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Current wavelength division multiplexing (WDM) systems use each wavelengthas a separate channel. Each channel may transport homogeneous or heterogeneoustraffic, such as synchronous optical network/synchronous digital hierarchy(SONET/SDH) over one wavelength, asynchronous transfer mode (ATM) over an-other, and perhaps time division multiplexing (TDM) voice, video, or Internet overanother. Moreover, WDM makes it possible to transfer traffic at different bit rates.Thus, depending on the application, one wavelength (or channel) may carry traffic atOC-3, OC-12, OC-48, or up to OC-192 rate, and another wavelength possibly at alower or even undefined rate (a rate defined by end-user equipment).
Apart from the number of services and traffic types, there are many key ques-tions in WDM: How many wavelengths can be multiplexed in a single fiber? Howare the wavelength channels monitored, managed, protected, and provisioned? Asoptoelectronic technology moves forward, it is possible to have a high density ofwavelengths in the same fiber. Thus, the term dense wavelength division multiplex-ing (DWDM) is used. In contrast, there are also low wavelength density WDM sys-tems, and a lower density yet, termed coarse WDM (CWDM).
Conventional single-mode fibers transmit wavelengths in the 1300 and 1550 nmrange and absorb wavelengths in the range of 1340-1440 nm range. WDM systems usewavelengths in the two regions of 1310 and 1550 nm. Special fibers have made it pos-sible to use the complete spectrum from 1310 nm to beyond 1600 nm. However,although new fiber technology opens up the spectrum window, not all optical compo-nents perform with the same efficiency over the complete spectrum. For example, er-bium-doped fiber amplifiers (EDFAs) perform best in the range of 1550 nm. Will otherfiber amplifiers perform as well, so that different amplifier types can be included?
180 Part IV Dense Wavelength Division Multiplexing
DWDM systems take advantage of advanced optical technology (e.g., tunablelasers, narrowband optical filters, etc.) to generate many wavelengths in the rangearound 1550 nm. ITU-T Recommendation 0.692 defines 43 wavelength channels,from 1530 to 1565 nm, with a spacing of 1000Hz, each channel carrying an OC-192 signal at 10 Obis. However, systems with wavelength channels of more than 43wavelengths have been introduced, and systems with many more wavelengths are onthe experimenter's workbench.
Currently, commercial systems with 16, 40, 80, and 128 channels (wavelengths)per fiber have been announced. Those with 40 channels have channel spacing of100 OHz, and those with 80 channels have a channel spacing at 50 OHz. Thischannel separation determines the width of the spectral (wavelength) narrownessof each channel, or how close (in terms of wavelength) the channels are. Forty-channel DWDM systems can transmit over a single fiber an aggregate bandwidthof 400 Obis (10 Gb/s per channel). It is estimated that at 400 Obis, more than 10,000volumes of an encyclopedia can be transmitted in 1 second.
The number of channels also depends on the type of fiber. A single strand of sin-gle-mode fiber can transmit over 80 km without amplification. Placing eight opticalamplifiers in cascade, the total distance is extended to over 640 km (this is typicalfor 80-channel systems at 10 Obis per channel).
There is a race among companies and experimenters to break new records;longer distances, more channels, and higher bit rates frequently make the news. Andthis trend is expected to continue until all limits of physics for this technology havebeen reached and pushed back.
Although DWDM technology is still evolving and technologists and standardsbodies are addressing many issues, systems are being offered with few tens of wave-lengths in the same fiber. However, it is reasonable to assume that in the near futurewe will see DWDM systems with several hundred wavelengths in a single fiber.Theoretically, more than 1000 channels may be multiplexed in a fiber. DWDM tech-nology with more than 200 wavelengths has already been demonstrated. Deviceswith 200 wavelengths per fiber at 40 Obis per wavelength with an aggregate band-width of 8 Tbls per fiber are feasible. Eight Tbls per fiber is an aggregate bandwidththat exceeds all needs today. Nevertheless, this bandwidth may become tomorrow'snorm if we extrapolate from the explosion in data traffic now in progress.
DWDM utilizes a large aggregate bandwidth in a single fiber by taking advan-tage of advanced optical technology that is able to launch and multiplex many wave-lengths in one fiber, switch wavelengths optically, and at the receiving end, demulti-plex and read each wavelength separately. In DWDM, each wavelength constitutesa separate channel capable of carrying traffic at a bit rate that may not be the sameon all channels. Because a channel does not exactly consist of a singular wavelengthbut of a narrow band around a center wavelength, each band is spaced from the nextby several gigahertz to provide a safety zone and avoid channel-wavelength over-lapping and thus cross-talk. This spacing is necessary for several reasons: lasersources and tunable filters may drift with temperature and time, optical amplifiers do
Chapter 14 DWDM Systems 181
not exhibit true flat gain over the wavelength range , spontaneous noise from EDFAsis cumulative, and there are often dispersion effects .
14.2 DWDM NETWORK TOPOLOGIES
Initially, DWDM started with one topology: point-to-point.A number of network topolo-gies in addition to direct point-to-point, are now considered, however: point-to-point withadd-drop capability, ring, fully mesh connected, and star.Except for point-to-point , thesetopologies may be mapped into a fiber-ring topology with many wavelengths. Dependingon topology and fiber length, optical amplification mayor may not be required. The needfor amplification and the number of amplifiers (if any) can be estimated from the distancebetween the transmitter and the receiver and according to system design parameters, suchas number of wavelengths (channels), channel width, channel separation, modulationtechnique, bit rate, fiber type, and other optical component characteristics.
Figure 14.1 illustrates a DWDM point-to-point topology with an optional ampli-fier. This topology is more suitable for long-haul ultrahigh aggregate bandwidth trans-port (where wavelengths are distinguished by different colors: wavelength multiplexersand demultiplexers are not shown). An optical add-drop multiplexer (OADM) may alsobe included on the link if one or more wavelengths are to be dropped and added.
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regime regime regime
Figure 14.1 A conceptual point-to-point DWDM system (also known as "big fat pipe") .
Figure 14.2 illustrates a five-node, fully connected mesh topology DWDMmapped onto a ring (a single ring is shown for clarity) , and a star network alsomapped onto a ring . Each interconnecting link is shown via a separate channel(wavelength), and in the fully connected case, four wavelengths are added/droppedat each node. The hub of the star network may receive a wavelength from a node onthe ring and selectively convert it to another wavelength destined to another node onthe same ring. This function is also referred to as broadcast and select.
Notice that although we make reference to nodes, the conventional communi-cations networks "node" may be replaced by "router," which is better suited to datanetworks. It should be pointed out, however, that modem networks transport a mix of
182 Part IV Dense Wavelength Division Multiplexing
Hub Node A/Hub
; --'. ..../
INode A I,
Node B - - Node C
8tar topology Fully connected
Figure 14.2 Fully connected and star WDM ring topologies; the fiber is shown as a dashed line (black),and interconnecting links are shown via separate channels (colored).
TDM (digital signal level n [D'Sn],SONET/SDH), ATM and Internet traffic. In DWDMapplications, a router that performs DSn and optical carrier level n (OC-n) grooming,optical multiplexing, switching, and provides real-time quality of service (QoS), startslooking like a traditional node. Therefore, although nodes and routers are conceptuallydifferent, for brevity we do not distinguish between the two. WDM networking exam-ines and evaluates these issues in detail, but this is beyond our scope.
14.3 DWDM APPLICABILITY
DWDM systems are applicable to many network types and on all network layers(Figure 14.3). For example, they are applicable to backbone networks (networks
Metro-area(081 to OC-12 rates)
IP/LAN Access & last mileto the home(080 to OC-3 rates)
o Node wlo OAOM ©~:=::=::~~--:~~Node w/OAOM
i High-speed optical node
- Optical link
Figure 14.3 WDM is applicable to all network layers, from access to backbone.
Chapter 14 DWDM Systems 183
that optically transport bulk ultrahigh bandwidth data at very high bit rates) cov-ering large geographical areas (intercontinental), and they interconnect continents(trans-oceanic). Intercontinental systems may have a point-to-point, mesh, star, orring topology, whereas transoceanic systems are predominantly point-to-point,possibly with a few add-drops. A submarine DWDM network may also be of thering or star type interconnecting a cluster of islands. DWDM systems are applica-ble to metropolitan areas interconnecting many high-rise buildings, to a campusenvironment, and to multilevel buildings. Finally, they are applicable to single-enterprise networks with fewer nodes, where a variety of traffic types (TDM,SONET/SDH, ATM, IP, etc.) merge to be transported to a higher level network.The ring or mesh topology is more suitable in such applications. One of the nodesis a hub providing connectivity with other networks. Notice that as we move fromthe access to backbone layers, bit rates increase because bandwidth is aggregated.Table 14.1 tabulates the legacy narrowband and broadband rates, and Table 14.2gives the SONET and SDH rates.
In DWDM applications, each node sources and terminates one (or more) wave-length(s) such that a fully interconnected mesh network or a star network is con-structed with a single fiber. Depending on the number of nodes and wavelengths, aWDM system is termed DWDM if there are many wavelengths and CWDM if thereare few wavelengths; typically, these are spaced at 50-500 GHz. Moreover, eachwavelength may transport different services, such as IP, SONET/SDH, or ATM. Asa result, practical systems are designed to handle one or more service types. For ex-ample, a family of products offered by Lucent Technologies known as WaveStar"
Table 14.1 Narrowband and Broadband Rates
Facility United States Europe Japan
DSO 64 Kb/s 64 Kb/s 64 Kb/s
DSI 1.544 Mb/s 1.544 Mb/s
El 2.048 Mb/s
DSlc 3.152 Mb/s 3.152 Mb/s
DS2 6.312 Mb/s 6.312 Mb/s
E2 8.448 Mb/s 32.064 Mb/s
DS3 44.736 Mb/s 34.638 Mb/s
DS3c 91.053 Mb/s
E3 139.264 Mb/s
DS4 274.176 Mb/s
184 Part IV Dens e Wavelength Division Multiplexing
Table 14.2 SONET/SDH Rates
Signa l Designation Line RateSON ET SDH Optical (Mb/s)
STS-I STM-O OC- I 51.84
STS-3 STM-I OC-3 155.52
STS-12 STM-4 OC- 12 622.08
STS-48 STM-16 OC-48 2,488.32
STS- l92 STM-64 OC- 192 9,953.28
STS-768 STM-254 OC-768 39,813.12
has been designed to handle any service at any bandwidth (Figure 14.4). Wave-Star OLS 400G multiplexes up to 80 wavelengths at OC-48 per fiber (80 x 2.5 =200 Gb/s per fiber) or up to 40 wavelengths at OC-I92 per fiber (40 x l 0 =400 Gb/sper fiber).
Similarly, products from other well-known companies support a range of wave-
Voice, data , video\ J
Opt icalVariable Fixed Wavestar
bandwidth band wid th acce ss(j) access
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Ic fabric 2.5G/10G.0::J -en(ij
Optical fabric IOLS 40G>
<ll:5: and 400G-- -
Figure 14.4 Example of a product family that handles any service at any bandwidth. (From LUCENTTechnologies, Bell Labs Technology, vol. 2, no. 2, p. 12, Reprinted with permission.)
Chapter 14 DWDM Systems 185
length channels at various rates . Nevertheless, DWDM network s have several unre-solved issues to address . Standards bodies are working toward drafting recommen-dations, but in the meantime each manufacturer offers systems with semiprivate so-lutions to meet market needs and to make an early entry. For example, operations,administration, maintenance, and provisioning (OAM&P), as well as dynamic wave-length assignment, are on the drawing board. Network management may also dif-fer from vendor to vendor along with reliability, latency, and quality-of-serviceagreements . Some use a supervisory (wavelength) channel for OAM&P (typicallyat 1310 nm or at about 1500 nm), but the wavelength, its bit rate, and the protocolhave not yet been standardized. Some use proprietary methods to enable transport-ing a mix of services and traffic types, although proposals have been made to pro-duce a standard. One such method is the digital wrapper, which, based on TDMprinciple , encapsulates an optical channel, with additional overhead. This overheadincludes OAM&P functions as well as forward error correction (FEC) to extend thetransported range by 4-5 dB (a practice similar to submarine applications: ITU-TRecommendation 0.975). Current systems support fixed-wavelength assignment foreach node or are manually reconfigurable . Dynamic wavelength assignment is an-other area to be standardized. Each network provider also offers a proprietary solu-tion for wavelength protection. The set of wavelengths is different from system tosystem, thus making interoperability almost impossible (Figure 14.5). Under the cir-cumstances, a description of DWDM standard networks is out of the question for thetime being . In Chapter 15 we will address these issues. Again, this is a current viewand the interested designer should be mindful of upcoming standards and recom-mendations .
IP ·- - oW·
Figure 14.5 Multinetwork interoperability. For a service to be provided end-to-end (dotted line)with the same guarantees; all networks must be compatible. leI is the intercarri er in-terface.