In the current competitive world, Quality of Service (QoS) is becoming, jointly with cost, a key indicator in selling and buying telecommunications services. At the same time, technology and liberalization trends are raising new types of concerns unknown with the Plain Old Telephony Services (POTS) using switched connections provided by a single monopoly supplier.

Nowadays, there are several standards describing QoS measurements but the questions of which indicators are to be monitored and which values they should meet are still open. ETSI EG 202 009-1 proposes a methodology for the identification of indicators relevant to the users that can be used either to monitor the QoS of Telecom services used by the private customers or for a Service Level Agreement (SLA) between a business customer and a supplier of Telecommunications services such as that proposed in ETSI EG 202 009-3 [i.22]. This part 3 gives guidelines on how to express explicit user's QoS requirements, prioritize the indicators, establish a preferred value for each of these indicators, while ETSI EG 202 009-2 [i.21] proposes QoS indicators for each service and each step of the Customer Relationship Course.

The present document was written to make available to the providers and users of any kind of telecom services (legacy network based or IP network based services) a common basis for mutual understanding about quality of service. It aims to assist users in identifying rationally their QoS requirements in terms of Service Level Objectives (SLO), helping the providers to better meet them for their mutual benefit.