 quality, bill correctness, Availability Coverage, availability of call center, service availability, Reliability Dropped calls ratio, number of billing complaints within a specific time period, etc. Security Fraud protection and prevention, etc. Simplicity Professionalism of help line (i.e. customer care), ease of software updates, ease of contract cessation procedure, Flexibility Ease of change in contract, availability of different billing methods including online and offline billing, etc. 1 Introduction Service provider QoS offered QoS delivered by the service by the service provider provider Customer's Customer's perception of requirements the QoS Customer Figure 1.7 QoS viewpoints. The ITU's performance model is based on four viewpoints of QoS initially defined in ITU-T G.1000 [12]. These fours viewpoints cover QoS from both customer and service aspects, as given in Figure 1.7: Customer's QoS requirements. This is the QoS level required by the subscriber, which may be specified in non-technical language also (e.g. good service is required), because the customer is interested not in how a given service is provided (e.g. by the telecom operator's network) but primarily in the obtained end-to-end QoS (expressed in terms they can understand, such as bitrates, data volume, etc.). QoS offered by the provider (or planned/targeted QoS). This is a statement made by the service provider (e.g. telecom operator) to customers about the QoS offered for a given service. This viewpoint is primarily used for a service level agreement (SLA), which serves as a bilateral agreement signed between the customer and the service provider. This QoS can be specified in terms understandable to the customer on one side and with technical terms for the purposes of implementation of such QoS on the side of the operator. This can also serve as a merit for subscribers to make the best choice from the given service provider's offerings. QoS achieved (i.e. delivered) by the provider. This viewpoint refers to the actual level of QoS achieved 