s are sent on their own channel, a separate frequency in AMPS and a separate time slot in GSM. During an AMPS call, signals are carried on the voice channel by in-band blank and burst inter- ruptions that interrupt the voice channel briefly and by out-of-band supervisory audio tones (SATs). During a GSM call, the signaling data are sent on the same digital chan- nel as the digitally encoded voice bits, and the digital carrier has extra frames for sig- naling data. 7.3 The GSM TDMA Channel The GSM radio carrier is a 200-kHz digital channel that carries eight conversations along with some signaling. This is a tougher channel than AMPS because it can tolerate more co-channel interference than AMPS. This difference will be discussed in Chap. 25. The GSM radio carrier is a 200-kHz digital channel using gaussian minimum phase shift keying (GMSK) modulation, a variant of quadraphase phase shift keying (QPSK). This 200-kHz carrier is also called the traffic channel (TCH). The rate of the carrier is 270 kbps. The data stream is divided into 4.615-ms frames, and each frame is divided into eight time slots of 148 bits each. Some of these frames are for signaling and syn- chronization, so each of the eight voice channels has a raw data rate of 22.8 kbps. The GSM voice channel uses error correction (as described in Sec. 17.2) to get higher accuracy at a lower bit rate. The resulting bit stream available for speech coding (as de- scribed in Chap. 18) is 13 kbps. The speech coder uses a mu-law coding and a speech- compression method called regular pulse excitation-long-term prediction (RPE-LTP). The original European version of GSM has 50 MHz of spectrum in the 900-MHz band, just like AMPS. And the forward link was 45 MHz higher in frequency than the reverse link, just like AMPS. This means that the classic GSM configuration supports 62 radio carriers, or 496 time-slotted channels for voice or signaling compared with 416 for AMPS. Other versions of GSM use the same channel architecture at higher 10ur usage here is technically c