.0 100.7 105.5 115.1 133.1 218.0 118.0 122.0 127.0 130.6 136.4 148.1 170.5 278.0 Introduction to Telecommunications Network Engineering Table 2.1 (continued) 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 174.6 179.7 186.2 270.4 277.1 285.7 367.2 375.2 385.9 464.5 474.0 486.4 Note in Table 2.1 that when high blocking probability is allowed, offered traffic may be higher than the number of available channels. A part of offered traffic is blocked and actual traffic, that part which is not blocked, naturally never gets a higher value than the number of channels in erlangs. Blockage probability can be calculated in many different ways. Table 2.1 is calculated according to erlang B formula, which assumes that blocked calls are immediately cleared and a subscriber waits and makes a new call later [3]. It gives slightly more optimistic results than the Poisson for- mula in (2.6). Erlang B formula is used in Europe and the Poisson formula in used in the United States for network planning. To compare the results of these two slightly different approaches, we consider an example where aver- age offered traffic A = 2 Erl. If the number of circuits n = 5, we get blocking probability P = 0.0367 according to erlang B formula instead of the 0.053 we got previously with (2.6). The erlang B formula is [2] as follows: (2.7) 2.13 Problems and Review Questions Problem 2.1 Describe how dialed digits are transferred from a subscriber's telephone to the local exchange. The Telecommunications Network: An Overview Problem 2.2 Explain how the telephone attenuates the speaker's voice from the micro- phone to the earphone. (Hint: Draw the current coming from the micro- phone in Figure 2.7 and imagine what happens to the magnetic field in the iron core of the transformer.) Problem 2.3 What is a 2W/4W hybrid and why is it needed at the end of the subscriber line? Problem 2.4 Explain how a 2W/4W hybrid prevents the signal from the network (receiv- ing pair) from looping back to the tran