avelength-division multiplexing (WDM) is currently being deployed in telecommunications networks in order to satisfy the increased demand for capacity brought about by both narrowband services and new broadband services such as high-speed Internet. While it is thought that WDM will ultimately evolve to interconnected rings or perhaps a mesh network, the objective of the Wavelength Switched Packet Network (WASPNET) project is to gain a more long-term understanding of how optical networks will develop. WASPNET is a WDM transport network that uses optical packet switching, resulting in greater flexibility, functionality, and granularity than possible with the current generation of WDM networks. These optical packets may be used to carry asynchronous transfer mode (ATM) or IP, for example, and the network is also designed to support synchronous digital hierarchy/synchronous optical network (SDH/SONET) traffic, thus permitting a smooth upgrade path. Optical packet switches [1-3] have attracted considerable research interest internationally due to their potential for overcoming projected difficulties with very large electronic switching cores, such as connection, pinout, and electromagnetic interference (EMI) problems. A key problem when designing packet switches of any kind is contention resolution, since multiple packets may arrive asynchronously at the same time to go to the same output. Buffering is often employed to solve this problem, but since optical random access memory (RAM) does not exist, delay lines (usually made of optical fiber) must be used to store optical packets and implement buffering. Various solutions to optical packet switching have been proposed, dictated by the buffering strategy [1]. Implement Medium to Large Buffers-The switches implemented by this technique may be cascaded to implement very large buffers, suitable for bursty traffic. Use No Buffers in the Switch Nodes, but Employ Deflection Routing-When multiple packets arrive destined for a given output, all but one are "deflected" to other outputs, to find their way to the destination by another route through the network. This not only provides fast and flexible routing, but also allows nodes to have no buffering. However, each packet transmitted from a node may be routed across a different path to the same destination. Some packets may wander within the network and waste bandwidth. Consequently, each packet will experience different propagation delays, and the traffic may not arrive at the destination node in sequence. Compromise by Using a Small Amount of Buffering with Deflection Routing-There are various such 2 x 2 buffered switches consisting of a chain of 2 x 2 switch devices and delay lines.

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This paper describes the development of an optical packet transport network, known as WAvelength Switched Photonic NETwork (WASPNET) -a collaboration between Strathclyde, Essex and Bristol Universities as well as BT, Fujitsu and GPT. One of its main objectives is to reduce packet contention at each node. Normally, this is resolved using node deflection routing or optical delay loops (i.e. the solution is focused at the node design strategy). However in WASPNET, this problem is considered not only as a node design problem but also as a network control and management issue. Although suitable node design can reduce packet loss performance, an appropriate network control can reduce the probability of contentions, hence, improve the network throughput and node cost. This suggests that the network management strategy also influences the node design. A possible network control methodology, the SCatteredWavelength-Path (SCWP), has been identified to support WASPNET implementation. The paper presents some of the comparison studies that were carried out. These include comparing its limitations, control complexity, packet loss performance and buffer requirements against another technique -the SHared-Wavelength-Path (SHWP). It highlights solutions to problems encountered by the SCWP. Although the studies performed were intended for WASPNET transport system, the fmdings are invaluable for those involved in WDM network design.

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Recently, many optical packet switches have been proposed, to overcome to potential problems of future large electronic switch cores. The need for buffering arises due to the unscheduled nature of packet arrivals at the switch inputs, and several strategies have evolved to implement this buffering. Examples of these are discussed in this paper, including the use of wavelength to assist in contention resolution; where contending packets can usually be transmitted on different wavelengths. Also, the implementation of multi-stage buffers is discussed, using differing technologies, both with and without deflection routing.