ronics. A separa- tion is used with a three-time slot offset so that the mobile will not have to send and receive at the same time. GSM FDMA/TDMA Combination To enable multiple access, GSM utilizes a blending of FDMA and TDMA. This combination is used to overcome the problems introduced in each indi- Figure 1-31 Frame = 4.62 ms TDMA framing and time slots. 0123456701234 577 us (.57ms) 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 vidual scheme. In the case of FDMA, frequencies are divided up into smaller ranges of frequency slots and each of these slots is assigned to a user during a call. Although this method will result in an increase of the number of users, it is not efficient in the case of high user demand. On the other hand, TDMA assigns a time slot for each user for utilizing the entire frequency. Similarly, this will become easily overloaded when encountering high user demand. Hence, GSM uses a two-dimensional access scheme. GSM uses the combined FDMA and TDMA architecture to provide the most efficient operation within the scope of price and reasonable data. The phys- ical channels are TDMA time slots and the radio channels are frequencies. This scheme divides the entire frequency bandwidth into several smaller pieces as in FDMA and each of these frequency slots is to be divided into eight time slots in a full-rate configuration. Similarly, 16 time slots will be in a half-rate configuration. Logical Channels GSM distinguishes between physical channels (the time slot) and logical channels (the information carried by the physical channels). Several recur- ring time slots on a carrier constitute a physical channel, which are used by different logical channels to transfer information-both user data and sig- naling. A channel corresponds to the recurrence of one burst every frame. It is defined by