Overview

There is no standard definition for QoS as it applies to real-time voice communications carried over an Ethernet LAN or IP WAN. As applied to a circuit switched PBX, QoS means consistent, reliable service delivery of control and communications signals in support of customer needs. This definition also can be used for LAN QoS in support of IP telephony. To enable LAN QoS requires all network elements, at all network layers, to work together to support a required level of traffic and service.

An IP-PBX by definition is not a virtual circuit switched communications system, like a traditional PBX, but rather a system that uses an IP network infrastructure. An IP network makes more efficient use of available bandwidth resources than does a circuit switched PBX and is designed to support the bursty nature of data communications traffic rather than the continuous traffic flow of real-time voice communications. IP networks can adapt to changing traffic conditions, but the level of service can be unpredictable. When used to support an IP-PBX system, the IP network must be properly designed and engineered to support the unique real-time traffic requirements of voice as opposed to less stringent data communications requirements.

QoS techniques manage bandwidth according to different application demands and network management settings but cannot guarantee a service level if resources are not available and allocated. Reserving resources for voice communications can seriously affect other network traffic. A priority for QoS network designers has been to ensure that best-effort traffic is available...