In 1968, the CCITT, now the International Telecommunication Union( ITU), introduced the basic PCM-30 system for Europe, in which 30 digital voice channels of 64 kbit/s each are combined in time division multiplex (multiplex). A further 2 channels are used to transmit synchronization, alarm and switching information. The transmission rate of this digitallink is 2,048 kbit/s. The 30 duplex channels are transmitted using the TDMA method with HDB3 coding. Each time frame lasts 125 µs. The PCM system is standardized in ITU-T recommendation G.732.
Based on PCM-30, the basic system with the first hierarchy level, there are several hierarchy levels. Each hierarchy level is formed from four systems of the hierarchy level below.
The number in the system designation indicates the number of user channels. The second hierarchy level is thus PCM-120, the third PCM-480, the fourth PCM-1920 and the fourth PCM-7680. The hierarchy levels also have the designations primary system (PCM-30), secondary system, tertiary system, quaternary system and quintary system. The transmission speeds of the individual stages are 8.448 Mbit/s (PCM-120), 34.368 Mbit/s (PCM-480), 139.264 Mbit/s (PCM-1920) and 564.992 Mbit/s for PCM-7680.
PCM transmission technology is used in the transmission links between local exch anges and long-distance exchanges for digital transmission of telephone channels at 64 kbps.
The basic PCM system PCM-30 is primarily used for connections in the local network, but is also used to some extent regionally and nationwide. The higher- order PCM systems, PCM-120 and PCM-480, are used at the regional network level, and PCM-1920 and PCM-7680 are used at the supraregional network level.
In North America in particular, PCM-24 is widely used as the standard configuration for trunk lines in telephone networks. This is a 1,544-kbit/s connection consisting of 24 individual PCM channels in a 125 µs time frame and is specified in the ITU standard G.733.