Overview of VoIP Systems

Abstract

In their simplest form, Voice over IP protocols simply enable two (or more) devices to transmit and receive real-time audio traffic that allows their respective users to communicate. In general, VoIP architectures are partitioned in two main components: signaling and media transfer. Signaling covers both abstract notions, such as endpoint naming and addressing, and concrete protocol functions such as parameter negotiation, access control, billing, proxying, and NAT traversal. Depending on the architecture, quality of service (QoS) and device configuration/management may also be part of the signaling protocol (or protocol family). The media transfer aspect of VoIP systems generally includes a comparatively simpler protocol for encapsulating data, with support for multiple codecs and (often, but not always) content security. A commonly used media transfer protocol is RTP [219]. There exits an RTP profile (named Secure RTP, or SRTP [131]) that supports encryption and integrity protection, but it is not yet widely used. The RTP protocol family also includes RTCP, which is used to control certain RTP parameters between communicating endpoints.