The architecture of GPRS is designed SO that signaling and high-level data protocols are system-independent. Only the low-level protocols in the radio interface must be changed to operate with the same services. The SGSN can be viewed as a packet-switched Mobile Switching Center (MSC); it delivers packets to mobile stations within its service area. SGSNs send queries to Home Location Registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS mobile stations in a given ser- vice area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobil- ity management functions such as mobile subscriber attach/detach and location management. The SGSN is connected to the base station sub- system via a Frame Relay connection to the packet control unit (PCU) in the BSC. GPRS requires modifications to numerous network elements. GPRS Subscriber Terminals A totally new subscriber terminal is required to access GPRS services. These new terminals will be backward compatible with GSM for voice calls. New terminals (TEs) are required because existing GSM phones do not handle the enhanced air interface, nor do they have the capability to pack- etize traffic directly. A variety of terminals will exist, as described in a pre- vious section, including a high-speed version of current phones to support high-speed data access, a new kind of personal digital assistant (PDA) device with an embedded GSM phone, and PC Cards for laptop computers. All these TEs will be backward compatible with GSM for making voice calls using GSM. GPRS BSS A software upgrade is required in the existing Base Transceiver Site (BTS). The Base Station Controller (BSC) will also require a software upgrade, as well as the installation of a new piece of hardware called a packet control unit (PCU). The PCU directs the data traffic to the GPRS network and can Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 