cated for GPRS. Information about whether the Mobile Communications network provides GPRS service and which channels (frequencies and time slots) are allocated for packet users is broadcast on the cell broadcast channel. HLR/GR stores information about the services and present location of its MSs (SGSN). SGSN, which corresponds to MSC/VLR, stores more accu- rate location of MSs and data related to their current service, such as the cur- rent IP address. It also performs security functions, such as authentication. When a GPRS user wants to access a packet-switched service, for exam- ple, the Internet, he or she performs GPRS attach. Then IP address is allo- cated for the MS and the user sees the Web page of his or her ISP's browser. The URL (see Section 6.6.11) of this default page is stored in his or her sub- scriber information in HLR/GR and transferred to SGSN at the time of GPRS attach. The GGSN in Figure 5.16 acts as a border router between GPRS and the Internet; the GPRS network looks the same as any other IP network from outside the Internet. The GGSN stores the routing table for all active IP addresses to be able to route packets to the correct SGSN. The SGSN then routes packets further to the cell where the destination MS is currently located. Physical radio channel, one time slot in each TDMA frame, transmits data blocks that occupy one time slot in four subsequent TDMA frames. Multiple users share each physical channel and each downlink packet con- tains identification of the destination MS. In the uplink direction, some data blocks are reserved for uplink channel requests from MSs. When an MS wants to transmit packet data it requests an uplink packet channel. Accord- ing to these requests, SGSN assigns uplink capacity to MSs. Each downlink data block contains MS identification that is allowed to transmit the next block in the uplink direction. We see that a mobile station can be connected to GPRS service con- tinuously, but it reserves capacity only if it needs to transmit or receive. Charging can be b