Quality of Service QoS

Transcription

1 Quality of Service QoS Issue in networks Issue in real time systems Issue in distributed multimedia. network real time Basic idea Quality of service gives a level of system performance assurance regarding traffic service requirements

2 This will be a very brief introduction QoS is major topic of research Bochmann Montreal Blair et al Lancaster Proposed ISO standard My notes in part based on informal publication of Ronald Wolak Two general aspects to QoS Service: application may set QoS levels Traffic: system may supply (guaranteed or expected) QoS levels.

3 QoS Types More precisely, there are two types of QoS currently available: Resource reservation QoS (Service) apportions network resources according to application's QoS request network's bandwidth management policy Prioritization QoS (Traffic) apportions network resources according to set bandwidth management policy, under which preferential treatment is given to applications with demanding transmission requirements.

4 QoS Types (continued) These two QoS types are complementary not competitive or mutually exclusive. They are designed to be used together to accommodate the operational requirements of different networks. Note So far we have been talking about QoS in general networks, not in distributed multimedia specifically

5 QoS in Distributed Multimedia QoS represents the quantitative characteristics qualitative characteristics of a distributed multimedia system, required to satisfy the functionality of an application. Application's functionality includes general user satisfaction correct presentation of multimedia artifact to the user Examples Start audio 5 seconds after video The rolling text and voice-over must be synchronized

6 Notes QoS depends on the application as well as the network, that is Different multimedia applications on the same distributed system often have different QoS parameters. Example: video game versus webpage It is difficult to separate the QoS parameters from other system parameters. Unlike other system parameters, QoS parameters are subject to negotiation between system components.

7 Multimedia QoS parameters We may distinguish five categories of distributed multimedia QoS parameters 1.Performance-oriented end-to-end delay and bit rate, jitter 2.Format-oriented video resolution, frame rate, and storage format 3.Synchronization-oriented skew between the beginning of sequences 4.Cost-oriented connection and data transmission charges and fees 5.User-oriented subjective image and sound quality

8 Note The relative significance of these five parameters depends heavily on the type of multimedia application to be distributed. For example End-to-end delay is less important for presentational applications than it is for conversational (i.e. interactive) ones.

9 Processing QoS Parameters. Several related activities are involved in processing QoS parameters in a distributed multimedia system 1.Assessing QoS requirements in relation to users' satisfaction with the quality of the application 2.Mapping assessment results to QoS parameters 3.Negotiating between system components to ensure that all components are able to meet requirements consistently

10 QoS may be dynamic in nature. Simple case When negotiation ends with agreement on requirements, the multimedia application can then be launched. Dynamic case(s), for example QoS requirements may change during a session negotiated parameters cannot be maintained due to network overload may require reassessment, remapping and/or renegotiation during a session

11 Summary QoS is the ability of a network element to have a level of assurance that its traffic and service requirements will be met. QoS parameters in a distributed multimedia system will vary over time. Therefore, distributed multimedia systems must continuously monitor actual QoS levels and block lower priority tasks when it becomes necessary. In this context, maintaining QoS is a complicated control problem.

12 Reference Vogel, A., Kerherve, B., Von Bochmann, G., & Gecsei, J. (1995). Distributed multimedia and QOS. IEEE Multimedia, 2(2),