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Network congestion
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<p>Network congestion in <a href="/facts/Data_networking/3w5RM99p">data networking</a> and <a href="/facts/Queueing_theory/8NDl56EM">queueing theory</a> is the reduced <a href="/facts/Quality_of_service/joz9VkUm">quality of service</a> that occurs when a network node or link is carrying more data than it can handle. Typical effects include <a href="/facts/Queueing_delay/Kbr94SPe">queueing delay</a>, <a href="/facts/Packet_loss/6fuIcFHk">packet loss</a> or the blocking of new connections. A consequence of congestion is that an incremental increase in <a href="/facts/Offered_load/oCVrze7p">offered load</a> leads either only to a small increase or even a decrease in network <a href="/facts/Throughput/6zAUsXqw">throughput</a>. </p><p><a href="/facts/Network_protocol/5uady7cB">Network protocols</a> that use aggressive <a href="/facts/Retransmission_(data_networks)/Em3pHg0f">retransmissions</a> to compensate for packet loss due to congestion can increase congestion, even after the initial load has been reduced to a level that would not normally have induced network congestion. Such networks exhibit two stable states under the same level of load. The stable state with low throughput is known as congestive collapse. </p><p>Networks use congestion control and congestion avoidance techniques to try to avoid collapse. These include: <a href="/facts/Exponential_backoff/pk2cAbqm">exponential backoff</a> in protocols such as <a href="/facts/CSMA%2fCA/hVMvKYeF">CSMA/CA</a> in <a href="/facts/802.11/ryEGu6Vq">802.11</a> and the similar <a href="/facts/CSMA%2fCD/N16l6pse">CSMA/CD</a> in the original <a href="/facts/Ethernet/nMfhU8tU">Ethernet</a>, <a href="/facts/Sliding_window/TvkveH2L">window</a> reduction in <a href="/facts/Transmission_control_protocol/WW4sPrt9">TCP</a>, and <a href="/facts/Fair_queueing/hwv3AIud">fair queueing</a> in devices such as <a href="/facts/Router_(computing)/Nh5cIyVf">routers</a> and <a href="/facts/Network_switch/CnvmhNYT">network switches</a>. Other techniques that address congestion include priority schemes which transmit some packets with higher priority ahead of others and the explicit allocation of network resources to specific flows through the use of <a href="/facts/Admission_control/amEBodmQ">admission control</a>. </p>