Voice over IP (VoIP)

Voice over IP (VoIP) is a voice over Data implementation in which voice signals are transmitted over IP networks. The phone instrument is the addressed device. It can be an IP telephone unit or a VoIP interface at a private Branch Exchange (PBX), which enables the phone system to access the IP network at a single point. A dial-plan map translates between PBX dial numbers and IP addresses; if a dialed phone number is found in the map, VoIP route the call to an IP host.

Advantages and Limitations of VoIP

VoIP is being rapidly adopted because it relies on the existing, robust router infrastructure of IP networks and the near-universal implementation of the IP protocol. It also eliminates per-call costs because it uses the existing internet connectivity channel. It does have some drawbacks. The major problem is that IPv4 does not provide for time-sensitive data. On a busy network, voice data is packet switched with other network data and delivery can slow down or become unreliable.

VoIP Protocols

A VoIP session may use one or more protocols, depending on the session parameters.

·         Secure Initiation Protocol (SIP): A signaling protocol for multimedia communication sessions. Used to initiate, modify, and terminate a session. SIP must work with other protocols because it is only responsible for the signaling portion of a communication session.

·         Session Description Protocol (SDP): The format for describing the content of a multimedia communication session.

·         Real-time Transport Protocol (RTP): Transmits audio or video content and defines the packet for delivery including the type of content, sequence numbering, time stamping, and delivery monitory. Has no specific UDP or TCP port number; rather a dynamic range of port numbers, which makes traversing firewalls difficult.

·         Real-time Transport Control Protocol (RTCP): Used primarily to monitor QoS in RTP transmissions. Acts as a partner to RTP to package and deliver data but does not transport data.