SIP Call Establishment

At a high level, SIP call establishment is very simple, as shown in Figure
8-9. The process starts with a SIP INVITE message, which is used from
the calling party to the called party. The message invites the called party to
participate in a session—a call. Included with the INVITE message is a session
description— a description of the media that the calling party wants to
use. This description includes the voice-coding scheme that the caller wants
to use, plus an IP address and a port number that the called party should
use for sending media back to the caller.
A number of interim responses to the INVITE may be sent, prior to the
called party accepting the call. For example, the caller might be informed
that the call is queued and/or that the called party is being alerted; that is,
the phone is ringing. Subsequently, the called party answers the calls,
which generates an OK response back to the caller. The OK response is
actually indicated by the status code value of 200 in the response. In the
example of Figure 8-9, the 200 (OK) response contains a session description,
indicating the media that the caller wants to use plus an IP address and
port number to which the caller should send packets.
Upon receipt of the 200 (OK) response, the caller responds with ACK to
confirm that the OK response has been received. At this point, media are
exchanged. These media will most often be coded speech, but could also be
other media such as video. Finally, one of the parties hangs up, which
causes a BYE message to be sent. The party receiving the BYE message
sends 200 (OK) to confirm receipt of the message. At that point, the call is
over.
All in all, SIP call establishment is quite a simple process. Of course, the
signaling could well pass via one or more proxy servers, in which case the
process becomes somewhat more complex. Nonetheless, it is clear that SIP
call establishment is much simpler than the equivalent H.323 process.