SIP Signalling

SIP signalling is based on the standard request/response flow. The sequence to follow is a simple example of a telephone call set-up procedure:

To set up a call session, the caller (or User Agent Client) sends a request with the SIP URL of the called party itself.

If the client knows the actual location of the other party it can send the request directly to their IP address, if not however the client can send it direct to a more local SIP network server.

This server will then attempt to resolve the called user's location and send the request on to them. There are many different ways it can do this, such as searching the DNS (Domain Name Server) or by accessing databases. Alternatively, the server may be a redirect type server that may return the called user location to the calling client for it to try directly.

Once found, the request is sent to the user and then several options become available. In the simplest form, the user's telephony client receives the request, that is, the user's phone rings. If the user takes the call, the client responds to the invitation with the designated capabilities best described the functions that the user needs to activate. The client software might support videoconferencing, for example, but the user may only want to use audio conferencing. Regardless, the user can always add functions - such as video conferencing, white-boarding, or a third user - by putting in a request to other users already using the link) of the client software and a connection is established. If the user declines the call, the session can be redirected to another user or voice mail server.

SIP also has two more distinct features. The first is a stateful SIP server's ability to split an incoming call so that several telephone extensions will ring together as a group. This feature could come into use when user is working between two locations (from home and an office, for example), or when someone is calling both a manager and secretary.

The second and more notable feature is SIP's unique ability to return different media types within a single session. An example of this could be a client calling an interior designer, who could then view images of different wallpaper types, complete an online order form - all within the same SIP communication session.

What is SIP?  | Background  | SIP Signalling  | Characteristics  | News  | Forum