y receive a busy tone, and they have to Introduction to Telecommunications Network Engineering try again. The essential question is this: How much higher should the capac- ity be for the subscribers to feel that the grade of service is acceptable? The starting point is how often subscribers are allowed to be blocked and receive a busy tone. This probability of blockage for an acceptable GoS is usually set to be in the range of 0.2% to 5%, which means that every 500th to 20th call is blocked during a busy hour. When the average traffic load is estimated to increase to a certain volume, the network operator should increase the network capacity to keep the blocking probability below the defined GoS level. The Poisson distribution is used as a probability model for these calcu- lations and it gives a probability for occurrence of X events when the average number of events is A according to this formula: P(x)=A"e-A (2.1) where e = 2.71828 and x! is the factorial of X, 1.2.3. X. Now the average number of occupied channels is A in erlangs and (2.1) gives the probability that X number of channels is occupied at a time when a subscriber makes a call. Blocking occurs if all n channels are occupied or there may even be a need for a larger number of channels. This probability is given by: P(xzn)=P(n)+P(n+1)+P(n+2)+ (2.2) On the other hand, one number of channels is always in use, giving this probability for P(0)+P(1)+P(2) + +P(n+1) ..=P(x<n)+P(xzn)= = 1 (2.3) and we change (2.2) into this form: P(x2n)=1-P(x<n) (2.4) Substituting (2.1) gives P(xZn)=1-[P(0)+P(1)+..+P(n-1)] (2.5) The Telecommunications Network: An Overview Now we have the Poisson formula, which is also known as the Molina lost calls held trunking formula, for blocking probability and it is as follows: A*e-A (2.6) To determine the grade of service, blocking probability, we compute (using the Poisson distribution) the probability that not one channel is free when a subscriber makes a call. For this we take a value of the average (total) offered traffic as A and c