TCP + RFC 1006
IP / IP Multicast
Intergrated / Differentiated Services Forwarding
Figure 2.2 H.323 protocol architecture
For basic call signalling and conference control interactions with H.323, the aforementioned components communicate using three control protocols:
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- H.225.0 Registration, Admission, and Status (RAS)
The RAS channel is used for communication between H.323 endpoints and their gatekeeper and for some inter-gatekeeper communication. Endpoints use RAS to register with their gatekeeper, to request permission to utilise system resources, to have addresses of remote endpoints resolved, etc. Gatekeepers use RAS to keep track of the status of their associated endpoints and to collect information about actual resource utilisation after call termination.
RAS provides mechanisms for user/endpoint authentication and call authorisation;
- H.225.0 Call Signalling
The call signalling channel is used to signal call setup intention, success, failures, etc, as well as to carry operations for supplementary services (see below). Call signalling messages are derived from Q.931 (ISDN call signalling); however, simplified procedures and only a subset of the messages are used in H.323.The call signalling channel is used end-to-end between calling party and called party and may optionally run through one or more gatekeepers (the call signalling models are later described in the ‘Signalling models’ Section).
Optimisations: Since version 3, H.225.0 supports the following enhancements:
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Multiple Calls - To prevent using a dedicated TCP connection for each call, gateways can be built to handle multiple calls on each connection.
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Maintain Connection - Similar to Multiple Calls, this enhancement will reduce the need to open new TCP connections. After the last call has ended, the endpoint may decide to maintain the TCP connection to provide a better call setup time for the next call.
The primary use of both enhancements is at the communication between servers (gatekeeper, MCU) or gateways.While, in theory, both mechanisms were possible before, beginning with H.323v3, the messages contained fields to indicate support for the mechanisms;
- H.245 Conference Control
The conference control channel is used to establish and control two-party calls (as well as multiparty conferences). Its functionality includes determining possible modes for media exchange (e.g., select media encoding formats that both parties understand) and configuring actual media streams (including exchanging transport addresses to send media streams to and receive them from). H.245 can be used to carry user input (such as DTMF) and enables confidential media exchange and defines syntax and semantics for multipoint conference operation (see below). Finally, it provides a number of maintenance messages. Also, this logical channel may (optionally) run through one or more gatekeepers, or directly between calling party and called party (please refer to the ‘Signalling models’ Section for details).
It should be noted that H.245 is a legacy protocol inherited from the collective work on multimedia conferencing over ATM, PSTN and other networks. Hence it carries a lot of fields and procedures that do not apply to H.323 but make the protocol specification quite heavyweight.
Optimisations:
The conference control channel is also subject to optimisations. Per default, it is transported over an exclusive TCP connection but it may also be tunnelled within the signalling connection P.16
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(H.245 tunnelling). Other optimisations deal with the call setup time.The last chance to start an H.245 channel is on receipt of the CONNECT message which implies that the first seconds after the user accepted the call, no media is transmitted. H.245 may also start parallel to the setup of the H.225 call signalling, which is not really a new feature but another way of dealing with H.245.Vendors often call this Early Connect or Early Media. Since H.323v2, it is possible to start a call using a less powerful but sufficient capability exchange by simply offering possible media channels that just have to be accepted.This procedure, called FastConnect or FastStart, requires less round-trips and is transported over the H.225 channel. After the FastConnect procedure is finished or when it fails, the normal H.245 procedures start.
A number of extensions to H.323 include mechanisms for more efficient call setup (H.323 Annex E) and reduction of protocol overhead e.g., for simple telephones (SETs, simple endpoint types and H.323.Annex F).
{ 2.2.1.3 Gatekeeper discovery and registration
An H.323 endpoint usually registers with a gatekeeper that provides basic services like address resolution for calling the other endpoints.There are two possibilities for an endpoint to find its gatekeeper:
- Multicast discovery
The endpoint sends a gatekeeper request (GRQ) to a well-known multicast address (224.0.1.41) and port (1718). Receiving gatekeepers may confirm their responsibility for the endpoint (GCF) or ignore the request
- Configuration
The endpoint knows the IP address of the gatekeeper by manual configuration.While there is no need for a gatekeeper request (GRQ) to be sent to the preconfigured gatekeeper, some products need this protocol step. If a gatekeeper receives a GRQ via unicast, it must either confirm (GCF) the request or reject it (GRJ).
When trying to discover the gatekeeper via multicast, an endpoint may request any gatekeeper or specify the request by adding a gatekeeper identifier to the request. Only the gatekeeper that has the requested identifier may reply positively. (see Figure 2.3)
Endpoint
Gatekeeper