users and occupied and at least one all 3 channels are occupied. of 3 channels is free Blocking probability: P(x<n) = P(0) + P(1) + P (2) P(x>=n) = P(3) + P(4) + P(x<n) = 1 - (P(0) + P(1) + P (2)] Figure 2.25 Probability of blocking. transmission lines, is 5 (n=5). = Then the probability of blockage is 5.3 % [P(x > 5) = 0.053]. This means that, on average, during the busy hour every ninteenth call is blocked, a busy tone is heard, and the subscriber has to redial. When the number of channels or servers n is high, precalculated tables like Table 2.1 are used for network planning. Such a table gives the required number of servers n when the GoS (= blocking probability in our study) and estimated offered traffic intensity A are given. For example, if the GoS is set to be 2% and offered traffic is 5 Erl (e.g., 100 subscribers with average offered traffic intensity of 50 mErl per subscriber), the network capacity should support at least 10 simultaneous calls (n = 10). If the capacity is smaller, for example, n = 9, we get 2% with offered traffic 4.34 Erl; an offered traffic 5 Erl would give a higher blocking rate. When we look at Table 2.1, we note that when the number of servers is small, offered traffic intensity is of the order of one-tenth of the maximum traffic intensity. For example, with two channels, offered traffic intensity at blocking probability 1% is only 150 mErl. Only one 9-minute call during an hour is allowed and both lines may be occupied on average only 4.5 minutes in an hour. The utilization of channels is less than 10%. When the number of servers is high, allowed average traffic intensity is close to the maximum or even higher. Even when most of the channels are occupied, some channels are still free for a new call because the number of channels is large. The Telecommunications Network: An Overview Table 2.1 Network Capacity Planning Blocking Probability, GoS 0.5% A 1.0% A 2.0% A 3.0% A 5.0% A 10% A 20% A 50% A 108.1 118.1 128.1 138.1 148.1 158.1 102.0 168.0 108.2 178.0 114.4 188.0 104.1 120.6 198