Dheeraj Nagpal (Training Report)

8/8/2019 Dheeraj Nagpal (Training Report)

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ABOUT THE COMPANY

The origins of the Global System for Mobile communications(GSM), as it is now known, date back to 1982 when CEPTestablished a co-ordinating group to oversee the developmentof a second generation cellular system. GSM was ultimatelydeveloped to become the worlds first digital cellular system.

The resultant ingenuity of the techniques adopted within GSMproduced a system which out-performed all aspects of firstgeneration cellular systems.

Importantly, GSM supports open interfaces (enabling operatorsto use infrastructure equipment from different suppliers) androaming between different network operators and countries.Also, GSM provides enhanced supplementary services andfeatures and the digital techniques used ensure that thesystem has a better spectral efficiency (enabling greatersubscriber numbers to receive service) than any analoguecellular system.

GSM was developed during the 1980s. It was designed as aEurope wide digital cellular radio system which wouldultimately replace first generation analogue cellular systemsthat were already experiencing capacity limitations in differentkey markets.

GSM system specifications were designed and agreed underthe control of the European Telecommunications StandardsInstitute (ETSI), with European telecoms administratorscontributing to create a truly pan-European system.

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In the early years of this specification work, the system wasoriginally referred to as Group Special Mobile, after theengineering working groups defining the system specifications.

This system is sometimes also referred to as GSM900, with theoperational frequency band at 900MHz

Work began in 1982 to establish the pan-European GSMsystem. Primary goals and objectives for this new digitalcellular radio system were:

Europe wide operation offering subscriber internationalroaming.

Digital transmission replacement for first generationanalogue systems.

High subscriber capacity handling capability. Open standards and system specification enabling multi-

vendor operation, supplier choice and minimal proprietaryinterfaces.

Security and data protection aspects to support anti-fraudmeasures.

The success of GSM is evident. GSM was originally consideredas a pan-European digital standard. It has expanded widely

outside Europe and is now used in countries such as Australia,New Zealand, South Africa, India, Saudi Arabia and Russia.Furthermore the re-banded variants, GSM1800 and GSM1900have broadened the appeal still further and enable operators toimplement high-capacity Personal Communications systems.

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GSM System Overview

WORLDWIDE GSM SUBSCRIBER GROWTH

GSM Variants

GSM900 was the original variant developed by ETSI.Since then two further GSM standards have beendeveloped, namely GSM1800 and GSM1900.

GSM1800 (formerly known as DCS1800) was developedin response to the need for a higher capacity version ofthe standard to address the perceived market for

Personal Communications Networks (PCNs) acrossEurope. The idea was to target the mass-market and beable to offer cellular communications at prices whichcould compete more readily with wireline services. Inessence GSM1800 is simply a re-banded version of 900.Much larger allocations of spectrum were possible inthe 1800MHz bands, enabling operators to cope withhigher traffic demands.

Standard Downlink Band(BTS to MS)

MHz

Uplink Band(MS to BTS)

MHz

GSM900 935 960 890 915

GSM1800 1805 1880 1710 1785

GSM1900 1930 1990 1850 1910

NETWORK ELEMENTS

The generic architecture of a GSM network is shown in thefollowing figure:

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BSS equipment is connected to a Mobile Switching Centre(MSC). In general terms, the MSC effectively constitutes theinterface between the radio system and other mobile and fixedtelecommunication networks. An MSC controls system networkand switching functions which include; call set-up (signalling,

control and switching), call routing, call termination andcollection of billing information. In order to support mobilesubscribers, an MSC must provide mobility managementutilities that are over and above those for a fixed networkswitch. To achieve this added functionality each MSC has anassociated Home Location Register (HLR), Visitor LocationRegister (VLR), Authentication Centre (AuC) and an EquipmentIdentity Register (EIR).

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Base Station Sub-System

General

The GSM Base Station Sub-System (BSS) comprises theinfrastructure elements which are responsible for the radiocellular aspects of the network, namely the Base TransceiverStation (BTS) and the Base Station Controller (BSC).

In essence the BSSs function is to connect the Network andSwitching Sub-System (NSS) to the Mobile Station (MS). Inaddition the BSS has connections to the Operations andMaintenance Sub-System (OSS) for configuration managementand fault/performance reporting purposes.

Base Transceiver Station (BTS)

The BTS is comprised of a GSM radio transmitter and receivertogether with signal processing and control equipment relatingto the air interface. In addition there will be a number ofantennas connected to the BTS by feeder cables.

The purpose of the Base Transceiver Station is to:

1. provide radio access to the mobile stations acting as anentry point for the mobile station into the fixed network;

2. manage issues related to the radio access, leaving theremainder of the fixed network to handle the call relatedissues such as addressing and routing;

While most other elements of the network may be positioned ina convenient location, the position of the BTS will depend onfactors that are related to provision of acceptable radio

coverage to subscribers. The location of the BTS has asignificant impact on the radio coverage mainly due to thepropagation conditions in the environment immediatelysurrounding the BTS and cell site selection is one of the criticalissues that must be considered when designing a cellular radionetwork such as GSM.

Interfaces

The BTS is connected to the BSC via the Abis interface. The

original intention for the GSM specification of the Abis interface

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was to ensure that base stations from any manufacturer wouldbe interoperable with a BSC from a different vendor. Due to thetiming of the specifications and commercial considerations, theAbis interface does not provide this level of interoperability,and generally, base stations from a manufacturer will only work

with BSCs from that manufacturer or from a closely associatedmanufacturer.

Functions

To keep the base station simple, the base station contains onlythe functions that are necessary to manage and control theradio interface at a cell site. These functions can be groupedinto seven categories:

1. RF Transceiver ControlThe RF transceiver control includes all functions related totransmission, reception, link quality measurements, timingmeasurements, power control, fault detection and linkfailure.

2. Base band Processing and Multiplexing

The base band processing and multiplexing is an associatedfunction to the transmission and reception, it includes all

functions related to modulation, equalization anddemodulation , channel coding and decoding, interleavingand deinterleaving, encryption and decryption.

3. Dedicated Channel Functions and Control

The dedicated channel functions are directed by the basestation controller, the BTS provides some support functionsincluding channel activation, assigning a channel, encryptionstart, handover detection.

4. Common Channel Functions and Control

The common channel functions relate to the differentcommon control channels used such as the BCCH and CCCH.

These functions include the scheduling and execution of thepaging messages, and the System Information messages.

5. Terrestrial Channel Functions and Control

The main function performed by the BTS is to provide a

blocking indication to the BSC.

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6. Synchronisation

The BTS is responsible for synchronising the mobile stationto the network.

7. Operations and Maintenance Functions

The BTS is linked directly to the Operations andMaintenance Centre, any equipment faults or alarms willbe observed immediately by the network operator.

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Base Station Controller (BSC)

The base station controller manages and controls basetransceiver stations (BTS). The BSC was introduced into the

GSM network to reduce the load on the MSC, and simplify theBTSs. Removing most of the radio control functions from theMSC allows the MSC to focus on call control and mobilitymanagement, the two main functions that the MSC isresponsible for.

Terrestrial Channel Management

The BSC is responsible for allocating the terrestrial channels onthe Abis interface from the BSC to the BTS. There is a 1-to-1

relationship between the terrestrial traffic channels on the Abisinterface, and the traffic channels on the radio interface.

When the BSC assigns a mobile station to a specific radiochannel it is also assigning a specific terrestrial circuit. TheMSC is responsible for allocating channels on the A interfacefrom the MSC to the BSC.

In the event of a fault on the A interface it is the responsibilityof the BSC to order the blocking of that circuit to the MSC. For

the Abis interface, it is the responsibility of the BTS to order theblocking of the

Channel Configuration Management

The channel configuration defines the use of the differentphysical and logical channels at a site. It is set by theoperations and maintenance center (OMC) and controlled bythe BSC. The OMC will download the current configuration tothe BSC which will then control and be responsible forassigning the radio channels.

Traffic and Dedicated Control Channel Management

There are three functions included here:

Frequency Hopping Management

The frequency hopping information is transferred from theOMC to the BSC. It is the responsibility of the BSC to transfer

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information is transmitted to the mobile station via the BCCHcarrier.

Channel Management

To allow the BSC to select a channel for a mobile station, the

BTS provides information related to channel blocking andinterference levels on the idle channels.

When assigning a channel, the BSC will download relevantparameters to the BTS such as channel type, channel codingtype, rate adaption and starting time.

Power Control

The mobile station receives the power control setting in theheader of the SACCH frames on the downlink, and returns

the actual power control setting in the header on the uplink.

Power control is regulated by the BSC, it is optional whetherthe power control is performed by the BTS or the BSC, andhowever, the BSC is responsible for the power controlalgorithm employed.

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The Network & Switching Sub-SystemThe Network and Switching Sub-System (NSS) sits between theBSS and other telecommunications networks (e.g. the PSTN).

The functions of the NSS are to manage communications

between subscribers connected to different BSCs, to locate andtrack mobiles in the GSM network for call-routing purposes andto provide connectivity to other networks, in particular thePSTN.

Key elements of the NSS include:

The Mobile switching Centre (MSC) incorporating the VisitorLocation Register (VLR)

The Home Location Register (HLR)

The Authentication Centre (AuC)

In practice a separate MSC called the Gateway MSC (GMSC)provides connection to external networks such as the PSTN.

The various parts of a GSM NSS are connected using Signallingsystem Number 7 (SS7).

MSC/VLR

The mobile switching centre (MSC) functions as an ISDN digital

exchange with some additional features to manage themobility associated with the GSM cellular system. A typical MSChas the following features and functions:

Call Control and Routing

The MSC is based on an ISDN digital exchange, all the callcontrol procedures and messages closely follow those definedfor ISDN switching equipment.

The MSC is responsible for the establishment and release of allconnections and for providing call routing via the gateway MSC.

MSC Interfaces

The MSC interfaces to the PSTN, ISDN, PSPDN as well as theinternal GSM interfaces to the BSS, the OMC, the locationregisters and the interworking function.

The internal interfaces are further defined in the systemarchitecture diagram in this section.

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Mobility Management Functions

The MSC is responsible for mobility management, Mobilitymanagement is the functions that are necessary because themobile station moves through the radio network and evenroams into other networks.

Radio Resource Management

The MSC is also involved in radio resource functions such ashandover and paging. When a mobile reaches the edge ofcoverage, and the adjacent cell is controlled from a differentBSC, then the MSC will become involved in the handoverprocess.

GMSC

A Gateway mobile switching centre (GMSC) is a device whichroutes traffic entering a mobile network to the correctdestination.

The GMSC accesses the network HLRs to find the location of amobile subscriber. It does not operate as an MSC; however anMSC can be assigned to act as a GMSC. The selection of theMSC which performs this routing function is up to the operator.

The operator coulddecide to designate more than one MSC asa GMSC.

SS7

SS7 is a common channel signalling system designed for usebetween public exchanges. The standard was first introduced inaround 1980 and has been evolving ever since.

SS7 operates over digital signalling channels and providesfacilities for transferring information even when there is noassociated call connection.

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GSM Services

Introduction

There are two categories of communications service in a GSMnetwork:

Teleservices, and

Bearer Services.

Teleservices

A Teleservice is a telecommunication service that is completelydefined including the terminal equipment functions. GSM

specifies the following 3 types of teleservices:

Telephony Services.

Short Message Services.

Fax Services.

Telephony Services

There are two telephony services offered by GSM:

Speech telephony is the main GSM telephony service. Itdefines an end-to-end speech transmission path betweenmobile and fixed users, and mobile to mobile users.

The speech telephony service provides the transmission ofspeech information and fixed network signalling tones.

Within the GSM system, all speech information istransported as a digital signal. This is a 64kbps PCM signalin the fixed network, and a 13kbps RPE-LTP signal acrossthe radio interface.

Emergency call is the second GSM speech telephony serviceprovided. The emergency call provides a standardised accessto the emergency services irrespective of the country thatservice is used in.

The Emergency call service is mandatory in GSMnetworks. They may be initiated from a mobile without aSIM, however, it is an option for the network operator

whether to accept these calls.

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Emergency calls can override any lock state that thephone may be in.

A standardised access to the emergency call (112) is usedin addition to the national emergency call code for thatcountry is used. If the national emergency code is used

the SIM must be present as this is not part of theemergency call teleservice.

Short Message Services

Short message service (SMS) is similar to a paging service,alpha-numeric messages can be sent from a short messageservice centre to mobile stations. Some types of SMS messagescan be sent from the mobile station to the SMS service centre.

Fax Services

GSM provides two types of FAX teleservice:

Speech and FAX: This service permits the user to switchbetween speech and FAX services during a call.

Automatic FAX: supports a Group3 FAX inAutocalling/Autoanswering mode only.

Bearer Services

Bearer services are the second type of basic service offered byGSM. The bearer servicediffers from the teleservices describedpreviously in that it only provides a basic transmissioncapability, the protocols and functions that occur in the dataterminal equipment are not defined. The bearer servicesprovide a range of different types of data transmission.

Alternate Speech and Data

Using this service, users will be able to alternate betweenspeech and data transmission. The data transmission part canbe the same as that provided by bearer services 21 to 34.

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New GSM Data Services

This section introduces some new and evolving GSM data

services.These services are not yet standardised, and so this sectionshould only be considered as an indication of how theseservices will appear when they are fully specified.

Three new services are considered:

The Generalised Packet Radio Service (GPRS)

Increase data throughput using data compression.

High Speed Circuit Switched Data

Generalised Packet Radio Service

GPRS is a set of new GSM bearer and teleservices that arecurrently being standardised within ETSI.

These new services will allow subscribers to send and receivedata in an end-to-end packet transfer mode without usingnetwork resources in circuit switched mode.

The GPRS mode of operation will permit a very efficient use ofnetwork resources for packet mode data applications.

This section outlines some of the basic features of the GPRSservice, because it is a new evolving technology, the specificsoutlined below may change as the specification processdevelops.

Data Compression

The basic proposal is to use V.42bis error correction protocol toincrease the user datathroughput from 9.6kbps to 19.2kbps oreven 38.4kbps. V.42bis is a data compression protocol capableof achieving compression rates up to 4:1.

A standard non-transparent 9.6kbps asynchronous GSM beareris used for the radio path.

For the fixed network, the V.42 error correction protocol is usedon top of a standard line modem such as V.32 between the far

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end and the IWF in the GSM network, this ensures a reliableconnection across the fixed network for the compressed data.

The data compression algorithm operates end-to-end. At thePSTN end, a V.32 or higher speed modem operates into asimilar modem in the IWF via the PSTN.

The V.42 protocol is used on top of V.32 to ensure a reliableerror-free link.

A non-transparent asynchronous GSM bearer service is usedacross the radio interface as it provides protection againsterrors over the radio link.

High Speed Circuit-Switched Data

ETSI is currently considering the issues concerned with highspeed circuit switched data (HSCSD). At this stage no definitetechnology decisions have been made, and so this section willreview some of the key issues that are being considered.

The basic proposals for HSCSD are to use multiple timeslots toachieve a higher aggregate data rate. The HSCSD proposals donot advocate significant changes to the air interface; the basicburst structure will remain the same.

A 2 timeslot solution will provide a user data rate of 19.2kbps,and an 8 timeslot solution a data rate of 76.8kbps.

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Operations & Maintenance

General

A GSM network contains a functional element known as theOperation Sub-System (OSS). The OSS performs a variety oftasks and requires interaction with other network elements inthe BSS and NSS.

The GSM specifications are not specific regarding operationsand maintenance (O&M) provision much is left up to theindividual equipment vendor/operator.

The overall objective of providing O&M functionality (ie theOSS) centrally is to improve the efficiency with which thenetwork can be configured and managed.

There are three key types of O&M functionality provided in atypical GSM network:

Network Operation and maintenance

Subscription Management

Mobile station management

Network Operations and Maintenance

This functionality is provided in order to enable the operatorspersonnel to remotely access each network element (eg MSC,BSC, HLR etc) in order to change configuration settings orreport faults. In practice this is achieved by providing computerworkstations called Operations and Maintenance Centres(OMCs) linked to the network elements. These OMCs can either

be stand-alone machines simply providing O&M functions for aspecific element or group of elements or they can be linkedand controlled by an integrated network management system.

Subscription Management

Subscription management comprises two elements:

Subscriber data management

Call charging

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Subscriber data management requires the HLR and dedicatedOSS machines. Customer information and subscription detailsare usually entered and stored on a dedicated system used bythe team responsible for customer management. This system isnot covered by the GSM specifications but it must interface

with the HLR in order to update the subscription informationstored in it (eg which services are allowed for a subscriber etc).

Call charging is also usually handled by a dedicated computersystem connected to the GSM network. In some cases the HLRsystem is used to record this billing information since it is alogical place to store it (see separate section on billing later).

Mobile Station Management

The Equipment Identity Register (EIR) is a database in the GSMnetwork which stores information about mobile stations. It isimportant to realise the distinction between the subscriberinformation stored in the SIM card and the mobile equipmentinformation relating to the physical mobile equipment.

The EIRs primary duty is to control access to the network bymobiles which have been stolen or are being used withoutauthorisation.

If a mobile is reported stolen the operator may flag it as eithergrey-listed or black-listed in the EIR. Black-listed mobilescannot gain access to the network. Grey-listed mobiles aremonitored in order to obtain information which could help trackit.

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Error Correction Coding

Channel coding is used in digital communication systems toovercome errors that are introduced by the noise and distortionthat is inherent in the system. The GSM system uses differentkinds of channel coding schemes, these include convolutionalcodes and cyclic linear block codes.

The basic principle of channel coding is to add bits to a datastream such that errors in that data stream can be corrected.

GSM uses two forms of error correction coding in aconcatenated format:

block coding, and

convolutional coding.

Block Codes

A block code consists of a sequence of data bits together with anumber of check bits. A simple example would be to take theoriginal X bits of data and repeat these as check bits twice. Thiswould enable us to correct one in three bit errors. In practicethe process is more complex, matrix multiplication is used togenerate the check bits.

The receiver puts the 64 bits into a matrix, performs the samemultiplication and compares the result with the transmittedresult of the matrix multiplication. Using some complexprocessing it can then deduce the most likely errors to haveresulted in any difference in the multiplication.

Convolutional Codes

Because the standard of a convolutional coder never receivesthe original signal (unlike block codes) it is never aware ofwhether there were any errors. This can mean that if there aremore errors than the code can correct, the decoder can beunaware of this and actually change bits that were not in errorin making what appears to be the correct decoding technique.With block codes, the receiver will determine that the matrixequation cannot be solved and will not attempt any furthercorrection, leaving the orginal signal unaltered.

Interleaving

By mixing up the bits before transmission and then un-mixing

them at the receiver it is possible to spread the errors over

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all 11 blocks, there will be 1 error in each block which caneasily be corrected. This is the principle of interleaving.

GSM interleaving is quite complicated. 456 bits of the inputstream are passed into the interleaver. This is exactly theamount of information transmitted in 4 burst periods (see

later). In order to get further gains, the information is actuallysplit into 8 sets of data, each filling up half of one burst period.

The next 456 bits is then treated in the same way, resulting inanother 8 sets of data to fill up the other half of the burstperiods. The burst periods are then divided into odd and evenbits, and the bits from the first 456 bits encoded on the evenbits whilst the bits from the second 456 bits are encoded ontheodd bits.

Burst FormattingBecause GSM is a TDMA system each user accesses thechannel for a limited period of time in a cyclical fashion. Theoverall TDMA system is very complex. This is basically becauseit must allow for a number of different channel types to betransmitted over the same resource. The key fact to rememberis that 8 users share one channel, transmitting cyclically. Therest is merely the decisions of the designers as to how best tosend the additional information required.

Ciphering

Prior to transmission the digital stream in encrypted to preventeavesdropping. This process is also known as ciphering.

Ciphering and deciphering in GSM are performed by exclusive-or-ing the 114 bits of each data burst with a 114 bit ciphersequence generated by the A5 algorithm. The uplink anddownlink use different cipher sequences.

The encryption process used is a form of public-key encryptionin which a key is passed between the MS and network (AuC) toagree the cipher sequence to be used but it is (virtually)impossible for anyone else to recreate the cipher sequence byonly knowing the key.

Modulation

Modulation achieves two things. The first is to translate thesignal to a frequency suitable for radio transmission. It doesthis by taking a sine-wave at the frequency of transmission andchanging, or modulating its characteristics in accordance with

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the information to be sent. The second function is to reduce theradio spectrum required to a minimum.

The digital waveform generated by the speech coder is in theform of a square wave, which has instantaneous changes intime. The modulation system takes this square wave and turns

it into a more gentle pulse shape essentially through a filteringprocess.

Amplitude modulation modulates the amplitude of the carrier inaccordance with the information to be transmitted. Frequencymodulation changes the frequency, whilst phase modulationchanges the phase in practice both result in very similareffects

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Features of GSM

Cellular telephone systems provide the MS subscriber and

network provider with many advantages over a

standard telephone network, but there are still many

drawbacks.

1. Compatibility

2. Noise Robust

3. Flexibility and Increased Capacity

a) Easily (RF) configured (Software Driven)

b) Half Rate

c) International Roaming

d) Better Frequency re-use

e) Multi-Band operation

1. Use of Standardized Open Interfaces

2. Improved Security and Confidentialitya) Encryption

b) ME authentication

c) Subscriber/SIM authentication

d) Frequency Hopping

1. Flexible Handover Processes

2. ISDN Compatibility

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TRANSMISSION MEDIA

There are three types of media that can be used intransmitting information in the telecommunications

world:

Coaxial cable ( actually an adaptation of copperwire )

Fiber

Wireless

1. CO-AXIAL CABLE

Co-axial cable basically consists of two concentric conductors

separated by a dielectric material as shown above. Co-axial

cable was initially developed as the backbone of analog

telephone networks where a single telephone cable would be

used to carry more than 10,000 voice channels at a time.

These systems operated in the range of 8.5 Mb/s to 274 Mb/s.

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CHARACTERISTICS OF CO-AXIAL CABLE

Two types of cables having 75 and 50 , impedance is

available.

Excellent noise immunity.

It has large bandwidth and low losses.

1. OPTICAL FIBER

Optical Fiber offers high capacity, low attenuation and is

insensitive to electromagnetic disturbances. Capacities from 2

Mbit/s to 1000 Mbit/s are common. The two wavelengthsmainly used in fiber optical systems are 1310 nm and 1550 nm

as the fiber attenuation is minimum at these two wavelengths.

Figure: 3 Types of Optical Fibre

Optical fiber is usually divided into two modes:

Multi mode

Single Mode

In a Multi mode fiber the different modes (propagation

directions) will not cover the same distance and will therefore

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not arrive at the receiver at the same time, which leads to

pulse broadening.

A single mode fiber suppresses all wave lengths but one, so

that only one mode can propagate in the fiber. Single modefiber is used extensively in telecommunication.

With a 1310 nm wavelength the maximum distance withoutrepeaters is around 50 km (single mode), and with 1550 nmthe corresponding distance is around 80 km (single mode).

A fiber cable usually consists of 24, 48 or 96 optical fiber.

CHARACTERISTICS OF OPTICAL FIBER CABLEHigher Bandwidth therefore can operate at higher

data rates.Reduced losses as the signal attenuation is low. Distortion is Reduced hence better quality is

assured. They are immune to electromagnetic

interferences. Small size and light weight.

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BASIC FIBER OPTIC COMMUNICATIONS

A basic communication system consists of a transmitter, a

receiver, and the medium, arranged as in figure below. The

information travels from the transmitter to the receiver over

the information channel. Information channels can be divided

into two categories; unguided (atmosphere) or guided (variety

of conducting transmission structures). Guided channels have

the advantages of privacy, no weather dependence. Fiber

waveguides have these advantages and have been already

discussed. Now let us have a look at the transmitters and

receivers and understand their functions

Transmitters

Optical transmitters convert electrical signals to optical signals,

which are transmitted through information channel (fiber). It

can transmit analog or digital signals. The basic elements that

may be found in transmitters are as follows:

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Transmitter Medium Receiver

Electrical Signal Electrical Signal


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Electronic Interfaces: It may be wires, connectors, or a pin

emerging from the packages, which receive electrical

signals.

Electronic processing circuitry: Which is to process inputelectrical signals to put them into a form suitable for

driving the light source. For example, conversion of

signals from voltage to current variations.

Drive Circuitry: The drive circuitry is required for biasing

the light source and depends on application

requirements, data format, and the type of source.

Light source: It is the device, which actually converts

electrical input signals to its optical equivalent. Most

common light sources are light emitted diodes (LED)

and Light Amplification by Stimulated Emissions of

Radiation (LASER) diodes.

Optical Interfaces: Through optical interface the optical

output of the light source is coupled to the fiber optic

cable. Common optical interfaces, connectors and fiber

pigtails.

Output sensing and stabilization: It is particularly required

in lasers maintain stable output power by way of

feedback mechanism.

Temperature sensing and control: temperature sensing

and control by deploying thermoelectric coolers in

transmitters can assure operation at a stable

temperature.

Receivers

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The job of the receiver is to receive the optical signals from the

fiber and convert the same to its electrical equivalent. The

basic elements of an optical receiver are given as under:

Detector: The detectors used in fiber opticcommunications are semiconductor photodiodes and

photo-detectors, which convert the received optical signal

into electrical form. Detectors in a fiber communications

system will either be an avalanche photodiode (APD) or a

PIN photodiode. The PIN device is cheaper, less

temperature sensitive, and requires lower reverse bias

voltage than the APD. However higher gain of APD over

PIN finds its use where receiver is to detect lower power.

Amplifier: The electrical output of the signal is required to

be amplified before it is sent for further processing.

Decision circuits: The decision circuit reproduces the

original signal from the electrical signal received from the

amplifier.

ADVANTAGES OF OPTICAL FIBER COMMUNICATIONS

In fiber-optic communication system signals are transmitted as

light, whereas conventional electronic communication system

relies on electrons passing through the wires. This is the crucial

operating difference between a fiber-optic communication

system and other systems. Due to this main difference fiber-

optic communication has some advantages over other

conventional systems. There are also some advantages due to

other attributes. We will discuss some of the advantages now.

Resource plentiful:

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The basic material for glass fibers is silicon dioxide, which isplentiful. Some optic fibers are made of transparent plastic,another readily available material.

High information carrying capacity:

One of the most important advantages of fibers is their abilityto carry large amounts of information. For example, a singlefiber can propagate data at 565 mbps rate, which contains7680 voice channels.

Less attenuation:Attenuation is the reduction of signal magnitude as it travelsalong the transmission path, normally measured in decibels perunit length. A typical fiber will attenuate an optical signal by

0.3dB/Km. whereas co-axial cable will attenuate a 100- MHzsignal by 22.6 dB/km.

Greater Safety:Optic fibers, glass or plastic, are insulators. No electric currentsflow through them, either owing to the transmitted signal orowing to external radiation striking the fiber.

Immunity to RFI:Fibers have excellent rejection of radio-frequency interference(RFI) RFI refers to interference caused by radio and televisionstations, radar, and other signals originating from electronicequipment.

No-cross talk:The optic wave within the fiber is trapped, none leaks outduring transmission to interfere with signals in other fibers.Conversely, light cannot couple into the fiber from its side. Weconclude that a fiber is well protected from interference andcoupling with other communications channels, whether theyare electrical or optical.

Higher Security:Fibers offer higher degree of security and privacy, becausefibers do not radiate the energy within them. It is difficult for anintruder to detect the signal being transmitted without theknowledge of users.

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LIMITATIONS OF OPTICAL FIBER

Sophisticated plants are required for manufacturing

optical fibers.

The initial cost incurred is high.

Joining the optical fiber is a difficult job.

Losses in fiber.

LOSSES IN FIBER

Signal attenuation is a major factor in the design of any

communications system. On one hand output power of the

transmitter cannot be increased beyond certain value due to

some technical limitation and on other hand receivers require

that their input power be above some minimum level. It means

transmission losses limit the total length of the path. Losses

occurring in glass fibers can be classified as under:

Losses due to absorption:

Even the purest glass will absorb heavily within specific

wavelength regions. This is a natural property of the glass

itself and is called intrinsic absorption. Intrinsic absorption is

very strong in the short wavelength, ultraviolet portion of the

electromagnetic spectrum and its peaks also occur in the

infrared region.

Losses due to scattering:Molecules move randomly through the glass in the molten

state, during manufacture the energy for the motion is

provided by the applied heat. As the liquid cools, the motions

cease. Upon reaching the solid state, the random, molecular

locations are frozen within the glass. This results in localized

variations in density and thus local variations in refractive

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index throughout the glass. The variation may be modeled as

small scattering objects embedded in an otherwise

homogeneous material. A beam of light passing through such a

structure will have some of its energy scattered by these

objects. It is clear that scattering severely restricts use of fibers

at short wavelengths. Below 0.8m, the loss owing to this

effect alone builds to a prohibitive value for long-distance

propagation. On the other hand, as the wavelength increases

the scattering loss diminishes.

Losses due to geometric effect:

Bending a fiber causes attenuation. It can be divided into twocategories, macro bending and micro bending. Macro bendingrefers to large scale bending, such as that which occursintentionally when wrapping the fiber on a spool or pulling itaround a corner while laying. As a practical example, 125 mdiameter fibers can be bent with radii of curvature as small as25mm with negligible loss. Microscopic bending often occurswhen a fiber is sheathed within a protective cable.

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DISPERSION IN FIBER

Dispersion is a phenomenon, which spreads the optical pulse asit travels down the length of an optical fiber. It is expressed interms of time per unit length. Dispersion is undesirable as itlimits the bandwidth or information carrying capacity of thefiber. Dispersion can be classified into following three types.

Modal Dispersion

When an optical pulse is propagating through a fiber theenergy is distributed among several modes which travel withdifference velocities with respect to wave guide axis. Somemodes (part of the wave arrive at the other end of the fiberbefore others, resulting spreading out of the wave form. Thiscan be understood better by taking a case as illustrated infigure below. Rays can enter the fiber at many different anglesto its axis. A ray that bounces back and forth within the coremany times will travel a slightly greater distance than one thatgoes straight through; meaning that it will arrive at the otherend of the fiber little later resulting spreading of the pulse.Modal dispersion is more prominent in multi-mode fibers.

1

2

3

Material Dispersion : The variation of refractive index withthe wavelength results in spreading of pulse. It is due tosome property of the material dispersion. In single-modefibers material dispersion is more prominent then othertypes of dispersions.

Waveguide Dispersion : Dispersions can be caused by the

structures itself. The effective refractive index for any

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particular mode varies with wavelength for a fixed filmthickness. The pulse spreading caused due to this effect iscalled waveguide dispersion.

Effect of Dispersion: As the pulse spread, they will overlap with

adjacent pulse and result will be transmission error. Hence toavoid overlapping the time duration between two consecutivepulses should be more that the amount spread for a given,path length. In other words pulse spreading restricttransmission speed.

1.

RADIO LINKRadio link is the name

for a microwave radio connection between two points. This is

actually a type of point-to-point wireless communication. The

radio frequencies used for RF links are in microwave range

therefore, RF links are also called as microwave links.

Radio link is typically available for 2, 4, 8, 16, 34, 140 and 155(SDH) Mbit/s capacities.

Advantages of microwave links:

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Very rapid installation

Average cost level

Sensitive to ambient disturbances

Less maintenance as compared to cables.

Limitations of microwave links:

Rain is the biggest problem for frequencies over 10

GHz.

Flat fading is the biggest problem for frequencies

under 10 GHz.

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(Synchronous Digital Hierarchy)SDH (Synchronous Digital Hierarchy) is an internationalstandard for high speed telecommunication overoptical/standardize networks which can transport digital signalsin variable capacities. It is a synchronous system which intendsto provide a more flexible, yet simple network infrastructure.

SDH (and its American variant- SONET) emerged from standard

bodies somewhere around 1990. These two standards create arevolution in the communication networks based on opticalfibers, in their cost and performance.

Before SDH

The development of digital transmission systems started in theearly 70s, and was based on the Pulse Code Modulation (PCM)method. In the early 80s digital systems became more andthere was a huge demand for some features that were notsupported by the existing systems. The demand was mainly to

high order multiplexing through a hierarchy of increasing bitrates up to 140 Mbps or 565 Mbps in Europe.

The problem was the high cost of bandwidth and digital devices.The solution that was created then was a multiplexingtechnique, allowed for the combining of slightly nonsynchronous rates, referred to as plesiochronous, which lead tothe term plesiochronous digital hierarchy (PDH).

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PDH (Plesiochronous Digital Hierarchy)

The Plesiochronous Digital Hierarchy (PDH) is a technologyused in telecommunications networks to transport large

quantities of data over digital transport equipment such asfibre optic and microwave radio systems.

Traditionally, digital transmission systems and hierarchies havebeen based on multiplexing signals which are plesiochronous(running at almost the same speed). Also, various parts of theworld use different hierarchies which lead to problems ofinternational interworking; for example, between those countriesusing 1.544 Mbit/s systems (U.S.A. and Japan) and those using the2.048 Mbit/s system.

To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, itsnecessary to demultiplex the signal all the way down to the 2Mbit/s level before the location of the 64 kbit/s channel can beidentified. PDH requires steps (140-34, 34-8, 8-2 demultiplex; 2-8,8-34, 34-140 multiplex) to drop out or add an individual speech ordata channel.

The PDH contains 4 basic bit rates:

Signal Digital Bit Rate Channels

E0 64 kbit/s One 64 kbit/s

E1 2.048 Mbit/s 32E0E2 8.448 Mbit/s 128E0

E3 34.368 Mbit/s 16E1

E4 139.264 Mbit/s 64E1

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Figure: PDH multiplexing by steps, showing add/dropfunction

Limitations of PDH Network

The main limitations of PDH are: Inability to identify individual channels in a higher-order bit

stream. Insufficient capacity for network management; Most PDH network management is proprietary. Theres no standardized definition of PDH bit rates greater

than 140 Mbit/s. There are different hierarchies in use around the world.

Specialized interface Equipment is required to interwork the two hierarchies.

Standards SDH has been standardized by ITU-T in 1988. In November 1988 the first SDH standards were approved.

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TERMINAL

MULTIPLEXER

STM-nSTM-mE1-E4 REGENERATORREG

ST

M-

n

ST

M-

n

SDH Elements:The most common SDH elements are: -

1. TERMINAL MULTIPLEXER:The terminal Multiplexer is used to multiplex local tributaries (lowrate) to the STM-N (high rate) aggregate. The terminal is used in thechain topology end element.

2. Regenerators

Regenerators, as the name implies, have the job of regeneratingthe clock and amplitude relationships of the incoming data signalsthat have been attenuated and distorted by dispersion. They derivetheir clock signals from the incoming data stream. Messages arereceived by extracting various 64 kbit/s channels (e.g. servicechannels E1, F1) in the RSOH (regenerator section overhead).Messages can also be output using these channels.

3. Add/drop multiplexers (ADM):

Plesiochronous and lower bit rate synchronous signals can beextracted from or inserted into high speed SDH bit streams bymeans of ADMs. This feature makes it possible to set up ringstructures, which have the advantage that automatic back-uppath switching is possible using elements in the ring in theevent of a fault.

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ADD-and-DROP MULTIPLEXER

with

LOCAL CROSS-CONNECT

CAPABILITY

ST

M-

n

E1

-

E4

ST

M-

n

ST

M-

n

LX

C

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Chain

Mesh

SDH Topologies LINEAR BUS CHAIN

The linear bus (chain) topology used when there is no need forprotection and the demography of the sites is linear.

The ring topology is the most common and known of the SDHtopologies it allows great network flexibility and protection.

MESH

The mesh topology allows even the most paranoid networkmanager to sleep well at nights because of the flexibilityand redundancy that it gives.

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Star(Hub)

Point-to-Point

STAR (HUB)

The Star topology is used for connecting far and less importantsites to the network.

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Ring

TerminalMultiplexer

Add-DropMultiplexerDigital Cross-Connect

Usage of SDH elements in SDH Topologies

The Terminal Multiplexer can be used to connect two sites in ahigh rate Connection

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The Add And Drop Multiplexer (ADM) is used to build the chain

topologies in the above picture. At the ends of the chainusually a Terminal Multiplexer is connected.

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SDH Protection

The SDH gives the ability to create topologies with protectionfor the data transferred.Following are some examples for protected ring topologies.

Atthis

picture we can see Dual Unidirectional Ring . The normal dataflow is according to ring A (red). Ring B (blue) carriesunprotected data which is lost in case of breakdown or itcarries no data at all.

In case of breakdown rings A & B become one ring without thebroken segment.

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The Bi-directional Ring allows data flow in both directions. Forexample if data from one of the sites has to reach a site which isnext to the left of the origin site it will flow to the left instead ofdoing a whole cycle to the right.

In case of

breakdown some of the data is lost and the important data isswitched. For example if data from a site should flow to its

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destination through the broken segment, it will be switched tothe other side instead.

SDH Hierarchy

STM = Synchronous Transport Module

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Regene

rator

Section

Multiplexer

Section

Multiplexer

Section

Path

Regene

rator

Section

T r i b u t ar i e s

SDH

Termin

al

Multiple

xer

TrafficAssemblyT r i b u t a

r i e s

SDH

Termin

al

Multiple

xer

SDH

Add &

Drop

Multiple

xer

SDHRegenerator

SDH

Regener

ator

Traffic

Disassembly

Regene

rator

Section

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SDH Frame Structure

The STM-1 frame is the basic transmission format for SDH. The frame

lasts for 125 microseconds therefore, there are 8000 frames per

second. The STM-1 frame consists of overhead plus a virtual

container capacity (see Figure 2). The first nine columns of each

frame make up the Section Overhead, and the last 261 columns

make up the Virtual Container (VC) capacity. The VC plus the

pointers (H1, H2, H3 bytes) is called the AU (Administrative Unit).

Carried within the VC capacity, which has its own frame structure of

nine rows and 261 columns, is the Path Overhead and the Container

(see Figure). The first column is for Path Overhead. Its followed by

the payload container, which can itself carry other containers.

STM-1 frame structure is shown in the Figure below.

The three main areas of the STM-N frame are indicated: SOH Administrative Unit pointer(s) Information payload

Fig. : - STM-1 frame structure.

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SDH Virtual Container

SDH supports a concept called virtual containers (VC). Through

the use of pointers and offset values, VCs can be carried in the

SDH payload as independent data packages. VCs are used to

transport lower-speed tributary signals below Figure 3

illustrates the location of a VC-4 within the STM-1 frame. Note

that it can start (indicated by the J1 path overhead byte) at any

point with in the STM-1 frame. The start location of the J1 byte

is indicated by the pointer byte values. Virtual containers can

also be concatenated to provide more capacity in a flexible

fashion.

Table 1 lists the names and some of the parameters of the

virtual containers.

Table 1. Virtual Containers (VC)

SDH Digital Bit Rate Size of VCVC-11 1.728 Mbit/s 9 rows, 3 columns

VC-12 2.304 Mbit/s 9 rows, 4 columns

VC-2 6.912 Mbit/s 9 rows, 12 columnsVC-3 48.96 Mbit/s 9 rows, 85 columns

VC-4 150.336Mbit/s 9 rows, 261columns

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Figure 3: Virtual container structure showing VC-4.

Once a container has been created, path overhead byte are

added to create a virtual container.

Path overheads contain alarm, performance and other

management information.

A path through an SDH network exists from the point where

a PDH signal is put into a container to where the signal is

recovered from the container.

2 Mbit/s PCM30 frame structure

The SDH frame rate is inherited from PCM.

As with PCM, the SDH has 8 bits per time slot.

As with PCM, the SDH frame rate in 125 us per frame.

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SDH Overhead The SDH standard was developed using a client/server

layer approach.

The layers have a hierarchical relationship, with each layer

building on the services provided by all the lower layers. This

section details the different SDH overhead information,

specifically:

Regenerator Section Overhead

Multiplex Overhead

Path Overhead

Regenerator Section Overhead The Regenerator Section Overhead contains only the

information required for the elements located at both ends of a

section. This might be two regenerators, a piece of line

terminating equipment and a regenerator, or two pieces of line

terminating equipment. The Regenerator Section Overhead is

found in the first three rows of Columns 1 through 9 of the

STM-1 frame (see Figure Below). Byte by byte, the Regenerator

Section Overhead is shown in Table 2.

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Figure : STM-1 Regenerator section overhead

Table : Regenerator Section Overhead

Byte DescriptionA1 and

A2

Framing bytes These two bytes indicate the

beginning of the STM-N frame. The A1, A2 bytes areunscrambled. A1 has the binary value 11110110, andA2 has the binary value 00101000. The framealignment word of an STM-N frame is composed of (3x N) A1 bytes followed by (3 x N) A2 bytes.

J0 Regenerator Section (RS) Trace message Its used totransmit a Section Access Point Identifier so that asection receiver can verify its continued connection to

the intended transmitter.Z0 These bytes, which are located at positions S[1,6N+2]to S[1,7N] of an STM-N signal (N > 1), are reserved forfuture international standardization.

B1 RS bit interleaved parity code (BIP-8) byte This is aparity code (even parity), used to check fortransmission errors over a regenerator section. Itsvalue is calculated over all bits of the previous STM-Nframe after scrambling, then placed in the B1 byte of

STM-1 before scrambling.E1 RS orderwire byte This byte is allocated to be used

as a local orderwire channel for voice communicationbetween regenerators.

F1 RS user channel byte This byte is set aside for theusers purposes; it can be read and/or written to ateach section terminating equipment in that line.

D1, D2,D3

RS Data Communications Channel (DCC) bytes These three bytes form a 192 kbit/s message channel

providing a message-based channel for Operations,Administration and Maintenance (OAM) betweenpieces of section terminating equipment. The channelcan be used from a central location or control,monitoring, administration, and other communicationneeds

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Multiplex Section Overhead

The Multiplex Section Overhead contains the information

required between the multiplex section termination equipment

at each end of the Multiplex section (that is, between

consecutive network elements excluding the regenerators).

The Multiplex Section Overhead is found in Rows 5 to 9 of

Columns 1 through 9 of the STM-1 frame (see Figure 4). Byte

by byte, the Multiplex Section Overhead is shown in Table 3.

Figure 4 : STM-1 Multiplex section overhead.

Table : Multiplex Section Overhead

Byte Description

B2

Multiplex Section (MS) bit interleaved parity code(MS BIP-24) byte This bit interleaved parity N x24 code is used to determine if a transmissionerror has occurred over a multiplex section. Itseven parity, and is calculated overall bits of theMS Overhead and the STM-N frame of the

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previous STM-N frame before scrambling.

K1 and K2

Automatic Protection Switching (APS channel)bytes These two bytes are used for MSP(Multiplex Section Protection) signaling betweenmultiplex level entities for bi-directional

automatic protection switching and forcommunicating Alarm Indication Signal (AIS) andRemote Defect Indication (RDI) conditions.

D4 to D12MS Data Communications Channel (DCC) bytes

These nine bytes form a 576 kbit/s messagechannel from a central location for OAMinformation(Control, maintenance, remote provisioning,monitoring, administration and other

communication needs).

S1

Synchronization status message byte (SSMB) Bits 5 to 8 of this S1 byte are used to carry thesynchronization messages. Following is theassignment of bit patterns to the foursynchronization levels agreed to within ITU-T

M1

MS remote error indication The M1 byte of anSTM-1 or the first STM-1 of an STM-N is used for a

MS layer remote error indication (MS-REI).Bits 2 to 8 of the M1 byte are used to carry theerror count of the interleaved bit blocks that theMS BIP-24xN has detected to be in error at the farend of the section. This value is truncated at 255for STM-N >4.

E2MS orderwire byte This orderwire byte providesa 64 kbit/s channel between multiplex entities foran express orderwire. Its a voice channel for use

by craftspersons and can be accessed atmultiplex section terminations.

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Path overhead

The path overhead (POH) plus a container forms a virtualcontainer. The POH has the task of monitoring quality andindicating the type of container. The format and size of thePOH depends on the container type.

A distinction is made between two different POH types:

VC-11/12 POH

The VC-11/12 POHis used for the low-order path. ATMsignals and bitrates of 1.544 Mbit/s and 2.048 Mbit/s are transported withinthis path.

VC-3/4 POH

The VC-3/4 POH is the high-order path overhead. This path isfor transporting 140 Mbit/s, 34 Mbit/s and ATM signals.

BIT RATE : STM-N

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Bit Rates :

International organization defined standardized bit rates:

STM1 155.52 Mbit/s

STM4 622.08 Mbit/s

STM16 2488.32 Mbit/s

STM64 9953.28 Mbit/s

STM = Synchronous Transport Module

SDH PointersSDH provides payload pointers to permit differences in the phase

and frequency of the Virtual Containers (VC-N) with respect to the

STM-N frame. Lower-order pointers are also provided to permit

phase differences between VC-1/VC-2 and the higher-order VC-

3/VC-4. On a frame-by-frame basis, the payload pointer indicates

the offset between the VC payload and the STM-N frame by

identifying the location of the first byte of the VC in the payload. In

other words, the VC is allowed to float within the STM-1 frame

capacity. To make this possible, within each STM-N frame, theres a

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pointer, known as the VC Payload Pointer that indicates where the

actual payload container starts. For a VC-4 payload, this pointer is

located in columns 1 and 4 of the fourth row of the Section

Overhead. The bytes H1 and H2 (two 8-bit bytes) of the Overhead

can be viewed as one value.

The pointer value indicates the offset in bytes from the pointer to

the first byte of the VC, which is the J1 byte. Because the Section

Overhead bytes are not counted, and starting points are at 3-byte

increments for a VC-4 payload, the possible range is: Total STM-1

bytes Section Overhead bytes = Pointer value range

For example:

(2430 81)/3 = 783 valid pointer positions

That is, the value of the pointer has a range of 0 to 782.

For example, if the VC-4 Payload Pointer has a value of 0, thenthe VC-4 begins in thebyte adjacent to the H3 byte of theOverhead; if the Payload Pointer has a value of 87, then theVC-4 begins in the byte adjacent to the K2 byte of theOverhead in the next row.

The pointer value, which is a binary number, is carried in bits 7through16 of the H1-H2 pointer word. The first four bits of theVC-4 payload pointer make provision for indicating a change inthe VC, and thus an arbitrary change in the value of thepointer. These four bits, the N-bits, are known as the New DataFlag. The VC pointer value that accompanies the New DataFlag will indicate the new offset.

Table: SDH Pointers

Byte Description

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H1 and H2 Pointer bytes These two bytes, the VC payload pointer,specify the location of the VC frame.Its used to align the VC and STM-1 Section Overheads in an STM-N signal, to

perform frequency justification, and to indicate STM-1 concatenation.

H3 Pointer action byte This byte is used for frequency justification.Depending on the pointer value,the byte is used to adjust the fill input buffers. The byte only carries validinformation in the event of negative justification, otherwise its not defined.

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Why SDH ?1. Simpler Multiplexing:

Low SDH level can be directly identified from higher SDH level

2. Simple D&I of traffic channels:Direct access to lower level systems without synchronization.

3. Allows mixing of ANSI & ETSI PDH systems.

4. SDH provides TMN (Telecommunication Network

Management).

5. Centralized Network Control

SDH ADVANTAGES:1. High Transmission Rates

Transmission rates of up to 10 Gbit/s can be achieved in

modern SDH systems. SDH is therefore the most suitable

technology for backbones, which can be considered as being

the super highways in todays telecommunications networks.

2. Simplified add & drop function

Compared with the older PDH system, it is much easier to

extract and insert low-bit rate channels from or into the high-

speed bit streams in SDH. It is no longer necessary to de-

multiplex and then re-multiplex the plesiochronous structure, a

complex and costly procedure at the best of times.

1. High availability and capacity matching

With SDH, network providers can react quickly and easily to

the requirements of their customers. For example, leased lines

can be switched in a matter of minutes. The network provider

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from a central location by means of a telecommunications

network management (TMN) system.

4. Reliability

Modern SDH networks include various automatic back-up and

repair mechanisms to cope with system faults. Failure of a link

or a network element does not lead to failure of the entire

network which could be a financial disaster for the network

provider.

5. Interconnection

SDH makes it much easier to set up gateways between

different network providers and to SONET systems. The SDH

interfaces are globally standardized, making it possible to

combine network elements from different manufacturers into a

network. The result is a reduction in equipment costs as

compared with PDH.

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Future of SDHRight now, SDH is the ideal platform for services ranging from

POTS, ISDN and mobile radio through to data communications(LAN, WAN, etc.), and it is able to handle the very latestservices, such as video on demand and digital videobroadcasting via ATM that are gradually becoming established.

Almost all new fiber-transmission systems now being installedin public networks use SDH or SONET. They are expected todominate transmission for decades to come, just as theirpredecessor PDH has dominated transmission for more than 20years (and still does in terms of total systems installed). Bitrates in long-haul systems are expected to rise to 40 Gbpssoon after the year 2000, at the same time as systems of 155Mbps and below penetrate more deeply into access networks.

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Bibliography

www.vodafone.in

Tektronix Tutorials

SDH Telecommunications Standard Primer

GSM Primer.

Daily Diary

Dheeraj Nagpal (Training Report)

Documents

Transcript of Dheeraj Nagpal (Training Report)