Clos Architecture in OPS

The need to transmit information in large volumes and in more compact forms is felt these days more than ever before. To provide the bandwidth necessary to fulfill the ever-increasing demand, the copper networks have been upgraded and nowadays to a great extend replaced with optical fiber networks. Though initially these were deployed as point-to-point interconnections, real optical networking using optical switches is possible today. Since the advent of optical amplifiers allowed the deployment of dense wavelength division multiplexing (DWDM), the bandwidth available on a single fiber has grown significantly.

Optical communication can take place in one of the two ways - either circuit switching or else packet switching. In circuit switching, the route and bandwidth allocated to the stream remain constant over the lifetime of the stream. The capacity of each channel is divided into a number of fixed-rate logical channels, called circuits. Optical cross connects (OXCs) switch wavelengths from their input ports to their output ports. To the client layer of the optical network, the connections realized by the network of OXCs are seen as a virtual topology, possibly different from physical topology (containing WDM links). To set up the connections, as in the old telephony world, a so called control plane is necessary to allow for signaling. Enabling automatic setup of connections through such a control plane is the focus of the work in the automatically switched optical network (ASON) framework. Since the light paths that have to be set up in such an ASON will have a relatively long lifetime (typically in the range of hours to days), the switching time requirements on OXCs are not very demanding.

It is clear that the main disadvantage of such circuit switched networks is that they are not able to adequately cope with highly variable traffic. Since the capacity offered by a single wavelength ranges up to a few tens of gigabits per second, poor utilization of the available bandwidth is likely. A packet switched concept, where bandwidth is effectively consumed when data is being sent, clearly allows more efficient handling of traffic that greatly varies in both volume and communication endpoints, such as in currently dominant internet traffic.

In packet switching, the data stream originating at the source is divided into packets of fixed or variable size. In this method the bandwidth is effectively consumed when data is being sent and so allows a more efficient handling of traffic that greatly varies in both volume and communication endpoints.

In the last decade, various research groups have focused on optical packet switching (OPS), aimed at more efficiently using the huge bandwidths offered by such networks. The idea is to use optical fiber to transport optical packets, rather than continuous streams of light. Optical packets consist of a header and a payload. In an OPS node, the transported data is kept in the optical domain, but the header information is extracted and processed using mature control electronics, as optical processing is still in its infancy. To limit the amount of header processing, client layer traffic (e.g., IP traffic) will be aggregated into fairly large packets.

To unlock the possibilities of OPS, several issues arise and are being solved today. To be competitive with the other solutions, the OPS cost node needs to be limited, and the architectures should be future proof (i.e., scalable). In this context, the work of Clos on multistage architectures has been inspiring.