Define Congestion.

Congestion: This is uneconomic to give sufficient equipment to carry all the traffic which could possibly be offered to a telecommunication system. Inside a telephone exchange this is theoretically possible for all subscribers to make a call concurrently. A situation can hence arise that each of the trunks in a group of trunks are busy, and therefore this can accept further calls. This type of state is termed as congestion. Into a message-switched system, calls which arrive throughout congestion wait in a queue till an outgoing trunk becomes free. Therefore, they are delayed but not lost. These systems are so termed as queuing systems or delay system.  A telephone exchange, in a circuit-switched system all attempts to make calls over a congested group of trunks is successful. These systems are so termed as lost-call systems. In a lost-call system the result of congestion is which the traffic in fact carried is less than the traffic offered to the system.