ions many signals must be sent along a single long-distance related to actual circuit length, for example, channel line, careful engineering is required to ensure that the extension. (Backup circuits can be leased for an addi- system will perform properly. tional charge. The length can vary when the backup is This type of multiplexing combines many digital bit used.) streams with relatively low bit frequency into a single bit The data communication terms "half-duplex" and stream with a relatively high bit frequency. It is, in "full-duplex" are closely related to, but not synonymous essence, a way for many slow communications channels with, the telephony terms "two-wire" and "four-wire. to "time share" a very fast channel. The advantage, of A half-duplex circuit is one that provides transmission in course, is that the cost per bit transmitted on a single fast two directions, but only in one direction at a time. channel is lower than on slower channels. In TDM, A full-duplex circuit is one that provides transmission in stations are allocated the entire bandwidth of frequencies both directions simultaneously. Obviously, and end-to-end but only for a small percentage of time (see Figure 3.1-8). four-wire circuit provides full-duplex capabilities. How- TDM is accomplished by simply interleaving data from ever, two-wire circuits can also be used for full-duplex several bit streams. This can be done on a bit basis or on communication by partitioning the available bandwidth a byte basis (called bit interleaving and byte interleaving). into separate frequency bands for each direction of During time interval 1, 8 bits from source channel 1 are transmission (derived four-wire). This technique is transmitted. During successive intervals, bytes from often utilized when a full-duplex data communications successive source channels are transmitted on the output circuit is desired over dial-up (two-wire) facilities. On channel. A complete set of values from each input the other hand, the existence of four-wire circuits does chann