What is SIP?

From Ekiga

What is SIP?

SIP stands for 'Session Initiation Protocol', it is defined by RFC3261 and provides a method for registering with VoIP Service (or 'Dial-tone') providers and initiating/receiving VoIP calls.

Ekiga is capable of using any service provider which supports SIP (for example Ekiga.net, FreeWorldDialup, SipGate, etc.).

Once a 'call' is created the audio (and optionally video) is set directly between caller and callee, although it is possible to use SIP proxies - where the call is routed through the proxy.

The audio/video is encoded/compressed using a codec. Different codecs are available within Ekiga, each provides a compromise between audio quality and bandwidth required.

SIP Numbers

A SIP number normally looks like an email address, 500@ekiga.net for example. The part before the '@' represents the user and the part after represents the service provider.

The user part may be either numeric or alphanumeric depending on provider. For example Ekiga.net uses alphanumeric usernames, but each has a numerical alias to enable dialing from a standard telephone key pad.

To dial a number on an application (such as Ekiga) just enter the SIP number. If the provider part is not entered, the application will assume the same provider as the user.

Using a telephone keypad it is not possible to entered alphanumerical characters, so it is not possible to enter the provider portion. To make calls to other providers the user can use a Peering prefix.

Another problem is that unlike a normal telephone, the full 'user' part of the number must be entered before the call is placed and the number can be any number of digits. The '#' can be used to indicate that the number is complete, another solution is to set up a dialplan (which interprets numbers as they are typed).

In order to differentiate a SIP number from an email address the 'sip:' URI should be used, for example sip:500@ekiga.net.