Voice over IP (VoIP)

Voice over IP, abbreviated VoIP, is the technology that carries telephone calls over IP-based data networks such as the internet instead of the traditional circuit-switched telephone system. The caller's speech is captured, digitised, compressed, and broken into packets that travel across the network like any other data, then reassembled and converted back into sound at the far end. Because it uses the same packet networks as other internet traffic, VoIP can lower the cost of calls and integrate voice with other services.

Several protocols make this possible, divided broadly into signalling and media transport. Signalling protocols set up, manage, and tear down calls. The Session Initiation Protocol (SIP) is a widely used signalling standard that locates participants and negotiates the parameters of a session, while the older H.323 family from the ITU provided an earlier framework for multimedia communication over packet networks. Once a call is established, the actual voice data is carried by separate media transport protocols designed for real-time streams.

The general idea of carrying voice over a packet network appears under related names depending on the underlying transport, such as voice over ATM, voice over DSL, and voice over cable, sometimes grouped under the umbrella term voice over packet. A practical challenge for all of them is quality: because packet networks can delay, reorder, or lose packets, voice traffic is sensitive to latency and jitter, so networks often give it priority to keep conversations clear. Despite these challenges, VoIP has become a standard way of providing telephony.

Frequently asked questions

How does VoIP work?

VoIP digitises speech, compresses it, and splits it into data packets that travel over an IP network, where they are reassembled and converted back into sound at the other end of the call.

What is SIP?

The Session Initiation Protocol is a signalling protocol used in VoIP to set up, manage, and end calls by locating participants and negotiating the session's parameters.

Why can VoIP call quality vary?

Packet networks can delay, reorder, or drop packets, and voice is sensitive to such delay and jitter, so networks often prioritise voice traffic to maintain clear calls.