Discuss grade of service. During busy hour, 1500 calls were offered to a group of trunks and 8 calls were lost. The average call duration was 120 seconds. Calculate the traffic offered, traffic lost, Grade of service and total duration of congestion.
(i) Grade of service: In loss systems, the traffic carried through the network is usually lower than the actual traffic offered to the network through the subscribers. The overload traffic is discarded and therefore it is not carried by the network. The amount of traffic discarded by the network is an index of the quality of the service offered through the network. It is termed Grade of Service (GOS) and is explained as the ratio of lost traffic to offered traffic. Offered traffic is the product of the average number of calls generated through the users and the average holding time per call. Therefore, GOS is given as
GOS = (A-A0)/A
Here
A= offered traffic A0 = carried traffic A-A0= lost traffic (ii) we know that
Traffic offered = A = Ch/T = (1500 x 2)/360 = 50 E
Traffic lost = (8 x 2)/(6/30) = (4 E)/15 = 0.26 E
Grade of service = Number of call lost/Number of calls offered
= 8/1500
Duration of congestion = Grade of service x 1 h
= (8 x 3600)/1500 = (96)/5 = 19.2 sec.