ans a bearer service such as frame relay, ATM, PSTN, PLMN or ISDN. The MSG is a specific type of MGW. Also connections to other networks are realized through MSGs. INCUMBENT OPERATOR INITIAL NETWORK Figure A3.3 shows, in a simplified way, the network structure of an existing metropolitan network. The network consists of two pairs of tandem/transit switches (TS) and around 25 local switches (LS). Every local switch has two routes for security reasons, one to each tandem/transit in a pair. MIGRATION STEPS Phase 1 As a first step, a telephony server and two MSGs could be introduced in the network. See Figure A3.4. This will be a first embryo to an ATM backbone network. Two of the tandem switches are connected to the two MSGs. Appendix 3: Development Tracks The vision Telephony Multimedia International servers server switches Multi-service gateways to other networks Multi-service network Telephony = Multi-service gateways Multi-service broadband access through access nodes Figure A3.2 A target network variant for an incumbent operator Transit traffic to other networks 12 LS's 12 LS's Figure A3.3 A simplified example of an existing metropolitan network With this in place, we can then start to connect some of the local switches to the MSGs and begin routing some voice calls over the ATM network. When this arrangement works perfectly, we can remove the old routes to the tandems (marked with crosses in Figure A3.4). We still have the same reliability as before in the network. We can also connect IP routers or other data sources to the MSGs and thus transfer data traffic through the ATM network. In this way we can verify that it is possible to mix voice and data traffic in the same network. Understanding Changing Telecommunications - Building a Successful Telecom Business Router Router 12 LS's 12 LS's Figure A3.4 First phase in the migration towards a multi-service network Phase 2 In the next step, we could add two more MSGs to expand the ATM network. We can connect the other two tandem switches to the two new MSGs and th