DWDM SYSTEMS CHAPTER 14.1 INTRODUCTION

Transcription

1 CHAPTER 14 DWDM SYSTEMS 14.1 NTRODUCTON Current wavelength division multiplexing (WDM) systems use each wavelength as a separate channel. Each channel may transport homogeneous or heterogeneous traffic, such as synchronous optical network/synchronous digital hierarchy (SONET/SDH) over one wavelength, asynchronous transfer mode () over another, and perhaps time division multiplexing (TDM) voice, video, or nternet over another. Moreover, WDM makes it possible to transfer traffic at different bit rates. Thus, depending on the application, one wavelength (or channel) may carry traffic at OC3, OC12, OC48, or up to OC192 rate, and another wavelength possibly at a lower or even undefined rate (a rate defined by enduser equipment). Apart from the number of services and traffic types, there are many key questions in WDM: How many wavelengths can be multiplexed in a single fiber? How are the wavelength channels monitored, managed, protected, and provisioned? As optoelectronic technology moves forward, it is possible to have a high density of wavelengths in the same fiber. Thus, the term dense wavelength division multiplexing (DWDM) is used. n contrast, there are also low wavelength density WDM systems, and a lower density yet, termed coarse WDM (CWDM). Conventional singlemode fibers transmit wavelengths in the 1300 and 1550 nm range and absorb wavelengths in the range of nm range. WDM systems use wavelengths in the two regions of 1310 and 1550 nm. Special fibers have made it possible 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 components perform with the same efficiency over the complete spectrum. For example, erbiumdoped fiber amplifiers (EDFAs) perform best in the range of 1550 nm. Will other fiber amplifiers perform as well, so that different amplifier types can be included? 179

2 180 Part V Dense Wavelength Division Multiplexing DWDM systems take advantage of advanced optical technology (e.g., tunable lasers, narrowband optical filters, etc.) to generate many wavelengths in the range around 1550 nm. TUT Recommendation 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 43 wavelengths have been introduced, and systems with many more wavelengths are on the 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 of 100 OHz, and those with 80 channels have a channel spacing at 50 OHz. This channel separation determines the width of the spectral (wavelength) narrowness of each channel, or how close (in terms of wavelength) the channels are. Fortychannel DWDM systems can transmit over a single fiber an aggregate bandwidth of 400 Obis (10 Gb/s per channel). t is estimated that at 400 Obis, more than 10,000 volumes of an encyclopedia can be transmitted in 1 second. The number of channels also depends on the type of fiber. A single strand of singlemode fiber can transmit over 80 km without amplification. Placing eight optical amplifiers in cascade, the total distance is extended to over 640 km (this is typical for 80channel 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. And this trend is expected to continue until all limits of physics for this technology have been reached and pushed back. Although DWDM technology is still evolving and technologists and standards bodies are addressing many issues, systems are being offered with few tens of wavelengths in the same fiber. However, it is reasonable to assume that in the near future we will see DWDM systems with several hundred wavelengths in a single fiber. Theoretically, more than 1000 channels may be multiplexed in a fiber. DWDM technology with more than 200 wavelengths has already been demonstrated. Devices with 200 wavelengths per fiber at 40 Obis per wavelength with an aggregate bandwidth of 8 Tbls per fiber are feasible. Eight Tbls per fiber is an aggregate bandwidth that exceeds all needs today. Nevertheless, this bandwidth may become tomorrow's norm if we extrapolate from the explosion in data traffic now in progress. DWDM utilizes a large aggregate bandwidth in a single fiber by taking advantage of advanced optical technology that is able to launch and multiplex many wavelengths in one fiber, switch wavelengths optically, and at the receiving end, demultiplex and read each wavelength separately. n DWDM, each wavelength constitutes a separate channel capable of carrying traffic at a bit rate that may not be the same on all channels. Because a channel does not exactly consist of a singular wavelength but of a narrow band around a center wavelength, each band is spaced from the next by several gigahertz to provide a safety zone and avoid channelwavelength overlapping and thus crosstalk. This spacing is necessary for several reasons: laser sources and tunable filters may drift with temperature and time, optical amplifiers do

3 Chapter 14 DWDM Systems 181 not exhibit true flat gain over the wavelength range, spontaneous noise from EDFAs is cumulative, and there are often dispersion effects DWDM NETWORK TOPOLOGES nitially, DWDM started with one topology: pointtopoint. A number of network topologies in addition to direct pointtopoint, are now considered, however: pointtopoint with adddrop capability, ring, fully mesh connected, and star.except for pointtopoint, these topologies may be mapped into a fiberring topology with many wavelengths. Depending on topology and fiber length, optical amplification mayor may not be required. The need for amplification and the number of amplifiers (if any) can be estimated from the distance between the transmitter and the receiver and according to system design parameters, such as number of wavelengths (channels), channel width, channel separation, modulation technique, bit rate, fiber type, and other optical component characteristics. Figure 14.1 illustrates a DWDM pointtopoint topology with an optional amplifier. This topology is more suitable for longhaul ultrahigh aggregate bandwidth transport (where wavelengths are distinguished by different colors: wavelength multiplexers and demultiplexers are not shown). An optical adddrop multiplexer (OADM) may also be included on the link if one or more wavelengths are to be dropped and added. Transmitters A1 Receivers f/) c (!) ~ u ~ '2. (!) Of/)......~... Electronic Photonic Electronic regime regime regime Figure 14.1 A conceptual pointtopoint DWDM system (also known as "big fat pipe"). f/) 0 CD _. < ::::: _. CD C'l ~ CD CD f/) ;:l. Figure 14.2 illustrates a fivenode, fully connected mesh topology DWDM mapped onto a ring (a single ring is shown for clarity), and a star network also mapped onto a ring. Each interconnecting link is shown via a separate channel (wavelength), and in the fully connected case, four wavelengths are added/dropped at each node. The hub of the star network may receive a wavelength from a node on the ring and selectively convert it to another wavelength destined to another node on the same ring. This function is also referred to as broadcast and select. Notice that although we make reference to nodes, the conventional communications networks "node" may be replaced by "router," which is better suited to data networks. t should be pointed out, however, that modem networks transport a mix of

4 182 Part V Dense Wavelength Division Multiplexing Hub.... ; / Node A/Hub ', Node E Node A \, 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), and nternet traffic. n DWDM applications, a router that performs DSn and optical carrier level n (OCn) grooming, optical multiplexing, switching, and provides realtime quality of service (QoS), starts looking like a traditional node. Therefore, although nodes and routers are conceptually different, for brevity we do not distinguish between the two. WDM networking examines and evaluates these issues in detail, but this is beyond our scope DWDM APPLCABLTY 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 P/LAN Access & last mile to the home (080 to OC3 rates)... Multiplexer o i Node wlo OAOM ~:=::=::~~:~~ Node w/oaom Metroarea (081 to OC12 rates) Highspeed optical node Optical link... ElectricallinkL::::::~"~:::::::::::::::::::~~ Backbone (>OC12 rates) Figure 14.3 WDM is applicable to all network layers, from access to backbone.

5 Chapter 14 DWDM Systems 183 that optically transport bulk ultrahigh bandwidth data at very high bit rates) covering large geographical areas (intercontinental), and they interconnect continents (transoceanic). ntercontinental systems may have a pointtopoint, mesh, star, or ring topology, whereas transoceanic systems are predominantly pointtopoint, possibly with a few adddrops. A submarine DWDM network may also be of the ring or star type interconnecting a cluster of islands. DWDM systems are applicable to metropolitan areas interconnecting many highrise buildings, to a campus environment, and to multilevel buildings. Finally, they are applicable to singleenterprise networks with fewer nodes, where a variety of traffic types (TDM, SONET/SDH,, P, etc.) merge to be transported to a higher level network. The ring or mesh topology is more suitable in such applications. One of the nodes is a hub providing connectivity with other networks. Notice that as we move from the access to backbone layers, bit rates increase because bandwidth is aggregated. Table 14.1 tabulates the legacy narrowband and broadband rates, and Table 14.2 gives the SONET and SDH rates. n DWDM applications, each node sources and terminates one (or more) wavelength(s) such that a fully interconnected mesh network or a star network is constructed with a single fiber. Depending on the number of nodes and wavelengths, a WDM system is termed DWDM if there are many wavelengths and CWDM if there are few wavelengths; typically, these are spaced at GHz. Moreover, each wavelength may transport different services, such as P, SONET/SDH, or. As a result, practical systems are designed to handle one or more service types. For example, 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 DS Mb/s Mb/s El Mb/s DSlc Mb/s Mb/s DS Mb/s Mb/s E Mb/s Mb/s DS Mb/s Mb/s DS3c Mb/s Mb/s E Mb/s DS Mb/s Mb/s

6 184 De ns e Wavelength Division Multiplexing Part V Table 14.2 SONET/SDH Rates Signa l Designation SDH SON ET Optical Line Rat e (Mb/s) STS STMO OC STS3 STM OC STS12 STM4 OC STS48 STM16 OC48 2, STS l92 STM64 OC 192 9, STS768 STM254 OC768 39, has been designed to handle any service at any bandwidth (Figure 14.4). WaveStar OLS 400G multiplexes up to 80 wavelengths at OC48 per fiber (80 x 2.5 = 200 Gb/s per fiber) or up to 40 wavelengths at OC92 per fiber (40 x l 0 =400 Gb/s per fiber). Similarly, products from other wellknown companies support a range of wave Voice, data, video \ (j) Variable bandwidth access Opt ical access ~ z ~ P fabric E * >en C Q) E Q) OJ <ll c <ll E '" 0 ~c.0 ::J en (ij u; Q) > <ll :5: J Fixed band wid th "'~~A"" D C D C bandwidth ma nager fabric STM fabric Optical fabric acce ss multiplexer 2.5G/10G OLS 40G and 400G Figure 14.4 Example of a product family that handles any service at any bandwidth. (From LUCENT Technologies, Bell Labs Technology, vol. 2, no. 2, p. 12, Repri nted with permission.)

7 Chapter 14 DWDM Systems 185 length channels at various rates. Neverthele ss, DWDM network s have several unresolved issues to address. Standards bodies are working toward drafting recommendations, but in the meantime each manufacturer offers systems with semiprivate solutions to meet market needs and to make an early entry. For example, operations, administration, maintenance, and provisioning (OAM&P), as well as dynamic wavelength assignment, are on the drawing board. Network management may also differ from vendor to vendor along with reliability, latency, and qualityofservice agreements. Some use a supervisory (wavelength) channel for OAM&P (typically at 1310 nm or at about 1500 nm), but the wavelength, its bit rate, and the protocol have not yet been standardized. Some use proprietary methods to enable transporting a mix of services and traffic types, although proposals have been made to produce a standard. One such method is the digital wrapper, which, based on TDM principle, encapsulates an optical channel, with additional overhead. Thi s overhead includes OAM&P functions as well as forward error correction (FEC) to extend the transported range by 45 db (a practice similar to submarine applications: TUT Recommendation 0.975). Current systems support fixedwavelength assignment for each node or are manually reconfigurable. Dynamic wavelength assignment is another area to be standardized. Each network provider also offers a proprietary solution for wavelength protection. The set of wavelengths is different from system to system, thus making interoperability almost impossible (Figure 14.5). Under the circumstances, a description of DWDM standard networks is out of the question for the time being. n Chapter 15 we will address these issues. Again, this is a current view and the intere sted designer should be mindful of upcoming standards and recommendations. P ow Figure 14.5 Multinetwork interoperability. For a service to be provided endtoend (dotted line) with the same guarantees; all networks must be compatible. le is the intercarri er interface.