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* Quality 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.
Approaches which provide quality
support are divided into
* Integrated services
* Differentiated services
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