In my previous blog post, I described the most important H.323 protocols, standards, and network elements. This time I'll be taking a look at how those network elements communicate.
As mentioned last time, H.225 RAS allows communication between endpoints and gatekeepers, using UDP ports 1718 and 1719. There are a number of different H.225 RAS messages, including:
Gatekeeper discovery (Gatekeeper Request [GRQ], Gatekeeper Confirm [GCF], and Gatekeeper Reject [GRJ]): used by endpoints to discover gatekeepers with which to register.
Registration and unregistration (Registration Request [RRQ], Registration Confirm [RCF], Registration Reject [RRJ], Unregister Request [URQ], Unregister Confirm [UCF], and Unregister Reject [URJ]): these messages can be used to register/unregister addresses with a gatekeeper and join a zone (a collection of H.323 network elements registered with a gatekeeper).
Admission control (Admission Request [ARQ], Admission Confirm [ACF], and Admission Reject [ACJ]): these messages are used for call admission control.
Call Disconnection (Disengage Request [DRQ], Disengage Confirm [DCF], and Disengage Reject [DRJ]): these are used to disconnect calls.
Bandwidth control (Bandwidth Request [BRQ], Bandwidth Confirm [BCF], and Bandwidth Reject [BRJ]): these can be used to change the amount of bandwidth during a call.
Endpoint location (Location Request [LRQ], Location Confirm [LCF], and Location Reject [LRJ]): as you can probably guess, these are used to locate endpoints.
Status information (Information Request [IRQ], and Information Request Response [IRR]), Information Request Ack [IACK], and Information Request NAck [INAK]): these can be used to find status information.
H.225 call signalling (as opposed to H.225 RAS) uses Q.931 messages (you may be familiar with these from studying ISDN) to setup, maintain, and tear down calls.
H.245 signaling is used for media control, including establishing master/slave relationships, exchanging H.323 terminal capabilities, and for logical channel signalling. Logical channels can be used to transport media traffic such as audio, video, and data.
Note that terminals (phones) A and B shown in intrazone call setup example are not themselves H.323 enabled. Instead terminals A and B connect to Cisco H.323 gateways (GWA and GWB), which are H.323 enabled, function as H.323 endpoints, and interact with a gatekeeper (GK1) in order to setup the call.
Once you have had a good look at those first two diagrams, and are comfortable with their content, you may like to take a look at this diagram, which shows H.225 call signalling and H.245 call control in more detail. This last example does not include any H.225 RAS (there is no gatekeeper involved in the call flow), and H.225 call signalling and H.245 call control is shown directly between two H.323 enabled PCs (H.323 terminals/endpoints).