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Basic DWDM Components

Heino Radmer, Optical Network Architect

02. Marts 2007

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Introduction

A short definition of a WDM system, which both applies toDense Wavelength-Division Multiplexing (DWDM), andCoarse Wavelength-Division Multiplexing (CWDM) is this:

In fiber-optic communications, wavelength-divisionmultiplexing (WDM) is a technology which multiplexes multipleoptical carrier signals on a single optical fiber by using differentwavelengths (Lambdas) of laser light to carry different signals.

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CWDM

The main characteristic of the recent ITU CWDM standard isthat the signals are not spaced appropriately foramplification by EDFAs.

This therefore limits the total CWDM optical span tosomewhere near 60 km for a 2.5 Gb/s signal, which is suitablefor use in metropolitan applications.

Systems from different vendors claim to span 80-120 km

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The DWDM system, TS

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The DWDM system, MUX

DWDM terminal demultiplexer

The terminal demultiplexer breaks the multi-wavelength signalback into individual signals and outputs them on separatefibres for client-layer systems (such as SONET/SDH) to detect.

1*M

UX

2*MUX

n*MUX

BAND

MUX/DEMUX

This shows both band

MUX/DEMUX, and the

smaller channel

MUX/DEMUX, which

multiplexes up to 8

channels at a time.

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The DWDM system, OADM

Optical Add/drop multiplexer (OADM)

Between multiplexing and demultiplexing points in a DWDM

system, there is an area in which multiple wavelengths exist.

It is often desirable to be able to

remove or insert one or more

wavelengths at some point along thisspan. An optical add/drop multiplexer

(OADM) performs this function. Rather

than combining or separating all

wavelengths, the OADM can remove

some while passing others on. OADMs

are a key part of moving toward thegoal of all-optical networks.

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The DWDM system, R-OADM

The R-OADM

The R-OADM is an extended version of the OADM so to speak.

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The DWDM system, OSC

Optical Supervisory Channel (OSC)

The OSC carries information about the multi-wavelength

optical signal as well as remote conditions at the opticalterminal or EDFA site.

The outofband Optical Supervisory Channel (OSC) allowsthe supervision of all the NEs along the WDM path; moreover it

gives some orderwires (data channel and voice channel) tothe users.

Out-of-band, means the OSC is using a different band thanthe DWDM system is normally running in, which normallywould be the U-band.

ITU standards suggest that thee OSC should utilize an OC-3signal structure, though some vendors have opted to use100 Megabit Ethernet or another signal format.

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Transponder

The transponder accepts input in the form of standardsingle-mode or multimode laser. The input can come fromdifferent physical media and different protocols and traffic

types.

Transponders entering

the 8 channel MUX

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The past of systems

The terms used in today's systems have changed, but thebasic building blocks still remain.

Naturally the techniques have been refined quite a bit sincethe beginning, but it still beats having many fibers and lotsof equipment to run traffic on those fibers.

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Todays DWDM system

In order to go the extra distance, and carry more traffic, theDWDM system have been boosted with RAMAN amplifiers,CMD and PMD compensation units.

The funny part of the RAMAN amplification is actually, that itbuilds on backscatter in the fiber, which is normally somethingyou would try to avoid.

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Future in DWDM

So the big question is, does DWDM have any future inoptical networking?

It is believed that future DWDM systems can multiplex up to15.000 or more channels on a single span.

So the answer to that question must be, YES.

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Questions

Questions welcome

Thank you!