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Optical Networks - _12_4 Buffering_136

Optical Networks - _12_4 Buffering_136 - 674 Photonic...

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674 Photonic Packet Switching process them very quickly. This may not leave much room for sophisticated header processing. See Problem 12.5 for an example. 12.4 Buffering In general, a routing node contains buffers to store the packets from the incoming links before they can be transmitted or forwarded on the outgoing links—hence the name store and forward for these networks. In a general store-and-forward network, electronic or optical, the buffers may be present at the inputs only, at the outputs only, or at both the inputs and the outputs, as shown in Figure 12.2. The buffers may also be integrated within the switch itself in the form of random access memory and shared among all the ports. This option is used quite often in the case of electronic networks where both the memory and switch fabric are fabricated on the same substrate, say, a silicon-integrated circuit, but we will see that it is not an option for optical packet switches. We will also see that most optical switch proposals do not use input buffering for performance-related reasons. There are at least three reasons a packet has to be stored or buffered before it is forwarded on its outgoing link. First, the incoming packet must be buffered while the packet header is processed to determine how the packet must be routed. This is usually a fixed delay that can be implemented in a simple fashion. Second, the required switch input and/or output port may not be free, causing the packet to be queued at its input buffer. The switch input may not be free because other packets that arrived on the same link have to be served earlier. The switch output port may not be free because packets from other input ports are being switched to it. Third, after the packet has been switched to the required output port, the outgoing link from this port may be busy transmitting other packets, thus making this packet wait for its turn. The latter delays are variable and are implemented differently from the fixed delay required for header processing. The lack of good buffering methods in the optical domain is a major impediment. Unlike the electronic domain, we do not have random access memory in the optical domain. Instead, the only way of realizing optical buffers is to use fiber delay lines, which consist of relatively long lengths of fiber. For example, about 200 m of fiber is required for 1 μ s of delay, which would be sufficient to store 10 packets, each with 1000 bits at 10 Gb/s. Thus usually very small buffers are used in photonic packet-switching networks. Note that unlike an electronic buffer, a packet cannot be accessed at an arbitrary point of time; it can exit the buffer only after a fixed time interval after entering it. This is the time taken for the packet to traverse the fiber length. This constraint must be incorporated into the design of PPS networks.
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12.4 Buffering 675 Figure 12.15 Example of a 2 × 2 routing node using a feed-forward delay line archi- tecture.
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Optical Networks - _12_4 Buffering_136 - 674 Photonic...

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