Computer Network | Quality of Service and Multimedia
Quality-of-Service (QoS) refers to traffic control mechanisms that seek to either differentiate performance based on application or network-operator requirements or provide predictable or guaranteed performance to applications, sessions, or traffic aggregates. Basic phenomenon for QoS means in terms of packet delay and losses of various kinds.
Need for QoS –
- Video and audio conferencing require bounded delay and loss rate.
- Video and audio streaming requires bounded packet loss rate, it may not be so sensitive to delay.
- Time-critical applications (real-time control) in which bounded delay is considered to be an important factor.
- Valuable applications should be provided better services than less valuable applications.
QoS Specification –
QoS requirements can be specified as:
- Delay Variation(Jitter)
- Error Rate
There are two types of QoS Solutions:
- Stateless Solutions –
Routers maintain no fine-grained state about traffic, one positive factor of it is that it is scalable and robust. But it has weak services as there is no guarantee about the kind of delay or performance in a particular application which we have to encounter.
- Stateful Solutions –
Routers maintain a per-flow state as flow is very important in providing the Quality-of-Service i.e. providing powerful services such as guaranteed services and high resource utilization, providing protection, and is much less scalable and robust.
Integrated Services(IntServ) –
- An architecture for providing QoS guarantees in IP networks for individual application sessions.
- Relies on resource reservation, and routers need to maintain state information of allocated resources and respond to new call setup requests.
- Network decides whether to admit or deny a new call setup request.
IntServ QoS Components –
- Resource reservation: call setup signaling, traffic, QoS declaration, per-element admission control.
- QoS-sensitive scheduling e.g WFQ queue discipline.
- QoS-sensitive routing algorithm(QSPF)
- QoS-sensitive packet discard strategy.
RSVP-Internet Signaling –
It creates and maintains distributed reservation state, initiated by the receiver and scales for multicast, which needs to be refreshed otherwise reservation times out as it is in soft state. Latest paths were discovered through “PATH” messages (forward direction) and used by RESV messages (reserve direction).
Call Admission –
- Session must first declare it’s QoS requirement and characterize the traffic it will send through the network.
- R-specification: defines the QoS being requested, i.e. what kind of bound we want on the delay, what kind of packet loss is acceptable, etc.
- T-specification: defines the traffic characteristics like bustiness in the traffic.
- A signaling protocol is needed to carry the R-spec and T-spec to the routers where reservation is required.
- Routers will admit calls based on their R-spec, T-spec and based on the current resource allocated at the routers to other calls.
Differentiated Service is a stateful solution in which each flow doesn’t mean a different state. It provides reduced state services i.e. maintaining state only for larger granular flows rather than end-to-end flows tries to achieve the best of both worlds.
Intended to address the following difficulties with IntServ and RSVP:
- Flexible Service Models:
IntServ has only two classes, want to provide more qualitative service classes: want to provide ‘relative’ service distinction.
- Simpler signaling:
Many applications and users may only want to specify a more qualitative notion of service.
Streaming Live Multimedia –
- Examples: Internet radio talk show, Live sporting event.
- Streaming: playback buffer, playback buffer can lag tens of seconds after and still have timing constraint.
- Interactivity: fast forward is impossible, but rewind and pause is possible.
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