Quality of service

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                In the field of 
computer networking
 and other 
packet-switched
 telecommunication networks, the 
traffic engineering
 termquality of service (QoS) refers to resource reservation control mechanisms rather than the achieved service quality. Quality of service is the ability to provide different priority to different applications, users, or data
flows
, or to guarantee a certain level of performance to a data flow. For example, a required 
bit rate
, 
delay
, 
jitter
, packet dropping probability and/or bit error rate may be guaranteed. Quality of service guarantees are important if the network capacity is insufficient, especially for real-time 
streaming multimedia
 applications such as 
voice over IP
, online games and 
IP-TV
, since these often require fixed bit rate and are delay sensitive, and in networks where the capacity is a limited resource, for example in cellular data communication. In the absence of 
network congestion
, QoS mechanisms are not required.
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 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 over-provisioning the capacity so that it is sufficient for the expected peak traffic load.
In the field of 
telephony
, quality of service was defined in the 
ITU
 standard X.902 as "A set of quality requirements on the collective behavior of one or more objects". Quality of Service comprises requirements on all the aspects of a connection, such as service response time, loss, signal-to-noise ratio, cross-talk, 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.
[1]
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. High QoS is often confused with a high level of performance or achieved service quality, for example high 
bit rate
, low 
latency
 and low 
bit error probability
. See also 
Relation to subjective quality measures
 below.
Relation to subjective quality measuresAn alternative and disputable definition of QoS, used especially in telephony and 
streaming video
 services, is a metric that reflects or predicts the subjectively experienced quality, for example the 
Quality of Experience
 (QoE) subjective business concept, the "user perceived performance" 
[2]
, the "degree of satisfaction of the user", the "number of happy customers" or the 
Mean Opinion Score
 (MOS). In this context, QoS is the cumulative effect on subscriber satisfaction of all imperfections affecting the service. This definition includes the application and the human in the assessment, and demands an appropriate weighting of diverse objective measures.
Quality of Service Experience
 (QoSE) on the other hand, is the actual measure of user’s experience with an operator in terms of delivered quality with or without reference to what is being promised. This differs from QoS as the former is defined only in the context of user experience but not 
Quality of Experience
 (QoE) because it is not subjective.


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