l system, it can be used with more com- plex synchronous modems capable of transmitting data at speeds up to 9600 bps on voice-grade telephone lines. It may also be used at much higher speeds on the newer DDS (dataphone digital service) circuits or satellite channels operating at speeds of 56,000 bps or higher. TDM's allow considerable flexibility in changing the number and/or the speed of channels as the requirements of the network change. This is possible because the TDM is a digital system inde- pendent of the data modem attaching it to the high-speed line. The small amount of buffering in the TDM allows it to regenerate data on each channel, which tends to reduce the overall error rate caused by end-to-end bit and character distortion. As previously mentioned, there are two types of TDM's, the bit TDM and the character TDM. The character TDM is primarily used when multiplexing asynchronous channels. The character TDM buffers a complete character before transmitting it down the high- speed line. It removes the start and stop bits from the character be- fore transmission, then adds the start and stop bits back to the char- acter during the demultiplexing process at the other end. Thus, in the case of a dumb terminal application, it is only necessary to trans- mit eight bits of data for every character received. The net result is that the high-speed line is used more efficiently and channel capacity on a given high-speed line is greater with character TDM than with bit TDM. The simplicity of the TDM technique results from the fact that the multiplexers communicate with each other, transmitting a constant stream of bits (bit TDM) or characters (character TDM) with a reg- ularly recurring SYNC character in a constant number of "time- 28 TELECOMMUNICATIONS slots" between SYNC characters. Each time-slot contains a prede- fined number of bits or characters for a specific channel, as shown in Fig. 2-7a. The fundamental principle of time-division multiplexing is that the frames (the data block following each SYNC chara