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In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections. A consequence of the latter two effects is that an incremental increase in offered load leads either only to a small increase in network throughput, or to an actual reduction in network throughput.〔(Al-Bahadili, 2012, p. 282) Al-Bahadili, H. (2012). (Simulation in computer network design and modeling: Use and analysis ). Hershey, PA: IGI Global.〕 Network protocols which use aggressive retransmissions to compensate for packet loss tend to keep systems in a state of network congestion, even after the initial load has been reduced to a level which would not normally have induced network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as congestive collapse. Modern networks use congestion control and congestion avoidance techniques to try to avoid congestion collapse. These include: exponential backoff in protocols such as 802.11 CSMA/CA and the original Ethernet, window reduction in TCP, and fair queueing in devices such as routers. Another method to avoid the negative effects of network congestion is implementing priority schemes, so that some packets are transmitted with higher priority than others. Priority schemes do not solve network congestion by themselves, but they help to alleviate the effects of congestion for some services. An example of this is 802.1p. A third method to avoid network congestion is the explicit allocation of network resources to specific flows. One example of this is the use of Contention-Free Transmission Opportunities (CFTXOPs) in the ITU-T G.hn standard, which provides high-speed (up to 1 Gbit/s) local area networking over existing home wires (power lines, phone lines and coaxial cables). RFC 2914 addresses the subject of congestion control in detail. ==Network capacity== A fundamental problem is that all network resources are limited, including router processing time and link throughput. For example: *A wireless LAN is easily filled by a single personal computer *Even on fast computer networks (e.g. Gigabit Ethernet), the backbone can easily be congested by a small number of servers and client PCs *The aggregate transmission from P2P networks have no problem filling an uplink or some other network bottleneck *Denial-of-service attacks by botnets are capable of filling even the largest Internet backbone network links, generating large-scale network congestion *In telephone networks (particularly mobile phones), a mass call event can overwhelm digital telephone circuits 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Network congestion」の詳細全文を読む スポンサード リンク
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