ll 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 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 protocol for the packet network and can establish and remove bearer connections over the packet network. The CSF handles the call signalling, that is, ISUP signalling to the Control Network M = Media gateway M3UA, SCTP see section 14.9.2 R = Router LE = Local exchange BICC/M3UA/SCTP/IP IPBCP Mobile systems: H.248/SCTP/IP/(MPLS) GCP and ISUP over ISUP/RTP/UDP/IP/(MPLS) M3UA/SCTP/IP/(MPLS) network Circuit Emulation Circuit Emulation voice/RTP/UDP/IP/MPLS IPBCP to set up a bearer between MGWs IPBCP is tunneled through H.248 and BICC Figure 14.13 Example of signaling protocols and signalling and voice transport in an IP core as defined in Figure 14.12 PSTN/ISDN network and BICC signalling in the BICC network. The call mediation node (CMN) can be used to route the BICC messages and may be useful in a large-scale BICC network with a large number of ISNs. Two other nodes have also been defined for the BICC architecture: the transit serving node (TSN) and the gateway serving node (GSN) The TSN is a node that handles BICC on both sides. A TSN may be used to offer intelligent network ba