Editing Quality of service (section)

==Definitions==
In the field of [[telephony]], quality of service was defined by the [[ITU]] in 1994.<ref name="E.800"/> Quality of service comprises requirements on all the aspects of a connection, such as service response time, loss, signal-to-noise ratio, [[crosstalk]], echo, interrupts, frequency response, loudness levels, and so on. A subset of telephony QoS is [[grade of service]] (GoS) requirements, which comprises aspects of a connection relating to capacity and coverage of a network, for example guaranteed maximum [[blocking probability]] and outage probability.<ref name="itu-t-2">[http://www.com.dtu.dk/teletraffic/handbook/telenook.pdf Teletraffic Engineering Handbook] {{webarchive |url=https://web.archive.org/web/20070111015452/http://oldwww.com.dtu.dk/teletraffic/handbook/telenook.pdf |date=January 11, 2007 }} ITU-T Study Group 2 (350 pages, 2.69&nbsp;MB)(It uses abbreviation GoS instead of QoS)</ref>

In the field of [[computer networking]] and other [[packet-switched]] telecommunication networks, [[teletraffic engineering]] refers to traffic prioritization and resource reservation control mechanisms rather than the achieved service quality. Quality of service is the ability to provide different priorities to different applications, users, or [[Traffic flow (computer networking)|data flows]], or to guarantee a certain level of performance to a data flow. For example, a required bit rate, [[network delay|delay]], [[delay variation]], [[packet loss]] or [[bit error rate]]s may be guaranteed. Quality of service is important for real-time [[streaming multimedia]] applications such as [[voice over IP]], [[multiplayer online game]]s and [[IPTV]], since these often require fixed bit rate and are delay sensitive. Quality of service is especially important in networks where the capacity is a limited resource, for example in cellular [[data communication]].

A network or protocol that supports QoS may agree on a [[traffic contract]] with the application software and reserve capacity in the network nodes, for example during a session establishment phase. During the session it may monitor the achieved level of performance, for example the data rate and delay, and dynamically control scheduling priorities in the network nodes. It may release the reserved capacity during a [[Clearing (telecommunications)|tear down]] phase.

A [[best-effort network]] or service does not support quality of service. An alternative to complex QoS control mechanisms is to provide high quality communication over a best-effort network by [[Overprovisioning (networking)|over-provisioning]] the capacity so that it is sufficient for the expected peak traffic load. The resulting absence of [[network congestion]] reduces or eliminates the need for QoS mechanisms.

QoS is sometimes used as a quality measure, with many alternative definitions, rather than referring to the ability to reserve resources. Quality of service sometimes refers to the level of quality of service, i.e. the guaranteed service quality.<ref>{{cite journal | doi = 10.1016/j.future.2008.11.001 | volume=25 | issue=7 | title=Real-time reconfiguration for guaranteeing QoS provisioning levels in Grid environments | year=2009 | journal=Future Generation Computer Systems | pages=779–784 | author=Menychtas Andreas}}</ref> High QoS is often confused with a high level of performance, for example high bit rate, low [[latency (engineering)|latency]] and low bit error rate.

QoS is sometimes used in application layer services such as telephony and [[streaming video]] to describe a metric that reflects or predicts the subjectively experienced quality. In this context, QoS is the acceptable cumulative effect on subscriber satisfaction of all imperfections affecting the service. Other terms with similar meaning are the [[quality of experience]] (QoE), [[mean opinion score]] (MOS), [[perceptual speech quality measure]] (PSQM) and [[perceptual evaluation of video quality]] (PEVQ).