is rare for any terminal to transmit continuously at its maximum operating speed. This is par- ticularly true of the interacting dumb terminals used with minicom- puter systems, which typically transmit only a line or perhaps a screen full of data before pausing. 2.8 STATISTICAL MULTIPLEXER In a typical application, it is rare to find a terminal transmitting con- tinuously at its maximum operating speed. The statistical multiplexer takes advantage of this and allows for a larger number of terminals to share a single phone line. Through the use of microcomputer tech- nology it is designed to statistically determine traffic and transmit variable length blocks according to the loading on individual chan- nels. In addition, it can check these transmitted blocks at the receive end for errors and automatically request retransmission in the event an error is detected. This is all accomplished without the need for any additional intelligence in the computer or terminal equipment in the network. A comparison is shown in Fig. 2-8 between a TDM and a statis- tical multiplexer. As shown, the TDM, in order to operate four chan- 4 Channels at 1200 bps 4 Channels at 1200 bps Computer Modem Modem 8 channels at 1200 bps 8 channels at 1200 bps Concentrator Concentrator Computer Modem Modem 4 channels at 2400 bps 4 channels at 2400 bps Concentrator Concentrator Computer Modem Modem Fig. 2-8. Comparison between a TDM and a statistical multiplexer. MODEMS AND MULTIPLEXERS 31 nels at 1200 baud, would require a 4800-baud high-speed link between the multiplexers. The statistical multiplexer on the other hand, may permit eight channels at 1200 baud to operate over the same 4800- baud data link, or even four channels at 2400 baud, operating over the 4800-baud data link. It is common in a statistical multiplexer application to have the aggregate speeds of the terminals to be equal to or even greater than the speed of the synchronous communication line between the multiplexers. This is permitted due to data buffering in each of the multiple