cter) are fixed in length and are transmitted continuously, since the receiving multi- plexer only knows which bits or characters belong to each channel as a function of their time relationship to the SYNC character. The number of bits or characters in each "time-slot" allows it to handle its assigned channel without losing data even when the channel is operating at maximum speed. But since the number of bits or char- acters is fixed, the TDM must transmit them even if the TDM is operating below its maximum speed or not operating at all. Since transmission is continuous, there is no possibility of retransmission even if the receiving multiplexer detects errors. The "idle" character used to fill a "time-slot," when no data character is ready for trans- mission, is also used to transmit channel interface condition infor- Channel 1 Channel 2 Channel 3 Channel 4 MUX (Transmit) DEMUX (Receive) Single conductor channel channel transmitters receivers Synchronized timing Fig. 2-7. TDM with fixed sequential scanning. MODEMS AND MULTIPLEXERS 29 mation, remote loopback commands, and diagnostic status information. Figure 2-7b shows that a TDM configuration with fixed sequential scanning samples a bit or character from each channel in order; channel 1 then 2, 3, etc. Thus all channels occupy the same amount of bandwidth on the high-speed line. If the highest speed terminal attached to the multiplexer is 300 bps, all channel allocations on the high-speed line will be at 300 bps regardless of the terminal speed, which may be lower. To counteract this deficiency, most TDMs on the market today use variable channel scanning. A variable scan multiplexer has the ability to make three samples for each 300-bps terminal, compared to one for 110 bps or two for 134.5-bps terminals. This permits more efficient use for high-speed line capacity. But, with or without variable channel scanning, each channel's time-slot is fixed and capable of supporting the channel operating at maximum speed continuously. Unfortunately for TDM efficiency, it 