8.1 Introduction

Teletraffic theory [1] is used to evaluate the number of lines required to link two switching offices with a specified quality or Grade of Service (GoS) expressed in terms of the blocking probability p. The total offered traffic A, the number of lines N, and the blocking probability p are related through the well-known Erlang-B formula

(8.1)

(8.2)

This formula is also used for radio systems. To illustrate the concept and the gain obtained by a trunked radio system with respect to a set of individual systems, a comparison is made between a system in which all communications are served by a single N-channel radio system and the case in which there exist N single-channel systems, each serving its own traffic ( A/ N). In both cases the same blocking probability p is assumed. A can be calculated using the expression

(8.3)

Comparing both results, it is clear that A is greater than A ¹ for N > 1. This is due to the nonlinearity of the Erlang formula.

It can be concluded that it is far more efficient to offer the whole traffic demand to a set of channels (trunked system) than to split the traffic and offer it to individual single-channel systems (Figure 8.1). The traffic-handling capability increment (trunking gain) is also apparent when the traffic per channel is...