here timeslots are dedicated to a user throughout the lifetime of the call irrespective of activity. In the Gb interface, the same user plane is used to send signalling and user data. Separate physical E1s are not required to be dedicated for signalling purposes. Access rates per user may vary without restriction from zero data to the maximum possible line rate, which is 64 X 32 = 2048 kbps in the case of an E1 trunk. The Gb interface can also be implemented over the IP. Routing Area The routing area (RA) in the GPRS is analogous to the location area (LA) in the GSM. A GPRS network is divided into several routing areas. An RA may consist of one or more cells as planned by a Gb interface IP Addressing planner. The location of an MS in the standby state is known to the SGSN only at a routing area level and is paged in the whole RA. A routing area identifier (RAI) identifies an RA. Non-GPRS cells are grouped in a null RA. When a mobile-terminated packet arrives at the SGSN, the MS is paged in the RA where it is located. For circuit switched services it is also paged in null RA. An RA is a subset of LA and cannot span across two LAs. An RA is served by a single SGSN. A combination of RAC and LAC should be unique: RAI = MCC + MNC + LAC + RAC Charging A charging gateway (CG) is responsible for collection of CDRs (call data records) from the GGSN and SGSN in a GPRS network. It is connected using a Ga interface. The operator may define whether charging will be done on an individual MS basis and/or a PDP context basis. The SGSN CDRs contain charging information for each MS related to the radio network usage. SGSN CDRs at least contain the following information: the amount of data sent/received by a MS on radio interface; usage of PDP addresses: how long an MS has used PDP addresses assigned to it; location of MS: HPLMN, VPLMN, etc. The GGSN CDRs contain charging information related to the external data network usage. The GGSN CDRs at least contain the following information: destination and source addresses; the amount of 