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.
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,
be used. In the packet-interleaved case, framing pulses mark the boundary between
packets. In the bit-interleaved case, if
input data streams are to be multiplexed,
a framing pulse is used every
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