orks required major investments accommodate the growth, while network operators did not want to invest too much in 'old telephony' networks. BICC can support PSTN/ISDN/PLMN services over a broadband connectivity network. It is specified by ITU-T in recommendation Q.1901 and allows any bearer to be used, such as IP, ATM and TDM. The BICC protocol allows operators to offer the complete set of PSTN/ISDN services with operator-grade quality, including all supplementary services, over a variety of packet networks. The BICC network is scalable. BICC supports all currently deployed services on circuit mode, ATM and IP networks, including third-generation wireless, yet does not limit the future introduction of new multimedia, multi-mode services and applications. Figure 12.9 shows the basic BICC network architecture. The general idea of a BICC network is the separation of call signalling on the one hand, and bearer signalling and user traffic on the other hand. The BICC protocol is used for call signalling functions, such as indicating the requested type of ISDN service and call routing, based on the dialled phone number. A bearer signalling protocol is used to set up a connection over the packet network to transport the speech or data payload. The bearer signalling pro- tocol depends on the type of packet network for example ATM AAL1, ATM AAL2, IP or TDM. The key network element of the BICC architecture is the interface serving node (ISN). The ISN is the gateway between the traditional PSTN/ISDN network and the BICC network. Three functions have been distinguished in an ISN: the bearer function (BF), the bearer control function (BCF), and the call serving function (CSF). The BF is a media gateway that converts the signals transported over time-division multiplexed (TDM) trunks in the PSTN/ISDN network to an appropriate form that can be transported over the packet network. Also functions like echo cancellation and transcoding may be present in the BF. The BCF controls the bearers by using the appropriate signalling proto