ifies a new VLR of its whereabouts. The VLR in turn uses the control network (which happens to be based on SS7) to signal the HLR of the MS's new location. Through this information, mobile-terminated (MT) calls can be routed to the user by the location information contained in the user's HLR. Defining the Channels As we look at the radio operation, a channel can be defined in different ways. Often times we hear a channel defined in radio frequency. Other times we hear the physical channel being described thinking that is radio 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 Chapter 1 frequency. Alas, the different definitions get in the way. For the definitions used, channels are defined by looking at the matrix: Radio channel is defined by the frequency used. Physical channels are indicative of the time slot that they occupy. Logical channels are defined by the function that they provide or serve. Frequencies Allocated In reality, GSM systems can be implemented in any frequency band. How- ever several bands exist where GSM terminals are available. Furthermore, GSM terminals may incorporate one or more of the GSM frequency bands listed in the following section to facilitate roaming on a global basis. Two frequency bands, of 25 MHz in each one, have been allocated by ETSI for the GSM system: The band 890 to 915 MHz has been allocated for the uplink direction (transmitting from the mobile station to the base station). The band 935 to 960 MHz has been allocated for the downlink direction (transmitting from the base station to the mobile station). However, not all countries can use all of the GSM frequency bands. This is due primarily to military reasons and to the existence of previous analog systems using part of the two 25-MHz frequency bands. Figure 1-21 shows the frequencies. Primary GSM When transmitting in a GSM networ