Congestion is a situation in Communication Networks in which too many packets are present in a part of the subnet causing performance degradation. Congestion in a network may occur when the load of the network (i.e. the number of packets sent over the network) is greater than the capacity of the network (i.e. the number of packets a network can handle.)
Congestion control mechanisms are divided into two categories; one category prevents the congestion from happening, and the other category removes congestion after it has taken place.