Minimum Number of H.225 Messages for H.323 Call

B.

To establish an H.323 call with bidirectional media, a minimum of three H.225 messages are required.

The H.225 protocol is used for call setup and call signaling in H.323. The three H.225 messages required for establishing a call with bidirectional media are:

1. ARQ (Admission Request): The ARQ message is sent by the calling endpoint to the gatekeeper. The ARQ message contains information such as the called party's address and the type of call being requested (audio, video, or data). The gatekeeper will either grant or deny the request based on various factors such as available resources, policy, and authentication.

2. ACF (Admission Confirm): If the gatekeeper approves the call request, it sends an ACF message back to the calling endpoint. The ACF message contains information such as the called party's address, the IP address and port number for RTP (Real-time Transport Protocol) and RTCP (RTP Control Protocol), and a call identifier.

3. Setup: The Setup message is sent by the calling endpoint to the called endpoint. The Setup message contains information such as the calling party's address, the type of call being requested (audio, video, or data), the called party's address, and the call identifier received in the ACF message. The called endpoint sends a Call Proceeding message back to the calling endpoint to indicate that it is processing the call setup request. Once the called endpoint is ready to establish the call, it sends a Connect message to the calling endpoint.

Therefore, a minimum of three H.225 messages are required to establish an H.323 call with bidirectional media, namely ARQ, ACF, and Setup. Options A, B, and D can be ruled out as they do not meet the minimum requirement of three messages. Option E is also incorrect as five messages are not required to establish an H.323 call with bidirectional media. Option C is the correct answer, as it correctly identifies the minimum number of H.225 messages required for call setup.