Data compression - to command a modem to start dialing Fax transmission Figure 6.14 Interfaces and operation of a voice-band modem. 6.4.1.2 Error Control Modems implement the physical layer channel from a terminal to the host at the other end. Errors may occur in the transmission channel between modems, for example, because of the noise on the subscriber line. Many modems send, in addition to the data, error check information and with the help of these they are able to detect and probably correct some bit errors. Both ends have to support the same error control protocol. One interna- tional standard for error correction in modems is Recommendation V.42 of the ITU-T. In addition to error detection and correction in modems, most com- munications software packages include error recovery functions imple- mented at higher layers end to end. For example, if TCP of TCP/IP is used and a received TCP data segment contains errors, errors are detected, and retransmission is requested by the far-end communications software where the TCP layer is implemented. 6.4.1.3 Data Compression Data compression makes it possible for the transmission rate at the interface between a computer and a modem to be much higher than the actual trans- mission rate through the network. For example, text can sometimes be Data Communications compressed by a factor of four or even more. Several methods of data com- pression are available. As a simple example of the compression of text infor- mation, we can imagine that the most common characters are not transmitted in ASCII form but with very short codes; less frequently needed characters would use longer bit strings. This principle would save some trans- mission capacity. One international standard for data compression is the V.42bis recommendation. Many proprietary standards are widely in use as well. 6.4.1.4 Facsimile Transmission Many modern modems include facsimile functionality that enables a user to send and receive faxes without printouts. These modems comply with the Group 3 fax recommendati