server in the H.323 standard. The gatekeeper has signalling channels open to the clients (but signalling also occurs between clients directly). See Figure 8.32. VoATM - VoATM systems (Figure 8.33) exist for mobile and fixed operators, but the approach is different. Service Implementation Central VolP control Central VolP control with VPN yellow SIP gatekeeper server, encrypted signalling (no speech encryption) IP CORE VPN yellow VPN yellow Figure 8.32 I-VPN and VoIP are added to the service offer of the actor Service layer Q.931 Telephony Telephony server server MEGACO H.248 ATM connectivity backbone Connectivity control broadband signalling Other operator's network Figure 8.33 VoATM network example Thus ATM (and IP) is used in new 3 G radio access networks for all types of services: voice, data and video. Thanks to the ongoing 3 G investments in the radio access networks the packetization in mobile networks may start in the access part. The 'mini-cell' standard AAL2 is favoured because the cell fill time for ordinary 53 byte cells becomes long. The reason is the low output bit rate from modern coders (below 10 kbit/s, for example for AMR coding). Understanding Changing Telecommunications - Building a Successful Telecom Business In fixed networks the modernization normally starts in the core network. The solutions allow an existing circuit-switched network - usually a network owned by a large telecom operator - to evolve smoothly. Typical characteristics that favour modernization are: High traffic load in backbone network Targeting one infrastructure for all traffic types Needs to assure full service transparency Long-term investment Highly meshed network The circuit mode traffic in fixed networks is usually PCM coded with 64 kbit/s and primarily AAL1 is used. Two new standard signalling protocols are used (see Figure 8.27): The BICC protocol, which is based on ISUP and is used for signalling between tele- phony servers. The H.248 protocol, which is used by the telephony server to control the media gate- ways. Th