Overview of H.323 Protocols

Figure 8-4 shows the H.323 protocol stack. Upon examination, we find a
number of protocols already discussed, such as RTP, TCP, and UDP. It is clear from the figure that the exchange of media is performed using RTP
over UDP and, of course, wherever there is RTP, there is also RTCP.
In Figure 8-4, we also find two protocols that have not yet been discussed
—namely H.225.0 and H.245. These two protocols define the actual messages
that are exchanged between H.323 endpoints. They are generic protocols
in that they could be used in any number of network architectures.
When it comes to the H.323 network architecture, the manner in which the
H.225.0 and H.245 protocols are applied is specified by recommendation
H.323.
H.225.0 is a two-part protocol. One part is effectively a variant of ITU-T
recommendation Q.931, the ISDN layer 3 specification, and should be quite
familiar to those with knowledge of ISDN. It is used for the establishment
and tear-down of connections between H.323 endpoints. This type of signaling
is known as call signaling or Q.931 signaling. The other part of
H.225.0 is known as Registration, Admission, and Status (RAS) signaling.
It is used between endpoints and gatekeepers and enables a gatekeeper to
manage the endpoints within its zone. For example, RAS signaling is used
by an endpoint to register with a gatekeeper and it is used by a gatekeeper
to allow or deny endpoint access to network resources.
H.245 is a control protocol used between two or more endpoints. The
main purpose of H.245 is to manage the media streams between H.323 session
participants. To that end, it includes functions such as ensuring that
the media to be sent by one entity is limited to the set of media that can be received and understood by another. H.245 operates by the establishment of
one or more logical channels between endpoints. These logical channels
carry the media streams between the participants and have a number of
properties such as media type, bit rate, and so on.
All three signaling protocols—RAS, Q.931, and H.245—may be used in
the establishment, maintenance, and tear-down of a call. The various messages
may be interleaved. For example, consider an endpoint that wants to
establish a call to another endpoint. Firstly, it may use RAS signaling to
obtain permission from a gatekeeper. It may then use Q.931 signaling to
establish communication with the other endpoint and set up the call.
Finally, it may use H.245 control signaling to negotiate media parameters
with the other endpoint and set up the media transfer. Figure 8-5 shows an
example of the interaction between the different types of signaling.