Dense Wavelength Division Multiplexing

Dense Wavelength Division Multiplexing February 1999

INTRODUCTION 1 DWDM TECHNOLOGY OVERVIEW 1 DWDM System Components 2 DWDM Lasers 2 Optical Multiplexer 3 Optical 3 Optical Amplifier 3 Competitive Technologies 4 BENEFITS OF DWDM SOLUTION 5 INTEGRATED DWDM FOR THE ASX-4000 6 DWDM-Capable OC-48c Port Cards 6 WMX-4 DWDM Multiplexer 7 APPLICATIONS 8 Extended Distance OC-48c ASX-4000 Interconnection 8 10Gbps ASX-4000 Interconnection Over Single Fiber Pair 9 Reduced Cost Interface to High-End DWDM Systems 10 CONCLUSION 10

INTRODUCTION One of the major trends in networking in late 1990 s has been a relentless growth in demand for bandwidth in both enterprise and service provider networks. Driving the need for more bandwidth is a combination of factors. More users are connecting as the commercial Internet offers a new online experience for consumers. Internet computing applications, including multi-tier distributed databases, interactive multimedia communication, and electronic commerce rely on the network and demand network resources. A new generation of high-speed Internet access is emerging to meet bandwidth demands and further amplify core bandwidth requirements. At the same time, competitive pressures make it imperative that networking costs be reduced even as the demand for capacity and new services increases. Successful companies are constantly on the lookout for new technologies which can provide a competitive edge and increase their cost effectiveness. Optical networking has emerged as a solution to the bandwidth crunch. In particular, one new optical technology Dense Wavelength Division Multiplexing (DWDM) promises to increase the capacity and performance of existing fiber optic backbones. DWDM offers a capacity upgrade solution with greater scalability and lower cost than available alternatives. FORE Systems flagship ATM backbone switch, the ForeRunner ASX-4000, offers an integrated DWDM solution that combines the benefits of DWDM transmission with the unique benefits of FORE s industry-leading switching technology, including Dynamic Protection Switching and Capacity Aware Routing. DWDM TECHNOLOGY OVERVIEW Most of today s high speed backbones consist of fiber optic links operating 2.5 gigabits per second (Gbps) or less. Wavelength Division Multiplexing (WDM) is a technique for increasing the information-carrying capacity of optical fiber by transmitting multiple signals simultaneously at different wavelengths (or colors ) on the same fiber. In effect, WDM converts a single fiber into multiple virtual fibers, each driven independently at different wavelengths. Systems with more than a small number of channels (two or three) are considered Dense WDM (DWDM) systems. Nearly all DWDM systems operate across a range of wavelengths in the 1550nm low-attenuation window. Systems with four to forty channels, with up to 10Gbps per channel, are commercially available today from several vendors, and the technology is advancing rapidly. Systems with 80 or more channels will soon be available. 1 Dense Wavelength Division Multiplexing rev: 99feb03

DWDM System Components Figure 1 is a block diagram of a DWDM system consisting of the following components: Optical transmitters (lasers) Optical multiplexer and demultiplexer Optical amplifier Optical receivers data 1 λ 1 λ 1 data 1 data 2 data 3 λ 2 λ 3 Mux λ 1, λ 2, λ 3, λ 4 Amplifier Demux λ 2 λ 3 data 2 data 3 data 4 λ 4 λ 4 data 4 Figure 1 Components of a DWDM System DWDM Lasers DWDM systems use high resolution, or narrowband, lasers transmitting in the 1550nm wavelength band. Operation in the 1550nm range provides two benefits: It minimizes optical power loss as the signal propagates along the fiber allowing much greater transmission distances with better signal integrity It permits the use of optical amplifiers to boost signal strength for extended distances. Optical amplifiers are much less costly than electrical amplifiers because they do not have to regenerate the individual optical signals. Narrowband transmit lasers are important for allowing close channel spacing and for minimizing the effects of other signal impairments (e.g. chromatic dispersion) which would otherwise limit the allowable distance before the signal must be regenerated electronically. The ITU has specified standard channel spacing plans to ensure interoperability between equipment from different vendors. In addition to interoperability, this standardization allows manufacturers to realize volume-based cost reductions by producing standard, rather than custom components. 2

Optical Multiplexer The optical multiplexer combines the transmit signals at different wavelengths onto a single optical fiber, and the demultiplexer separates the combined signal into its component wavelengths at the receiver. Several technologies are currently used for optical multiplexing and demultiplexing, including thin-film dielectric filters and various types of optical gratings. Some multiplexers are constructed as completely passive devices, meaning they require no electrical input. Passive optical multiplexers behave essentially like very high precision prisms to combine and separate individual colors of the WDM signal. Like prisms, most passive optical devices are reciprocal devices, meaning they function in the same way when the direction of the light is reversed. Typically the multiplexing and demultiplexing functions are provided by a single device, a WDM multiplexer/demultiplexer. Some multiplexers have the ability to transmit and receive on a single fiber, a capability is known as bi-directional transmission. Optical The optical receiver is responsible for detecting the incoming lightwave signal and converting it to an appropriate electronic signal for processing by the receiving device. Optical receivers are very often wideband devices, i.e. able to detect light over a relatively wide range of wavelengths (from about 1280-1580nm). This is the reason why some seemingly incompatible devices can actually interoperate. For instance, directly connecting two otherwise compatible network interfaces with different transmitter wavelengths is usually not a problem, even though one end may be transmitting at 1310nm and the other at 1550nm! Optical Amplifier An amplifier is sometimes used to boost an optical signal to compensate for power loss, or attenuation, caused by propagation over long distances. While typically not required on links of less than about 65km, optical amplifiers represent an important advancement for WDM. Before the development of optical amplifiers, the only way to boost an optical signal was to regenerate it electronically, that is, convert the optical signal to an electrical signal, amplify it, convert it back to an optical signal, and then retransmit it. Electronic regeneration of a WDM signal would require a separate regenerator for each wavelength on each fiber. A single optical amplifier, on the other hand, can simultaneously amplify all the wavelengths on one fiber. This allows the cost of signal amplification to be spread over several users or applications. An additional benefit of the optical amplifier is that as a strictly optical device, it is a protocol- and bit rate-independent device. That is, an optical amplifier operates the same way regardless of the framing or bit rate of the optical signal. This allows a great deal of flexibility in that an optically amplified link can support any 3 Dense Wavelength Division Multiplexing rev: 99feb03

combination of protocols (e.g. ATM, SONET, Gigabit Ethernet, PPP) at any bit rate up to a maximum design limit. Competitive Technologies Historically, network managers have had two alternatives for increasing the capacity of a fiber optic transmission plant: use more fiber, or operate the same fiber at a higher bit rate. Figure 2 is a schematic showing the available options for increasing transmission capacity from 2.5Gbps to 10Gbps. data 1 data 1 data 2 data 2 data 3 data 3 data 4 data 4 Figure 2 (a) Space Division Multiplexing: four fibers, four 2.5Gbps lasers data data Figure 2 (b) Time Division Multiplexing: one fiber, one 10Gbps laser data 1 λ 1 λ 1 data 1 data 2 data 3 λ 2 λ 3 Mux λ 1, λ 2, λ 3, λ 4 Amplifier Demux λ 2 λ 3 data 2 data 3 data 4 λ 4 λ 4 data 4 Figure 2 (c) - Wavelength Division Multiplexing: one fiber, four 2.5Gbps lasers Where sufficient dark (unused) fiber is available to meet currently foreseeable needs, the use more fiber approach is straightforward and perhaps the lowest cost solution. Otherwise, installing new fiber can be a very costly undertaking, especially when new conduit must be installed, or in densely populated metropolitan areas. Even in situations where dark fiber is available, it is sometimes more cost effective to achieve higher utilization of each fiber rather than lighting up more fiber. This is especially true with longer distance links where signal amplification (or dispersion compensation) is required, since each fiber would require a separate amplification (or compensation) device. 4

The second, or higher bit rate, option requires that the signals be multiplexed electronically, e.g. by Time Division Multiplexing (TDM), to the new higher bit rate for transmission. SONET (Synchronous Optical Network) is currently the most commonly implemented multiplexing standard for high-speed optical signals, with bit rates which increase stepwise by factors of four: 155Mbps, 622Mbps, 2.5Gbps, and 10Gbps. An unfortunate drawback of the TDM approach is that it requires a serviceaffecting, all-at-once upgrade to the new higher rate. Network interfaces must be replaced by units with four times their capacity, whether or not all the capacity is immediately required. FORE s DWDM approach, by contrast, allows non-service affecting, incremental capacity upgrades in 2.5Gbps increments from 2.5Gbps to 10 Gbps as demand increases. Another drawback of the higher bit rate approach is that signal distortions (due to dispersion and fiber non-linearities) become a limiting factor at much shorter distances as bit rate increases. Dispersion effects, which lead to smearing of the signal pulses, are several times greater at 10Gbps than at 2.5Gbps over standard single mode fiber. Since it is individual channel bit rate which determines the amount of dispersion, a DWDM link can span a greater distance before electronic signal regeneration is required than an equal capacity single-channel link. (Dispersion limits are typically on the order of 1000km at 2.5Gbps compared to about 200km at 10Gbps with standard single mode fiber.) A third limitation of the TDM solution is that it constrains the capacity of the fiber to the speed of the available electronics. The highest transmission rate in commercially available electronics is 10Gbps, while the capacity of the fiber is orders of magnitude higher. Electronic components capable of operating at this speed are costly to construct, operate and maintain. With DWDM, electrical components continue to operate at the channel bit rate (i.e. 2.5Gbps) while the multiplexing is done in the optical domain. Current DWDM technology allows 40 or more channels on a single fiber, or over 100Gbps per fiber. BENEFITS OF DWDM SOLUTION In summary, the advantages offered by DWDM include the following: Minimizes fiber usage by converting each fiber into multiple virtual fibers Extends the non-regenerated distance limit compared to an equal-capacity single laser solution Provides greater scalability with incremental, in-service capacity upgrades and shorter provisioning time. Scalable from a single channel through 40 or more channels on a single fiber. Additionally, DWDM is the only commercially available technology currently capable of delivering more than 10Gbps on a single fiber. 5 Dense Wavelength Division Multiplexing rev: 99feb03

DWDM systems can be transparent to changes in bit rates and protocols running over them. INTEGRATED DWDM FOR THE ASX-4000 The ForeRunner ASX-4000 backbone ATM switch offers a unique integrated DWDM solution targeted at campus, interoffice and metropolitan networks. The components of this solution consist of DWDM-capable OC-48c port cards and a four-channel DWDM multiplexer/demultiplexer. As an integrated system this package supports dual-fiber (transmit/receive pair) switch interconnects at up to 10Gbps. Figure 3 shows typical DWDM usage. 2 OC-48c SMLR A 2 OC-48c SMLR B WMX-4 10 Gbps! OC-48c Port Card A at 1559.0 and 1560.6 nm OC-48c Port Card B at 1555.8 and 1557.4 nm Figure 3 ASX-4000 + WMX-4 = 10Gbps Fiber Pair Interconnects DWDM-Capable OC-48c Port Cards The ASX-4000 DWDM-capable port cards are 2-port OC-48c cards with wavelength-tuned narrowband Single Mode Long Reach (SMLR) lasers. Two different versions of the port card are available, each with lasers tuned to two different fixed wavelengths in the ITU 200GHz channel plan. (Total of four distinct operating wavelengths: 1555.8nm, 1557.4nm, 1559.0nm, and 1560.6nm.). The DWDM-capable port cards are compatible with the standard OC-48c port cards for the ASX-4000 and may be installed in any available slot as a direct 6

replacement for the standard cards. No special configuration is required since the lasers are factory-tuned to the proper operating wavelengths. All FORE OC-48c port cards use wideband receiver optics, making it possible to directly connect DWDM ports of different colors without using the DWDM multiplexer, or to directly connect DWDM (155X nm) and non-dwdm (1310nm) ports. This feature allows customers the flexibility to purchase and use DWDMcapable port cards today, with the option of utilizing DWDM in the future. 192.7 THz (1555.75 nm) 192.3 THz (1558.98 nm) 192.5 THz (1557.36 nm) 192.1 THz (1560.61 nm) Port card A Port card B Figure 4 DWDM-capable OC-48c Port Cards WMX-4 DWDM Multiplexer The WMX-4 is a four-wavelength passive optical multiplexer/demultiplexer tuned to the wavelengths of the ASX-4000 DWDM-capable port cards. The WMX-4 includes four single-color transmit/receive port pairs and one DWDM trunking port pair. Figure 5 is a front panel view of the WMX-4. Port 1 Port 2 Port 3 Port 4 Trunk Port Figure 5 WMX-4 Optical Multiplexer 7 Dense Wavelength Division Multiplexing rev: 99feb03

The WMX-4 provides both multiplexing and demultiplexing functions. The multiplexing module combines the four single-color transmit signals from the ASX-4000 into a four-color DWDM signal for transmission on the trunking port. The demultiplexer module splits the DWDM signal received on the trunking port into its component colors and forwards each to the proper port. The small size of the WMX-4 (1.75 high) allows it to be mounted directly above the ASX-4000 in a standard 19 rack (see Figure 6). As a strictly passive optical device, the WMX-4 offers very high reliability and simple implementation. No software configuration is required to use the WMX-4. Adding a channel is as simple as connecting a single mode fiber from the ASX-4000 to the multiplexer at each end of the link. Figure 6 Rack Mounted ASX-4000 With WMX-4 APPLICATIONS Typical applications of the DWDM-capable port cards and WMX-4 multiplexer include: Direct ASX-4000 port-to-port interconnects (without WMX-4) at distances up to 65+km (or 100+km with an optical amplifier) Point-to-point DWDM switch interconnects at 5-10Gbps over a single fiber pair Connection to higher density third-party DWDM equipment, bypassing wavelength conversion equipment 8 Extended Distance OC-48c ASX-4000 Interconnection The ASX-4000 DWDM-capable ports may be connected together directly (without the WMX-4 mux) to extend the reach for switch-to-switch fiber interconnection.

Because optical signals in the 1550nm band suffer less attenuation than signals in the 1310nm range, this configuration extends the allowable transmission distance before amplification is required. Typical non-amplified distance is approximately 65+km with DWDM-capable cards, compared to 40km for the standard (non- DWDM) SMLR OC-48c port cards. With an optical amplifier, the distance between DWDM-capable port cards can be extended to 100km or beyond. In this configuration, DWDM ports of different colors (or even non-dwdm OC- 48c ports) can be mixed and matched because FORE s wideband receiver optics allows each receiver to see lightwaves of any wavelength. 10Gbps ASX-4000 Interconnection Over Single Fiber Pair The ASX-4000 is a 40Gbps non-blocking ATM switch targeted at enterprise and service provide backbones. Backbones built around such high capacity switches typically require a very high speed transmission network. It is not unusual for a network design to require 10Gbps switch-to-switch connection capacity. The ASX-4000/WMX-4 package provides a cost-effective 10Gbps switch interconnection over a single fiber pair. As a passive optical device, the WMX-4 offers very high reliability and requires no management or provisioning effort. FORE s Capacity Aware Routing ensures that ATM connections will be distributed over all four wavelengths for maximum link utilization. FORE s Dynamic Protection Switching provides network-level resiliency capable of rerouting ATM connections on a 10 millisecond time scale. For maximum reliability, the ASX-4000 will provide 1+1 hardware protection for all data path elements and support for SONET Automatic Protection Switching (APS) compliant with the Bellcore GR-253-CORE specification. OC-48c 10 Gbps WMX-4 WMX-4 ASX-4000 ASX-4000 Figure 7 Typical Point-to-Point DWDM Connection 9 Dense Wavelength Division Multiplexing rev: 99feb03

Reduced Cost Interface to High-End DWDM Systems Use of ITU channel spacing provides compatibility between the ASX-4000 DWDM-capable port cards and high-end equipment from other DWDM suppliers. FORE has already demonstrated interoperability between the ASX-4000 port cards and equipment from other leading DWDM suppliers. By integrating the precision-tuned narrowband DWDM lasers into the ASX-4000, the FORE solution avoids the need for wavelength conversion devices typically required with high-end DWDM systems. Eliminating the wavelength conversion devices means reducing the number of lasers required on a given DWDM link, providing a significant savings in the cost of equipment. Currently available high-end DWDM systems are capable of multiplexing 40 or more 2.5Gbps signals, for a single fiber capacity of at least 100Gbps. Though equipment costs increase with the number of channels, these systems are achieving a per-fiber capacity that is well beyond the range available from SONET equipment (at any cost) for the foreseeable future. CONCLUSION DWDM is available today from several vendors, including FORE Systems, and offers the ability to scale transmission capacity without downtime, forklift upgrades or lengthy provisioning times. Benefits of FORE s integrated ATM/DWDM solution include the following: DWDM-capable port cards extend maximum distance for direct ASX-4000 switch interconnection at OC-48c rate (without WMX-4) to 65+km without amplification, 100+km with amplification Flexible DWDM port cards may be used with or without WMX-4, and interoperate with both 1550nm and 1310nm OC-48c port cards Passive WMX-4 mux provides very high reliability WMX-4 reduces fiber costs by maximizing fiber utilization (up to 10Gbps per fiber pair) Ease of implementation transparent to ASX-4000 signaling and routing protocols; no need for management or configuration Allows incremental, in-service capacity upgrades from 2.5Gbps to 10Gbps in 2.5Gbps increments with zero-wait provisioning Use of standard wavelengths allows interoperability with third-party DWDM equipment and an upgrade path to higher capacity DWDM systems Reduced cost when connecting to third-party DWDM muxes by eliminating the wavelength conversion equipment usually required Compatible with off-the-shelf optical amplifiers 10 Copyright 1998 FORE Systems, Inc. All rights reserved. FORE Systems, ForeRunner, PowerHub, ForeThought, ForeView and AVA are registered trademarks of FORE Systems, Inc. All Roads Lead To ATM, Application Aware, ASN, ATV, CellChain, CellPath, CellStarter, EdgeRunner, FramePlus, ForeRunnerHE, ForeRunnerLE, Intelligent Infrastructure, I 2, MSC, Netpro, Networks of Steel, Network of Steel, StreamRunner, TACtics Online, TNX, Universal Port, VoicePlus and Zero Hop Routing are trademarks of FORE Systems, Inc. All other brands or product names are trademarks of their respective holders. Please be advised that technical specifications set forth in this product literature are correct as of September 1998 and are subject to change by FORE Systems, Inc. at any time without notice.

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