What is DWDM?

Dense Wavelength Division Multiplexing (DWDM) is anoptical multiplexingtechnology used to increase bandwidth overexisting fiber networks.DWDM works by combining and transmittingmultiple signals simultaneouslyat different wavelengths on the samefiber. The technology createsmultiple virtual fibers, thus multiplyingthe capacity of the physicalmedium.

DWDM provides ultimate scalability and reach for fiber networks.Withoutthe capacity and reach of DWDM systems, most Web 2.0 andcloud-computing solutions today would not be feasible. Establishingtransport connections as short as tens of kilometers to enablingnationwide and transoceanic transport networks, DWDM is the workhorse ofall the bit-pipes keeping the data highway alive and expanding.

WDM has revolutionized the cost per bit of transport. Thanks toDWDM,fiber networks can carry multiple Terabits of data per secondoverthousands of kilometers – at cost points unimaginable less than adecadeago. State-of-the-art DWDM systems support up to 192 wavelengthson asingle pair of fiber, with each wavelength transporting up to100Gbit/scapacity – 400Gbit/s and one Terabit/s on the horizon.

Dense wavelength division multiplexing (DWDM) refers originally tooptical signals multiplexed within the 1550 nm band so as to leveragethe capabilities (and cost) of erbium doped fiber amplifiers (EDFAs),which are effective for wavelengths between approximately 1525–1565 nm(C band), or 1570–1610 nm (L band). EDFAs were originally developed toreplace SONET/SDH optical-electrical-optical (OEO) regenerators, whichthey have made practically obsolete. EDFAs can amplify any opticalsignal in their operating range, regardless of the modulated bit rate.In terms of multi-wavelength signals, so long as the EDFA has enoughpump energy available to it, it can amplify as many optical signals ascan be multiplexed into its amplification band (though signal densitiesare limited by choice of modulation format). EDFAs therefore allow asingle-channel optical link to be upgraded in bit rate by replacing onlyequipment at the ends of the link, while retaining the existing EDFA orseries of EDFAs through a long haul route. Furthermore,single-wavelength links using EDFAs can similarly be upgraded to WDMlinks at reasonable cost. The EDFA’s cost is thus leveraged across asmany channels as can be multiplexed into the 1550 nm band.

DWDM systems

At this stage, a basic DWDM system contains several main components:
WDM multiplexer for DWDM communications

A DWDM terminal multiplexer. The terminalmultiplexer contains a wavelength-converting transponder for each datasignal, an optical multiplexer and where necessary an optical amplifier(EDFA). Each wavelength-converting transponder receives an optical datasignal from the client-layer, such as Synchronous optical networking[SONET /SDH] or another type of data signal, converts this signal intothe electrical domain and re-transmits the signal at a specificwavelength using a 1,550 nm band laser. These data signals are thencombined together into a multi-wavelength optical signal using anoptical multiplexer, for transmission over a single fiber (e.g., SMF-28fiber). The terminal multiplexer may or may not also include a localtransmit EDFA for power amplification of the multi-wavelength opticalsignal. In the mid-1990s DWDM systems contained 4 or 8wavelength-converting transponders; by 2000 or so, commercial systemscapable of carrying 128 signals were available.
An intermediate line repeateris placed approximately every 80–100 km to compensate for the loss ofoptical power as the signal travels along the fiber. The‘multi-wavelength optical signal’ is amplified by an EDFA, which usuallyconsists of several amplifier stages.
An intermediate optical terminal, or optical add-drop multiplexer.This is a remote amplification site that amplifies the multi-wavelengthsignal that may have traversed up to 140 km or more before reaching theremote site. Optical diagnostics and telemetry are often extracted orinserted at such a site, to allow for localization of any fiber breaksor signal impairments. In more sophisticated systems (which are nolonger point-to-point), several signals out of the multi-wavelengthoptical signal may be removed and dropped locally.
A DWDM terminal demultiplexer.At the remote site, the terminal de-multiplexer consisting of anoptical de-multiplexer and one or more wavelength-convertingtransponders separates the multi-wavelength optical signal back intoindividual data signals and outputs them on separate fibers forclient-layer systems (such as SONET/SDH). Originally, thisde-multiplexing was performed entirely passively, except for sometelemetry, as most SONET systems can receive 1,550 nm signals. However,in order to allow for transmission to remote client-layer systems (andto allow for digital domain signal integrity determination) suchde-multiplexed signals are usually sent to O/E/O output transpondersprior to being relayed to their client-layer systems. Often, thefunctionality of output transponder has been integrated into that ofinput transponder, so that most commercial systems have transpondersthat support bi-directional interfaces on both their 1,550 nm (i.e.,internal) side, and external (i.e., client-facing) side. Transponders insome systems supporting 40 GHz nominal operation may also performforward error correction (FEC) via digital wrapper technology, asdescribed in the ITU-T G.709 standard.
Optical Supervisory Channel (OSC).This is data channel which uses an additional wavelength usuallyoutside the EDFA amplification band (at 1,510 nm, 1,620 nm, 1,310 nm oranother proprietary wavelength). The OSC carries information about themulti-wavelength optical signal as well as remote conditions at theoptical terminal or EDFA site. It is also normally used for remotesoftware upgrades and user (i.e., network operator) Network Managementinformation. It is the multi-wavelength analogue to SONET’s DCC (orsupervisory channel). ITU standards suggest that the OSC should utilizean OC-3 signal structure, though some vendors have opted to use 100megabit Ethernet or another signal format. Unlike the 1550 nmmulti-wavelength signal containing client data, the OSC is alwaysterminated at intermediate amplifier sites, where it receives localinformation before re-transmission.

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