ls. (The H.323 usage of Q.931 is not compatible with Q.931 as used in an ISDN network.) H.245 is used for control signaling or media negotiation and capability exchange between terminals. T. .120 is used for multipoint graphic communications. H.323 audio codecs are specified in the ITU G.7xx series. Video codecs are specified in the H.26x series. H.323 also ref- erences two IETF protocols, RTP and RTCP, for the media transport which are described in Sections 7.2 and 7.3. The H.235 recommendation covers privacy and encryption, while H.450 covers supplementary services such as those com- monly found in the PSTN (e.g., call forwarding, call hold, and call park). Table 8.2 Protocols Referenced by H.323 Protocol Description H.225 Registration, admission, and status (RAS) and call signaling H.245 Control signaling (media control) T.120 Multipoint graphic communication G.7xx Audio codecs H.26x Video codecs Real-time transport protocol (RFC 3550) RTP control protocol (RFC 3550) H.235 Privacy and encryption H.450 Supplementary services SIP: Understanding the Session Initiation Protocol 8.2 Example of H.323 Figure 8.2 shows a basic call flow involving two terminals and a gatekeeper. The flow shows the interaction between the various elements and the various proto- cols used to establish the session. The call begins with an exchange of H.225.0 RAS messages between the calling terminal and the gatekeeper. All RAS messages are transported using UDP. It is assumed that both terminals have already regis- tered with the gatekeeper using the Registration Request (RRQ) message. The calling terminal sends an Admission Request (ARQ) message to the gatekeeper containing the address of the called terminal and the type of session desired. The address could be specified as an H.323 alias, E.164 telephone number, e-mail address, or a URL. The gatekeeper knows about all calls in the zone it controls; it decides if the user is authorized to make a call and if there is enough bandwidth or other resources available. In this example, there is