Explain CONGESTION.

CONGESTION: This is uneconomic to provide sufficient equipment to carry entire traffic that could possibly be given to a telecommunication system. Inside a telephone exchange this is theoretically possible for every subscriber to make a call at the same time. Therefore a situation can arise while all the trunks in a group of trunks are busy, and hence this can accept further calls. This 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. This system is therefore called queuing systems or delay system.  As a telephone exchange, in a circuit-switched system all attempts to make calls over a congested group of trunks are successful. These systems are therefore termed as lost-call systems. The result of congestion, in a lost-call system is that the traffic actually carried is less than the traffic given to the system.