"B". Each of these central offices has the theoretical capacity to handle ten thousand individual telephone numbers. (Central office "A", with prefix "123", has available 10,000 numbers from 123-0000 to 123-9999; central office "B", with prefix "124", the same.) If all 10,000 subscribers in "A" were to simultaneously call 10,000 subscribers in "B", then it would be necessary to have 10,000 lines to connect the two towns. However, the odds of that happening are remote, as the number of simultaneous phone calls is usually much lower. Erlang-B is a common formula that predicts the optimal number of trunk lines actually needed under normal conditions. This concept has been simply applied to radio user groups, to determine the optimal number of channels needed, under normal conditions, to accommodate a given number of users. In the event of a widespread emergency such as a major earthquake, many more users than normal will attempt to access both the telephone and radio systems. In both cases once the trunking capacity of the systems is fully used, all subsequent users will receive a busy signal. In such a case management of communications becomes critical with only very local communications sharing simplex (non-system) frequencies, and longer distance communications sharing pre-planned trunking talkgroups and governing use of the resources to essential communications. In our example of police dispatch, different talkgroups are assigned different system priority levels, sometimes with 'preemption' capability, attempting to ensure that communication between critical units is maintained. Differences from conventional two-way radio 'Trunked' radio systems differ from 'conventional' radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while 'trunking' radio systems use a pool of channels which are available for a great many different groups of users. For example, if police communications are configured in such a way that twelve conventional channels are required to permit citywide dispatch based upon geographical patrol areas, during periods of slow dispatch activity, much of that channel capacity is idle. In a trunked system, the police units in a given geographical area are not assigned a dedicated channel, but instead are members of a talk-group entitled to draw upon the common resources of a smaller pool of channels. Advantages of Trunking Trunked radio takes advantage of the probability that with any given number of users, not everyone will need channel access at the same time, therefore fewer discrete