ilable now with almost every GSM network. Chapter 14 Wireless WANs This IP-based connectivity supports a wide range of enterprise and consumer applications and provides users with throughput rates of up to 40Kbps, with the convenience of being able to connect from anywhere. GPRS customers enjoy advanced, feature-rich data services such as color Internet browsing, e-mail on the move, powerful visual communications such as video streaming, multimedia mes- sages, and location-based services. For operators, the adoption of GPRS is a fast and cost-effective strategy that not only supports the real first wave of mobile Internet services but also represents a big step toward 3GSM (i.e., W-CDMA) networks and services. GPRS has been integrated into GSM standards releases since Release 97. At the end of 2005, there were a total of 290 live GPRS networks. GPRS is a packet-switched solution that works by overlaying a packet-based air interface on the eight time slots used for GSM transmissions. GPRS is an always-on service; it enables continuous flows of IP data packets over the system for applications such as Web browsing and file transfer. In theory, GPRS supports transmission speeds of up to 172.2Kbps. In practice, due to various interference issues that affect wireless communications, the experienced speeds are substan- tially lower than this. Figure 14.5 shows the main elements of the GPRS network. Basically, two core modules are required: a gateway GPRS support node (GGSN) and a serving GPRS support node (SGSN). The GGSN acts as a gateway between the GPRS network and a public data network such as an IP network or perhaps even an X.25 network. The GGSN also connects to other GPRS networks to facilitate GPRS roaming. The SGSN provides packet routing to and from the SGSN service area for all users in that ser- vice area. It provides the packet-switched link to mobile stations. A few additional changes need to take place, including the addition of the packet control units, which, for the example shown in Figure 14.5, are h