mber of subscribers connected to a local exchange is usually very large and on average only a small portion of them place external calls at the same time. The principle of how to find the capacity, that is, the number of lines n in our example, that is economically feasible but acceptable from subscribers' points of view is explained next. 2.12.3 Traffic Intensity and the Erlang The measure of traffic intensity for circuit-switched connections is called the erlang in honor of the Danish mathematician A. K. Erlang, the founder of traffic theory. The erlang unit is defined as (1) a unit of telephone traffic specifying the percentage of average use of a line or circuit (one channel) or (2) the ratio of time during which a circuit is occupied and the time for which the circuit is available to be occupied. Traffic that occupies a circuit for 1 hour during a busy hour is equal to 1 erlang. Consider these examples: If the traffic intensity of a subscriber line is 1 erlang, the line is occu- pied for 60 minutes in an hour. If a subscriber line is in use 6 minutes out of an hour (on average), the traffic intensity is 6 minutes/60 minutes or 100 mErl. The maximum traffic intensity of a 2-Mbps (30 PCM channels) line system is 30 erlangs, that is, all channels are in use 60 minutes during the busy hour. The typical average busy-hour traffic volume generated by one sub- scriber is in the range of 10 to 200 mErl. Low values are typical for residential use and high values for business subscribers. 2.12.4 Probability of Blocking The problem in traffic engineering is determining the capacity if the average offered traffic intensity is known (or estimated). The term offered traffic refers to the average generated total traffic including the traffic that is blocked in the system. Clearly the capacity should (at least usually) be higher than offered traffic; otherwise, many users would not be able to get service because all lines would be occupied all the time (on average). If all trunk lines are occupied, new users are blocked, the