An important characteristic of a circuit-switching device is whether it is blocking or nonblocking. Blocking occurs when the network is unable to connect two stations because all possible paths between them are already in use. A blocking
network is one in which such blocking is possible. Hence a nonblocking network
permits all stations to be connected (in pairs) at once and grants all possible connection requests as long as the called party is free. When a network is supporting
only voice traffic, a blocking configuration is generally acceptable, because it is
expected that most phone calls are of short duration and that therefore only a fraction of the telephones will be engaged at any time. However, when data processing
devices are involved, these assumptions may be invalid. For example, for a data
entry application, a terminal may be continuously connected to a computer for
hours at a time. Hence, for data applications, there is a requirement for a nonblocking or “nearly nonblocking” (very low probability of blocking) configuration.
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