If there are no switching paths free in the network, the call requested will be denied, where the subscriber is said to be blocked and the network is called the Blocking Network. In a blocking network, the number of simultaneous switching paths is less than the maximum number of simultaneous conversations that can take place. The probability that a user may get blocked is called the Blocking Probability. A good design should ensure low blocking probability.

Traffic

The product of the calling rate and the average holding time is defined as the Traffic Intensity. The continuous sixty-minute period during which the traffic intensity is high is the Busy Hour. When the traffic exceeds the limit to which the switching system is designed, a subscriber experiences blocking.

Erlang

The traffic in a telecommunication network is measured by an internationally accepted unit of traffic intensity known as Erlang (E). A switching resource is said to carry one Erlang of traffic if it is continuously occupied through a given period of observation.