discussion lead by:samrat kulkarni

Nov 8, 2000 CS590F Purdue University

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

• 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


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?


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


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


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.


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.


Figure:A wavelength cross connect node


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.


•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.

Figure:optical channel

OTN Frame Format


WDM Network Infrastructure


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


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.


Table:Time responses of various APS mechanisms


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


The wavelength packing problem


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.


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.

discussion lead by:samrat kulkarni

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