H.323 Fast Connect Procedure

It is clear from Figure 8-5 that H.323 call establishment and release can be
quite cumbersome. Moreover, it is possible for a given gatekeeper to choose
to be in the path of all call signaling and H.245 signaling in addition to RAS
signaling. In such a scenario, the number of messages exchanged can be
very large, which can extend the call setup time beyond acceptable limits.
In order to speed things up, H.323 includes a procedure known as Fast Connect,
a method that can significantly reduce the amount of call establishment
signaling. The Fast Connect Procedure is depicted in Figure 8-6.

The Fast Connect procedure involves the setting up of media streams as
quickly as possible. To achieve this, the Setup message can contain a faststart
element within the User-User information element. The faststart element
is actually one or more Open Logical Channel request messages
containing all the information that would normally be contained in such
requests. It includes reverse logical channel parameters if the calling endpoint
expects to receive media from the called endpoint.
If the called endpoint also supports the procedure, then it can return a
faststart element in one of the Call Proceeding, Alerting, Progress, or Connect
messages. That faststart element is basically another OLC message,
which appears like a request to open a bidirectional logical channel. The
included choices of media formats to send and receive are chosen from those
offered in the faststart element of the incoming Setup message. The calling
endpoint has effectively offered the called endpoint a number of choices for
forward and reverse logical channels, and the called endpoint has indicated
those choices that it prefers. The logical channels are now considered open
as if they had been opened according to the procedures of H.245.
Note that a faststart element from the called party to the calling party
may be sent in any message up to and including the Connect message. If it
has not been included in any of the messages, then the calling endpoint
shall assume that the called endpoint either cannot or does not want to support
faststart. In such a case, the standard H.245 methods must be used.
The use of the Fast Connect procedure means that H.245 information is
carried within the call signaling messages and no separate H.245 control
channel exists. Therefore, bringing a call to a conclusion is also faster. The
call is released simply by the sending of the call-signaling Release Complete
message. When used with the fast connect procedure, this has the effect of
closing all of the logical channels associated with the call and is equivalent
to using the procedures of H.245 to close the logical channels.