 distribution of holding times. Full availability means that any much smaller. Moreover, routes are dimensioned to cater for the call can use any trunk on the route, provided that it is free. traffic forecast at the end of a planning period for which plant is Equation 29.1 is well suited to calculate the GOS, given A and N. provided. Thus, if traffic grows in accordance with the forecast, the However, the dimensioning problem is: given A and the GOS, GOS will initially be much better than the specified value and determine N. Equation 29.1 is ill-suited for this. Fortunately, tables increase towards it over the provisioning period. However, if traffic are available giving the traffic capacity of different numbers of grows more quickly than was forecast, the specified GOS will be trunks for various grades of service. An example (Bear, 1988) is reached during the provisioning period and towards the end of it the shown in Table 4. By looking down the column for the specified GOS will be much worse than that specified. This situation has grade of service until the required traffic is exceeded, one can read occurred often, since telecommunications has expanded so rapidly across to determine the number of trunks that is needed. For in recent years. example, to handle 20 erlangs of traffic with one lost call in 100, the Of course, a subscriber is interested in the grade of service of a table shows that 30 trunks are required. Table 29.4 applies to complete connection which often consists of several routes con- lost-call system. Tables are also published for queuing systems nected in tandem, whereas Equations 29.1 and 29.2 give the grade (Hunter, 1988). of service for a single link in such a connection. However, if a Table 29.4 Traffic capacity table for full availability groups (continued on next page) Number of 1 lost call in Number of 1 call lost in trunks trunks 50 (0.02) 100 (0.10) 50 (0.02) 100 (0.01) (0.005) E (0.001) E, (0.005) E (0.001) E 0.020 0.010 0.005 0.001 0.015 0.046 29/18 Telephone networks Table 29.4 (co