If the number of data packets in a network exceeds a certain value, then the network becomes overloaded and its performance drops. This is the case in buffer overflow, when the buffer in the receiving data station overflows and the buffer storage space runs out.
Congestion causes the network to take more time to handle and remove overload situations, congestion, and conflicts. Congestion controls avoid such bottlenecks.
Congestion control should be implemented in a distributed manner or moved to subnets. Such control mechanisms ensure that sufficient storage capacity is available for data buffering at the transition points between the subnets.
One way of controlling overload is to reserve buffer memory, other methods work with flow control and use a windowing technique to control the data flow, and still other methods are based on rejecting data packets as soon as the buffer memory is full. The latter method for overload control is called retransmission method. It has the disadvantage that the network is additionally loaded by the rejected data packets.