In a standard H.323 call flow between two endpoints (T1 and T2), two separate, reliable signaling channels are established directly between them. These channels use the Transmission Control Protocol (TCP). The first TCP connection is for H.225.0 call signaling, which handles call setup and teardown (using messages like Setup, Connect, Release Complete). The second TCP connection is for the H.245 control channel, which is used for capability negotiation, opening and closing logical media channels (for RTP streams), and other control messages. Therefore, a total of two TCP connections are required between the endpoints for a complete H.323 call.

1. ITU-T Recommendation H.323 (06/2006): Packet-based multimedia communications systems.

Section 7.1

"Call signalling channels": This section defines the two distinct channels: "The Call Signalling Channel is a reliable channel used to convey H.225.0 call control messages... The H.245 Control Channel is a reliable channel used to convey H.245 messages." In the context of IP networks

a "reliable channel" is implemented using TCP. The standard call flow diagrams illustrate these as separate connections established between the endpoints.

2. Davidson

J.

& Peters

J. (2000). Voice over IP Fundamentals. Cisco Press.

Chapter 5

"H.323

" Section "H.323 Call Between Two Terminals in the Same Zone": This chapter details the call setup process

stating: "The H.225 signaling connection is established on TCP port 1720... After the H.225 call setup is complete

an H.245 session is established on a dynamically assigned TCP port. The H.245 session is used to exchange the capabilities of the H.323 terminals." This explicitly confirms two separate TCP connections.

3. Huawei HCIA-UC V4.0 Training Material: H.323 Protocol Basics.

The official training material for the corresponding certification path describes the H.323 protocol suite and call establishment process. The section on basic call flows illustrates the establishment of the H.225.0 call signaling channel (TCP) followed by the establishment of the H.245 control channel (TCP) between the two communicating endpoints.