Optical Networks - _12_1 Optical Time Division Multiplexing_133

Optical Networks - _12_1 Optical Time Division Multiplexing_133

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658 Photonic Packet Switching We start this chapter by describing techniques for multiplexing and demultiplex- ing optical signals in the time domain, followed by methods of doing synchronization in the optical domain. Synchronization requires delaying one stream with respect to the other if they are misaligned in time. In this context, we will also study how tunable optical delays can be realized. We then discuss various solutions for dealing with the buffering problem. We conclude the chapter by discussing burst switching, a variant of PPS, and some of the experimental work that has been carried out to demonstrate the various aspects of PPS. 12.1 Optical Time Division Multiplexing At the inputs to the network, lower-speed data streams are multiplexed optically into a higher-speed stream, and at the outputs of the network, the lower-speed streams must be extracted from the higher-speed stream optically by means of a demultiplex- ing function. Functionally, optical TDM (OTDM) is identical to electronic TDM. The only difference is that the multiplexing and demultiplexing operations are per- formed entirely optically at high speeds. The typical aggregate rate in OTDM systems is on the order of 100 Gb/s, as we will see in Section 12.6. OTDM is illustrated in Figure 12.4. Optical signals representing data streams from multiple sources are interleaved in time to produce a single data stream. The interleaving can be done on a bit-by-bit basis as shown in Figure 12.4(a). Assuming the data is sent in the form of packets, it can also be done on a packet-by-packet basis, as shown in Figure 12.4(b). If the packets are of fixed length, the recognition of packet boundaries is much simpler. In what follows, we will assume that fixed-length packets are used. In both the bit-interleaved and the packet-interleaved case, framing pulses can be used. In the packet-interleaved case, framing pulses mark the boundary between packets. In the bit-interleaved case, if n input data streams are to be multiplexed, a framing pulse is used every n bits. As we will see later, these framing pulses will turn out to be very useful for demultiplexing individual packets from a multiplexed stream of packets. Note from Figure 12.4 that very short pulses—much shorter than the bit interval of each multiplexed stream—must be used in OTDM systems. Given that we are interested in achieving overall bit rates of several tens to hundreds of gigabits per second, the desired pulse widths are on the order of a few picoseconds. A periodic train of such short pulses can be generated using a mode-locked laser, as described in Section 3.5.1, or by using a continuous-wave laser along with an external modu- lator, as described in Section 3.5.4. Since the pulses are very short, their frequency
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12.1 Optical Time Division Multiplexing 659 Packet-multiplexed stream Bit-multiplexed stream 0 11 1 01 1 1 00 Bit- interleaved time division multiplexer (a) 0 1 1 1 Packet- interleaved time division multiplexer (b) Framing pulses Framing pulses t Figure 12.4 (a) Function of a bit-interleaved optical multiplexer. (b) Function of a
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Tech.

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Optical Networks - _12_1 Optical Time Division Multiplexing_133

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