138 CHAPTER 6 Non-Blocking Networks
A B C D
E F G H
H D F A
B A E C
Figure 6.20 A TDM switch accepts and generates time-domain multiplexed streams and switches the con-
tents of input time slots into the contents of output time slots. For example, channel C on 0.2
(input 0, time slot 2) is switched to 1.3.
a cross connect or a grooming switch, switches in time as well as space to map input
STS-1 time slots onto output STS-1 time slots.
Figure 6.20 shows an example of TDM switching for the simpliﬁed case of two
inputs and outputs with four time slots each. This example includes both unicast
connections — for example, the B on 0.1 (input 0, time slot 1) is switched to 1.0
(output 1, time slot 0) — and multicast connections — the A on 0.0 is switched to
both 0.3 and 1.1. The VC2002 performs a similar function, but on 72 inputs with
48 time slots each.
TDM grooming switches form the bulk of most metropolitan and long-haul voice
and data communication networks.
Most of these networks are implemented as in-
terconnected sets of SONET rings (because of the restoration properties of rings).
Grooming switches act as cross-connects that link multiple rings together and as
add-drop multiplexers that multiplex slower feeds onto and off of the ring.
The conﬁguration of a TDM switch typically changes slowly except during pro-
tection events. New STS-1 circuits are provisioned at a rate of at most a few per
minute. However, when a fault occurs in the network (for example, a backhoe cuts
a ﬁber bundle), a large fraction of the 3,456 connections in a VC2002 may need to
be switched within a few milliseconds to avoid interruption of service.
A TDM switch can take advantage of the fact that the input and output streams
are multiplexed by implementing the ﬁrst and third stages of a Clos network in
the time domain, as shown in Figure 6.21. The ﬁgure shows how the 2-input by
4-time slot example of Figure 6.20 can be realized without the complexity of an
8 × 8 crossbar switch. Instead, a three-stage Clos network in which m=4, n=4 and
12. A single 51.83-Gbits/s STS-1 may contain 28 1.5-Mbits/s T1 or DS1 channels, each of which in turn
carries 24 64-Kbits/s DS0 channels, each of which carries one phone call.
13. Although people think of the Internet as being implemented with packet routers, these routers exist only
at the edges and are connected by TDM circuits between these endpoints.