k, the mobile station to the base sta- tion uses an uplink. The reverse channel direction is the downlink from the base station to the mobile station. GSM uses the circa 900-MHz band. The frequency band used is 890 to 915 MHz (mobile transmit) and 935 to 960 MHz (base transmit). The duplex channel enables the two-way communi- cations in a GSM network. Because telephony was the primary service, a full-duplex channel is assigned with the two separate frequencies in a 45-MHz separation. To give the maximum number of users access, each band is subdivided into 125 carrier frequencies spaced 200-kHz apart, using FDMA tech- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Introduction to GSM Figure 1-21 The uplink and downlink frequencies. mobile to base station 890-915 MHz base station to mobile 935-960 MHz GSM frequencies initially set with 25 MHz (transmit and receive) spaced apart by 45 MHz. Figure 1-22 45-MHz Spacing Spectrum bands for Creates Duplex Pairing primary GSM. UPLINK DOWNLINK Channel 0 not used. Acts as guardband. niques. The spectrum assignment is shown in Figure 1-22. Only 124 chan- nels are used, where channel 0 is reserved and held as a guard band against interference from the lower channels. Each of these carrier frequencies is further subdivided into time slots using TDMA. The frequency bands are usually split between two or more providers who then build their networks. The channels are set at the 200 kHz each. The International Telecommuni- cation Union (ITU), which manages the international allocation of radio spectrum (among other functions), allocated the bands for mobile networks in Europe. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the we