Evolution of Optical Transport Technologies:
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Evolution of Optical Transport Technologies: From SONET/SDH to WDM (IEEE Comm. June 2000) Dirceu Cavendish, C&C Research Laboratories. Discussion lead by:Samrat Kulkarni. Contents. Introduction (Motivation) SONET technology WDM technology SONET and WDM integration Summary. Introduction.

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Discussion lead by:Samrat Kulkarni

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Discussion lead by samrat kulkarni

Evolution of Optical Transport Technologies:From SONET/SDH to WDM(IEEE Comm. June 2000)Dirceu Cavendish, C&C Research Laboratories

Discussion lead by:Samrat Kulkarni

CS590F Purdue University


Contents

Contents

  • Introduction (Motivation)

  • SONET technology

  • WDM technology

  • SONET and WDM integration

  • Summary

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Introduction

Introduction

  • Currently the most wide-spread technology for digital transmission is SONET/SDH based--optical fibers carry a single digital signal per fiber per direction.

  • Wavelength division multiplexing(WDM) multiplies the bandwidth capacity of an optical transmission medium many folds --several digital signals using several wavelengths are multiplexed without any interference among them.

  • Motivation of article:Answer the following questions

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Introduction cont d

Introduction(cont’d)

  • How is the new WDM technology going to interoperate with the legacy SONET/SDH equipment?

  • Is the WDM technology likely to replace SONET/SDH entirely?

  • If so, what is the likely roadmap for this transition?

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Overview of sonet

Overview of SONET

  • SONET clients organize data into SONET signals in various ways

  • ATM over SONET:ATM forum defined a SONET interface

    --Cells mapped back to back into the SPE

    --Receiver ATM equipment relies on ATM header CRC to establish synchronization

  • IP over SONET:

    --IP/ATM/SONET:IETF(RFC 1483), IP encapsulation over ATM

    --IP/PPP/HDLC/SONET:IETF(RFC 1619)

    PPP:Point-Point Protocol provides link error control and initialization

    HDLC:Provides framing, and maps frames into the SPE.

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Discussion lead by samrat kulkarni

CS590F Purdue University


Discussion lead by samrat kulkarni

CS590F Purdue University


Overview of sonet cont d

Overview of SONET(Cont’d)

  • Organized as multiple inter-connected rings.

  • Advantage of a ring topology is the APS feature.

  • Due of large bandwidth capacity, OAM&P is an important factor.

  • SONET has dedicated overhead bytes for protection at various levels.

  • 1+1 protection:Signal transmitted on two non-intersecting fiber paths from a source to a destination.

  • 1:1 protection:Signal transmitted on only the Working section, uses Protection section only during failure.

Figure:A SONET network

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Wdm technology

WDM Technology

  • Principle:Multiplex several signals using different wavelengths over the same optical fiber.

  • Spectrum used in the band of 1300nm or 1500nm.

  • Technology possible—advances in optical components such as Distributed Feedback Lasers(DFBs), Erbium Doped Fiber Amplifiers(EDFAs) and photo detectors.

  • Current technology—100 optical channels can be multiplexed into a single fiber.

  • Advantage:Cost effectively increase the bandwidth of already installed fibers many fold.

  • Number of wavelengths etc. is being standardized for interoperability and avoidance of interference.

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Wdm components

WDM Components

  • DFB lasers:transmitters for each wavelength

  • Optical amplifiers:(EDFA)

  • OADMS(optical add/drop multiplexers)

  • Optical Switch(OS):Enables switching in the optical domain.

  • Wavelength Converter(WC): converts signal at given wavelength to signal at another wavelength maintaining same digital content.

  • Above enable to build a wavelength cross-connect node(WXC) which is the heart of a WDM network

  • WXCs have been researched for a number of years, but are not yet matured to be commercially available at the time of this writing.

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Discussion lead by samrat kulkarni

Figure:A wavelength cross connect node

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Optical transport networks

Optical Transport Networks

  • OTNs are WDM systems providing transport services via light paths.Light paths have bandwidths up to several gigabits per second.

  • An OTN is made up of WXCs, plus a management system for setup and teardown of light paths.

  • Flexible because of most of the optical components are transparent to signal encoding(i.e. only at the boundary of the optical layer/electronic layer is the signal encoding important).

  • Thus OTNs providing transparent optical services to support various technologies such as SONET,ATM, IP is a likely scenario in the future.

  • The optical layer is subdivided into three sub layers:

    • The optical channel layer which interfaces with OTN client(for e.g. SONET)

    • The optical multiplex layer network, which multiplexes various channels

    • The optical transmission section layer which provides optical signal transmission.

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Discussion lead by samrat kulkarni

OTN Frame Format

Figure:optical channel

  • Overhead bytes used for OAM&P functions(like SONET)

  • Additional functions such as provisioning of dark fibers (reservation of a particular wavelength between end-points for a user)

  • FEC over optical transmission increases maximum span length, and reduces the number of repeaters

  • Several optical channels can be multiplexed (OMS) and transmitted

  • Draft of optical channel format of NNI and UNI is currently under progress.

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Wdm network infrastructure

WDM Network Infrastructure

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Sonet sdh and wdm integration

SONET/SDH and WDM Integration

  • Network operators are just finishing migration from PDH(pleisiochronus digital hierarchy) to SONET/SDH.

  • Unlikely to embrace any new technology that does not interact with SONET/SDH.

  • Optical Internetworking Forum, ITU-T study group 15, SONET Interoperability Forum are working on these issues.

  • Hence only a gradual migration possible from SONET to WDM networks

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Issues for sonet wdm integration

Issues for SONET/WDM integration

  • SONET Frame Encapsulation should be easily done is OCh frame.

  • WDM and SONET layers should be completely decoupled and the SONET device need not be aware of the WDM used to transport its signal.

  • However the above reduces flexibility in WDM networks(For e.g., last-hop wavelength should be the same as that used by the SONET interface if no WC exists.)

  • Problem of Multi-layer APS

    --Multi layer environment—should have ascending response timescale.

    --Unnecessary failure recovery at higher layers causes route instability and congestion.

    --Fault persistence checks at higher layers/artificially slowing down recovery mechanisms are some solutions.

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Discussion lead by samrat kulkarni

Table:Time responses of various APS mechanisms

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Integration issues cont d

Integration Issues(Cont’d)

  • Wavelength Packing problem:

  • --Given a traffic matrix between ingress/egress optical termination points, how should the various wavelengths at the ingress nodes be populated to minimize network cost?(Current research topic)

  • Network management issues:

  • --Management Information Blocks(MIBs) for various optical sub layers(fiber type, maximal rates, wavelength conversion capabilities, protection control, power levels etc.) have to be defined

  • --Major telecommunication companies have joined to produce a Common Object Request Broker Architecture(CORBA) NMS to provide integrated management architectures for various network types, including SONET, WDM and ATM

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Discussion lead by samrat kulkarni

The wavelength packing problem

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Evolution toward an all optical transport network

Evolution toward an all optical transport network

  • Likely to occur gradually.

  • First WXC devices connected to existing fibers(e.g. transatlantic ones)

    and interface with SONET and FDDI.

  • Optical sub networks connected by SDH or ATM equipment(optical signal brought to electronic domain, sophisticated protection mechanisms feasible).

  • As SONET devices depreciate new optical access solutions will appear squeezing SONET layer off the protocol stack.Lightpaths are cleared from electronic-optical conversion.Failure detection becomes challenging in a transparent optical layer.

  • SONET functionalities are transferred to layer above(IP/ATM) or below(WDM).

  • Issues with Packets packed directly into OCh frames.

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Summary

Summary

  • Large investment in SONET/SDH technology has just begun to pay-off.

  • WDM technology provides an alternative for fiber access and transport services.

  • WDM promises of unlimited bandwidth and fast protection capabilities are not enough to entice network operators to retire SONET/SDH equipment entirely.

  • The deployment of WDM devices must be economically well justified, as well as well planned.

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