Quality of service (QoS) mechanism controls the performance, reliability and usability of a telecommunications service. Mobile cellular service providers may offer mobile QoS to customers just as the fixed line PSTN services providers and Internet service providers may offer QoS. QoS mechanisms are always provided for circuit switched services, and are essential for non-elastic services, for example streaming multimedia. It is also essential in networks dominated by such services, which is the case in today's mobile communication networks. Mobility adds complication to the QoS mechanisms, for several reasons: A phone call or other session may be interrupted after a handover, if the new base station is overloaded. Unpredictable handovers make it impossible to give an absolute QoS guarantee during a session initiation phase. The pricing structure is often based on per-minute or per-megabyte fee rather than flat rate, and may be different for different content services. A crucial part of QoS in mobile communications is grade of service, involving outage probability (the probability that the mobile station is outside the service coverage area, or affected by co-channel interference, i.e. crosstalk) blocking probability (the probability that the required level of QoS can not be offered) and scheduling starvation. These performance measures are affected by mechanisms such as mobility management, radio resource management, admission control, fair scheduling, channel-dependent scheduling etc. Factors affecting QoS Many factors affect the quality of service of a mobile network. It is correct to look at QoS mainly from the customer's point of view, that is, QoS as judged by the user. There are standard metrics of QoS to the user that can be measured to rate the QoS. These metrics are: the coverage, accessibility (includes GoS), and the audio quality. In coverage the strength of the signal is measured using test equipment and this can be used to estimate the size of the cell. Accessibility is about determining the ability of the network to handle successful calls from mobile-to-fixed networks and from mobile-to-mobile networks. The audio quality considers monitoring a successful call for a period of time for the clarity of the communication channel. All these indicators are used by the telecommunications industry to rate the quality of service of a network. Measurement of QoS The QoS in industry is also measured from the perspective of an expert (e.g. teletraffic engineer). This involves assessing the network to see if it delivers the quality that the network planner