Advances in Photonic Packet Switching: An Overview <s> Wavelength Conversion <s> Operation without packet alignment of an all-optical packet switch is proposed and predicted feasible through a detailed traffic analysis. Packet alignment units are eliminated resulting in a simple switch architecture while optimal traffic performance is maintained through the flexibility provided by WDM. <s> BIB001 </s> Advances in Photonic Packet Switching: An Overview <s> Wavelength Conversion <s> Wavelength conversion has been shown to reduce the probability of blocking in both circuit-switching and packet-switching wavelength routed optical networks (WRONs). The effectiveness of the blocking reduction depends on the topology, and is known to be best for meshed topologies, where the average number of hops per path is large. This paper shows that by exploiting wavelength conversion, routing without buffers, known as hot-potato, becomes an interesting option for packet switching WRONs with meshed topologies, such as Manhattan Street (MS) Network and ShuffleNet (SN). The results show that, by using more than 4 wavelengths, a 64 node MS or SN network can work at full load with a hop delay within one hop from its lowest achievable value. We also show that using delay-line routing buffers at the node is a much more effective way of reducing blocking than using wavelength conversion. <s> BIB002
Optical buffering and deflection routing could be regarded as deflection in general, one in time domain and the other in space domain. With today's enabling technology in WDM, the wavelength domain presents us one more dimension of solution. Both buffering and deflection have their advantages and disadvantages: buffering offers a better network throughput but involves more hardware and controls, deflection is easier to implement but cannot offer an ideal network performance. When combined with wavelength conversion, their disadvantages could be overcome or minimized, therefore giving the network designer more choice and flexibility. In this section we will examine some interesting combinations. In a switch node applying wavelength conversion and buffering, the input stage demultiplexes wavelength channels and the wavelength converters locate available wavelengths for certain output ports. The nonblocking space switch selects the output port or the appropriate delay-line. Here the buffer may consist of a series of delay-lines with different lengths. Wavelength conversion combined with optical buffering can also be incorporated in an asynchronous network. An example of an optical packet switch block without packet alignment is described in BIB001 . It is very similar to the broadcast and select architecture proposed in the KEOPS project, except that it is expanded for WDM operation with wavelength conversion. Since the switch is made of optical gates, it is fully non-blocking and can be configured incrementally making the architecture ready for asynchronous operation. Wavelength conversion has been shown to reduce the number of optical buffers or reduce packet loss probability. When the nodes are provided with a number of optical receivers/transmitters equal to the number of wavelengths, hot-potato routing in conjunction with wavelength conversion becomes an interesting option for mesh topologies such as Manhattan Street network and ShuffleNet BIB002 . It has been shown that delay-lines are more effective to solve contention than wavelength conversion. However, wavelength conversion provides noise suppression and signal reshaping. Therefore, whether to use wavelength conversion or not depends on the specific network. In a network with only a small number of wavelengths, buffering might be more desirable. In a network with a large number of wavelengths and full wavelength conversion, buffers may not be necessary. There are several possible combinations of optical buffering and wavelength conversion with store-andforward routing or deflection routing. It presents an open research problem to decide which scheme offers a low implementation cost, low packet delay, low packet loss ratio, high network throughput, etc., depending on the network specifications.