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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
Nov 8, 2000
CS590F Purdue University
Contents
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Introduction (Motivation)
SONET technology
WDM technology
SONET and WDM integration
Summary
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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)
• 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
• 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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Overview of SONET(Cont’d)
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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
• 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
• 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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Figure:A wavelength cross connect node
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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
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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
•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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Table:Time responses of various APS mechanisms
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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
• 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
• 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.
Nov 8, 2000
CS590F Purdue University