National-scale networks likely to be opaque

National-scale networks likely to be opaque

"Transparency" has become a buzzword in networking discussions. But complete transparency may prove neither feasible nor desirable.

E.L. GOLDSTEIN, J.A. NAGEL, J.L. STRAND, R.W. TKACH, AT&T Labs--Research

Wavelength-division multiplexed (wdm) point-to-point transmission is fast emerging as the structural cornerstone upon which national-scale core communications will be built. What is unknown, however, is whether the nodes of these wdm networks will be transparent--meaning that signals will propagate from source to destination through intervening nodes without optoelectronic conversion. We argue that civilian communication needs will in fact drive the core network not to transparent, but to opaque form,1 with critical reliance on an emerging class of small, inexpensive optoelectronic converter: the transponder.

Two reasons can be cited for this: First, transparency is on balance a liability in national-scale networks, and second, the alternative to transparency--opacity--is on balance a virtue in such networks. These facts lead one to a broad class of opaque optical-crossconnect networks whose optical-transmission, network-management, and interoperability features make them--unlike transparent systems--eminently engineerable on a national scale.

Pros and cons of transparency

There are two venerable arguments for building transparent wdm networks. The first concerns format indepen- dence--transparent wdm networks can transport analog or digital or unspecified modulation formats on separate wavelengths of a single network. This is true in principle and constitutes an elegant feature of wdm optics. It is our working assumption, however, that the case for network transparency will not rest on this form of format independence. Global momentum toward digital intensity-modulated systems in general, and toward Synchronous Optical Network (sonet) and Synchronous Digital Hierarchy protocols and bit rates in particular, has become overwhelming. It is critical that a single network be capable of transporting diverse services--sonet, Asynchronous Transfer Mode (atm), and Internet protocol (IP)-based--at a restricted set of specified standard bit rates.

The second argument concerns node bypass--transparent wdm networks readily allow express signals to bypass extensive electronic processing in intermediate nodes. This is true for nodes consisting of transparent fixed-wavelength add/drop multiplexers (see Fig. 1a). It is a relatively small step to add reconfigurability (see Fig. 1b). From a transmission point of view, it is a substantial yet natural step to consider networks of transparent, reconfigurable wavelength-selective crossconnects (see Fig. 1c). All of the depicted network elements provide node bypass, with its attendant prospects for cost reduction.

Transparency is thus an appealing feature because it allows signals to propagate at high power levels through long chains of losses. But transparency also lets small degradations do likewise. This exposes the system to a familiar but ever-expanding menagerie of cumulative performance degradations. The causes of these degradations include chromatic and polarization-mode dispersion; optical-fiber nonlinearities; polarization- dependent loss; multipath interference; wavelength-misalignment of lasers and wdm filters; wdm-filter passband-narrowing; component crosstalk; noise- accumulation in ideal, flat-gain optical amplifiers; and a variety of gain-shape, cross-saturation, and compression effects in fiber amplifiers.

These accumulating impairments have recently been chronicled in voluminous literature (see Refs. 2-6, for instance) to which we add the following three observations:

Impairments scale with network reach--First, even when considered in isolation, the above effects impose on the components of transparent networks stringent requirements that scale with reach. Inband crosstalk alone, for example, in a network corrupted by a single leakage path and operating at 2.5 Gbits/sec per wavelength, requires component crosstalk values smaller than -20 dB.4 This in itself is not a trivial demand, but it steadily tightens with system reach. Thus, a network corrupted by 32 leakage paths--not a large number in a national-scale system--requires component crosstalk values smaller than -40 dB (see Fig. 2).7 For each of the other impairments listed above, a transparent network will suffer similar accumulation, and thus a similarly relentless tightening of component requirements.

Transparency constrains capacity--Transparency in wdm networks in general carries either performance costs or dollar costs or both, and these are offset only by savings in optoelectronic converters. This is evident from the accumulation of spontaneous-emission noise alone. Consider a transparent network whose amplifiers are ideal, with quantum-limited noise figures of 3 dB, flat-gain spectra that precisely compensate fiber loss, perfectly equalized input-signal power-levels, and fairly aggressive amplifier output-power levels of +7 dBm/channel. Assume an operating margin of 6 dB. In such an ideal system, due to spontaneous noise-accumulation alone, the supportable per-channel capacity declines with system length as shown in Fig. 3. Thus, a network with ideal amplifiers spaced at 80 km, operating at 10 Gbits/sec per channel, will suffer intolerable noise buildup after about 3200 km if the nodes are loss-free; finite node loss will necessitate proportionate reach reduction. Further cumulative impairments--transient cross-saturation, intolerance to loss-variations, and non-flat gain spectra--suffered by real amplifiers will tighten these constraints.6

If you wish to scale a transparent network beyond these limits, there are only two options: Accept degraded noise performance or decrease the repeater spacing below 80 km, as indicated in Fig. 3. Network operators have thus far resisted this because of associated costs.

Impairments interact--Finally, the above comments consider only a single impairment acting alone. In a deployed national-scale transparent network, however, a number of cumulative impairments will act in concert. Recent research shows that this raises two serious concerns.

First, it has been found that two types of cumulative degradation that are individually benign can impose unexpectedly severe impairments when acting in concert. For example, the nonlinear-index-induced broadening in a long-haul network segment, when transparently passed to a conventional-fiber local- exchange network, has been observed to produce severe dispersive error floors. These arise even in a system operating at less than 15% of its theoretical dispersion limit.8 Moreover, such floors have been observed even for signals whose nonlinear broadening was wholly indiscernible using laboratory-grade optical-spectrum analyzers. This latter observation suggests that it will be difficult to create signal-quality interface standards that can both guarantee performance in transparent long-haul networks and be verifiable using field- deployable optical diagnostic equipment. If this is so, it is unclear how national-scale transparent networking would be made to work in practice.

Transparency in fact constrains the upgradability of the network in two ways. First, as noted above, a transparent network has, by definition, lower capacity than a regenerated network. Second, a transparent net- work must standardize technology choices at the outset and cannot exploit, for example, developments in broader amplifier bandwidths or narrower channel spacings as they become available. Opaque networks by contrast can employ the best technology available in each system that is added, with no barrier to interconnecting systems with different technologies. The various transmission systems entering a node need not use the same channel wavelengths, number of channels, or even modulation formats. This easy adoption of new technology and kind of "technology transparency" represents one of the most powerful features of opaque optical networking.

We expect the above considerations to have the following impact. Both fixed and reconfigurable varieties of transparent wavelength-add/drop multiplexing (Figs. 1a and 1b) are currently on the brink of deployment over limited (not national-scale) domains. Reconfigurable wavelength-selective crossconnect networks (Fig. 1c), however, even without wavelength translation, are expected to prove feasible on a national scale only by sacrificing either network cost or network performance or both. Moreover, if transparent wavelength-translation is needed in such networks, this will require adding a feature that has not yet been demonstrated in plausible form in the laboratory.

Opacity considerations

An obvious but often overlooked fact is that the full variety of functional network elements in Fig. 1 is readily implemented in opaque form. This is indicated in Fig. 4, which illustrates an opaque reconfigurable crossconnect that operates, for concreteness, at an OC-48 (2.5-Gbit/sec) line rate.

Opacity in this form exhibits exactly one bad feature: It requires numerous transponders--optoelectronic converters that transform, in the example, long-reach 1.5-micron wdm signals at OC-48 rates into cross-office OC-48 standards-compliant form at 1.3 microns, and vice versa. These transponders are a current feature of many commercial wdm systems, providing adaptation of sonet equipment to the particular wavelengths required. Notwithstanding their virtues, transponders are admittedly less than ideal in two respects. First, they are not free--although their precipitously dropping price trajectories, due partly to miniaturization, show no signs of leveling off. Second, transponders retime and reshape and are thus bit-rate-specific: Increasing the bit rate requires transponder upgrade.

In all other respects, opacity is no vice in optical-crossconnect networks. Opaque crossconnects, such as that in Fig. 4, arrest cumulative transmission impairments through effectively complete signal-waveform cleanup. They facilitate performance-monitoring and fault-detection by monitoring, as desired, any of various sonet signal-quality indicators (e.g., loss-of-signal and loss-of-frame) and parity-check bytes, and communicating the results to the optical-crossconnect system. Moreover, opaque optical crossconnects ease the task of fault location since, by contrast with transparent architectures, their transponders offer a means of generating the alarm-insertion signals upon which downstream alarm-suppression ordinarily relies.9-11 Beyond this, transponders not only ease the task of topology self-identification by providing a way to insert identifiers, but also provide the function of wavelength-translation at no incremental cost.1

Finally, and perhaps more important than all of the preceding considerations, multipoint wdm networks will not be deployable unless they are segmented into subsystems that terminate on open, nonproprietary signal interfaces. In fact, transparent wdm long-haul networks embody no such interfaces; moreover, for reasons mentioned above, the fundamental transmission-performance knowledge that would be required to specify such interfaces is not currently within reach. Opaque networks, by contrast, are fundamentally segmented into precisely such open interfaces by the 1.3-micron cross-office ports on their transponders. Since such open interfaces arise at the boundary between each wdm transport system and the optical crossconnects on which its endpoints terminate, the critical issue of multivendor interoperability effectively vanishes.

Although it is possible to arrange opaque optical crossconnects in various network topologies, mesh-configured networks provisioned with suitable spare restoration capacity are expected to provide a particularly robust platform for supporting a diversity of services. These services could include both circuit-switched and packet-switched offerings, at information rates up to and including the line rate itself, with flexible, resource-efficient restoration. Because such a network is indifferent to whether its constituent wavelengths carry sonet, atm, or IP-based services, it offers precisely the form of format transparency that is of chief practical interest.

The technological heart of such a network is an N ¥ N space switch--either optical or electronic--with port count N expected to number initially in the tens in core long-haul-network applications, with growth to hundreds anticipated. Although this exceeds the capabilities of current prototype switches, the scaling of port count is not expected to pose a serious hurdle. This is in part because partitioning the network into point-to-point interoffice links frees it of accumulating impairments, thus greatly relaxing requirements on the switch fabric. Moreover, large port counts may be realized in less than fully connected crossconnects.12 As a result, network-element cost will likely be dominated not by the switch fabric itself, but by its associated control system. This system will in turn be simplified by the coarse (OC-48- or OC-192-level) granularity at which the traffic is managed and restored.

Summary

Even when considered individually, the transmission performance, cost, and network management obstacles associated with transparency make it unlikely that a transparent, reconfigurable wavelength-selective crossconnect network could be engineered on anything approaching a national scale. When these considerations are taken collectively, the case seems quite overwhelming. By contrast, the corresponding virtues of transponder-based systems in general, and of networks with open interfaces in particular, suggest that opaque wdm optical-crossconnect-based networks will likely provide the functional value of their transparent counterparts while being feasible on a national or global scale. u

Acknowledgments

The approach described here owes much to the thinking of Tom Afferton, Krishna Bala, Laurel Clark, Bob Doverspike, Lara Garrett, Ezhan Karasan, Lih Lin, Adel Saleh, and Jane Simmons.

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Fig. 4. An opaque reconfigurable crossconnect may be either optical or electrical in function.