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714 SIGNALING FOR VoIPFigure 21.6-4. BCTP encapsulation header (From Rec. Q.1990. R reproduced with the kindpermission of ITU).Messages are first encapsulated by the sending BCF using the Bearer-ControlTunneling Protocol (BCTP) [33], a generic tunneling procedure for transportingbearer-control messages. Encapsulation inserts a header with the format shown inFig. 21.6-4. The resulting BCTP PDU is then sent from the BCF to the CSF usingthe H.248 tunneling mechanism (not discussed here [16]), and from the CSF tothe peer CSF using the APM procedure (see previous section). At the destinationCSF, the process is reversed. The tunneling process is shown in Fig. 21.6-5.ATM Networks. ATM networks use ATM protocols for bearer channel setup, suchas UNI, PNNI, and B-ISUP, discussed in Chapter 22. Tunneling is not used.21.6.6 BICC Call SetupBICC call setup involves the interaction of all entities in the serving node architectureof Fig. 21.6-2 and the interworking of all BICC protocols.An example of call setup is shown in Fig. 21.6-6. In the example bearer channelsare set up from the originating (preceding) SN, and all SN components communicatevia protocol messages, except for the MCF–MMSF interface, assumed to beinternal. The example shows a call setup sequence between two adjacent SNs,Figure 21.6-5. IPBCP tunneling.