Introducción y objetivo del presente trabajo

H.323 Gateway is used to translate H.323 messages into XGSP XML messages and it also hides the complexities of H.323 protocol from XGSP Session Server. XGSP Session Server does not need to understand the H.323 protocol. H.323 terminal also does not need to understand XGSP messages, since H.323 Gateway translates XGSP XML messages into H.323 binary messages and sends H.323 terminals necessary information based on the received responses from XGSP Session Server.

When a session is activated H.323 Gatekeeper keeps the alias name of the session, which is the sessionID of the active session. In addition to that, H.323 Gatekeeper also keeps H.323 terminal registrations. So when a H.323 terminal wants to join a XGSP session it needs to use the sessionID of the active session when calling H.323 MCU. H.323 Gatekeeper translates this sessionID into H.323 MCU address and routes the call to the registered H.323 MCU.

3.3.1.1

H.323 Call Setup and Termination

In order to establish a call between two endpoints, information needed is signaling destination address, media capabilities and media transport addresses at which both endpoints can receive RTP packets. When H.323 call is received by H.323 MCU, it parses the information received from client and exchanges XGSP XML messages with XGSP Session Server. Figure 3-15 illustrates a H.323 call setup with XGSP Session Server. This procedure has three important steps: H.225 call setup, H.245 capability exchange and audiovisual logical channel creation.

The first step in H.323 call is the call setup. At this step when H.323 Gateway receives a call setup from a H.323 terminal, it sends a JoinSession (Figure 3-6) message with the sessionID of the active session to tell XGSP Session Server that a H.323 terminal wants to join the XGSP session. If XGSP Session Server permits the client, it sends a SessionSignalResponse (Figure 3-8) as a reply while setting the Result field to OK and setting the media description of the session to let the client know what media formats are supported by the XGSP session. This step is repeated for each media type, audio and video as seen in Figure 3-15. At the end of this step H.323, Gateway maintains session media description for audio and video and H.323 terminal is considered as made a successful connection.

In the second step, H.323 Gateway and H.323 terminal exchanges the media capability. Client chooses one of the media formats supported by XGSP session. Usually XGSP Session Server provides only one media format which is supported by all clients in videoconferencing sessions, for instance for audio ULAW and for video H.261 formats.

In the next step, H.323 terminal makes a request to establish channel for one of the media types, i.e., audio. H.323 Gateway also sends H.323 terminal an open channel request for incoming stream. H.323 Gateway sends another JoinSession message to XGSP Session Server with media type and format provided by H.323 terminal. The same procedure is repeated for other media types, i.e. video. During the JoinSession procedure, XGSP Session Server maintains the state of the call. So it expects that the second JoinSession request will include the media capability of the H.323 terminal. It also waits for a fixed amount of time between audio and video channel establishments. If within that amount of time, H.323 Gateway does not send JoinSession message for the other media type, it concludes that the H.323 terminal will only receive and/or send one type of media.

When H.323 terminal wants to leave the session, the signaling between H.323 terminal and H.323 Gateway takes place according to H.323 protocol. H.323 Gateway sends LeaveSession (Figure 3-7) message to XGSP Session Server for the channels opened and XGSP Session Server unsubscribes H.323 terminal from media servers.

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3.3.1.2

H.323 Terminal Video Selection

XGSP Session Server provides audio and video stream lists in the session. Stream lists can be obtained as explained in section 3.2.8. Also a H.323 console can be launched from GlobalMMCS web interface. This console is a basic audio/video control client for H.323 terminal. Using this console, video stream displayed on H.323 terminal can be switched.

The video selection for XGSP clients is explained in section 3.2.6. For H.323 terminals, H.323 console can make the video selection requests. When a video stream is selected, H.323 console sends a VideoSourceSelection (Figure 3-9) message to XGSP Session Server. When a new video stream is selected and the request is processed by XGSP Session Server, H.323 terminal needs to be notified for this selection so that it can refresh its incoming video stream. For this reason, XGSP Session Server sends a VideoSwitch message as shown in Figure 3-16 to H.323 Gateway to notify H.323 terminal. Then H.323 Gateway notifies H.323 terminal to start video switching.

Figure 3-16: Representation of VideoSwitch XML message