Until this point, we have focused on packet-switched networks, but for this chapter,
we shift our attention to circuit-switched networks. In circuit-switched networks,
connections between a particular source and destination are ﬁrst set up, then held
as data ﬂows continuously through the connection, and then ﬁnally torn down.
Historically, non-blocking, circuit-switched networks were ﬁrst associated with the
telephone system. A call placed from one person to another represented a single con-
nection request. For the call to go through, an unused path through the network had
to be found and then allocated for that call. If the telephone system was non-blocking,
then the call would always succeed as long as the recipient’s phone was not in use.
More precisely, a network is said to be non-blocking if it can handle all circuit re-
quests that are a permutation of the inputs and outputs. That is, a dedicated path can
be formed from each input to its selected output without any conﬂicts (shared chan-
nels). Conversely, a network is blocking if it cannot handle all such circuit requests
In this chapter, we examine two types of non-blocking networks. First, a network
is strictly non-blocking if any permutation can be set up incrementally, one circuit at
a time, without the need to reroute (or rearrange) any of the circuits that are already
set up. If any unused input can be connected to any unused output without altering
the path taken by any other trafﬁc, then the network is strictly non-blocking.
In contrast, a network is rearrangeably non-blocking (or simply rearrangeable)
if it can route circuits for arbitrary permutations, but incremental construction of
1. Chapter 12 includes a detailed description of both packet and circuit switching.
2. The U.S. phone system is obviously blocking to any person who has ever received the “all circuits are busy”