mitted in bursts and the carrier frequency is changed after each burst. There are 79 carrier frequencies with 1-MHz spacing over which the transmission frequency hops. Each piconet uses a different pseu- dorandom hopping sequence over the 79 carriers. Several piconets can oper- ate in the same area simultaneously because their signals interfere only at a time when they happen to occupy the same frequency channel. The modulation rate of Bluetooth is 1 Mbps, which all devices and both transmission directions in the piconet share. If we compare WLAN and Bluetooth technologies we see that WLAN is a system for a work group (LAN) and Bluetooth is for only a single user (PAN). The number of devices in the Bluetooth network is very limited and data rate available for each device is quite low. 5.5 GSM As an example of a digital cellular network, we introduce the structure and operation of the GSM network. The European digital cellular system GSM was developed by CEPT during the 1980s, and this work was continued by ETSI. The acronym GSM came originally from the stan- dardization working team, but GSM is presently understood to mean Global System for Mobile Communications. Two other cellular networks are based on GSM technology: the European DCS-1800, which operates in the 1.8-GHz band, and the American GSM-1900, which operates in the 1.9-GHz band. Our discussion in this section is valid for all of these networks. In GSM, unlike in analog mobile networks, subscription and mobile equipment are separated. Subscriber data are stored and handled by a sub- scriber identity module (SIM), which is a smart card belonging to a subscriber. With this card the subscriber can use any mobile telephone equipment just if it were his or her own. The radio equipment is called mobile equipment (ME) and we can say that the mobile station consists of two parts, ME and SIM; that is: MS = SIM + ME. 5.5.1 Structure of the GSM Network A simplified architecture for the GSM network is presented in Figure 5.9. For a more detailed look at the structur