fic as well as its mean value. Methods for determining the numbers of high-usage and tandem circuits required have been developed (Wilkinson, 1956; Wall- strom, 1969; Rapp, 1964). Results obtained from analytical formulae may be inaccurate in practical cases, particularly when complex alternative-routing schemes are involved, because the assumptions have been over- simplified. The alternative is to obtain results by means of a com- puter simulation of the system under study (Bear, 1988). Public network operators have extensive computer programs for this pur- pose. Other programs, which may be used in planning private networks are available commercially. Total traffic (Earlangs) 29.10 References Figure 29.15 Trunk occupancy for full availability groups of various sizes (groups designed to give same grade of service Andrews, F.T. and Hatch, R.W. (1971) National Telephone Network Transmission Planning in the American Telephone and Tele- graph Company, Trans. IEEE COM-19, p. 302. overload. For small groups of trunks the former criterion determines the number required, whereas the latter criterion determines the Banerjee, V., Rabundrakumar, K., and Szczech, B.J. (1989) London Code Change, Brit. Telecommun. Eng. 8, (3), p. 134. number of trunks when the traffic is high and the number of trunks large. For example, if it is specified that the grade of service shall Barron, D.A. (1959) Subscriber Trunk Dialling, Proc. IEE 106B, p. be 0.002 but not deteriorate beyond 0.01 when the traffic increases Bealby, A.G. (1990) Network Administration Implementation Pro- by 10%, then Figure 29.16 shows that the former criterion is the gramme, Brit. Telecom. Eng. 8 (4) p. 212. more stringent when the number of trunks is less than 70 but the Bear, D. (1988) Principles of Telecommunication Traffic Engineer- overload criterion applies far more than 70 trunks. Many public ing, Peter Peregrinus. network operators therefore use traffic tables based on such dual CCITT (1989a) Recommendation E163, Numbering Plan for the criteria. BT adopted t