he PLMN as if the PLMN were a router in the PDN, i.e. by using IP or X.25. A terminal system that has a fixed connection to a PDN can send and receive PDUs (e.g. IP-PDUs) at will. When the access is over a mobile connection, a number of additional activities are required (these are all performed by the GPRS process control system): An MS is not always connected to the PLMN, i.e. most of the time the PLMN is not aware of the MS. The MS is roaming around, just checking which PLMN and PDCH are most suitable to use, if the MS user would like to be connected. The MS is said to be in the idle state. When the user wants to use GPRS, an attachment procedure is executed over i1.0 (in the GPRS service access protocol, GMM, see Fig. 4.11). The first part of this procedure establishes a logical connection (identified by a temporary logical link identity, TLLI) between the PLMN and the MS over the radio interface. The TLLI uniquely identifies the MS amongst all other MSs that are attached to the PLMN within the same routing area (a routing area is one or a number of cells). The MS is now in the attached state. The second part of the procedure connects the user to the external PDN, consid- ering a specific PDN, a specific type of PDN and an address of the PDN (i.e an IP or an X.25 address). This is called activation and is also handled over the GMM protocol. The MS users define their needs in parameters called Access Point Name (the desired PDN, e.g. the Internet or some private packet net- work), PDPtype (IP or X.25) and PDPaddress (the user's own address in the PDN; this can be a fixed, subscribed address, or a temporary address from the PLMN). The MS is now in the active state. 4.4 General Packet Radio Service, a Case Study From this point users can communicate with other hosts on the PDN as if they had a fixed access to the PDN. The burden on the process control system does not stop here, however. Since the MS is still moving around, the process control system must keep track of the current position. Part of this job is pe