unt of activity factor for the subscriber is 3/60 = 5%. Call traffic is measured in units of erlang (E), named after Agner Erlang (1878-1929) a Danish mathematician who founded the field of teletraffic engineering. One erlang of traffic is defined as the amount of traffic that occupies one telephone link. In the above example the subscriber is generating 0.05 E of traffic [4]. The traffic amount depends on two factors. One is the length of a phone call, known as the holding time. This is a random process, and is defined by the average length of a call (usually denoted using u) and its probability distribution. The other is the time between two call attempts, known as call inter-arrival. This also is a random process defined by its average (usu- ally denoted using A) and probability distribution. The amount of traffic (p) is calculated by dividing average holding time by average inter-arrival time: ==///. The holding time and inter-arrival processes are illustrated in Figure 3.17, where different u's show the amount of time a call occupies a multiplex channel and A's show the time between the arrival of two phone calls as seen by the exchange. Figure 3.17 shows that at least three circuits are required for this system if no call is to be blocked. Blocking probability is of great importance in telephony as it is a measure of the quality of the telecommunications network. Network subscribers will not be satisfied with the service if too many calls are blocked, and may migrate to a different operator, which clearly has business implications. Blocking probability P is defined as the probability that an attempted call cannot find a free multi- plexed channel and is therefore blocked. It is calculated from the following Erlang-B formula: Broadband Telecommunications Technologies and Management Call i is Call i is started ended Call i+1 is Call i+1 is Call i+4 is Call i+4 is started ended started ended Call i+2 Call i+2 is is started Call i+3 is ended Call i+3 is started ended Figure 3.17 Holding time and inter-arrival p