By Loudon Blair, Director of Network Architecture, Ciena Corp.
Today's core traffic consists largely of STS-1, -3, -12 and -48 circuits aggregated to OC-48 or OC-192 data rates for transmission across a WDM optical network. As the intelligent optical networking industry matures, service providers are being offered the opportunity to manage this data at multiple levels of granularity, ranging from the full contents of a single fiber, to single wavelengths (or bands of wavelengths), to STS-level circuits. While the choice of switching granularity within any network depends on service type and the availability of the appropriate switching technology, the optimum choice ultimately depends on aggregate traffic demand.
A core network solution that is able to meet the key requirements of operational simplicity and flexibility at minimal cost will depend upon the chosen network architecture and the availability of mature technology to support that architecture. Eliminating electrical regenerators and improving utilization of wavelengths are two methods that can be used to make more effective use of network resources to help lower capital expenditures. The general merits of different optical switching technologies are outlined elsewhere¹ while the implications of managing bandwidth of varying levels of granularity are reviewed here for three different network architectures. The network solutions described are based on the general classifications of (i) opaque, (ii) fully transparent and (iii) partially transparent.
Opaque Architecture
With an opaque architecture, optical signals are electrically regenerated at every switch in the network. While this can result in short, non-regenerated transmission distance requirements, a carrier may need to perform multiple optical-to-electrical-to-optical (OEO) conversions for each channel across the network, leading to a high potential capital cost.
If the opaque switch offers an intelligent STS level grooming fabric, significant cost savings may be achieved. Growing traffic demands can be served by fewer wavelengths if, at each core switch, grooming is performed to maximize the utilization of each wavelength by packing multiple demands into a single, shared wavelength. In addition, the total number of optical switch ports required to support the reduced channel count will drop. Thus, the additional cost of electrical regenerators at a switch node may be compensated by more efficient wavelength utilization. Plus, by embedding software intelligence into such a switch, the service bandwidth can be effectively de-coupled from the infrastructure bandwidth. For example, an STS-level grooming switch could enable new services such as "wavelength binding," "transparent service multiplexing" and "flexible concatenation."
However, if the opaque switch comprises an optical or electrical crossbar fabric and is incapable of STS level grooming, then the above benefits cannot be realized, other than wavelength conversion, which is inherent through the use of electrical regeneration.
Fully Transparent Architecture
In the fully transparent approach, bandwidth of any granularity less than a single wavelength is groomed at the edge of the network. STS level switches are used solely to multiplex and de-multiplex data to and from wavelengths so that, in the core of the network, bandwidth is managed at the wavelength level (or integers thereof).
At first glance, the transparency of this approach is appealing. Taken in isolation, the all-optical network takes advantage of ultra-long haul transmission techniques to minimize the cost of point-to-point transport by eliminating the need for intermediate electrical regenerators. Also, the all-optical switch appears to offer potential cost savings over an opaque switch solution, again through the elimination of electrical regenerators at the network facing ports. Assuming that this architecture does not incur additional costs in order to overcome the significant engineering challenges, the combined cost advantage of all-optical transport plus all-optical switching appears compelling.
Upon review however, it is important to note these cost savings are, in fact, achieved for an optimized case where the bandwidth demand between two points in the network fully utilizes a provisioned integer number of wavelengths between those two points. Any service that requires less than a single wavelength's worth of capacity will inefficiently fill that wavelength and lead to under-utilization of the network capacity. Since efficient packing of wavelengths cannot be achieved through transit grooming in the all-optical network, any cost savings achieved through the elimination of electrical regeneration devices in the transport layer is offset by additional WDM line cards required to support under-utilized wavelengths. Consequently, more wavelengths will lead to an increase in the number of required optical switch ports at each network node.
Partially Transparent Architecture
A partially transparent architecture benefits from both opaque and transparent design approaches. By collapsing the network into regions of limited transparency, it is possible to benefit from efficient wavelength packing at opaque switches with STS level granularity, yet still achieve further cost reductions by eliminating electrical regeneration between opaque switches. Further benefits include relaxed ultra-long haul transmission distance requirements and a reduced optical switch/OADM hop count between opaque switch nodes. Such a scenario, perhaps described as an "all-optimal" architecture, would still maintain the advantage of service/infrastructure bandwidth de-coupling supported by the embedded intelligence of the STS grooming switch.
Of course, the benefit of core grooming will diminish as service bandwidths increase and the aggregate traffic demand between any two locations efficiently fills wavelengths at the ingress to the network. Today, where the average utilization of a 10 Gbps wavelength is relatively low, the STS level grooming switch offers significant port (cost) reduction benefits. Looking forward, it will be important to evolve this architecture to manage data at the granularity of a wavelength. The partially transparent network architecture described above offers this evolutionary path, working in step with current availability of mature and intelligent optical technologies.
About the Author
Loudon Blair is the Director of Network Architecture for Ciena Corporation (Linthicum, MD). For more information, visit www.ciena.com.
¹ Rick Dodd, "Optical switches: Is one technology better than the other for carriers today?", Lightwave, January 2000. For the full article, go to: http://lw.pennnet.com/Articles/Article_Display.cfm?Section=Archives&Subsection=Display&ARTICLE_ID=55599