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H.323 Entities and Responsibilities
Figure 2 shows the different entities that can be involved in an H.323 conference. A gatekeeper is an optional element in an H.323 conference that provides call control services to the H.323 endpoints such as address translation, admission control, bandwidth control, and zone management. The gatekeeper may also provide other optional functions such as call authorization and call accounting information. A gatekeeper cannot be specified as a destination in a call. Address translation is the method by which an alias address (e.g., e-mail address) is translated to a transport address. Admission control is a way of limiting H.323 access to a network, or to a particular conference using Request, Admission, and Status (RAS) messages defined in Recommendation H.225.0. A gatekeeper also manages bandwidth allocation to H.323 endpoints using RAS messages. Zone management defines the scope of entities over which a gatekeeper has control. These include endpoints, gateways, and MCUs. In addition, a gatekeeper can allow secure access to the conference using various authentication mechanisms. Q.931 and H.245 messages can be routed through the gatekeeper, and statistical information about the calls in progress can be collected. Gatekeepers may also perform telephony service operations such as call forwarding and call transfer.
H.323 endpoints can interact with each other directly in a point-to-point or multipoint conference if no gatekeeper is present. When a gatekeeper is present, all endpoints have to register with it.
A gateway operates as an endpoint on the network that provides real-time, two-way communication between H.323 terminals on the packet-based network and other ITU terminals on a switched-circuit network, or to another H.323 gateway. Other ITU terminals include those complying with Recommendations H.310 (B-ISDN), H.320 (ISDN), H.321 (ATM), H.322 (GQoS-LAN), H.324 (GSTN), H.324M (Mobile), and POTS. The gateway provides the appropriate translation between transmission formats (for example, H.225.0 of an H.323 endpoint to/from H.221 of an H.320 endpoint) and between communication procedures (for example, H.245 of an H.323 endpoint to/from H.242 of an H.320 endpoint). This translation is specified in Recommendation H.246. The gateway also performs call setup and clearing on both the network side and the Switched-Circuit Network (SCN) side. Translation between video, audio, and data formats may also be performed in the gateway. In general, the purpose of the gateway is to complete the call in both directions between the network endpoint and the SCN endpoint in a transparent fashion.
The MCU is an endpoint on the network, which provides the capability for three or more terminals or gateways to participate in a multipoint conference. It may also connect two terminals in a point-to-point conference, which may later develop into a multipoint conference. The MCU consists of two parts: a mandatory Multipoint Controller (MC) and optional Multipoint Processors (MP). The MC provides the capability for call control to negotiate with all terminals to achieve common levels of communication. It is this element that is required for all multipoint conferences. The MP allows mixing, switching, or other processing of media streams under the control of the MC. The MP may process a single media stream or multiple media streams depending on the type of conference supported. In the simplest case, an MCU may consist only of an MC with no MPs.
The following section provides an overview of the basic operation of an H.323 endpoint in a
point-to-point conference without a gatekeeper and then with a gatekeeper.
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