e same protocol can also be used in core networks connecting mobile systems. The migration from a circuit mode core network to an ATM core network is exemplified in Appendix 3. For large ATM networks a hierarchical network structure can be adopted. The structure is similar to a modern transit layer design in circuit-mode networks, but the capabilities are of another order of magnitude. See Figure 8.34. Interactive Class Interactive traffic can be mixed with other classes, such as streaming. MGW hub MGW A MGW B MGW A MGW B MGW A MGW B Figure 8.34 A hierarchical ATM structure Service Implementation According to Chapter 5 the most critical needs are round-trip delay and accuracy. Short round-trip delay is also requested by conversational services. Interactive games are a typical application, and also ordinary WWW services. The round-trip delay issue might be somewhat problematic for TCP traffic in mobile networks. See Chapter 7. 8.9 CONVERGENCE BETWEEN FUNDAMENTAL TECHNICAL PLANS 8.9.1 NUMBERING/ADDRESSING PLAN Regarding terminology the term 'addressing' is more common within the datacom indus- This section on numbering/addressing is more basic than task oriented. A goal is to illustrate the consequences of the present convergence between the IP world and the telecom world (telecom in a narrow sense). The convergence is very clear regarding numbering: two numbering plans are used, the E.164 for PSTN/ISDN/PLMN/ATM and IP numbering for IP networks (IPv4 or IPv6). Numerous cases of interworking will occur. Within IETF and ITU-T work is proceeding on mapping E.164 numbers to URLs that can be used for SIP, HTTP and SMTP. The numbering area has grown a lot and it offers several challenges for the plan- ner. One is the scarcity of IPv4 addresses, which is counteracted by the use of network address translation (NAT) using addresses which are valid in a local area, without reduc- ing the overall IPv4 address space. NAT translates between authorized addresses on the Internet and private addresses, provides a type of proxy/fir