Grade of Service (GOS)

      In loss systems, the traffic carried by the network is generally lower than the actual traffic offered to the network by the subscribers.

      The overload traffic is rejected and hence is not carried by the network.

      The amount of traffic rejected by the network is an index of the quality of the service offered by the network.

      This is termed Grade of Service (GOS) and is defined as the ration of lost traffic to offered traffic.

      Offered traffic is the product of the average number of calls generated by the users an average holding time per call.

      On the other hand, the actual traffic carried by the network is called the carried traffic and is the average occupancy of the servers in the network.

      GOS is given by

GOS= A – AO

               A

Where

            A         = Offered traffic

            AO      = Carried traffic

            A-AO  = Lost traffic

      The smaller the value of grade of service, the better is the service.

      The recommended value for GOS in India is 0.002 which means that two calls in every 1000 calls or one call in every 500 calls may be lost.

      Usually, every common subsystem in a network has an associated GOS value.

      The GOS of the full network is determined by the highest GOS value of the subsystems in a simplistic sense.

      A better estimate takes into account the connectivity of the subsystems, such as parallel units.

      Since the volume of traffic grows as the time passes by the GOS value of a network deteriorates with time

      In order to maintain the value within reasonable limits. Initially the network is sized to have a much smaller GOS value than the recommended one so that the GOS value continues to be within limits as the network traffic grows.

      The blocking probability PB is defined as the probability that all the servers in a system are busy.

      When all the servers are busy, no further traffic can be carried by the system and the arriving subscriber traffic is blocked.

      At the first instance, it may appear that the blocking probability is the same measure as the GOS.

      The probability that all the servers are busy may well represent the fraction of the calls lost, which is what the GOS is all about.

      However, this is generally not true.

Example:

                                    In a system with equal number of servers and subscribers. The GOS is zero as there is always a server available to subscriber.