VoIP

Voice-over-IP (VoIP) implementation enables users to carry voice traffic (for example, telephone calls and faxes) over an IP network.

There are 3 main causes for the evolution of the Voice over IP market:

• Low cost phone calls
• Add-on services and unified messaging
• Merging of data/voice infrastructures

A VoIP system consists of a number of different components: Gateway/Media Gateway, Gatekeeper, Call agent, Media Gateway Controller, Signaling Gateway and a Call manager

The Gateway converts media provided in one type of network to the format required for another type of network. For example, a Gateway could terminate bearer channels from a switched circuit network (i.e., DS0s) and media streams from a packet network (e.g., RTP streams in an IP network). This gateway may be capable of processing audio, video and T.120 alone or in any combination, and is capable of full duplex media translations. The Gateway may also play audio/video messages and performs other IVR functions, or may perform media conferencing.

Coders are used for efficient bandwidth utilization. Different coding techniques for telephony and voice packet are standardized by the ITU-T in its G-series recommendations: G.723.1, G.729, G.729A etc.

The coder-decoder compression schemes (CODECs) are enabled for both ends of the connection and the conversation proceeds using Real-Time Transport Protocol/User Datagram Protocol/Internet Protocol (RTP/UDP/IP) as the protocol stack.

Quality of Service

A number of advanced methods are used to overcome the hostile environment of the IP net and to provide an acceptable Quality of Service. Example of these methods are: delay, jitter, echo, congestion, packet loss, and missordered packets arrival. As VoIP is a delay-sensitive application, a well-engineered, end-to-end network is necessary to use VoIP successfully. The Mean Opinion Score is one of the most important parameters that determine the QoS.

There are several methods and sophisticated algorithms developed to evaluate the QoS: PSQM (ITU P.861), PAMS (BT) and PESQ.Each CODEC provides a certain quality of service. The quality of transmitted speech is a subjective response of the listener (human or artificial means). A common benchmark used to determine the quality of sound produced by specific CODECs is the mean opinion score (MOS). With MOS, a wide range of listeners judge the quality of a voice sample (corresponding to a particular CODEC) on a scale of 1 (bad) to 5 (excellent).