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

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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

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  • Introduction (Motivation)
  • SONET technology
  • WDM technology
  • SONET and WDM integration
  • Summary

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  • 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
  • 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


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

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

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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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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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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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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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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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  • 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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