d in Ref. [2]. In the following we introduce the reference architecture which will realize the type of services presented above and illustrated in Figure 9.4. Supplementary services Value added Toolkits: multimedia non-call services related (SIP) services CAMEL Circuit (e.g. telephony, (e.g. eMail,MM teleservices chat, www, News, SIM ATK whiteboards) etc...) Telephony "Internet tools" SoLSA etc.. Other Bearer Circuit services Bearer SMS,UUS, services Figure 9.4 Service classification [2]. Towards IP Based Networks RELEASE 00 REFERENCE ARCHITECTURE While the standardization groups have split ROO in R4 and R5 in order to achieve its specification pragmatically in phases, here all for practical purposes we will refer to them as ROO. Hence, all forthcoming notation based on Ref. [7] is addressed as ROO. To achieve access independence and to maintain a smooth interoperation with wireline terminals across the Internet, ROO aims conformance as far as possible with IETF Inter- net standards for cases where an IETF protocol has been selected, e.g. SIP. To support VoIP, the architecture assumes that the standard includes a minimum set of mandatory codecs and minimum set of mandatory protocol options. Specifications in Ref. [7] out- line the principles of the reference architecture; thus, here we provide mainly an over- view of the architecture and its components. 9.2.1 Overview of Release 00 Architecture Figure 9.5 provides a generic view of the ROO architecture. Notice that the following interfaces correspond also to the ROO reference architecture: E interface - between MSCs (including MSC server/MGW); G interface - between VLRs, G interface; Gn interface between SGSNs, Gm interface - between CSCF and UE; Gs interface (op- tional) between MSC (or MSC server) and SGSN. Alternative Legacy mobile Applications & Access signalling network Services*) Network Multimedia R-SGW IP Networks HSS *) T-SGW GERAN PSTN/ UTRAN Legacy/Extemal MSC Server GMSC server T-SGW *) Applications HSS *) & Services *) R-SGW *) Signalling Interface *