The protocols behind VoIP

VoIP systems most often make use of four pieces of network architecture: QoS, multicast, SIP and H.323. The first and most important realisation about these pieces is they're not from the same rule book: it's a bit like linking together the smell of asparagus, tax rebates, luck and sunshine, because they can all be experienced by a human.

The vital VoIP service

All four of the vital VoIP service and traffic types are related to a network, but to see how disconnected they actually are let's start with the two protocols SIP and H.323: one comes from the ITU (International Telecommunication Union), the other from the IETF (Internet Engineering Task Force). Both have a wider scope than the use to which they're put by VoIP, and neither come with handy testers showing you a red or a green light in any given situation.

An entire concept for using any IP network, up to and including the whole of the internet, for a kind of transport that goes against the grain of the rest of the system. Everything about networks is about making sure that packets reach their intended target; multicast is about ensuring packets go to every target in the scope of the network. VoIP systems use multicast for sharing group status data around a population of phones, which is a highfalutin way of saying that it helps you to see that the switchboard is ringing.

A data structure for initiating sessions of messages between two parties. In VoIP, SIP appears mostly as a means of communicating and authenticating with a proxy server (also called SIP Proxy) that puts call participants into contact with one another.

The simplest definition in the VoIP framework

This has the simplest definition in the VoIP framework: it ships packets of audio data between VoIP resources, although not always in the same format and not in absolutely every implementation.

Finally, we need to mention Power over Ethernet (PoE), which isn't a protocol as such but a means of delivering a voltage over a network. PoE is a convenient way of running small devices on the end of network cables, where a huge deployment of little fat power transformers under every desk would be both cumbersome and prone to failure (as well as spectacularly un-green).