Level 2 Corresponding interface for Network (e.g wireless GSM, UMTS network etc.) System control H.245 control SRP/LAPM System control procedures User interface Modem control V.25ter H.324M area Figure 6.2 The ITU Rec. H.324 model. Adapted to wireless from what was originally meant for fixed networks. Service Components in UMTS The H.324 terminal has an operation mode for use over ISDN links. Annex D in the H.324 recommendations defines this mode of operation as H.324/I [3]. H.324/I offers direct inter-operability with the H.320 terminals, H.324 terminals on the GSTN, H.324 terminals operating on ISDN, and voice telephones. For seamless data communications between UMTS and PSTNs, the UMTS call control mechanism takes into account V.8bis messages. These messages get interpreted and converted into UMTS messages and V.8bis, respectively. The latter contains identifica- tion procedures and selection of common modes of operation between data circuit- terminating equipment (DCE) and between data terminal equipment (DTE). Essential V.8bis features include: flexible communication mode selection by either the calling or answering party; enabling automatic identification of common operating modes; enabling automatic selection between multiple terminals sharing common tele- phone channels; friendly user interface to switch from voice telephony to a modem based communi- cations. 6.4.1.3 Enabling Packet Switched Video Telephony The H.323 ITU-T protocol standard for multimedia (and IP telephony) call control en- ables PS multimedia communications in UMTS. The standard: employs a peer-to-peer model in which the source terminal and/or GW is the peer of the destination terminal and/or GW, treats gateways (GW) and terminals alike; requires GWs and terminals to provide their own call control/processing functions; provides multiple options for voice, data and video communications; it may employ a gatekeeper function to provide telephone number-to-IP address translation, zone admission control and other resource management functions.