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Optical Networks: A Practical Perspective - Part 68

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Optical Networks: A Practical Perspective - Part 68. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 640 Photonic Packet Switching Figure 12.17 Example of a 2 x 2 routing node using a feedback delay line architecture. for a common output port arrive simultaneously one of them is switched to the output port while the others are switched to the recirculating buffers. In the context of optical switches the buffering is implemented using feedback delay lines. In the feedback architecture of Figure 12.17 the delay lines connect the outputs of the switch to its inputs. With two delay lines and two inputs from outside the switch is internally a 4 x 4 switch. Again if two packets contend for a single output one of them can be stored in a delay line. If the delay line has length equal to one slot the stored packet has an opportunity to be routed to its desired output in the next slot. If there is contention again it or the contending packet can be stored for another slot in a delay line. Recirculation buffering is more effective than output buffering at resolving contentions because the buffers in this case are shared among all the outputs as opposed to having a separate buffer per output. The trade-off is that larger switch sizes are needed in this case due to the additional switch ports needed for connecting the recirculating buffers. For example in HK88 it is shown that a 16 x 16 switch requires a total of 112 recirculation buffers or about 7 buffers per output to achieve a packet loss probability of 10 6 at an offered load of 0.8. In contrast we saw earlier that the output-buffered switch requires about 25 buffers per output or a total of 400 buffers to achieve the same packet loss probability. In the feed-forward architecture considered earlier a packet has a fixed number of opportunities to reach its desired output. For example in the routing node shown in Figure 12.14 the packet has at most three opportunities to be routed to its correct destination in its arriving slot and the next two immediate slots. On the other hand in the feedback architecture it appears that