SM hierarchy. The BSC is an important level in GSM because it can do its own handovers. (AMPS calls them handoffs, but GSM calls them handovers.) This means that the GSM BSC has to have some switching powers independent of the MSC. 7.2 The TDMA Channel Concept In analog frequency division multiple access (FDMA), the concept of the radio channel is clear. A channel is a frequency range allocated to carrying a modulated signal from a transmitter to a receiver. The continuous time of the voice signal is represented by con- tinuous time in the modulated radio carrier. This is the same for broadcast FM 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. TDMA Wireless Telephony (GSM) Key Radio Concepts and AMPS cellular, except that AMPS cellular adds the idea of a duplex channel-two separate frequencies used for bidirectional conversation. In GSM, the channel is more complicated than in AMPS in two important ways. First, it is a digital channel, where the voice signal is coded into a sequence of binary digits and the digits are sent over the radio link. Second, eight digital voice streams combined into a single radio carrier. Each voice channel gets one of eight time slots. In TDMA, we maintain the distinction between the carrier, which includes the entire dig- ital radio link, and the channel, which only includes one time slot of a carrier. Therefore, a voice channel, which in FDMA referred to a separate radio carrier with its own frequency range, in TDMA refers to a time slot within a multichannel carrier with a larger frequency range. The two directions are still two different frequencies, but their time slots are staggered so that the user terminal never has to transmit and receive at the same time. The digital medium also changes the way control signals are managed in GSM. When a call is not in progress, in both AMPS and GSM control signal