The Case for Opaque Multiwavelength Optical Networks

Wavelength-division multiplexing (WDM) now seems certain to play a leading role in public telecommunications networks of the future. What is unclear, however, is whether these WDM networks will be "transparent," i.e., whether their signals will travel from source to destination through a sequence of intervening nodes with little or no optoelectronic conversion along the way. In this work we consider "opaque" WDM networks, in which network elements include optoelectronic conversion and signal regeneration along the path, thus preventing accumulation of impairments. There are two chief arguments for building transparent WDM networks. The first is the argument from "format-transparency"the ability to transport signals in analog or digital or unspecified modulation formats on separate wavelength channels. This is a remarkable feature of transparent WDM networks. However, given the current momentum in public networks towards digital formats in general, and towards SONET and ATM in particular, it seems unlikely that the case for network-transparency will rest on formabtransparency alone. The second argument for transparency turns on the ability of a transparent WDM network to provide high-speed nodebypass and to reconfigure in response to failures and changing traffic conditions. Here we argue that opaque WDM networks also offer these same advantages, but without the complexity and cost associated with transparency. Moreover, we show that in opaque VJDM networks, wavelength-translation can be added nearly for free. The argument for transparency is often made by referring to some variant of the node architecture in Fig. 1, which contains a wavelength add-drop multiplexer (W-ADM) with fixed optical routing. We assume that the W-ADM resides in an arbitrary ring or bus network, and that each wavelength transports a highspeed signal at rates on the order of OC-48 or above. The transparent node of Fig. l a has a weighty virtue: for wavelengths 1-4, which simply transit the node, no high-speed tra.nsmitters or receivers are needed. This offers potential cost savings over the opaque node of Fig lb. However, the transparency of Fig. la also carries costs. The optical amplifiers will require gain-equalization, which they will not require in opaque implementations. Fiber dispersion and nonlinearity will often make it difEcult or impossible to use existing, installed fiber. Component-crosstalk requirements will be stringent. The network's wavelength-multiplexers must have broad, flat passbands to avoid the transmissionspectrum-narrowing that results from cascaded non-flat filters. Moreover, passband-narrowing, passbandmisalignment, and fiber dispersion will impose tight requirements on the frequency-registration and spectral purity of the laser transmitters. Finally, transparency introduces a host of new operations and management challenges, including the tasks of network monitoring and partitioning transmission impairments. Trans parent networks will thus require new approaches to creating operations and management systems. To summarize, transparency does offer a substantial benefithigh-speed nodebypassbut it offers this at substantial cost. Node-bypass does not require transparency. The opaque node of Fig. l b provides the benefits of node bypass without incurring any of the casts listed above. This is chiefly because the digital receiver/transmitter pairs in channels 1-4 of Fig. l b prevent signal impairments from acm"mting through the network, thus alleviating the tight performance requirements that must otherwise be placed on the network's optical components. Of course, opacity also carries a cost, namely, the cost of an added digital receiver/transmitter pair for each wavelength transiting the node. It is open to argument whether the cost of these optoelectronic converters will be more or less than the costs of transparency enumerated above. However, when wavelength-translation is added, the advantage turns clearly in favor of opacity. One implementation of a transparent W-ADM with wavelength-translation is shown in Fig. 2a. It requires both optical switches and optical wavelength-converters. Practical wavebengthconverters, however, have yet to be demonstrated. An opaque W-ADM with wavelength reconfiguration and translation is shown in Fig. 2b. It merely requires an electrical crassbar switch operating at the line rate per wavelength (e.g. OC-48) to implement wavelength-translation. The cost of such electrical switches is expected to be small compared with the cost of optical wavelength-converter. Moreover, opaque networks avoid the host of transparency difficulties listed above. We conclude that when wavelength-translabtion is needed in WDM networks, the advantage tips strongly toward opacity. For larger reconfigurable, wavelength-converting WDM crossconntcts used in arbitrary mesh networks the situation is much the same. Figure 3 represents an opaque reconfigura.ble wavelength-converting crossconnect with optical input and output ports. The opaque crossconnect terminates 4 input fibers and 4 output fibers. Each fiber transports 8 wavelengths. The network element of Fig. 3 then performs the following function: it crossconnects the 8 . 4 = 32 input WDM channels with the 32 output channels. The sole restriction is the usual one: the mapping must be one-bone. Note that wavelength-translation again presents a substantial practical challenge for transparency, but comes naturally with opacity. This is clear from Fig. 3, which shows an opaque WDM crossconnect that provides reconfigurable routing with full wavelength-translation, all of which is purchased with a modest amount of optoelectronic conversion and electronic crosspoint switching. Once again, a transparent implementation requires both optical switches and optical wavelength-converters.