ons of the ITU-T and transmit facsimile informa- tion in digital format at 9,600 or 14,400 bps. We can envision facsimile equipment as a combined scanner and a modem. Faxes and fax modems also perform compression since runs of O's (blank paper) are very common. The Group 4 fax is designed to use a 64-Kbps B-channel but it has not become popular because of low penetration of ISDN service. It can commu- nicate with a Group 3 fax, in which case the bit rate for a Group 3 fax will be used [5]. 6.4.1.5 Dial-Up Modems All modern modems are able to transmit multifrequency signaling tones to the telephone network to establish a connection. Voice-band modems include signaling functionality similar to that of a telephone and an external telephone is not needed for dial-up. 6.4.1.6 Operation of a Voice-Band Modem Connection Modems operate at various data rates over a voice-band telephone channel. Modern modems support many different data rates and they can adapt the transmission data rate to the quality of the channel. In Chapter 4 we saw that the maximum transmission data rate depends on the bandwidth and noise of the channel. If the S/N is degraded (noise level increases), the data rate has to be decreased to keep the transmission error rate low enough. Modems are also able to adapt their data rate and error control scheme to the capability of the other end. To do this, they exchange control data sequences during con- nection establishment. The analog signal from a modem is PCM coded into a 64-Kbps data stream at the subscriber interface of a local telephone exchange. The absolute maximum capacity of the transmission channel through the telephone Introduction to Telecommunications Network Engineering network can never exceed 64 Kbps. Some quantizing noise is introduced in the quantizing process of PCM, as we learned in Chapter 3, and it reduces the end-to-end data rate from the maximum of 64 Kbps. The present highest rate modems operating at 33.6 Kbps are quite close to the theoretical maxi- mum when we consider quantizin