Optic Fiber Network Planning

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Project P709

Planning of Full Optical NetworkDeliverable 2

Basic factors influencing optical networks

Volume 5 of 5: Annex D Interaction between the optical and theclient layer

Suggested readers:

PNOs studying potential upgrade possibilities for their SDH networks

System engineers and network planners

Experts on standard bodies of ITU-T SG13 (Q 19), SG15 (Q 16, 17, 20) andETSI TM-1WG2/WG3

Researchers engaged in the field of optical transmission networks andtechnologies

For full publication

May 1999


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1999 EURESCOM Participants in Project P709

EURESCOM PARTICIPANTS in Project P709 are:

Finnet Group

Swisscom AG Deutsche Telekom AG

France Tlcom

MATV Hungarian Telecommunications Company

TELECOM ITALIA S.p.a.

Portugal Telecom S.A.

Telefonica S.A.

Sonera Ltd.

This document contains material which is the copyright of certain EURESCOMPARTICIPANTS, and may not be reproduced or copied without permission.

All PARTICIPANTS have agreed to full publication of this document.

The commercial use of any information contained in this document may require alicense from the proprietor of that information.

Neither the PARTICIPANTS nor EURESCOM warrant that the information containedin the report is capable of use, or that use of the information is free from risk, and

accept no liability for loss or damage suffered by any person using this information.This document has been approved by EURESCOM Board of Governors fordistribution to all EURESCOM Shareholders.


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Deliverable 2 Volume 5: Annex D Interaction between optical and clinet layer

1999 EURESCOM Participants in Project P709 page i (v)

Preface

(Prepared by the EURESCOM Permanent Staff)

The advances in optical fibre transmission technology over the past years have keptpace with the demand for increased bandwidth. In particular the introduction of theWDM technology enables Telecom Operators to upgrade the capacity of theirnetworks by an order of magnitude. The evolution of photonics makes thedevelopment of optical switching and routing structures in the core and metropolitanpart of the transport network possible.

As a consequence, the development of an optical network infrastructure will enablethe flexible, reliable and transparent provision of transport services for any type oftraditional and innovative services and applications. Taking into consideration thecurrent trends, the objective of network planning is to find the best possible balancebetween network implementation cost, network flexibility, network availability and

survivability, subject to service requirements and topological constraints.The aim of the P709 EURESCOM Project is to investigate a number of alternativestrategies for the planning of the optical transport network - with massive deploymentof WDM, OADM, and small size OXC- that will be used in a middle term future.

This is the second Deliverable (D2) of P709. D2 summarises the most importantfactors that have to be taken into account when preparing the planning of opticalnetworks. Restoration and protection techniques implemented in optical networks areassessed in terms of requirements, constraints on network planning and upgrading, aswell as their interaction with client layer functionalities. A study of resource allocationand impact on network planning and upgrading is also presented.

We should remind the reader that the first P709 Deliverable (D1) provided an

overview over network architectures, which potentially may be used in the future andD3 will give an analysis of the existing network planning methods, plus guidelines forplanning future optical networks.

The present Deliverable (D2) is a very useful study for Optical Network planners &system engineers, and experts on Standard Bodies of ITU-T SG15 and ETSI TM1(WG2 & WG3).


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Volume 5: Annex D Interaction between optical and clinet layer Deliverable 2

page ii (v) 1999 EURESCOM Participants in Project P709


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1999 EURESCOM Participants in Project P709 page iii (v)

Table of Contents

Preface.............................................................................................................................i

Table of Contents..........................................................................................................iii

Abbreviations................................................................................................................. v

1 Introduction ............................................................................................................ 1

2 Overview of SDH architectures.............................................................................. 22.1 What is a network architecture? ...................................................................22.2 SDH network layers..................................................................................... 42.3 Classes of SDH network architectures .........................................................42.4 Trail protection.............................................................................................5

2.4.1 Linear VC trail protection ...............................................................52.4.2 Linear Multiplex Section protection................................................5

2.4.3 MS Dedicated Protection Rings ......................................................52.4.4 MS Shared Protection Rings ........................................................... 6

2.5 Subnetwork connection protection...............................................................72.5.1 General characteristics of SNC protection ...................................... 72.5.2 SNC protection with inherent or non-intrusive monitoring ............8

3 Overview of optical architectures.........................................................................103.1 What is an optical network architecture? ................................................... 103.2 Classes of architectures ..............................................................................103.3 Capabilities and limitations of optical network architectures .................... 123.4 Architectures selected in P709...................................................................123.5 Coloured Section Ring...............................................................................12

3.5.1 General ..........................................................................................123.5.2 Description .................................................................................... 133.5.3 Protection ...................................................................................... 143.5.4 Functional model...........................................................................153.5.5 Implementation.............................................................................. 153.5.6 Design rules...................................................................................17

3.6 Optical Multiplex Section Shared Protection Ring.................................... 203.6.1 General ..........................................................................................203.6.2 Description .................................................................................... 203.6.3 Functional model...........................................................................213.6.4 Implementation.............................................................................. 223.6.5 Design rules...................................................................................24

3.7 MWTN mesh..............................................................................................243.7.1 General ..........................................................................................243.7.2 Description .................................................................................... 243.7.3 Functional model...........................................................................253.7.4 Implementation.............................................................................. 263.7.5 Design rules...................................................................................28

3.8 Conclusion ................................................................................................. 29

4 Dimensioning methods......................................................................................... 304.1 Dimensioning methods for multiple-ring OMS-SPRing or SDH

SPRing networks........................................................................................ 304.1.1 Types of suitable networks............................................................30

4.1.2 Routing problems in multiple-ring networks ................................31


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page iv (v) 1999 EURESCOM Participants in Project P709

4.1.3 The dimensioning method .............................................................324.1.4 Application to OMS-SPRing .........................................................34

4.2 Optical dimensioning methods ...................................................................354.2.1 Optical node structures in a meshed network ................................364.2.2 Dimensioning methods ..................................................................374.2.3 Application to a network example.................................................40

5 Optical/Electrical layers : management possibilities ............................................415.1 Management information processing .........................................................415.2 Possible interaction between SDH overhead and optical overhead............43

5.2.1 Constraints and limitations ............................................................435.2.2 Use of unallocated SDH overhead Bytes for optical

overhead.........................................................................................45

6 Dimensioning methods for optical/electrical layers..............................................496.1 The traffic grooming problem or how to minimise the number of

terminal equipment in the network.............................................................49

6.2 Dimensioning method based on simulated annealing.................................536.3 Application of the tool to network examples..............................................546.3.1 Results for 1+1 protection .............................................................546.3.2 Results for SPRing protection .......................................................576.3.3 Comparison between 1+1 and SPRing protection.........................58

7 Conclusion ............................................................................................................61

References ....................................................................................................................62


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1999 EURESCOM Participants in Project P709 page v (v)

Abbreviations

AIS Alarm Indication Signal

APS Automatic Protection SwitchingDAt Dispersion Accommodation (in optical transmission section

layer)

DAc Dispersion Accommodation (in optical channel layer)

DCC Data Communication Channel

EMF various Equipment Management Functions

FFS For Further Study

LOS Loss Of Signal

MCF Message Communication Function

Mod Modulation/demodulation (O/E conversion)

OA&M Operation, Administration and Maintenance

OCH Optical CHannel section

OCH/Client_A Optical CHannel/Client Adaptation function

OCHOH Optical CHannel OverHead

OCH_T Optical CHannel Termination function

OM Optical Multiplexing

OMS Optical Multiplex Section

OMS/OCH_A Optical Multiplex Section/Optical CHannel Adaptation function

OMSOH Optical Multiplex Section OverHead

OMS_T Optical Multiplexer Section Termination function

opt. DCC Optical Data Communication Channel

OSC Optical Supervisory Channel

OTS Optical Transmission Section

OTSOH Optical Transmission Section OverHead

OTS/OMS_A Optical Transmission Section/Optical Multiplex SectionAdaptation function

OTS_T Optical Transmission Section Termination function

RDI Remote Defect Indication

SDH Synchronous Digital Hierarchy

TMN Telecommunication Management Network

WA Wavelength Assignment

WDM Wavelength Division Multiplex


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1999 EURESCOM Participants in Project P709 page 1 (62)

1 Introduction

The scope of this Annex consists in proposing optical/electrical based architecturesand describing the management capabilities as well as dimensioning methods. Sucharchitectures correspond to the ones depicted in Deliverable 1.

The second chapter concerns an overview of SDH architectures. It describes differentSDH layers and their functionalities. Functional models are described according toITU-T recommendations namely G805 and G803. Different protections schemes aregiven and compared.

An overview of optical architecture is presented in the third chapter. The concept ofoptical architectures is given and classification methods as well. The optical networklayers are detailed and the architectures selected in Deliverable 1 of P709 aredeveloped. General information, functional models, proposed implementations as wellas design rules are given.

The dimensioning methods chapter provides a description of different architecturesmainly optical based ones as well as optical meshed networks. The dimensioningproblem is presented for each architecture under studies assumptions. Algorithms fordimensioning MS-SPRing and OMS-SPRing based networks are presented andtheoretical references provided.

Concerning the meshed optical networks, the presentation focuses on nodedimensioning according to the assumptions described in Annexes A and B of thisDeliverable. Numerical results are also provided on the European network describedin Annex C of this Deliverable.

Thereafter, in addition to P615 project, a brief analysis of the managementpossibilities of the optical layers in the conjunction with the client SDH electrical

layer is given. The informations management model used to control the optical layeris described. Finally, the interactions between SDH layer capabilities and the opticalone are also described.

In the previous chapters, dimensioning optical networks is presented. As SDH isclient layer of the optical layer and according to the subject covered by this Annex,such two layers should be dimensioned together. Dimensioning are different from theones used for the pure SDH networks or the pure optical networks. However, adaptedalgorithms for dimensioning optical/electrical networks are presented. Finallyevaluated networks under given assumptions are presented.


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page 2 (62) 1999 EURESCOM Participants in Project P709

2 Overview of SDH architectures

The chapter starts by defining the concept of network architecture. Then, the layerednetwork structure of the SDH is presented and the classes of SDH networkarchitectures originating from the use of different protection techniques areindentified. In the following two sections of the chapter, these architectures are lookedat in more detail.

2.1 What is a network architecture?

A network architecture is an abstract description of a networks functionalities andconfiguration. Network functionalities that are usually used for distinguishing networkarchitectures are traffic routing and traffic protection functionalities. The configurationof the network depends on the number of nodes and their interconnections.

An architecture potentially consists of a vertical and a horizontal description. The

vertical description uses the concept of network layers to identify and represent thedifferent network functionalities and their relationships. The horizontal descriptionuses the concept of network domains (subnetworks) to identify and representadministrative/physical partitions within a network layer. The following figureillustrates the concepts of network layers and domains.

T1304500-95/d06

Specific pathlayer network

Specific pathlayer network

Transmission medialayer network

Subnetworks Links

Access group A layer network

Layering view(client/server association)

Partitioning view

(a) Layeringconcept (b) Partitioningconcept

Figure 1. Orthogonal views of layering and partitioning

It should be noted that each layer can have its independent topology. At the physicallevel the topology coincides with the physical disposition of the cables implementing


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the network. However, the topologies at other layers will be virtual (or logical)topologies, defined by the interconnectivity between the network nodes.

Network architectures are formally represented by functional models. Functionalmodels follow the definitions and rules presented in the following ITU-T

Recommendations : G.805 Generic Functional Architecture of Transport Networks (General

rules for all kinds of networks)

G.803 Architectures of Transport Networks based on the SynchronousDigital Hierarchy (SDH) (Application of G.805 to SDH networks)

Draft G.otn Architecture of Optical Transport Networks (Application ofG.805 to optical networks)

In a functional model we can find the different network functionalities allocated todifferent network layers. The layers contain transport processing functions, andinteract vertically according to a client/server relationship. Within each network layerthere may be different subnetworks interconnected by links in a horizontalrelationship. The following figure presents an illustrative functional model.

T1304480-95/d04

Trail

AP AP

Trailtermination

Network connection

TCPSNC

CPLink connection

Trailtermination

TCP

Client to

serveradaptation

Clientlayer

network

Trail

Client toserver

adaptation

APAP

Trailtermination

Trailtermination

Serverlayer

network

TCP

SNC

CP

LC

CP

LC

CP

LC

CP

SNC

TCP

Figure 2. Example of a functional model

The functional model of a network is especially important from a networkmanagement point of view, since it is especially adapted to the TMN model. It is alsoimportant to provide an abstract description of the network functionalities, which canthen be implemented in different ways using physical equipment.


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2.2 SDH network layers

In order to understand SDH network architectures we must be acquainted with itsnetwork layer model. This model is represented in the Figure 3, consisting of three

main layers : the circuit, path and transmission media layers. The layers can be dividedin sub-layers, as shown in the figure.

At the circuit layer we can have 2 Mb/s, 34 Mb/s or 140 Mb/s circuits, which aremapped into Virtual Containers (VC) in the path layers (ex: 2 Mb/s into VC-12, 34Mb/s into VC-3 and 140 Mb/s into VC-4). The path layer is split in two sub-layers,since some VCs of lower capacity can be multiplexed in higher capacity VCs. Thisresults in lower- and higher-order paths.

The transmission media layer can be divided in a physical media layer, which dependson the kind of transmission media used (optical fibre, coaxial cable, radio link) and asection layer, associated to the netwok elements responsible for transmission: themultiplexers and regenerators.

Physical Media Layer

VC-3

VC-11

Circuit Layer NetworksCircuitLayer

PathLayer

TransmissionMediaLayer

Regenerator Section Layer

Multiplex Section Layer

VC-4

VC12 VC-2 VC-3

SectionLayer

Higher Order

Path Layer

Lower OrderPath Layer

Figure 3. Layered network model of the SDH

Each layer is supervised for its quality of service and must be managed in anintegrated way with the rest of the layers. As such, each layer associates overheadinformation to the signals received from its client layer, which allows theimplementation of supervision and control functionalities.

2.3 Classes of SDH network architectures

SDH network architectures will be treated in this chapter according to the kind ofprotection technique used. There are two classes of protection techniques:

SDH trail protection (at the section or path layer)


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SDH subnetwork connection protection (at any path layer, on any physicalstructure)

Trail protection is used to protect Multiplex Section Trails or VC Trails. It can beapplied to rings or point-to-point links. MS protection protects against failures in

specific Multiplex Sections, thus protecting all VCs simultaneously. VC trailprotection, on the other hand, is an end-to-end protection technique, and can beapplied to individually chosen VCs.

Subnetwork Connection Protection can be used to protect a portion of a path betweentwo Connection Points (CP) or between a CP and a Termination Connection Point(TCP) or the full end-to-end path between two TCPs. It differs from VC trailprotection in the technique used to monitor the quality of the protected signal.Contrary to trail protection techniques, SNC protection does not have inherent signalquality monitoring capabilities, which results in the need to use specific monitoringtechniques. SNC protection schemes are classified according to the monitoringtechnique adopted.

Protection techniques can also be bidirectional or unidirectional, as well as revertiveor non-revertive. Bidirectional protection switches both directions of traffic toprotection channels, even if only one traffic direction is affected by a failure.Unidirectional protection switches only the affected traffic direction to protectionchannels in case of a unidirectional failure. Revertive protection switches the trafficback to the normal channels after correction of a failure. Non-revertive protectionkeeps traffic running on protection channels after correction of a failure.

In the following sections we will analyse in more detail the different SDH protectionarchitectures.

2.4 Trail protection

2.4.1 Linear VC trail protection

Linear VC trail protection is a dedicated protection technique, which means that anappropriate amount of capacity is available, over a separate physical route, forprotection of the normal traffic. This technique can be used on rings and meshes,either on the LO or HO path layers. It is applied to VCs individually (not all VCs needto be protected) and protects traffic on an end-to-end basis. Linear VC trail protectioncan operate unidirectionally or bidirectionally. In the later case, extra traffic can becarried on the protection path.

2.4.2 Linear Multiplex Section protection

Linear MS protection can be dedicated or shared. In the shared case, an MS trail withprotection capacity is available to protect one of a set of working MS trails. Linear MSprotection allows the simultaneous protection of all VCs in an MS trail in a point-to-point link. This kind of protection technique can also be unidirectional orbidirectional, allowing extra traffic to be carried over the protection trail in thebidirectional case.

2.4.3 MS Dedicated Protection Rings

A basic MS Dedicated Protection Ring (MS-DPRings) consists of two fibres

supporting counter-rotating traffic. Only one fibre supports working traffic, the other


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being dedicated to protection. MS-DPRings require an Automatic ProtectionSwitching (APS) Protocol to operate. However, the detailed operation of these rings isstill under study by ITU-T.

In the event of a failure (span or node failure), the node immediately before the failure

re-routes the traffic from the working fibre to the protection fibre. The traffic thencirculates around the ring, in the opposite direction over the protection fibre, untilreaching the node immediately after the failure. This node routes the traffic back to theworking fibre. Thus, all traffic is restored except for the case of a node failure, wheretraffic to/from the failed node is lost.

A B C

DEF

A B C

DEF

Figure 4. a) Schematic of a 6 node SDH ring network which employs two

unidirectional fibres. One of the fibres supports working traffic (grey) and the

other supports dedicated protection channels (white). Bi-directional traffic

(diverse routes) is shown between nodes A and D under normal operating

conditions; b) The same ring and traffic, but now with a cable cut type failure

between nodes B and C (one multiplex section). Traffic is ring-switched on to

(and off) the dedicated protection fibre by the nodes adjacent to the cut

2.4.4 MS Shared Protection Rings

MS Shared Protection Rings can have two or four fibres. The two-fibre version will be

described here. Each of the two fibres (one carrying clockwise and the other counter-

clockwise traffic) has a capacity which is divided equally between working channels

(time-slots) and protection channels (time-slots). The working channels on one fibre

are associated with the protection channels on the other fibre and vice versa. Only one

set of section overheads are carried on each fibre. In a two fibre scheme, where the

ring is rated at STM-16, each span can carry eight AU-4s of priority (protected) traffic

on the working channels. Under normal conditions the protection channels can be usedto carry additional low-priority traffic (e.g. PSTN) which is dumped when protection

capacity is required. Traffic will normally be routed on the most direct route to its

destination node, although sometimes the longer routing will be preferred due to

capacity limitations and in order to balance the traffic load on the ring. Connections

are made through SDH ADMs at the path layer level according to the destination

node.

In the event of a span failure, priority traffic is switched from the working channels of

one fibre to the protection channels of the other fibre. This operation, conducted at the

node immediately preceding the broken span, causes the traffic to change direction

and go the long way around the ring until it reaches the node on the other side of thebroken span. The traffic is then switched back on to the working channels of the

original fibre where it continues to its destination node. Bidirectional switching and an


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APS protocol are essential for the proper operation of the protection scheme in an MS-SPRing.

Figure 5a depicts a 6 node ring under normal conditions with bi-directional trafficbetween nodes A and D while Figure 5b shows the same traffic under span failure

conditions.Note that multiplex section shared protection can also be used to protect againstintermediate node failures. In the case of multiple cable cuts, the ring splits into two ormore segments, and service can be maintained between any two nodes providing theyare in the same segment.

A B C

DEF

A B C

DEF

Figure 5. a) Schematic of a 6 node SDH ring network which employs two

unidirectional fibres. The capacity of each fibre is divided between working

(grey) and protection (white) channels. Bi-directional traffic is shown between

nodes A and D under normal operating conditions; b) The same ring and traffic,

but now with a cable cut type failure between nodes B and C (one multiplex

section). Traffic is ring-switched on to (and off) the protection channels at thenodes adjacent to the cut

2.5 Subnetwork connection protection

2.5.1 General characteristics of SNC protection

Subnetwork Connection Protection is a linear protection technique that can be used to

protect a portion of a path between two Connection Points (CP), or between a CP and

a Termination Connection Point (TCP,) or the full end-to-end path between two TCPs.

Not all VCs in a MS have to be protected by this technique, as it can be applied to

individual VCs. SNC is typically used to protect segments of a VC trail that traversesseveral administrative domains, as for example when two nodes in belonging to

different network operators are linked by a VC trail. The following figure illustrates

this case:


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VC trail

SNC Protection SNC Protection SNC Protection

Administrative borders

between operators

Figure 6. Use of SNC protection to protect a VC trail on a segment by segment

basis

SNC protection can be applied to rings, meshes or mixed topologies. Both uniform ordiverse routing of traffic is allowed. Only unidirectional protection switching isallowed so far, since bidirectional switching would require a dedicated overhead,which is not available, for supporting an APS protocol. SNC protection also supportsreversive and non-reversive modes of operation.

2.5.2 SNC protection with inherent or non-intrusive monitoring

Contrary to trail protection techniques, SNC protection does not have inherent signalquality monitoring capabilities, which results in the need to use specific monitoringtechniques. SNC protection schemes are classified according to the monitoring

technique adopted. Currently, there are two kinds of monitoring techniques for SNCprotection described in [1]: inherent monitoring (SNC/I) and non-intrusive monitoring(SNC/N)

In SNC/I the protection switching criteria are derived from the server layerinformation. For example, a Server Signal Fail (SSF) indication can be used by theSNC protection system to initiate a protection switching action. Thus, the failuredetection is performed by the server layer and the protection switching is performedby the client layer. This kind of protection protects traffic against failures in the serverlayer. If SNC/I is used to protect a VC-4, the server layer is the MS layer. If the pathto protect is a VC-12, the server layer is a HOP (e.g.: VC-4).

SNC/N implements a trail termination in the path layer itself. This trail termination is

only able to read the path overhead to monitor the path status, not being able to alterin any way the bytes in this overhead. Thus the name of the technique. SNC/N is ableto protect the path against failures in the server layer, and failures and degradations inthe client layer. The failure detection and the protection switching are performed buthe client layer.

The comparison of the different SDH protection architectures allows the followingconclusions to be drawn:

For uniform or adjacent node traffic paterns the MS-SPRing provides morecapacity than MS-DPRings or path protected rings.

For hubbed traffic paterns the most appropriate architectures are the MS-DPRing

and the path protection.


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Path protection can be applied to all transmission rate (STM-n, with n=1,4,16)and can be used with all topologies where two physically independent trails areavailable.

The APS protocol of MS-SPRings is optimised for AU-4 operation (in Europe).

Therefore, add & drop should preferentially be performed at the VC-4 level. MS-SPRings are only advantageous, when compared to MS-DPRings, when

working at the STM-16 level, because at the STM-4 level the number of availableAU-4s is not enough to provide real benefits.

Two-fibre MS-SPRings cannot operate at the STM-1 level.

After this overview of SDH architectures, the second section will present an overviewof optical architectures.


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3 Overview of optical architectures

The chapter starts by defining the concept of optical architecture. Then, aclassification method for architectures, based on the allocation of the main networkfunctionalities to network layers, is proposed. In the following sections of the chapter,the optical architectures selected by P709, as well as OAM aspects of the opticallayers are looked at in more detail.

3.1 What is an optical network architecture?

An optical network architecture is an abstract description of a network where some orall functionalities are allocated to the optical layers of the layered model. An opticalarchitecture inserts three new network layers between the regenerator section layer ofthe SDH layered model and the physical media layer (optical fibre): the OpticalChannel (OC) Layer, the Optical Multiplex Section (OMS) Layer and the Optical

Transmission Section (OTS) Layer. Presently, it is expected that an optical networkarchitecture supports several types of client technologies in the upper layers. Theresulting layered model would look as in the following picture:

IP

ATM

SDH PDH

WDMFigure 7. Interrelation of client layers and the WDM layer

3.2 Classes of architectures

Many different functionalities can possibly be considered as criteria for classifyingarchitectures. Here, only the traffic routing and traffic protection functionalities of thenetwork will be taken into consideration.

Traffic routing in a network is directly related to the networks physical and/or logicaltopologies. This functionality is carried-out by Cross-Connects (XCs) or Add-Drop

Multiplexers (ADMs), and can be performed electrically or optically. Morespecifically, routing can be done in the following network layers :

SDH Low Order VC or SDH High Order VC layers

Optical Channel layer using optical switch or optical cross-connect

Advanced networks supporting high volumes of traffic must ensure that networkoutages, with the associated traffic and revenue losses, are minimised. To achieve this,restoration and/or automatic protection techniques are implemented in the network.Restoration consists of re-routing traffic from damaged links via any availablecapacity in a different route to the same destination. It is usually performed manuallyor with the intervention of a centralised system. Protection is built into the equipment,

relies on pre-assigned capacity in diversely routed links and is performed


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automatically by the equipment whenever a network damage occurs. Modernnetworks have already implemented electrical protection techniques. With thedevelopment of advanced optical technology, network protection can also beimplemented optically. As such, protection can be achieved in the following networklayers :

SDH Low Order VC or SDH High Order VC (path protection)

SDH Multiplex Section layer (MSP: Multiplex Section Protection)

Optical Channel layer using optical switch or optical cross-connect

Optical Multiplex Section layer using optical switch

Considering that the above mentioned network functionalities can be implemented inthe electrical or optical layers of the architecture, independently of the networktopology, we have four classes of network architectures:

1. Electrical Routing/Electrical Protection Switching

2. Optical Routing/Electrical Protection Switching

3. Electrical Routing/Optical Protection Switching

4. Optical Routing/Optical Protection Switching

According to the work of previous EURESCOM Projects P413 and P615, these fourclasses of architectures represent a logical evolutionary path from todays technology(Class 1, SDH-based) to future all-optical technology (Class 4), with intermediatesteps in Classes 2 and 3. This evolution is illustrated graphically in the followingfigure.

timePresent

1. Electrical Protection

and

Electrical Routing

(e.g.: present-day SDH systems)

2. Electrical Protection

and

Optical Routing

(e.g.: Coloured Section Rings)

3. Optical Protection

and

Electrical Routing

(e.g.: optically protected ring)

4. Optical Protection

and

Optical Routing

(e.g.: all-optical network)

SDH protection

not available

SDH protection

available

Figure 8. Time scale for the introduction of optical functionalities

It should be noted that in general, in an optically routed network, routing will be partlydone in the electrical layers, by the SDH equipment (finer granularity), and partly in

the optical layers, by fixed or re-arrangeable wavelength routing (coarser granularity).Therefore, classifying an architecture as optically routed puts the emphasis on theadditional degree of freedom allowed by routing traffic in the optical domain. On theother hand, a basic difference between optical and electrical routing should be pointedout: electrical routing is implemented logically (using routing tables in the SDHequipment), while optical routing is really a physical operation.


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3.3 Capabilities and limitations of optical network architectures

The following conclusions were reached by EURESCOM Project P615 based on afunctional comparison of optical, SDH and mixed architectures.

Optical routing allows an additional degree of freedom (flexibility) for routingtraffic in a network.

The routing connectivity in an optical network is more limited than in SDH whenwavelength conversion is excluded. Fortunately, this does not decrease thecapacity efficiency in WDM ring networks.

When an optical network carries SDH traffic, then optical channel (OCH)protection is functionally equivalent to existing SDH protection. Therefore, thereis little interest for OCH protection right now.

Optical multiplex section (OMS) protection, having simultaneous protection of

all wavelengths, can replace the SDH protection systems of all the channels,providing potentially a huge reduction of equipment and cost reduction inexchange for the reduced level of protection.

3.4 Architectures selected in P709

Project P709 in its Deliverable 1 selected a number of optical network architecturesfor further study in the other Deliverables. The selected architectures were thefollowing :

Coloured Section Ring (CS-Ring)

Optical Multiplex Section Shared Protection Ring (OMS-SPRing)

MWTN Optical Cross Connect-based Mesh (MWTN Mesh)

In the following sections, this chapter will present detailed information on these threearchitectures. The information is taken from Annex A of EURESCOM Project P615sDeliverable 1. For each architecture the information presented includes a generaldescription, a functional model, proposed implementations and design rules.

3.5 Coloured Section Ring

3.5.1 General

fibres: 2physical topology: ring (max. number of nodes limited by number of

wavelengths and by SDH protection mechanism)

logical topology: mesh

routing: SDH path layer followed by optical channle layer

protection: Linear MS (1+1 MSP assumed)

span failure: yes

multiple span failure: only if both spans are in the same SDH MS

intermediate SDH node failure: no if node is used for o/e/o redirection


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intermediate opt. node failure: yes (looks like span failure to SDH nodes)

3.5.2 Description

The Coloured Section Ring, invented and patented by France Telecom, employswavelength division multiplexing to improve the capacity of standard SDH two-fibrerings. This architecture is an example of optical routing and electrical protection.

The standard SDH node (an SDH ADM) is supplemented by a two-channel OADM ortwo single-channel OADMs as shown in Figure 9. The OADMs do not possess anycross-connect functionality. Each node on the ring (see Figure 10a) is directlyassociated with two other nodes using two unique wavelengths. Any other nodes onthe ring can be reached by going through either of these two directly linked nodes.They in turn will be linked to other nodes, etc. Note that Figure 10a) depictswavelengths linking adjacent nodes, however, in order to maximise the capacity of thering, it is preferable to have direct links on the busiest routes (e.g. say A to D, and A toF), and indirect on the quieter routes (i.e. traffic from A to B, C and E would route via

D or F). Traffic undergoes o/e/o wavelength conversion at the directly-linked nodesand at any other intermediate nodes preceding the destination node.

OADM 1 OADM 2

east westSDH ADM

Figure 9. Schematic of add & drop node used in the coloured section ring.

Working traffic is shown by the thick lines and protection traffic is shown by the

thin lines, however, their capacities are identical. Here, the OADM functionality

has been broken into two separate single channel OADMs, but could also be

fulfilled by a 2-channel OADM. The OADMs by themselves have no cross-

connect functionality, but through signals are switched to different wavelengthsby the SDH ADM

The size of the ring in terms of number of nodes is directly limited by the number ofwavelengths available (one wavelength per node), therefore a sixteen wavelengthsystem can support 16 (or fewer) nodes. An additional restriction may occurdepending upon the particular SDH protection mechanism employed.

With this basic scheme, the maximum capacity that can be allocated between any twonodes on the ring is limited to two optical channels (two wavelengths), say 2 x 2.5Gbit/s for an STM-16 system. Of course, this extreme situation would prohibit thisparticular pair of nodes communicating with any other nodes on the ring, and ingeneral we would limit the capacity between any two nodes to a level much less than


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two channels. However, if additional capacity is needed at one site, there is no reasonwhy we cannot site several optical nodes at the same geographic site.

Traffic entering the ring at an SDH ADM is connected through a particular interface(east or west in a simple two-port ADM) according to the destination address. At this

point, the signal is allocated to a wavelength which ensures it is only connected to oneof the other SDH ADMs on the ring. If this node is not the final destination, traffic isconnected through to the other optical interface of the SDH ADM and is reallocated toanother wavelength. This process repeats until a connection is made between theoriginating and destination nodes. Note that although optical routing is used to bypassmany of the en-route SDH ADMs, SDH path layer routing is used at the otherintermediate nodes where opical/eletrical/optical wavelength conversion occurs.

3.5.3 Protection

The coloured section ring uses linear MSP to protect traffic. Dedicated protection(1+1) with single-ended switching has been assumed for the effects of this discussion.

In the event of a span failure, traffic is protected by duplicate traffic which is sent thelong way around the ring on the same wavelength (the protection route), see Figure10b. Therefore the SDH ADMs require duplicate optical interfaces for both directions(east & west) and switches on the receive side to allow connection either to theworking or protection routes.

In general the scheme cannot protect against multiple span failures, but where onewavelength is used to cross several spans and the second span failure occurs on thesame multiplex section as the first span failure, then traffic will remain protected. Thecoloured section ring can also be affected by intermediate node failures, e.g. nodeswhere wavelength conversion occurs using the SDH ADM. These failures cannot beprotected against by the multiplex section protection. At other intermediate nodes,

traffic would not be affected by failure of the SDH ADM, but would be affected bycomplete OADM failure. However, this latter type of failure may be protected by themultiplex section protection scheme.

A B C

DEF

OADM

SDH ADM

A B C

DEF

Figure 10. a) Coloured Section Ring under normal operating conditions.

Protection traffic is not shown, b) Coloured Section Ring showing protection

mechanism. Protection traffic for the broken span is shown by thin green line


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3.5.4 Functional model

The basic functional architecture of the CS ring node is illustrated in Figure 11. Thearchitecture of the 1+1 MSP is based on information provided in ITU-T Standards

G.783 and G.841. The MSPC ellipse represents the switch required to receive traffic,and also the permanent connections to send duplicate traffic for protection purposes.

MS

RS

OC

OMS

OTS

HP

EASTWEST

MSPC

Figure 11. Functional architecture of a coloured section ring node showing the

duplicate protection circuit (shaded), and the MSP switch on the receive side

The MSPC switch (receive side) is activated by a Signal Degrade or Signal Failurealarm which is normally detected in the trail termination function in the MultiplexSection layer (lower termination function in the MS layer, below the MSPC). An APSprotocol is not used unless dual ended switching is required (i.e. 1:1 protection insteadof 1+1, see G.841).

3.5.5 Implementation

The Coloured Section Ring associates optical routing on a 2-fibre ring with linear

SDH multiplex section protection. It is an example of an architecture with opticalrouting and electrical protection.

This architecture is based on an SDH ring where ADMs are optically interconnected.The basic principle is to associate a wavelength with each SDH Multiplex Section(between two ADMs regardless of their physical location in the ring), using MultiplexSection Protection. Protection is done in the SDH layers using linear MS protection.Working and Protection Multiplex Sections are diversely routed on the ring utilisingthe same wavelength on different fibres (Figure 12).

The benefits of CS-Ring architecture rely on an increased transmission capacity byimplementing WDM and on a new level of flexibility to define a node logical order toreduce the transit traffic through the nodes.


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Working Section 1

Protection

Section 1

OADM

Node 1Node 2

Node 3Node 4

Section 2

Section 3

Section 4

Figure 12. Scheme of a four node Coloured Section Ring

Wavelength routing is used in fixed filtering configuration and wavelength reuse isadopted in order to reduce the number of implemented wavelengths. The functionalityof the OADM is described in Figure 13. In a CS-Ring, two wavelengths are assignedin each node. The most efficient and cost effective technology for a two wavelengthadd-drop device in the nm range channel spacing is the multilayer technology.

AddDrop n* i

N* iN* i

Figure 13. Functionality of an Optical Add Drop Multiplexer with spectral reuse

Table I shows the insertion loss at the different ports of an OADM with two drop oradd wavelengths in case of fixed filtering and spectral reuse.

Number of add or drop wavelengths input - output input - drop port ; add port - output

2 wavelengths 2 dB 1 : 1.5 dB ; 2 dB2 : 2 dB ; 1.5 dB

Table I - Insertion loss of a two wavelength multilayer OADM without

connectors

To implement a CS-Ring, an optical layer is needed for wavelength routing. On astandard ADM basis, two extra line boards fitted with MS dedicated wavelength areimplemented for protection in each ADM. The standard boards are replaced bydedicated wavelength short distance line boards for working state. Figure 14 shows adetailed implementation on the optical layer in node 2 assuming splices between thefilters. Optical connectors are used at each pigtail of the drop or add port and theinput/output fibre section. The insertion loss for a wavelength going through the


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optical layer of a node is 2 dB and the insertion loss of working signal drop/add port is1.5 dB (2 dB for protection signals).

Multilayer Filter

OADM ; InsertionLoss : 2 dB Optical Connector

Figure 14. Detailed implementation of filters at node 2 of a CS-RING

In terms of ring fibre span, the size of the CS-Ring architecture is limited by theprotection link power budget. To overcome the standard 26 or 28 dB power budget,optical amplification can be used according to the needs. In order to amplify only theprotection links, the implementation of optical amplification in the middle of the nodeoptical layer is suggested according to Figure 15. Note that the position of the filtersused for protection signals is different from the previous case. For architecturecomparisons, we assume that the optical layer has no chromatic, no polarisation effectand optical crosstalk gives no penalty on transmission.

OA

Figure 15. Detailed implementation of filters with optical amplification

As the optical layer is not reconfigurable, supervisory wavelength is not needed as faras there are no in line amplifiers. An embedded channel in the SDH overhead can beused to transmit supervisory informations to the manager.

3.5.6 Design rules

General contents of the design rules

In general the design rules should contain information of the physical limitations ofthe architectures to be studied. The physical limitations (architecture by architecture)can be described with the following specifications :

maximum number of nodes that can be included into the given architecture


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maximum length of a span between two neighbouring nodes without and withoptical amplification (OA) implemented in the nodes.

maximum length of the protection path

On the other hand there are not only physical but different kinds of capacitylimitations that can be identified in an architecture. These capacity limitations candepend on the applied technology or on the architecture itself, thus it should bespecified architecture by architecture.

The capacity limitations can be described with the following specifications:

Maximum number of available wavelengths in the architecture (if it is limitedonly by the current state of the technology and not the given implementation ofthe architecture, it should be noticed so)

Maximum number of the available wavelengths in one node (if it is limited onlyby the current state of the technology and not the given implementation of thearchitecture, it should be noticed so)

Maximum capacity of an optical channel (i.e. maximum capacity that can becarried by one wavelength)

The physical limitations and the capacity limitations are studied in this chapter. Themaximum number of available wavelengths in each architecture and per node isspecified.

The general assumptions adopted to calculate the ring maximum length are :

fibre attenuation : 0.28 dB/km (according to G.692)

an optical connector at each end of a fibre section : 0.3 dB mean loss and 0.1 dBstandard deviation. As many connectors are used in a ring, a statistical basis is

adopted (ETSI recommendation M 1009) assuming 3 times the standard deviationto calculate the related insertion losses. In the Tables, all results are rounded atthe upper integer number of dB.

Using OA at each node, two extra connectors are considered to enable theimplementation of the amplifier board.

A 28 dB optical budget is considered enabling transmission rate up to STM-16with joint engineering. With OA in each node, the protection span can beconsidered as a multi-wavelength link with in-line amplifiers for the relatedwavelengths. To evaluate the span length, we considered an attenuation rangebetween the OAs from 22 dB to 33 dB. According to commercial products, a 33dB attenuation range is feasible with only one in-line amplifier and 22 dBattenuation range is feasible with typically 6 in-line OAs and 8 wavelengths.

The span lengths with OA reported in Table II and Table III are rather pessimisticbecause they are based on G 692 recommendation and commercial system capabilitiesusing +13 dBm or +15 dBm output level Optical Amplifiers.

Physical limitations

In CS-Rings, the primary parameter is the number of nodes that is equal to the numberof wavelengths. This number is limited either by the maximum number of nodes in anSDH ring (16) or the maximum number of wavelengths in a wavelength multiplexerallowed by the current state-of-the-art technology; we assume a node implementationaccording to Figure 16.


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OADM ; InsertionLoss : 2 dB

Fibre 1

Fibre 2

Optical Connector

OADM

OADM

OADM

OADM

Optical Amplifier

Figure 16. Node configuration of CS-Ring without and with Optical amplification

The working path is always shorter than the protection path, therefore the latter shouldbe considered in calculation of the maximum span lengths.

This is shown in the Table II; the result is rounded.

Number of nodes 3 4 5 6 7 8 9 10 to 16

Max. span length without OA. 37 21 14 9 5.9 3.8 2.2 no

longest protection path without OA 75 64 55 46 35 26 17 no

Max. span length with OA at each node 107 92 89 82 77 76 75 for further

study

Table II - Physical limitations of a CS-Ring (length in km)

Table II is filled up to 9 nodes which is the maximum number of nodes achievablewithout OA. with the defined parameters and the physical implementation depicted inFigure 15. With OA in a 9 node CS-Ring, 8 OA are implemented in a protection path.Besides undersea transmission, no commercial equipments are available for a link

longer than around 650 km including more than 6 OA. The length limitation is due tothe maximum level of chromatic dispersion (12800 ps) compatible with STM-16transmission.

Capacity limitations

The number of wavelengths in a CS-Ring is identical to the number of nodes. For thetime being, the size of a CS-Ring with OA including more than 9 nodes requiresfurther studies because this implementation implies more than 9 wavelengths which iscommercially available but we need also links with more than 8 in-line amplifiers.This implies an accumulation of spontaneous emission and a degradation of SNRwhich must be calculated.

Regarding the flexibility of wavelength routing to define a logical order of the nodes,we must take into account the physical implementation to know how many nodes arereally crossed by the protection path. Using this flexibility, the protection path lengthmay be reduced in some cases and thus the span length.

According to Table II results, the CS-Ring full span with OA is in the range 320 km (3nodes) to 675 km (9 nodes) which is the maximum length of a STM-16 path on G 652fibre due to chromatic dispersion. Larger size CS-Ring will imply the use of a reducedchromatic dispersion fibre or chromatic compensation.

For ring comparisons in Deliverable No. 2 of EURESCOM P615, the span length ofeach span is 10 km. According to the results in Table II, no optical amplification isneeded in a 5 node CS-Ring and 2 nodes must be fitted with OA in a 8 node CS-Ring.


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3.6 Optical Multiplex Section Shared Protection Ring

3.6.1 General

fibres: 2

physical topology: ring


routing: optical and SDH path layer

protection: optical multiplex section shared protection(dual-ended switching)

span failure: yes

multiple span failure: yes

node failure: yes

3.6.2 Description

Protection at the optical multiplex level could be beneficial. It would enable protectionof all optical channels simultaneously, with a low number of optical switches, giving ita cost advantage over electrical protection. Of course, such an advanced functionalityrequires a proper management system. Although OMS-protection does not protectagainst defects in the network layers, like multiplexing / demultiplexing, it protectsagainst cable-cuts, which is an increasingly important issue for public networkoperators.

A B C

DE

1, 3, 5, ...2, 4, 6, ...

a)

A B C

DE

Fibre failure

b)

Figure 17. Two-fibre OMS-shared protection ring architecture. a) working state,

b) protection state

This section covers one specific example, namely, the Two-Fibre Optical MultiplexSection - Shared Protection Ring (OMS-SPRing). In this architecture, the capacity oneach of the two fibres is shared1 by working and protection traffic. For example, asixteen-wavelength WDM system would have eight channels allocated for workingtraffic and eight for protection traffic on each of the two fibres. In the event of afailure, working traffic on one fibre would be switched over to the protection capacity

1 An alternative to shared protection is dedicated protection when a dedicated fibre is used for

protection traffic only.


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on the other fibre. Therefore the eight working channels on one fibre have the samewavelengths as the eight protection channels on the other fibre and vice versa. Theoperation of this protection mechanism is illustrated in Figure 17.

Additionally, the OMS-SPRing provides optical traffic routing based on wavelength

selection by the nodes.


When optical multiplex section protection (i.e. between OADMs) is considered in aring architecture, then, realistically, dual-ended switching must be used. Single-endedswitching would result in duplicate traffic generated for each OMS which wouldinterfere with itself on the long protection routes unless wavelength conversion wasemployed at each node. Dual-ended switching should also enable the implementationof wavelength reuse.

The functional model for this 2-fibre, OMS-SPRing is shown in Figure 18. It should

be noted that this diagram has been simplified between the HP and OC layers, since itshows only one or two channels being added and dropped, whereas one channel wouldbe used for each of the other nodes on the ring in order to fully exploit optical routing.

RS

HP

MS

HPC

SharedOMSP2 fibres

OMS

OTS

EASTWEST

OC

OMSPC

Figure 18. Two-fibre OMS-SPRing node with cable cut on east side

The functional model shown here is based on the SDH MS-SPRing functional modeldepicted in G.841. The standard G.681 implies that the OMS and OTS functionsshould handle the same number of wavelengths. This model appears to violate thisrestriction in G.681, but it is believed that it circumvents G.681 by using functionshaving an identical channel capacity in the OMS and OTS layers, but that the OMS

Adaptation function is only part-populated, e.g. it would be an OMSA_16 function in


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the case of a 16 wavelength system, but it would only accommodate working channelsor protection channels, not all 16.

The challenge in EURESCOM Project P615 was to study the functional model of thenode shown in Figure 18, and convert it to a physical picture of a ring network. It

should be noted that switching occurs on both sides of a cable cut (dual-endedswitching).


This part deals with the implementation of a two-fibre OMS-SPRing with a meshedlogical node connectivity: each node has a link with all the other nodes on a dedicatedwavelength. The transmission capacity on each of the two fibres is shared betweenworking and protection traffic. Therefore, in a 7 node ring, 6 wavelengths are neededper fibre. OADMs are implemented in each node for wavelength routing and dualended optical switching enables protection in case of cable cut. Management of theoptical layer is not considered in this chapter. Figure 19 presents a typical

implementation of the optical layer in a node. Wavelength allocation is studied inDeliverable No. 2 of EURESCOM P615. Opto-mechanical 22 swiches are suited forprotection as far as insertion losses are low (1 dB typical) and switching speed is in the20 ms range.

The proposed node configuration includes one OADM per fibre and a 22 cross-baroptical switch implemented at each side of the node. Optical amplifiers can be used.The best place for the amplifiers seems to be between the OADM and optical switchin order to use both optical amplifiers in protection state to cope with the extra losses.

OS1 OS2EDFA

OADM

OADM

Figure 19. Configuration of a node with a cross-bar optical switch

An alternative configuration for the node is depicted in Figure 20. It includes oneOADM per fibre and two 21 optical switches.

OS1 OS2

OS4 OS3

EDFA

OADM

OADM

Figure 20. Configuration of a node with 21 optical switch


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In both nodes, only half of the fibre capacity is used to transport working traffic, theother half being reserved for protection.

Regarding the optical amplification, the best location seems to be before the opticalswitches, but it should be noted that insertion losses are increased in the protected

state.In a 5 node architecture, 3 wavelengths are needed in each node for a full logicalmesh. Insertion losses are 2 dB for a wavelength through an OADM. For 5 nodearchitecture comparisons, we assume 2 dB insertion losses between the add and dropports.

Figure 21 and Figure 22 depict the node configuration in working and protectionswitched states in order to compare them on the insertion loss basis.

1dB

OADM

OADM

1dB2dB

2dB 2dB

2dB2dB

2dB

3 dB

4dB3 dB

Figure 21. Cross-bar node configuration in a 5 node OMS SP ring (Working state)

1dB

OADM

OADM

1dB

2dB

2dB 2dB

2dB2dB

2dB6dB

Cable Break

Figure 22. Cross-bar node configuration in a 5 node OMS SP ring (Protection state)

Multilayer filters are adopted for architecture comparison with 1 dB loss per droppedchannel because they are available and they exhibit the lower insertion losses andcosts than grating devices.


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3.6.5 Design rules

The configuration shown in Figure 23 was used to obtain the information presented inTable III on physical limitations of OMS-SPRings.

OS1 OS2

Figure 23. Schematic implementation of a node in an OMS-SPRing

Number of nodes 4 5 6 7 8 9 to 16

Max span length without OA. 4 2 - - -

longest protection path without OA 16 11 - - - -

Max. span length with OA 92 78 59 51 36 for further study

Table III - Physical limitations of OMS-SPRing (in km)

According to Table III, the ring full span is in the range of 368 km (4 nodes) to 288km (8 nodes). The reduction in size as the number of nodes is increasing is due to anincreasing number of wavelengths used in the ring. This limitation could be overcomeby the implementation of a more powerful OA used only for protection links. TheOMS-SPRing with 9 to 16 nodes needs further studies by means of software tools tosimulate an optical amplified WDM network.

3.7 MWTN mesh

3.7.1 General

fibres: 2 per link

physical topology: mesh


routing: optical + SDH

protection: OC or SDH path

MS failure: yes

multiple MS failure: yes

node failure: yes

3.7.2 Description

The Multi-Wavelength Transport Network (MWTN) Project, part of the RACE

program, set out to develop ideas for a future broad-band flexible transport network


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employing the optical network layers. MWTN was seen from the outset as a broad-band network designed to overlay existing networks. The fundamental building blockof the MWTN architecture is the Optical Cross-Connect (OXC) node (Figure 24).Optical cross-connects are particularly useful for meshed network architectures, wherenodes have to route traffic from three or more directions.

The MWTN OXC node has the ability to redirect (route) traffic according towavelength. Inbound traffic is selected first by its incoming route, and second bywavelength. Optical cross-connects are then used to redirect this traffic onto outgoingroutes. Therefore each optical channel can be selected and redirected as required. Inaddition, optical channels may be added or dropped using a conventional digital cross-connect (DXC).

Traffic is transferred between the OXCs and the DXC via tuneable receivers andtransmitters. The DXC may also be used for fine granularity processing (down to theVC-12 level), such as grooming, AU routing, and monitoring. The DXC can also beused to perform wavelength conversion, since all-optical wavelength conversion is not

included in the OXC functionality. Both the OXCs and the DXC can be used to restoreservice in the event of a network failure.

DXCTUNEABLE Tx

OSSs

Rx

ADDDROP

TUNEABLE FILTERS

Figure 24. Reconfigurable OXC proposed by the MWTN consortium. The node

shown handles traffic from three directions. Both pre- and post-amplification

(triangles) are used to boost signals. Optical channels are selected by tuneable

filters and power equalised (not shown) before cross-connection. Traffic may be

added / dropped or groomed using the DXC in conjunction with the tuneable

transmitters (Tx) and receivers (Rx)

For a WDM system employing n wavelengths, the node requires n OXCs to have fullcross-connect functionality. Each OXC must have an input (and corresponding output)port for each fibre accessing the node with additional ports for each DXC.


The functional model of the MWTN mesh comprises routing functionalities in theelectrical and optical layers. The DXC switch is located above the SDH MultiplexSection layer but below the Higher-Order Path functions. The OXC function is locatedbetween the OMS and OC functions because the optical channels can be selectedusing tuneable filters.


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MS

RS

OC

OMS

OTS

HP

Figure 25. Functional model for the MWTN OXC illustrated in Figure 24. OC

protection has not been shown in this diagram, but could be represented by an

expanded OC layer with additional termination and adaptation functions, alongwith an OCPC matrix


Optical nodes are studied in European Projects as MWTN, OPEN, METON,PHOTON or MEPHISTO to develop ideas for a future broad band flexible transportnetwork designed to overlay the existing networks. Optical Cross Connects (OCC) areparticularly useful for meshed network architectures where nodes have to route trafficfrom different directions. Taking into account analysis made within EURESCOMP615 and an overview of the other Projects dealing with OCC, P615s choice wasguided by these functionalities :

a non-blocking reconfigurable node

a reliable configuration (solution including several medium size switch matricesis preferred)

switching speed

optical properties (insertion loss, crosstalk etc.)

commercial availability

According to these guidelines, P615 proposed two OCC configurations. The first one,depicted in Figure 26, is reconfigurable, has low losses with no regeneration and no

wavelength conversion. It is based on the first stage for demultiplexing input channels,the middle stage for switching and the final stage for multiplexing. Optical amplifier isimplemented at the output of the OCC.


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input fibre 1

Supervisory and management

input fibre m

from local node to local node

demultiplexer space switch multiplexer O.A.

Figure 26. MWTN node configuration for meshed architecture comparison

If n wavelengths are used per fibre and m fibres are connected at the input of the OCC,n switches are needed ; each matrix has m input and output ports related to the numberof fibres. An extra matrix is used for dropping or adding channels for the local node.Assuming three input and output fibres and 8 wavelengths per fibre, the availableoptical components are grating type devices with 3 dB typical insertion loss in micro-optic technology plus 1 dB for addressing wavelength supervisory. An opto-mechanical matrix has 2 dB insertion loss. For an input - output channel, a typicalinsertion loss is 11 dB including 1 dB for optical connectors. Narrowing effect ofcascaded nodes is out of the scope of this chapter. With 16 wavelength per fibre,typical insertion loss is 12 dB. The OCC depicted in Figure 26 is suited for a smallsize meshed architecture with an end to end restoration policy because there is nowavelength conversion.

One wavelength is dedicated for supervisory in order to get fault, reconfiguration andprotection/restoration management.

In case of a larger size network with subnetwork protection/restoration, the OCC nodeconfiguration depicted in Figure 27 is proposed. This configuration can be taken as afurther step after Figure 26 configuration, taking into account MWTN and OPENProjects, but P615 suggested the implementation of the wavelength translation withelectrical conversion because not all optical converters were mature.

In the first part of the OCC, the wavelength is chosen with a tuneable filter then routed

through space switches. Implementation of wavelength converters enables each signalto reach an output whatever is the input wavelength. Assuming m input - output fibresand n wavelength in each fibre, the first stage of the OCC is m optical couplers (1n)followed by a switching stage including n space switches (mm). Wavelength isselected by tuneable filters (micro-optic Fabry-Perot filters : 2 dB typical insertion lossincluding wavelength locking). The final stage is a wavelength converter and anoptical multiplexer. Assuming 4 fibres with 8 wavelengths per fibre, total insertionloss before detection is around 16 dB. Insertion loss of the final stage is 4 dB beforeoptical amplification. Assuming 16 wavelengths per fibre, insertion losses arerespectively 19 dB and 4.5 dB.


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AOWl converter

SpaceSwitch

Tunablefiltre

input fibre 1

input fibre m

from local node

to local node

Supervisory and management

1

m

n

1

Figure 27. Optical Cross Connect with wavelength conversion

Optical Cross Connects are proposed in 2 configurations. The first one (Figure 26)includes only passive devices with 11/12 dB insertion losses and is suited for smallsize meshed networks. The other one (Figure 27) includes wavelength converters withelectrical conversion and the insertion losses are 16 dB before detection and 4 dB afterwavelength conversion.

3.7.5 Design rules

The configuration of the node, adopted for architecture comparison, is defined inFigure 28. According to the traffic demand considered in Deliverable No. 2 ofEURESCOM P615, the size of the network is defined on a number of fibre basis andnumber of wavelength per fibre.

input fibre1

Supervisoryand management

input fibre3

demultiplexer space switch multiplexer O.A.

n*STM-1

AddDrop

8(1/2 sdh LTE)

Figure 28. MWTN node configuration for meshed architecture comparison


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The maximum fibre span is 100 km. As only 8 wavelengths are used per fibre, P615assumed the implementation, in each node, of only one Optical Amplifier in each fibrelink. No in-line amplifiers were needed.

The implementation described in Figure 28 exhibits 3 fibres with 4 wavelengths per

fibre There are also add/drop possibilities for local node and the total number ofoptical channels crossing the optical cross-connect is 16. The related cost is 16*(theaverage cost of one optical channel in an OCC) and we must add the cost of the lineterminal equipments and of the electrical DXC4/4. For a bi-directional link, this costmust be multiplied by 2. So, for the Optical Cross-Connect, an average cost is definedas a number of optical channels crossing the OCC.

3.8 Conclusion

This chapter defined what is meant by optical network architecture: a networkarchitecture that implements some or all network functionalities in the optical layers ofthe layered model. Choosing the traffic routing and protection functionalities asclassification criteria, the chapter then proposed a classification method forarchitectures, taking into account whether the chosen network functionalities wereallocated to the electronic or optical layers of the functional model.

Then, the network architectures selected by EURESCOM Project P709 were identifiedand, for each one, a detailed description was presented. The description included anoverview of the architectures functionalities, a functional model, a discussion ofimplementation options and basic design rules.

The optical and electrical architectures are now well known. The next section willstudy the problem of dimensioning and some methods to solve it will be proposed.


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4 Dimensioning methods

4.1 Dimensioning methods for multiple-ring OMS-SPRing or SDH

SPRing networks

This section considers a way of dimensioning simple networks, which are made-up ofa collection of rings. The rings can be OMS-SPRing optical rings, or SDH MS-SPRings.

The suggested dimensioning method is based on planar graph theory. It isunfortunately only applicable to networks where all the traffic generating nodes can bedrawn so that they appear on the edge of the network, however this includes manysimple networks which comprise a few rings. It also gives the minimum possiblenumber of rings required to carry the traffic, and thus gives an optimal solution.

4.1.1 Types of suitable networks

The used of shared protection rings in telecommunication networks is currentlyincreasing due to some attractive characteristics of SDH MS-SPRing rings:

The network is built up of small scaleable elements (ADMs) which can becheaper than SDH digital cross connects needed for mesh networks.

Shared protection rings are usually more efficient in terms of fibre usage than1+1 protection in rings.

Although restoration in mesh networks can be more bandwidth-efficient, it isslower with a state-of-the-art restoration time of one minute. It generally provides

a lower quality of service than MS-SPRing protection where cable breaks causeless than 50ms of downtime.

Many telecommunication operators are therefore choosing to deploy backbones basedon interconnected MS-SPRings. Routing in MS-SPRing based networks is at the sametime more complex than routing across a single 1+1 protected ring, but is also moreamenable to analytical techniques, since routing a demand involves only choosing theworking path only (the protection path being implicit). With 1+1 protected meshnetworks the protection as well as the working path must be chosen, resulting ingreater complexity. In effect, using MS-SPRing based networks bring us back to theproblem finding sets of unprotected paths across a network, a problem which hasbeen covered extensively in the literature, with regards not only to

telecommunications.The principles of the MS-SPRing are also applicable to WDM rings such as the OMS-SPRing, and we therefore consider in this contribution ways to dimension both opticaland SDH shared protection rings.

An example of a network composed of multiple rings, which is suitable fordimensioning using the proposed method, is shown in Figure 29. The keyrequirements of such a network are that:

The network is planar, i.e. can be drawn on a plane such that no links cross oneanother.

All the nodes where traffic originates or terminates are on the exterior

boundary of the network.


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Any nodes which are inside the network boundary must have no trafficterminating, and must also be even (i.e. the sum of link capacity adjoining thatnode must be even). In a network made of rings, this is always the case.

Unfortunately this can rule out many large networks, but if the ring design is

performed carefully, many smaller networks (less than ~20 nodes) meet theserequirements. As shown in Figure 29, rings can be chosen to intersect one another.

Once the geographical placement of rings have been determined, and the ringcapacities (e.g. STM16 MS-SPRing, STM64 MS-SPRing, 16 or 40 OMS-SPRing),then the number of stacked rings required to carry the given traffic demands must becalculated. The next section outlines the possible problems in such an approach.

1

2

3

45

f

Figure 29. Example of a multiple ring network suitable for dimensioning using

the proposed method

4.1.2 Routing problems in multiple-ring networks

Figure 30 shows a demand between two nodes named Start and Finish. A numberof possible routes can be chosen. For example, the Start node sits on both rings 4and 5, so either ring could carry the first leg of the traffic. Three possible paths are

shown. One uses rings 1,2 and 4, the other two both use rings 1 and 5, but travellingon opposite sides of the rings. If the demand is larger than 1 VC4 (or wavelength) itmay also be split and several paths be used at the same time.


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1

2

3

4

5

f

Start

Finish

Figure 30. Possible routes for a given demand

A certain ring usage choice will impact on the in-ring routing, and consequently on thering loading and free capacity. Similarly the choice of the gateway node between ringscan impact on the total numbers of rings needed to route all the traffic within thenetwork.

Faced with all these possible routings and ring choices, it can be difficult to determinethe minimum number of rings required to transport a given set of demands.

The dimensioning method proposed here consists of:

Estimating the number of stacked rings required for each geographical ringplacement

The model then calculates whether this number of rings can carry all the trafficdemands

The number of rings needed can then be increased or reduced until the smallestpossible number required has been found

The proposed method can also be extended to find the routings, but this is notexplained in this contribution.

4.1.3 The dimensioning method

The dimensioning method is based upon the Okamura-Seymour theorem [2], and itsextension due to A. Frank [3].

The Okamura-Seymour theorem states that ifG is a planar graph with a set of nodes Vand edgesE, andHis a set ofkdemands {si,ti}, i=1,, k, then if:

All siand ti are on the exterior boundary of the graph


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The graph is even (the sum of the edge capacity adjoining a given node, plus thesum of the demands originating or terminating at the same node, must be even forall nodes in V)

Then provided that for any cut which separates the network into 2 connected

subnetworks, the sum of the edge capacity removed to perform the cut must besuperior of equal to the sum of the demands spanning the cut (see Figure 31 for anexample).

To be more precise, letXbe a subset ofV. Let D(X) be the set of all edges with oneend inXand the other in V-X, and let (X) be the set of all demands with one terminalinXand one in V-X. If |D(X)| represents the sum of the capacity of all edges in D(X),and |d(X)| represents the sum of size of all demands in (X), then there is enoughcapacity in the network to carry all the demands if and only if:

For allXV, |D(X)| - |(X)| is even and non-negative (1)

It is obvious that this condition is required, but in Reference [2] it is proved that it is

sufficient.

Fibre infrastructure with capacity in VC4s Demands in VC4s

A cut of the network with capacityD(X) of 32 VC4s

Demands across the cut with(X) bandwidth 32 VC4s

(a) (b)

(c) (d)

16

16

16

16

16

16

16

16

16

167

9 4

16

3

3

6

6

7

9

16

Set X

Figure 31. Example of making a cut of a network to check if sufficient

transmission capacity exists. In this case the cut condition is just satisfied, the cut

is tight

Two important conditions limit the applicability of the theorem in telecommunicationnetworks:

The parity or evenness condition

The requirement that all nodes be on the edge of the network.

The necessity of the parity condition is demonstrated in Figure 32a. It can be seen thatall nodes are on the exterior boundary, the cut criterion is satisfied (there is enoughcapacity to transport 2 VC4s both demands across any cut of the network), but theparity condition is not satisfied therefore the demands cannot be routed.

The necessity of all demands originating or terminating on the outside boundary isshown Figure 32b: all nodes are even in this case, and the cut criterion is satisfied, but

still the demands cannot be routed.


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Capacity = 1 VC4

on all edges

2 demands

of 1 VC4 each

(a)(b)

Figure 32. Examples demonstrating the necessity of the conditions

The restriction due to the parity criterion can be alleviated in two ways:

By makin

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