s it is important that an investment in an ATM network can be retained, if the telephony service is supposed to migrate from ATM to IP as a long-term goal, when the volumes and revenues for the data and multimedia services increase. Let us take a look at a possible migration path from ATM to IP over ATM. Appendix 3: Development Tracks Router Router 12 LS's 12 LS's Router Router Figure A3.8 Phase 5 in the migration process Router Router 12 LS's 12 LS's Router Router Figure A3.9 The result of phase 5 in the migration process Understanding Changing Telecommunications - Building a Successful Telecom Business Telephony server upgraded and reused H.248 H.248 H.248 H.248 network network MSGs upgraded and reused Figure A3.10 Migration from ATM to IP connectivity Step 1 The signalling system BICC CS1 (bearer independent call control - capability set 1) is used as call control protocol between the telephony servers. The media gateway control is designed using the standardized H.248 protocol. The bearer control employs standard- ized ATM signalling (UNI, PNNI and A-INI). The signalling from the circuit-switched network (e.g. DSS1 or ISUP) is transported transparently through the ATM network to the telephony server via AAL1 circuit emulation. Step 2 BICC CS2 will be used. The H.248 protocol will still be used for the control of media gateways from the telephony server. The basic applied principle is still that the call control and bearer control are separated in the network. Media gateways must convert the narrowband signalling information into IP packets and send them through the IP network. See Figure A3.10. Upgrade Issues The migration from ATM to IP connectivity can be done by means of some additional hardware, and change of software packages in the server and in the media gateways. The ATM backbone network can be reused to carry IP over ATM for voice as well as for data and multimedia services. Examples of changes are the BICC signalling and the IP address handling. Appendix 4 Dimensioning Media Gateways and Associated 