## A

where A = carried traffic (equation 8.4) A = offered traffic A - Aq = lost traffic.

The smaller the value of grade of service, the better is the service. The recommended GOS is 0.002, i.e. 2 call per 1000 offered may lost. In a system, with equal no. of servers and subscribers, GOS is equal to zero.

GOS is applied to a terminal to terminal connection. But usually a switching centre is broken into following components

(a) an internal call (subscriber to switching office)

(b) an outgoing call to the trunk network (switching office to trunk)

(c) the trunk network (trunk to trunk)

(d) a terminating call (switching office to subscriber).

The GOS calculated for each component is called component GOS. The overall GOS is in fact approximately the sum of the component grade of service.

Example 8.3. During a busy hour, 1400 calls were offered to a group of trunks and 14 calls were lost. The average call duration has 3 minutes. Find (a) Traffic offered (b) Traffic carried (c) GOS and (d) The total duration of period of congestion.

Sol.