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674 SIGNALING FOR VoIPFigure 21.2-4. H.323 channels.Call Signaling Channel. This channel transports Q.931 protocol messages(Chapter 10), as amended by Rec. H.225. It provides signaling communicationbetween H.323 endpoints for basic call-control messages (call setup and release)and to obtain the transport address of the control channel. It uses TCP transport.Control Channel. This channel transports H.245 protocol messages [3]. It providessignaling communication between H.323 endpoints to control (open and close)Logical Channels and to negotiate and modify media parameters. This channelmay also be used for authentication and encryption, according to Rec. H.235 [4].It uses TCP transport.Logical Channel. This channel transports RTP/RTCP messages (Section 20.4.1)for media (bearer) flows. It consists of a RTP and a RTCP connection betweentwo H.323 endpoints, set up via H.245 messages on the Control Channel. Therecan be one bidirectional channel or multiple unidirectional channels in a connection,allowing for asymmetric media communication. This channel uses UDP transport.21.2.3 H.323 Call-Control Messages and ParametersH.323 signaling messages are bit-oriented and formats for most of them are specifiedwith the ASN.1 notation (Section 16.2 and [5]), which makes it impractical todescribe bit-level structures in concise form. This section describes those messagesat a fairly high level and the reader interested in the details is referred to the relevantITU-T recommendations.RAS Messages. These messages use the RAS Channel and the most importantones are listed in Table 21.2-2.ARQ is sent by an endpoint to request, and ACF by the GK to grant, admission tothe network for a call.