 one location to another by transmission facilities or systems using a multitude of media (paired cable, coaxial cable, analog or digital radio systems, optical fibres, satellite communica- 21.2.1 Frame organisation tion). The synchronous time division multiplexers typically are used as For each digital channel there is assigned a group of b bits, called a termination equipment of such carrier systems. These multiplexers word which corresponds to a digital message element (data). The are better known as T1 or E1 multiplexers, depending on the groups belonging to the same channel are transmitted at a frequency multiplexing scheme they use. T1 is generally used in America and fs, equal to the sampling frequency. Thus, the bit rate of channel i is based on the PCM system that was originally designed by AT&T. can be calculated as in Equation 21.2. The name T1 was derived from the identification number of the committee set up by the American National Standards Institute Di = f. b (21.2) (ANSI). Europe, on the other hand, uses the E1 system, which is based on CEPT (Conference of Post and Telecommunications) recommendations. The delay with which CEPT has undertaken the In the case of digital PCM telephony, these words are composed definition of a primary digital PCM system has allowed it to profit of 8 bits and called octets. Hence the bit rate of each channel equals 64000bit/s. from the experience of the American systems. On the international level, the two types of system coexist al- When n digital channels are assembled into a time division multi- though they are incompatible, and are a subject of two recommen- plex, the collection of the n words of b bits (and eventually auxiliary dations of the CCITT. Although both systems will be discussed later bits which are added to them), within a period given by Equation in this chapter, Table 21.1 compares their main characteristics. The 21.3, constitutes a frame. Table 21.1 Primary European and American systems (21.3) E1 (European T1 (American Although the structure of the 