measures are not considered here. We instead con- centrate only on the evaluation of the blocking probability. In Figure 2.24 blocking occurs if more than n subscribers make external calls at a time. For the probability of unsuccessful calls, operators define the target value, the highest probability of an unsuccessful call that they assume to be acceptable for their customers. The smaller this probability is, the more capacity they have to build into the network. Another factor we could use to define GoS is how long the subscriber has to wait until the service becomes available. We could design the network to keep customers in a queue until, for example, a transmission channel becomes free. This factor is also essential to those who plan the telephone service where a person answers incoming calls (e.g., switchboard service of an enterprise, customer service telephone). 2.12.2 Busy Hour Network capacity planning is based on the so-called busy hour traffic inten- sity, and at other times the GoS is typically much better. Busy hour is an hour in the year when the average traffic intensity gets the highest value. To be accurate, the busy hour is determined by first selecting the 10 working days in a year with the highest traffic intensity; four consecutive 15-minute peri- ods (of those 10 days) with the highest traffic intensity make up the busy hour. The basic goal is to find a minimum capacity that gives the defined grade of service. Figure 2.24 shows a local exchange with a number of sub- scribers and a much smaller number n of trunk lines to the next exchange. If more than n subscribers make an external call at a time, some of them are Subscriber Trunk lines lines, n channels Local telephone exchange Figure 2.24 Local exchange and blocking. The Telecommunications Network: An Overview blocked and they have to try again. The number of external calls varies in a random manner and to be sure that blocking never occurs n should be equal to the number of subscribers. This is a far too expensive solution because the nu