It is uneconomic to provide adequate equipment to carry all the traffic which could possibly be offered to a telecommunication system. In a telephone exchange it's theoretically possible forevery subscriber to make a call concurrently. A situation can so arise when all the trunks in a group of trunks are busy and so it can accept further calls. This state iscalled congestion. In a message-switched system, calls that arrive during congestion wait in a queue until an outgoing trunk becomes free. So they are delayed however not lost. Such systems are thus known as delay system or queuing systems . In a circuit-switched system, like a telephone exchange, all attempts to make calls over a congested group of trunks are successful. Such systems are hence termed as lost-call systems. In a lost-call system result of the congestion is that traffic actually carried is less than traffic offered to the system.