Quality of service

Once users are provided with the functionality that they require of a service, such as the file service in a distributed system, we can go on to ask about the quality of the service provided. The main nonfunctional properties of systems that affect the quality of the
service experienced by clients and users are reliability, security and
that is retrieving a film from a video server and presenting it on the user’s screen. For another at a fixed rate. For example, a movie service might consist of a client program streams of data that are required to be processed or transferred from one process to
–time-critical data Some applications, including multimedia applications, handle ability to meet timeliness guarantees, as discussed in the following paragraphs.
responsiveness and computational throughput, but it has been redefined in terms of The performance aspect of quality of service was originally defined in terms of Reliability and security issues are critical in the design of most computer systems.
recognized as a further important aspect of service quality.
to meet changing system configurations and resource availability has been Adaptability performance satisfactoryresult the successive frames of video need to be displayed to the user within
some specified time limits.
In fact, the abbreviation QoS has effectively been commandeered to refer to the ability of systems to meet such deadlines. Its achievement depends upon the availability of the necessary computing and network resources at the appropriate times. This implies
a requirement for the system to provide guaranteed computing and communication resources that are sufficient to enable applications to complete each task on time (for example, the task of displaying a frame of video).
The networks commonly used today have high performance – for example, BBC iPlayer generally performs acceptably – but when networks are heavily loaded their performance can deteriorate, and no guarantees are provided. QoS applies to operating systems as well as networks. Each critical resource must be reserved by the applications that require QoS, and there must be resource managers that provide guarantees.Reservation requests that cannot be met are rejected. 

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