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Optical Networks: A Practical Perspective - Part 67

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Optical Networks: A Practical Perspective - Part 67. 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. | 630 Photonic Packet Switching Biréfringent fiber Figure 12.10 Block diagram of a soliton-trapping logical AND gate. Figure 12.11 Illustration of the operation of a soliton-trapping logical AND gate a Only one pulse is present and very little energy passes through to the filter output. This state corresponds to a logical zero. b Both pulses are present undergo wavelength shifts due to the soliton-trapping phenomenon and most of the energy from one pulse passes through to the filter output. This state corresponds to a logical one. will not be selected by the filter. Thus the filter output has a pulse logical one only if both pulses are present at the input and no pulse logical zero otherwise. 12.2 Synchronization 631 Figure 12.12 The function of a synchronizer a The two periodic pulse streams with period T are out of synchronization the top stream is ahead by AT. b The two periodic streams have been synchronized by introducing a delay AT in the top stream relative to the bottom stream. 12.2 Synchronization Synchronization is the process of aligning two pulse streams in time. In PPS networks it can refer either to the alignment of an incoming pulse stream and a locally available clock pulse stream or to the relative alignment of two incoming pulse streams. Recall our assumption of fixed-size packets. Thus if framing pulses are used to mark the packet boundaries the framing pulses must occur periodically. The function of a synchronizer can be understood from Figure 12.12. The two periodic pulse streams with period T shown in Figure 12.12 a are not synchronized because the top stream is ahead in time by AT. In Figure 12.12 b the two pulse streams are synchronized. Thus to achieve synchronization the top stream must be delayed by AT with respect to the bottom stream. The delays we have hitherto considered for example while studying optical multiplexers and demultiplexers have been fixed delays. A fixed delay can be achieved by using a fiber of the appropriate length. .