DWDM

DenseWavelength

DivisionMultiplexing

February 1999

INTRODUCTION 1

DWDM TECHNOLOGY OVERVIEW 1DWDM System Components 2

DWDM Lasers 2Optical Multiplexer 3Optical Receiver 3Optical Amplifier 3

Competitive Technologies 4

BENEFITS OF DWDM SOLUTION 5

INTEGRATED DWDM FOR THE ASX-4000 6DWDM-Capable OC-48c Port Cards 6WMX-4 DWDM Multiplexer 7

APPLICATIONS 8Extended Distance OC-48c ASX-4000 Interconnection810Gbps ASX-4000 Interconnection Over Single FiberPair 9Reduced Cost Interface to High-End DWDM Systems10

CONCLUSION 10

1Dense Wavelength Division Multiplexing

rev: 99feb03

INTRODUCTION

One of the major trends in networking in late 1990s has been a relentless growthin demand for bandwidth in both enterprise and service provider networks.Driving the need for more bandwidth is a combination of factors. More users areconnecting as the commercial Internet offers a new online experience forconsumers. Internet computing applications, including multi-tier distributeddatabases, interactive multimedia communication, and electronic commerce rely onthe network and demand network resources. A new generation of high-speedInternet access is emerging to meet bandwidth demands and further amplify corebandwidth requirements.

At the same time, competitive pressures make it imperative that networking costsbe reduced even as the demand for capacity and new services increases. Successfulcompanies are constantly on the lookout for new technologies which can provide acompetitive edge and increase their cost effectiveness.

Optical networking has emerged as a solution to the bandwidth crunch. Inparticular, one new optical technology Dense Wavelength Division Multiplexing(DWDM) promises to increase the capacity and performance of existing fiberoptic backbones. DWDM offers a capacity upgrade solution with greaterscalability and lower cost than available alternatives.

FORE Systems flagship ATM backbone switch, the ForeRunner ASX-4000, offersan integrated DWDM solution that combines the benefits of DWDM transmissionwith the unique benefits of FOREs industry-leading switching technology,including Dynamic Protection Switching and Capacity Aware Routing.

DWDM TECHNOLOGY OVERVIEW

Most of todays high speed backbones consist of fiber optic links operating 2.5gigabits per second (Gbps) or less. Wavelength Division Multiplexing (WDM) is atechnique for increasing the information-carrying capacity of optical fiber bytransmitting multiple signals simultaneously at different wavelengths (or colors)on the same fiber. In effect, WDM converts a single fiber into multiple virtualfibers, each driven independently at different wavelengths. Systems with morethan a small number of channels (two or three) are considered Dense WDM(DWDM) systems. Nearly all DWDM systems operate across a range ofwavelengths in the 1550nm low-attenuation window.

Systems with four to forty channels, with up to 10Gbps per channel, arecommercially available today from several vendors, and the technology isadvancing rapidly. Systems with 80 or more channels will soon be available.

2DWDM System Components

Figure 1 is a block diagram of a DWDM system consisting of the followingcomponents:

Optical transmitters (lasers) Optical multiplexer and demultiplexer Optical amplifier Optical receivers

Mux

l1

l2

l3

l4

Dem

ux

l1

l2

l3

l4

l1, l2, l3, l4

Transmitter

Transmitter

Transmitter

Transmitter

Receiver

Receiver

Receiver

Receiver

Amplifier

data 1

data 4

data 3

data 2

data 1

data 4

data 3

data 2

Figure 1 Components of a DWDM System

DWDM LasersDWDM systems use high resolution, or narrowband, lasers transmitting in the1550nm wavelength band. Operation in the 1550nm range provides two benefits:

It minimizes optical power loss as the signal propagates along thefiber allowing much greater transmission distances with better signalintegrity

It permits the use of optical amplifiers to boost signal strength forextended distances. Optical amplifiers are much less costly thanelectrical amplifiers because they do not have to regenerate theindividual optical signals.

Narrowband transmit lasers are important for allowing close channel spacing andfor minimizing the effects of other signal impairments (e.g. chromatic dispersion)which would otherwise limit the allowable distance before the signal must beregenerated electronically.

The ITU has specified standard channel spacing plans to ensure interoperabilitybetween equipment from different vendors. In addition to interoperability, thisstandardization allows manufacturers to realize volume-based cost reductions byproducing standard, rather than custom components.


rev: 99feb03

Optical MultiplexerThe optical multiplexer combines the transmit signals at different wavelengthsonto a single optical fiber, and the demultiplexer separates the combined signalinto its component wavelengths at the receiver. Several technologies are currentlyused for optical multiplexing and demultiplexing, including thin-film dielectricfilters and various types of optical gratings. Some multiplexers are constructed ascompletely passive devices, meaning they require no electrical input. Passiveoptical multiplexers behave essentially like very high precision prisms to combineand separate individual colors of the WDM signal. Like prisms, most passiveoptical devices are reciprocal devices, meaning they function in the same waywhen the direction of the light is reversed.

Typically the multiplexing and demultiplexing functions are provided by a singledevice, a WDM multiplexer/demultiplexer. Some multiplexers have the ability totransmit and receive on a single fiber, a capability is known as bi-directionaltransmission.

Optical ReceiverThe optical receiver is responsible for detecting the incoming lightwave signal andconverting it to an appropriate electronic signal for processing by the receivingdevice. Optical receivers are very often wideband devices, i.e. able to detect lightover a relatively wide range of wavelengths (from about 1280-1580nm). This isthe reason why some seemingly incompatible devices can actually interoperate.For instance, directly connecting two otherwise compatible network interfaces withdifferent transmitter wavelengths is usually not a problem, even though one endmay be transmitting at 1310nm and the other at 1550nm!

Optical AmplifierAn amplifier is sometimes used to boost an optical signal to compensate for powerloss, or attenuation, caused by propagation over long distances. While typically notrequired on links of less than about 65km, optical amplifiers represent animportant advancement for WDM.

Before the development of optical amplifiers, the only way to boost an opticalsignal was to regenerate it electronically, that is, convert the optical signal to anelectrical signal, amplify it, convert it back to an optical signal, and thenretransmit it. Electronic regeneration of a WDM signal would require a separateregenerator for each wavelength on each fiber. A single optical amplifier, on theother hand, can simultaneously amplify all the wavelengths on one fiber. Thisallows the cost of signal amplification to be spread over several users orapplications.

An additional benefit of the optical amplifier is that as a strictly optical device, it isa protocol- and bit rate-independent device. That is, an optical amplifier operatesthe same way regardless of the framing or bit rate of the optical signal. Thisallows a great deal of flexibility in that an optically amplified link can support any

4combination of protocols (e.g. ATM, SONET, Gigabit Ethernet, PPP) at any bitrate up to a maximum design limit.

Competitive Technologies

Historically, network managers have had two alternatives for increasing thecapacity of a fiber optic transmission plant: use more fiber, or operate the samefiber at a higher bit rate. Figure 2 is a schematic showing the available options forincreasing transmission capacity from 2.5Gbps to 10Gbps.

Transmitter Receiver



Transmitter Receiverdata 1

data 4

data 3

data 2

data 1

data 4

data 3

data 2

Figure 2 (a) Space Division Multiplexing: four fibers, four 2.5Gbps lasers

ReceiverTransmitterdata data

Figure 2 (b) Time Division Multiplexing: one fiber, one 10Gbps laser

Mux

l1

l2

l3

l4

Dem

ux

l1

l2

l3

l4

l1, l 2, l 3, l 4

Transmitter

Transmitter

Transmitter

Transmitter

Receiver

Receiver

Receiver

Receiver

Amplifier

data 1

data 4

data 3

data 2

data 1

data 4

data 3

data 2

Figure 2 (c) - Wavelength Division Multiplexing: one fiber, four 2.5Gbps lasers

Where sufficient dark (unused) fiber is available to meet currently foreseeableneeds, the use more fiber approach is straightforward and perhaps the lowest costsolution. Otherwise, installing new fiber can be a very costly undertaking,especially when new conduit must be installed, or in densely populatedmetropolitan areas. Even in situations where dark fiber is available, it issometimes more cost effective to achieve higher utilization of each fiber rather thanlighting up more fiber. This is especially true with longer distance links wheresignal amplification (or dispersion compensation) is required, since each fiberwould require a separate amplification (or compensation) device.


rev: 99feb03

The second, or higher bit rate, option requires that the signals be multiplexedelectronically, e.g. by Time Division Multiplexing (TDM), to the new higher bitrate for transmission. SONET (Synchronous Optical Network) is currently themost 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 service-affecting, all-at-once upgrade to the new higher rate. Network interfaces must bereplaced by units with four times their capacity, whether or not all the capacity isimmediately required. FOREs DWDM approach, by contrast, allows non-serviceaffecting, incremental capacity upgrades in 2.5Gbps increments from 2.5Gbps to10 Gbps as demand increases.

Another drawback of the higher bit rate approach is that signal distortions (due todispersion and fiber non-linearities) become a limiting factor at much shorterdistances as bit rate increases. Dispersion effects, which lead to smearing of thesignal pulses, are several times greater at 10Gbps than at 2.5Gbps over standardsingle mode fiber. Since it is individual channel bit rate which determines theamount of dispersion, a DWDM link can span a greater distance before electronicsignal regeneration is required than an equal capacity single-channel link.(Dispersion limits are typically on the order of 1000km at 2.5Gbps compared toabout 200km at 10Gbps with standard single mode fiber.)

A third limitation of the TDM solution is that it constrains the capacity of the fiberto the speed of the available electronics. The highest transmission rate incommercially available electronics is 10Gbps, while the capacity of the fiber isorders of magnitude higher. Electronic components capable of operating at thisspeed are costly to construct, operate and maintain. With DWDM, electricalcomponents continue to operate at the channel bit rate (i.e. 2.5Gbps) while themultiplexing is done in the optical domain. Current DWDM technology allows 40or 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 morechannels on a single fiber. Additionally, DWDM is the only commerciallyavailable technology currently capable of delivering more than 10Gbps on asingle fiber.

6 DWDM systems can be transparent to changes in bit rates and protocolsrunning over them.

INTEGRATED DWDM FOR THE ASX-4000

The ForeRunner ASX-4000 backbone ATM switch offers a unique integratedDWDM solution targeted at campus, interoffice and metropolitan networks. Thecomponents of this solution consist of DWDM-capable OC-48c port cards and afour-channel DWDM multiplexer/demultiplexer. As an integrated system thispackage supports dual-fiber (transmit/receive pair) switch interconnects at up to10Gbps. Figure 3 shows typical DWDM usage.

2 OC-48cSMLR B

2 OC-48cSMLR A

10 Gbps!

WMX-4

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 CardsThe ASX-4000 DWDM-capable port cards are 2-port OC-48c cards withwavelength-tuned narrowband Single Mode Long Reach (SMLR) lasers. Twodifferent versions of the port card are available, each with lasers tuned to twodifferent fixed wavelengths in the ITU 200GHz channel plan. (Total of fourdistinct operating wavelengths: 1555.8nm, 1557.4nm, 1559.0nm, and 1560.6nm.).

The DWDM-capable port cards are compatible with the standard OC-48c portcards for the ASX-4000 and may be installed in any available slot as a direct


rev: 99feb03

replacement for the standard cards. No special configuration is required since thelasers are factory-tuned to the proper operating wavelengths.

All FORE OC-48c port cards use wideband receiver optics, making it possible todirectly connect DWDM ports of different colors without using the DWDMmultiplexer, or to directly connect DWDM (155X nm) and non-DWDM (1310nm)ports. This feature allows customers the flexibility to purchase and use DWDM-capable port cards today, with the option of utilizing DWDM in the future.

192.7 THz(1555.75 nm)

192.5 THz(1557.36 nm)

192.3 THz(1558.98 nm)

192.1 THz(1560.61 nm)

Port card A Port card B

Figure 4 DWDM-capable OC-48c Port Cards

WMX-4 DWDM MultiplexerThe WMX-4 is a four-wavelength passive optical multiplexer/demultiplexer tunedto the wavelengths of the ASX-4000 DWDM-capable port cards. The WMX-4includes four single-color transmit/receive port pairs and one DWDM trunkingport 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

8The WMX-4 provides both multiplexing and demultiplexing functions. Themultiplexing module combines the four single-color transmit signals from theASX-4000 into a four-color DWDM signal for transmission on the trunking port.The demultiplexer module splits the DWDM signal received on the trunking portinto 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 abovethe ASX-4000 in a standard 19 rack (see Figure 6). As a strictly passive opticaldevice, the WMX-4 offers very high reliability and simple implementation. Nosoftware configuration is required to use the WMX-4. Adding a channel is assimple as connecting a single mode fiber from the ASX-4000 to the multiplexer ateach 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 multiplexerinclude:

Direct ASX-4000 port-to-port interconnects (without WMX-4) at distances upto 65+km (or 100+km with an optical amplifier)

Point-to-point DWDM switch interconnects at 5-10Gbps over a single fiberpair

Connection to higher density third-party DWDM equipment, bypassingwavelength conversion equipment

Extended Distance OC-48c ASX-4000 InterconnectionThe ASX-4000 DWDM-capable ports may be connected together directly (withoutthe WMX-4 mux) to extend the reach for switch-to-switch fiber interconnection.


rev: 99feb03

Because optical signals in the 1550nm band suffer less attenuation than signals inthe 1310nm range, this configuration extends the allowable transmission distancebefore amplification is required. Typical non-amplified distance is approximately65+km with DWDM-capable cards, compared to 40km for the standard (non-DWDM) SMLR OC-48c port cards. With an optical amplifier, the distancebetween 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 FOREs wideband receiver opticsallows each receiver to see lightwaves of any wavelength.

10Gbps ASX-4000 Interconnection Over Single Fiber PairThe ASX-4000 is a 40Gbps non-blocking ATM switch targeted at enterprise andservice provide backbones. Backbones built around such high capacity switchestypically require a very high speed transmission network. It is not unusual for anetwork design to require 10Gbps switch-to-switch connection capacity.

The ASX-4000/WMX-4 package provides a cost-effective 10Gbps switchinterconnection over a single fiber pair. As a passive optical device, the WMX-4offers very high reliability and requires no management or provisioning effort.

FOREs Capacity Aware Routing ensures that ATM connections will be distributedover all four wavelengths for maximum link utilization.

FOREs Dynamic Protection Switching provides network-level resiliency capableof rerouting ATM connections on a 10 millisecond time scale. For maximumreliability, the ASX-4000 will provide 1+1 hardware protection for all data pathelements and support for SONET Automatic Protection Switching (APS)compliant with the Bellcore GR-253-CORE specification.

OC-48c

10 Gbps

ASX-4000

WMX-4

ASX-4000

WMX-4

Figure 7 Typical Point-to-Point DWDM Connection

10

Reduced Cost Interface to High-End DWDM SystemsUse of ITU channel spacing provides compatibility between the ASX-4000DWDM-capable port cards and high-end equipment from other DWDM suppliers.FORE has already demonstrated interoperability between the ASX-4000 port cardsand 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 typicallyrequired with high-end DWDM systems. Eliminating the wavelength conversiondevices 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 ormore 2.5Gbps signals, for a single fiber capacity of at least 100Gbps. Thoughequipment costs increase with the number of channels, these systems are achievinga per-fiber capacity that is well beyond the range available from SONETequipment (at any cost) for the foreseeable future.

CONCLUSION

DWDM is available today from several vendors, including FORE Systems, andoffers the ability to scale transmission capacity without downtime, forklift upgradesor lengthy provisioning times.

Benefits of FOREs integrated ATM/DWDM solution include the following:

DWDM-capable port cards extend maximum distance for direct ASX-4000switch interconnection at OC-48c rate (without WMX-4) to 65+km withoutamplification, 100+km with amplification

Flexible DWDM port cards may be used with or without WMX-4, andinteroperate 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

Copyright 1998 FORE Systems, Inc. All rights reserved. FORE Systems, ForeRunner, PowerHub, ForeThought, ForeView and AVA areregistered trademarks of FORE Systems, Inc. All Roads Lead To ATM, Application Aware, ASN, ATV, CellChain, CellPath, CellStarter,EdgeRunner, FramePlus, ForeRunnerHE, ForeRunnerLE, Intelligent Infrastructure, I2, 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 otherbrands or product names are trademarks of their respective holders. Please be advised that technical specifications set forth in thisproduct literature are correct as of September 1998 and are subject to change by FORE Systems, Inc. at any time without notice.


rev: 99feb03

Documents

DWDM