is based on service description parameters. Service description parameters are for example bearer type (e.g. ATM/AAL2), codec (e.g. AMR) and B- number in telephony-oriented networks. If the coding is changed along the path a trans- coder is linked into the path. In packet-oriented networks the necessary packet call set-up information is called packet data protocol (PDP) context. It includes for example num- bering and QoS information. IP addresses for packet traffic to/from attached devices are often provided by a RADIUS server connected to the GGSN node in UMTS. A RADIUS server also has AAA functions. Other security functions are creation of tunnels, which is one of the main protection methods in WCDMA/UMTS. Tunnelling protocols are added to the packet stack. See the central part in Figure 8.15 starting with IP and ending with PPP, which are extra protocols. The tunnels form virtual private networks. The resources in the radio access network are called radio access bearers (RABs). An early version has the following RABs: for real-time services an ATM AAL2 path is available (conversational RAB for AMR speech) and also nx64 kbit/s channels (n starting with 1). As UMTS progresses with issues like QoS, it will be possible to also provide real-time packet-based services based on IP. A common IP path for all services is seen as the ultimate solution. The 1 X 64 kbit/s bearer service is called conversational RAB for 64 kbit/s multimedia. Service Implementation A combination of several of these 64 kbit/s bearers is possible by using the multi- call service. The main use is for circuit-based multimedia (including streaming services) according to H.324 M. The service can be connected to IP networks (using H.323), ISDN networks (using H.320 or H.324I), and to other networks with H.324 M. For non-real time services a much cheaper best effort packet mode is acceptable and therefore preferable. To start with the packet services will be 'best effort services. Both point-to-point and point-to-multipoint services will be possibl