Network congestion is the situation in which an increase in data transmissions results in a proportionately smaller increase, or even a reduction, in throughput.
Throughput is the amount of data that passes through the network per unit of time, such as the number of packets per second. Packets are the fundamental unit of data transmission on the Internet and all other TCP/IP (transmission control protocol/Internet protocol) networks, including most LANs (local area networks).
Congestion results from applications sending more data than the network devices (e.g., routers and switches) can accommodate, thus causing the buffers on such devices to fill up and possibly overflow. A buffer is a portion of a device's memory that is set aside as a temporary holding place for data that is being sent to or received from another device. This can result in delayed or lost packets, thus causing applications to retransmit the data, thereby adding more traffic and further increasing the congestion.
Congestive collapse is the situation in which the congestion becomes so great that throughput drops to a low level and thus little useful communication occurs. It can be a stable state with the same intrinsic load level that would by itself not produce congestion. This is because it is caused by the aggressive retransmission used by various network protocols to compensate for the the packet loss that occurs as a result of congestion, a retransmission that continues even after the load is reduced to a level that would not have induced congestion by itself.
Network congestion is somewhat analogous to road congestion. One technique that has been used with some success to deal with road congestion is metering, in which the rate of vehicles entering a road or area is restricted by signals. Many economists advocate pricing (i.e., charging to use the roads, with higher prices when congested) as a more efficient congestion reduction technique. Such congestion pricing has become much easier to put into practice in recent years as a result of advances in electronics technology.
However, there are also some important differences. For example, it can be much easier and less costly to increase capacity in communications networks than in road networks. Also, communications network traffic can be compressed in many cases, whereas road traffic cannot. In addition, communications network traffic can often be rerouted at essentially zero cost (when alternative routes exist), whereas large costs (particularly in terms of lost time) can be incurred from rerouting road traffic.
Various techniques have likewise been developed in attempt to minimize congestion collapse in communications networks. In addition to increasing capacity and data compression, they include protocols for informing transmitting devices about the current levels of network congestion and having them reroute or delay their transmissions according to congestion levels.
Created November 28, 2005.