our approach
IP over WDM II - 33
core. This also helps to deal with the discrete nature of bandwidth in circuit switched networks,
several small bandwidth LSPs can be tunnelled in one gigantic WDM wavelength (describe by
only one label) optimizing bandwidth usage and loosening the discreteness of the bandwidth in
circuit-switched domain.
A natural hierarchy exists in GMPLS networks due to an architectural restriction
mandating an LSP to begin and terminate in similar switching equipment (e.g. PSC LSP cannot
terminate in a WDM transparent switch). This architectural aspect will shape the architecture of
GMPLS networks as can be seen in the Figure 8. In this figure, similar to Figure 6, each cloud
represents a different multiplexing level with LSP grain getting coarser towards the inner clouds.
Notice, the forwarding adjacencies changing according switching technologies. The hierarchical
structure, bundling and forwarding adjacencies will account to the high scalability of GMPLS
networks though introducing complex questions in routing and signaling protocols
An LSP that starts in a packet switching node (a router) might be nested with several
others to form a new TDM LSP. This same LSP can be nested once again in a wavelength LSP,
which in its turn can be nested in a fibre LSP that will travel with some other fibres in the same
cable, thus forming a fibre and bundle LSPs. In what concerns routing protocols, this nesting will
help shorten routing databases and reducing the traffic in LSA flooding updates. Some extra
demands are introduced though, because no longer an adjacency in control channel (e.g. between
a PSC node and a TDM node) will represent a routing adjacency. Despite this tunnelling the
network continuous to act in a peer-to-peer configuration, each node is fully aware of the entire
path though meeting scalability concerns.
Chapter 4 - Our approach
34 - IP over WDM II
PSC TDM λSC FSC
Bundle
Combining packetsinto one LSP
Combining loworder LSPs to
fill a f ibre
FA-PSC
FA-TDMFA-LSC
PSC TDM λSC FSC
Bundle
Combining packetsinto one LSP
Combining loworder LSPs to
fill a f ibre
FA-PSC
FA-TDMFA-LSC
Figure 8 - LSP hierarchical structure traditional in GMPLS networks.
Link bundling is one other technique, which presents useful in reducing even further
routing databases and traffic updates. By gathering several LSP our tunnels, which share
common characteristics, it is possible to achieve a large reduction in LSA number and size. One
suitable case is several optical fibres in the same cable. Though some degree of detail will be lost
in the bundling process it will be clearly outweighed by the scalability enhancements brought to
the link state protocols.
As early discussed it might be unpractical to attribute an IP address to every interface
connecting to nodes. Unnumbered links are used in such situations. The link is referred using a
set containing the node ID (usually its IP address) and the link ID, an ID attributed by each node,
unique in the node’s scope. Link Management Protocol will be used to synchronize the IDs used
in each end of the link and discovery of who connects to whom. Link State Protocols have been
extended to cope with the new identification set.
Link management (LMP)
LMP runs between data plane adjacent nodes and is used to manage TE links.
Specifically, LMP provides mechanisms to maintain control channel connectivity, verify the
physical connectivity of the data-bearing links, correlate the link property, and manage link
failures. A unique feature of LMP is that it is able to localize faults in both opaque and
transparent networks.
Chapter 4 - Our approach
IP over WDM II - 35
LMP is defined in the context of GMPLS, but is specified independently of the GMPLS
signaling specification since it is a local protocol running between data-plane adjacent nodes. As
a result, LMP can be used in other contexts with non-GMPLS signaling protocols.
LMP control channel management is used to establish and maintain control channels
between nodes. Control channels exist independently of TE links, and can be used to exchange
MPLS control-plane information such as signaling, routing, and link management information.
Managing links
An "LMP adjacency" is formed between two nodes that support the same LMP
capabilities. Multiple control channels may be active simultaneously for each adjacency. A
control channel can be either explicitly configured or automatically selected, however, LMP
currently assume that control channels are explicitly configured while the configuration of the
control channel capabilities can be dynamically negotiated.
For the purposes of LMP, the exact implementation of the control channel is left
unspecified. The control channel(s) between two adjacent nodes is no longer required to use the
same physical medium as the data-bearing links between those nodes. For example, a control
channel could use a separate wavelength or fiber, an Ethernet link, or an IP tunnel through a
separate management network.
A consequence of allowing the control channel(s) between two nodes to be physically
diverse from the associated data-bearing links is that the health of a control channel does not
necessarily correlate to the health of the data-bearing links, and vice-versa. Therefore, new
mechanisms have been developed in LMP to manage links, both in terms of link provisioning
and fault isolation.
LMP for DWDM Multiplexer
In an all-optical environment, LMP focuses on peer communications (e.g. OXC-to-
OXC). Is possible to obtain important link information between two OXCs in the DWDM
Terminal multiplexers at the edge of the network. Exposing this information to the control plane
can improve network usability by further reducing required manual configuration and also by
greatly enhancing fault detection and recovery.
DWDM extensions to LMP are defined for use between and OXC and the DWDM muxs.
Fault detection is particularly an issue when the network is using all-optical switches (OXC).
Once a connection is established, OXCs have only limited visibility into the health of the
Chapter 4 - Our approach
36 - IP over WDM II
connection. Even though the OXC is all-optical, long-haul DWDM muxs typically terminate
channels electrically and regenerate them optically, which presents an opportunity to monitor the
health of a channel between OXCs. DWDM extensions to LMP can then be used by the DWDM
mux to provide this information to the PXC.
In addition to the link information is also possible to OXC and DWDM mux to exchange
other types of information, such as information regarding alarm management and link
monitoring.
Traffic Engineering
Traditionally, a TE link is advertised as an adjunct to a "regular" OSPF or IS-IS link, i.e.,
an adjacency is brought up on the link, and when the link is up, both the regular IGP properties
of the link (basically, the SPF metric) and the TE properties of the link are then advertised.
However, GMPLS challenges this notion in three ways:
- First, links that are non-PSC may yet have TE properties; however, an OSPF adjacency
could not be brought up directly on such links.
- Second, an LSP can be advertised as a point-to-point TE link in the routing protocol, i.e.
as a Forwarding Adjacency (FA); thus, an advertised TE link need no longer be between two
OSPF direct neighbours. Forwarding Adjacencies (FA) are further described in a separate
section.
- Third, a number of links may be advertised as a single TE link (e.g. for improved
scalability), so again, there is no longer a one-to-one association of a regular adjacency and a TE
link.
Thus we have a more general notion of a TE link. A TE link is a logical link that has TE
properties, some of which may be configured on the advertising LSR, others which may be
obtained from other LSRs by means of some protocol, and yet others which may be deduced
from the component(s) of the TE link.
An important TE property of a TE link is related to the bandwidth accounting for that
link. GMPLS will define different accounting rules for different non-PSC layers. Generic
bandwidth attributes are however defined by the TE routing extensions and by GMPLS, such as
the unreserved bandwidth, the maximum reservable bandwidth, the maximum LSP bandwidth.
Chapter 4 - Our approach
IP over WDM II - 37
It is expected in a dynamic environment to have frequent changes of bandwidth
accounting information. A flexible policy for triggering link state updates based on bandwidth
thresholds and link-dampening mechanism can be implemented.
TE properties associated with a link should also capture protection and restoration related
characteristics. For instance, shared protection can be elegantly combined with bundling.
Protection and restoration are mainly generic mechanisms also applicable to MPLS.
It is expected that they will first be developed for MPLS and later on generalized to
GMPLS.
A TE link between a pair of LSRs doesn't imply the existence of an IGP adjacency
between these LSRs. A TE link must also have some means by which the advertising LSR can
know of its liveness (e.g. by using LMP hellos). When an LSR knows that a TE link is up, and
can determine the TE link's TE properties, the LSR may then advertise that link to its GMPLS
enhanced OSPF or IS-IS neighbors using the TE objects/TLVs. We call the interfaces over
which GMPLS enhanced OSPF or ISIS adjacencies are established "control channels".
Conclusions
Signaling will probably the first GMPLS technology to be deployed in
telecommunications networks due to the significant overhead reduction in provisioning. It is
expected that in this early stages there will be no routing protocols running and much of the
topological configurations will be set manually at each node.
However, routing protocols are the core of distributed control architecture, and GMPLS
aims just to achieve that. Link state protocols will be fundamental to ease network management,
fasten network deployment and provide high degrees of resilience in future telecommunications
networks.
Chapter 5 – The proposed architecture
38 - IP over WDM II
Chapter 5 – The proposed architecture
The study of GMPLS protocols and its extensions is the main stream of the work done
since we started this project. It has been done considering two key aspects. One is that the
control plane to develop assumes an OXC fiber switch and wavelength switch capable (FSC and
λSC). The other was that the primary objective is to switch and route IP traffic.
Once defined the major guidelines we realized there was not enough time to develop the
complete control plane. It is necessary to create new OSPF, RSVP, LMP and OXC controller
software.
Considering different functionalities and some test possibilities, it is possible to get some
results by developing the OXC controller and the new RSVP. So we focus all our efforts in those
two components while keeping in mind the OSPF, LMP and TE integration.
OXC controller and RSVP are covered in following topics, but its important to note that
the complete control plane implementation lies on a extensive protocol integration. The OSPF
and LMP development must follow considerations presented in previous chapters.
OXC Controler
Our initial approach towards designing a full-blown control plane, according to what was
stated in chapter 4, focused primarily in developing the essential components to demonstrate our
router – namely the control program, the interface to the electronic switch controller and the
interface to the RSVP signaling module.
The first and most essential component to be designed is, of course, the OXC controller.
It works as a digital high-level controller. Its data structures keep the current configuration
settings, which are set downwards via the serial communication interface (serial port) and
upwards via the messages received from RSVP.
The switch is described using three different structures. The simplest one is the lambda,
which describes the status of an individual wavelength. Apart from simple status and failure
counters variables it includes two pointers, one for the destination wavelength (within switch
scope) – lambda structure – and one other to the destination port.
Chapter 5 – The proposed architecture
IP over WDM II - 39
Failure Counter
Delta
Failure Counter
Lambda ID
Active
Operative
Status
In tunning
Dest Lambda
Dest Port
Lambda
Failure Counter
Delta
Failue Counter
Port ID
Num. Lambdas
Operative Status
Num. Lambdas
Active
Lambdas Array
Port
Input/ Output
Next Hop Address
Wave length step
Max Wave length
Operative Status
Num. Input Ports
Num Lambdas
Num. Output Ports
Ports Array
Optical Switch
Lambda Converter
Min Wave length
Figure 9 - Control structures.
The port structure describes each switch port in a similar way to the lambda structure. It
differs from it as it specifies the port type (input or output), the remote node address (in a
sockaddr_in or similar structure), number of wavelength (total and active). The last element is an
array of lambda structures, describing the status of each port’s wavelength, as above-mentioned.
Finally, the optical switch structure stores information respective to the optical switch
technological characteristics, such as wavelength conversion capabilities, wavelength range and
step, input and output port number. It gathers all the ports in a single array, input ports firstly
placed, followed by the output ports.
The full picture can be seen above where is described a simple four by four switch, with
two wavelengths per port. Only one connection is represented: from port one, wavelength one,
bound to port three, wavelength one.
Chapter 5 – The proposed architecture
40 - IP over WDM II
Lamb
PortPort
Port
Lamb
Optical Switch
Port
Port Array
Input Ports
Output
LambLamb Lamb
Lambda
Lambda
Lambda
Port 1 Lambda
Port 2 Lambda Array
Port 3 Lambda
Port 4 Lambda
OXC controller
Figure 10 - OXC Controller Data Structures, full picture.
Output Port
Input Lambda
Input Port
Previous Entry
Output Lambda
Next Entry
FIB Entry
FIB
Hash seed
FIB Hash Table
Hash Table Size
0
FIB Hash Table
[...]
Hash Size-1
Figure 11 - FIB data structures.
The Forward Information Base is quite simple in this kind of switching architecture. It
must only contain origination and destination port and wavelength. Our view led as to a very
simple yet effective implementation. Instead of building a full set of data structures, most of
them replicating the ones allocated by the controller, we decided to use only pointers to the
original controller structures. The FIB itself is laid out as double linked bucket hash table. Some
discussion still remains about what hash seed to use, our present view is that the combination
Chapter 5 – The proposed architecture
IP over WDM II - 41
input port, input lambda and output port should prove effective in most circumstances. Figure 11
shows a detailed view of the data structures. Figure 12 gives a broader view of the data structure
usage. Figure 13 represents the full control plane, FIB data structures and their interaction in the
configuration stated above.
FIB Entry
FIB
Hash
[...]
Hash size-
0
FIB Entry
FIB Entry
Forwarding Information Base
Figure 12 - Forwarding Information Base Hash Table.
Lambd
Lambd
Lambd
OXC Controller
Data
Forward Informat
ion
Figure 13 - OXC controller and FIB joint operation.
Chapter 5 – The proposed architecture
42 - IP over WDM II
The data structures above mentioned are allocated, populated and used by the main
program. Two interfaces were designed at this initial stage. One regarding the communications
interface with the RSVP module and the other one interfacing with the fabrics.
Unix sockets were our choice to interface the RSVP, due to its simplicity, bidirectional
operation and similarity with Netlink sockets. Netlink sockets are used in Linux to allow a user
space program to send and receive data from a procedure in the kernel. As our program might be
ported in the near future to kernel space as a driver module, should a PCI card be used to control
the fabrics, it is rather important to account this future development at this time.
mb
mb
Main Program
Unix Socket
Interface
Serial
Port File Descipto
r
Communications Thread
Serial Port Communications
Thread
RSVP module
Serial Port Device
Fabrics
RS-232
Figure 14 - Overall program diagram.
The serial port was a sensible choice for a fabrics control channel for its simplicity and
readiness of implementation. An existing Stop and Wait protocol was adapted to meet our
requirements; simplicity was the key word as it had to be implemented in a microcontroller at the
fabrics side.
Both sides of the interfaces use threads to allow full asynchronous behaviour in read and
write operations. Multi-threading was preferred to Unix signals, as the formers are easy to
Chapter 5 – The proposed architecture
IP over WDM II - 43
implement and offer more predictable behaviour. Synchronization using mutexes is left for
further study.
The global operation mode is depicted in Figure 14.
Our software is still in a rather early development state. We could even say it is pre-alpha
release, so to speak. Only the main components were implemented and even in this small part of
the full architecture proposed there is much to be defined and tested. Though, we do believe it
settles a good start towards future developments. Porting to PCI control card and subsequent
control plane implementation in kernel space are reasonably easy to achieve. The most
problematic aspect of this task would be to integrate the FIB with the routing and neighbour
structures inside the kernel, which was also evaluated. All the protocols specified could run in
user space, exception made perhaps to LMP.
RSVP
The work under this topic was defining the changes to implement in RSVP in order to
support GMPLS.
Some issues must be considered in advance. The objective is the RSVP adaptation to
support OXC functions and integrate GMPLS specifications. Thus a wise approach is to use an
RSVP package already developed and tested and change to meet our demands. As justified
before, traffic engineering is a key component, so the use an RSVP-TE packet must be
considered.
Two kinds of changes must be made: OXC communication functions and RSVP’s
signaling messages.
OXC controller communication functions.
OXC controller and RSVP communication is performed using the Forward Information
Base (FIB). FIB retains adjacency and link information from LMP and switch characteristics
and current configuration from OXC controller. RSVP forwards signaling messages using FIB
information in the same way that OXC controller uses it to configure physical optics
components.
The work to do under this topic is developing RSVP to FIB read/write functions. Using
these functions RSVP will be able to setup the OXC and to process signaling messages
according to its capabilities.
Chapter 5 – The proposed architecture
44 - IP over WDM II
Note that the FIB data structure is explained under OXC controller topic.
RSVP signaling messages
RSVP messages must prepared to support new objects and new fields inside current
objects. Based on current drafts the new objects to be considered are a generalized label request,
a generalized label, suggested label and label sets.
1. Generalized label request object
A Generalized label request object is set by the ingress node, transparently passed by
transit nodes, and is used by the egress node. The message should contain as specific an LSP
Encoding Type and the Switching type may be updated hop-by-hop
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Length Class-Num (19) C-Type (4 | TBA)
LSP Enc. Type Switching type G-PID
Figure 15 - Generalized Label Request object format.
A node processing a PATH message containing a Generalized Label Request must verify
that the requested parameters can be satisfied by the interface on which the incoming label is to
be allocated, the node itself, and by the interface on which the traffic will be transmitted.
The node may either directly support the LSP or it may use a tunnel. In either case, each
parameter must be checked. Nodes must verify that the type indicated in the Switching Type
parameter is supported on the corresponding incoming interface. If the type cannot be
supported, the node generates a PathErr message with a "Routing problem/Switching Type"
indication.
The G-PID parameter is normally only examined at the egress. If the indicated G-PID
cannot be supported then the egress generates a PathErr message, with a "Routing
problem/Unsupported L3PID" indication.
Bandwidth encodings are carried in the SENDER_TSPEC and FLOWSPEC objects.
Other bandwith/service related parameters are transported in the object are ignored and
transported transparently.
Chapter 5 – The proposed architecture
IP over WDM II - 45
2. Generalized label object
The Generalized Label travels in the upstream direction in Resv messages carrying
generalized label information.
As seen in Figure 16 this object is very simple including only one field, the label. The
interpretation of this field depends on the type of link over wich label is used. When using fiber
and wavelengths, as the present case, label indicates fiber or lambda to be used, from sender’s
perspective, and is 32 bit long.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Length Class-Num (16) C-Type (2 | TBA)
Label
Figure 16 - Generalized Label Object format.
Values used in this field have local significance and the receiver may need to convert the
received value to a value with that has local significance. Values may be configured or
determined using FIB information from LMP.
The recipient of a Resv message containing a Generalized Label verifies that the values
passed are acceptable. If values are unacceptable then the recipient generates a ResvErr message
with a "Routing problem/MPLS label allocation failure" indication.
Note that the presence of both a generalized and normal label object in a Resv message is
a protocol error and should treated as a malformed message by the recipient.
3. Suggested label object
The Suggested Label object is used to provide a downstream node with the upstream
node’s label preference. This permits the upstream node to start configuring it’s hardware based
on the proposed label before the label is communicated by the downstream node.
The format of a Suggested_Label object is identical to a generalized label. It is used in
Path messages. A Suggested_Label object uses Class-Number TBA and the C-Type of the label
being suggested.
As an optimisation object is possible in a previous approach to ignore it. It is important
however to support the data structures and option values from the beginning.
Chapter 5 – The proposed architecture
46 - IP over WDM II
4. Label Set object
The Label Set object is used to limit the label choices of a downstream node to a set of
acceptable labels. This can be used on a per hop basis.
There are four cases where Label Set is useful in the optical domain as referred in
“Chapter 4 - Our approach”. Label Set is used to restrict label ranges that may be used for a
particular LSP between two peers. The receiver of a Label Set must restrict its choice of labels
to one which is in the Label Set. Much like a label, a Label Set may be present across multiple
hops.
The use of Label Set is optional, but its importance in optical domain implies that it must
be implemented from the start.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Length Class-Num (TBA) C-Type (1)
Action Reserved Label Type
Subchannel 1
…
Subchannel N
Figure 17 - Label Set object format .
In Figure 17 is presented Label Set object format. Action field defines the type of the
object defined in subchannel fields.
Depending on the “Action” value those fields could represent a label list or a label range.
“Action” value also defines if the list or range is to include or exclude. A Label Set is defined via
one or more Label_Set objects. Specific labels can be added to or excluded from a Label Set via
Action zero (0) and one (1) objects respectively. Ranges of labels can be added to or excluded
from a Label Set via Action two (2) and three (3) objects respectively. When the Label_Set
objects only list labels/subchannels to exclude, this implies that all other labels are acceptable.
The absence of any Label_Set objects implies that all labels are acceptable. A Label Set
is included when a node wishes to restrict the label(s) that may be used downstream.
On reception of a Path message, the receiving node will restrict its choice of labels to one
which is in the Label Set. If the node is unable to pick a label from the Label Set or if there is a
Chapter 5 – The proposed architecture
IP over WDM II - 47
problem parsing the Label_Set objects, then the request is terminated and a PathErr message
with a "Routing problem/Label Set" indication is generated.
On reception of a Path message, the Label Set represented in the message is compared
against the set of available labels at the downstream interface and the resulting intersecting Label
Set is forwarded in a Path message. When the resulting Label Set is empty, the Path must be
terminated, and a PathErr message, and a "Routing problem/Label Set" indication is generated.
When processing a Resv message at an intermediate node, the label propagated upstream
MUST fall within the Label Set.
Note, on reception of a Resv message a node that is incapable of performing label
conversion has no other choice than to use the same physical label (i.e. wavelength) as received
in the Resv message. In this case, the use and propagation of a Label Set will significantly
reduce the chances that this allocation will fail.
Note also that the node is capable of performing label conversion may also remove the
Label Set prior to forwarding the Path message.
Communication interface
One point to be defined is how to implement the communication interface between the
host holding the control plane implementation and the OXC fabrics.
Physical optics, thermal control and a digital electronic controller are the major blocks
inside the OXC fabrics. Thermal control is used to tuning physical optics block in order to switch
wavelengths from port to port. The digital electronic controller is the “intelligent” component
inside OXC fabrics. Its function is to setup and monitoring physical optics according to
information from control plane.
In order to implement the information exchange between control plane and digital
electronics controller, a communication interface definition is required.
Our first approach, considering a test lab implementation, was to use the host serial port.
A more professional approach has been evaluated. The use of a PCI board interface seems to be
a versatile and efficient solution, specially considering evolving to an edge router configuration.
Using a PCI board.
In order to implement this solution some changes are required.
Chapter 5 – The proposed architecture
48 - IP over WDM II
In the first place it is necessary to design a PCI board. Which functions could be
implemented there? Could the software be simplified by transferring some functions to
hardware? Could some of the functions actually defined for the digital electronic controller
inside OXC fabrics be moved to PCI board? A complete study and evaluation must be performed
in order to reach a functional effective solution.
Some changes might also be essential to introduce in the OXC Controller.
In the OXC controller software there are also some changes needed. It is necessary to
implement different functions and new PCI driver modules in kernel space. This is quite more
complex than the use of serial port functions implemented in user space.
Conclusions
The OXC controller software must be defined according to the needs and capabilities of
the OXC fabrics.
OSPF, LMP and RSVP are of more complex to implementation. As we should not start
by reinventing the wheel, we found reasonable to use open software packages already available
an implement only the modifications needed. This kind of approach was tested with RSVP and
proved appropriate.
Our OXC design intends to define an IP Optical Router. A complete IP Optical router
should be fully capable of supporting packet, wavelength and fiber switching/routing.
Considering fully passive optical equipment, packet switching is not yet possible, a first
step, due to those technological issues, is to implement wavelength and fiber switching/routing.
As GMPLS implements a unique integrated control plane, a fully functional OXC should
be able to exchange information with all types of GMPLS nodes. This includes the reception and
forwarding of GMPLS Path and Resv messages, which could be originated in an SONET/SDH
multiplexer, an ATM switch or even an IP MPLS Edge Router upgraded to a GMPL control
plane.
Our OXC goes beyond protocol stacks and framing as it appears completely transparent
to upper layer protocols. In this context it makes no sense to define it as an IP Router. What
makes it comparable to IP traditional router is its GMPLS control plane, which uses protocols
bred in the IP world but extended to support non-IP traffic.
Chapter 6 – Conclusion
IP over WDM II - 49
Chapter 6 – Conclusions
The evolution of optical networks takes us towards simplification and standardization of
the control structures. The trend is to carry IP, with the least overhead possible, in the optical
medium. Gigabit Ethernet will play a dominant role in this approach and latest developments in
this technology just confirm that.
As communication switched networks become more complex, centralized control
systems strive to keep up with the growing pressure for fast network deployment and
reconfiguration. Different equipments have different switching capabilities, and control protocol
stacks must be able to deal with all those differences in the same network or even in the same
link. GMPLS with its peer decentralized control model and traffic engineering extensions
promises to ease network management in a more efficient and cost-effective way.
GMPLS assumes itself as a solution to implement a complete and integrated control plane
with many different operation models.
The need of overhead reduction in provisioning makes signaling the first GMPLS
technology to be deployed in telecommunications networks. In early stages much of the
topological configurations will be set manually at each node, however, routing protocols are the
core of distributed control architecture. Link state protocols will be fundamental to ease network
management, fasten network deployment and provide high degrees of resilience.
The switching domains concept must be always present. According to GMPLS
specifications all nodes must be able to interact with its neighbours at control plane level no
matter which are their switching capabilities. Thus two approaches are possible:
- If the node is able to intercept and understand data flows traversing it, more complex
functions could be performed based on higher-level information. More valuable data
could also be shared with the entire network.
- If the node does not understand those data flows, it acts transparently based on
configuration messages receive by its neighbours. When acting transparently the path
configuration messages either are manually defined or start in “non-transparent” nodes
following the path based in a hop-by-hop basis.
The use of OSPF and LMP protocols and the independent control plane concept present
in GMPLS, allows the automatic routing and provisioning to be used in transparent nodes.
Chapter 6 – Conclusions
50 - IP over WDM II
An OXC fiber and wavelength switch capable (FSC and λSC) acts transparently
regarding data flow type crossing it. It switches WDM channels independently of what is
transported in them.
Considering this, once the control plane is fully implemented, our OXC will act as a
network core WDM Optical OXC integrated in the GMPLS distributed architecture. Performing
automatic, on client demand, provisioning, wavelength routing and routing information
distribution in peer-to-peer model.
It will be able to switch GMPLS WDM channels no matter what type of data being
carried, such as IP over Gigabit Ethernet frames or ATM cells over SDH frames, integrated in
distributed control architecture.
References
IP over WDM II - 51
References
Papers
[1] Ghani, N., Dixit, S., Wang, T., On IP-over-WDM Integration, IEEE communications Magazine, March 2000.
[2] Banerjee, A., Drake, J., Lang, J.P., Turner, B., Kompella, K., Rekhter, Y, GMPLS: An Overview of Routing and Management Enhancements, IEEE communications Magazine, January 2001.
[3] Jerran, N., Farrel, A., MPLS in Optical Networks, http://www.dataconnection.com, October 2001
[4] Banerjee, A., Drake, J., Lang, J.P., Turner, B., Kompella, K., Rekhter, Y, GMPLS: An Overview of Signaling Enhancements and Recovery Techniques, IEEE communications Magazine, July 2001.
[5] Hernandez-Valencia, E., Scholten, M., Zhu, Z., The Generic Framing Procedure (GFP): An overview, IEEE communications Magazine, May 2002.
[6] Awduche, D., Rekhter, Y, Multiprotocol Lambda Switching: Combining MPLS Traffic Engineering Control with Optical Crossconnects, IEEE communications Magazine, March 2001.
[7] O. Aboul-Magd, et. al., Automatic Switched Optical Network (ASON) Architecture and Its Related Protocols", Internet Draft, draft-ietf-ipo-ason-01.txt, June 2002.
[8] Matsuura, N., Katayama, M., Shiomoto, K., Requirements for using RSVP-TE in GMPLS signaling, Internet Draft, draft-matsuura-gmpls-rsvp-requirements-01.txt, June 2002.
[9] Ashwood-Smith, P. et al, Generalized MPLS - Signaling Functional Description, Internet Draft, draft-ietf-mpls-generalized-signaling-08.txt, April 2002.
[10] Ashwood-Smith, P. et al, Generalized MPLS Signaling - RSVP-TE Extensions, Internet Draft, draft-ietf-mpls-generalized-rsvp-te-07.txt, April 2002.
[11] Lang, et al. Link Management Protocol, Internet Draft, draft-ietf-mpls-lmp-02.txt, March, 2001.
[12] Kompella, K., et al, Routing Extensions in Support of Generalized MPLS, Internet Draft, draft-ietf-ccamp-gmpls-routing-00.txt, September, 2001.
[13] Kompella, K., Rekhter, Y., Signaling Unnumbered Links in RSVP-TE, Internet Draft, draft-ietf-mpls-rsvp-unnum-01.txt, February 2001.
[14] 10 Gigabit Ethernet Alliance, 10 gigabit Ethernet – an introduction, September 2000
References
52 - IP over WDM II
[15] Acterna Communications, Packet Over Sonet/SDH Pocket Guide, 2000
[16] Agilent Technologies, An overview of ITU-T G709, Optical Transport Network
[17] Optical Internetworking Forum, OIF UNI 1.0 – Controlling Optical Networks, 2001
[18] Papadimitriou, Dimitri, Enabling Generalised MPLS Control for G.709 Optical Transport Networks, Alcatel, October 2001
[19] Kompella, et al., OSPF Extensions in support of Generalized MPLS, IETF, draft-ietf-ccamp-ospf-extensions.txt, 2000
[20] Fredette, Andre, et al., Link Management Protocol (LMP) for DWDM Optical Line Systems, IETF, draft-ietf-ccamp-lmp-wdm-00.txt, February 2002
[21] Mannie, Eric, et al., Generalized Multi-Protocol Label Switching (GMPLS) architecture, IETF, draft-ietf-ccamp-gmpls-architecture-01.txt
[22] Rosen, E., et al., Multi-Protocol Label Switching Architecture, IETF,rfc-3051,2001
[23] Xu, Y., et al, A Framework for Generalized Multi-Protocol Label Switching (GMPLS), IETF, draft-many-ccamp-gmpls-framework-00.txt, January 2002
[24] Papadimitriou, Dimitri, et al., GMPLS signalling extensions for G.709 Optical Networks Control, IETF, draft-ietf-ccamp-gmpls-g709-02.txt
[25] Linux How-To’s at www.linuxdoc.org
Kernel How-To
Linux Networking Overview How-To
Linux IPMasquerade How-to
Linux IPChains How-To
[26] Herrin, Glenn, Linux IP Networking – A guide to the Implementation and Modifications of the Linux Protocol Stack, May 2000
[27] Rusling, David A., The Linux Kernel, January 1999
Poster
Metropolitan GMPLS Network
ATM MPLS Network
Metropolitan SDH Network
All Optical Network / OTN 10 Gigabit EthernetMPLS Network
OXC - Optical CrossConector
SDH Ring
OXC
OXC OXC
IP Network
IP Network
E-LSR
Router IP
IP Network
IP Network
Fabrics
IP over WDMDesigning an IP Optical Router
Supervisors• Henrique Salgado (UOSE)• Manuel Ricardo (UTM)
Students• Igor Terroso (UOSE)• Joel Carvalho (UOSE)
The road to the OXC
Router IP
• Bruno Leite (UTM)• Fernando Pinto (UTM)
http://www.fe.up.pt/~ee97159/pstfc
Control Plane
Voice Network
TDMTDM
Router IP w/ TDM
λSR
λSR
λSR
λSR
E-LSR
E-LSR
LSR
ATM
ATM
ATM
ATM
Cellular Network
Cellular Network
IP Network
Router IP
LSR
LSR
λSR
Voice Network
OXC
OpticalLayer
SDHATM
IP
IPGbE
OpticalLayer
Traffic Enginnering Control
FIB
OSPF - TETETopologyDB
OXC controller
LMP
RSVP - TE
Path and Wavelength selection
Thermal controller
Digital Electronic controller
Optical components
IPGbE
WDM Layer
CommunicationsInterface
OpticalLayer
SDH
ATM
IP
IP
Traditional SDH approaches.- IP/ATM/SDH- POS (IP/PPP/HDLC/SDH)
Traditional SDH based approaches, withWDM layer.
IP over WDM overlayapproach
Direct MPλSapproach
WDM Adaptation Layer
HDLC
IP
GbE
IP
SDH
ATM
IP
WDM / OTNOptical Layer
Gigabit Ethernetover SDH Aproach
PPP
IP
HDLC
PPP IP
GbEGFP
WDM
PPP Eth
IP
GFP
PPP Eth
ATM
IP
Protocol Stack Evolution for IP over WDM SolutionsFabrics pictures
HDLC
OpticalLayer
SDH
PPPIP
SDHWDM Adapt.
Optical Layer
HDLCPPPIP
WDM Layer
IP
GFPPPPEth
IP
GbESDH
WDM Adapt.
Optical Layer
SDH
IPATM
WDM Adapt.
Optical Layer
OXC
λSR
Objectives• Developing OADM Architectures• Enhancement of the OADM Architecture
• Define GMPLS router architecture• Specify and develop router control plane
Optical Add – Drop Multiplexer 2
Drop 2
λ2
Add
Input
λ1, λ2, λ3
Output
λ1, λ3λ1
λ2λ2
λ3
Optical Add – Drop Multiplexer 1
Drop 1
λ2
Add
Input
λ1, λ2, λ3
λ2
Output
λ1, λ3
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ivity
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-8
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0
OXC optical components Peltier controller – Power circuit Peltier controller - Analogical
Reducing the protocol stack:Eliminating multiple framing levels
Reducing overhead
Use of IP as the convergence layer
Using WDM as Transport Layer:Simple thin optical layer
Use of additional sub-layer for carrier-class reliability
Input signals
Bar-state Output
Input 2 crossingGrating transmission / reflection
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-70
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Output 1Output 2
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-10
-5
0
A
B
Tra
nsm
issi
on(d
B)
λ (nm)
1546 1548 1550 1552-80
-70
-60
-50
-40
-30
-20
-10
0
Output 1Output 2
Tra
nsm
issi
on(d
Bm
)
λ(nm)
Input signal – WDM source A – Output signal B – Dropped signal 1 C – Dropped signal 1 D – Dropped signal 2 λM = λ2 λM between two channels
Final OXC Architecture• Reduced crosstalk
• High performance levels
• Good channel selectivity
• High scalability
Implementation focused on building OXC controller and the Forwarding Information Base
OXC controller
Main program and two threads
Main program manages OXC according to what is written in the FIB by the RSVP module, via UNIX socket read by communications thread
Main program responds to RSVP module via UNIX socket
Serial port chosen to control the OXC fabrics due to simple implementation – simple Stop and Wait data link protocol adapted
Threads act as listeners, providing fully asynchronous communication (preferred for simplicity to UNIX signals)
FIB implemented as a double linked bucket hash table
Each entry contains pointers to OXC controller data structures (conceptually: input port, input wavelength, output port, output wavelength)
Hash function key – input port, output port, input wavelength
RSVP
New objects and messages required:
. Generalized label request object . Generalized label object
. Sugested label object . Label Set Object
Output 1
Output 2
Optical Cross – Connect 2
Input 1
λ2, λ3
Input 2
λ1
λM
λM
Patent pending
Optical Cross – Connect 1
Input 2
Output 1
Output 2
Input 1
λ1, λ2, λ3
λM
US Patent 5940551
Published article: I. Terroso, J.P. Carvalho, O. Frazão, M. Ricardo, H.M. Salgado,
“Avaliação de duas arquitecturas de OADM Baseadas em circuladores ópticos e redes de Bragg em fibra óptica.”, Física 2002Published article: J.P. Carvalho, I. Terroso, O. Frazão, M. Ricardo, H.M. Salgado, “Comutador óptico (OXC)Baseado em circuladores ópticos e numa rede de Bragg em fibra óptica.”, Física 2002
Submitted Patent: “Comutador óptico (OXC) 2x2 portas para sistemas demultiplexagem em comprimento de onda e escalável a NxN portas”
RS
-232
Lambda
Lambda
Lambda
Main Program
UnixSocket
Interface
SerialPort FileDesciptor
CommunicationsThread
Serial PortCommunications
Thread
RSVP module
Serial PortDevice
Project objective reached