uter technology to provide greater efficiency and automatic retransmission. 26 TELECOMMUNICATIONS Frame Frame Modem Modem Transmission Terminals Terminals Fig. 2-6. Time division multiplexing. Time-slots are allotted to channels on dedicated basis whether or not "live" data are being carried. At any given time, majority of slots may not carry useful data but contain fill characters. These three multiplexing techniques are compared on the basis of general criteria as shown in Table 2-1. Frequency division multi- plexing (FDM) does not score well because it is not a digital tech- nique. It is inefficient because the channels have to be separated across the band to prevent cross-talk and this, or course, wastes bandwidth. FDM is inflexible because a change in channel speed or number of channels may demand that the center frequencies of all channels be redefined. Channel capacity is also physically limited by the telephone channels 3000-Hz bandwidth resulting in a capacity of only six channels at 300 bps. Since the FDM is not capable of mul- tiplexing two channels at 1200 bps, it scores very poorly as a device for handling high speed channels. One strong point of the FDM, however, is that since each channel Table 2-1. Multiplexing Technique Comparisons COMPARISON CHARACTER FACTOR Efficiency Excellent Excellent Channel capacity Excellent Flexibility Excellent Reliability Excellent Excellent MODEMS AND MULTIPLEXERS 27 uses a different channel within the bandwidth, individual channels can be dropped at different points along the same telephone line. This allows multiplexing of one or two terminals in one city or lo- cation and extending the line to the next city or location to pick up one or two more terminals. There can be as many drops along the line as there are channels along the capacity of the FDM system. Thus the FDM provides an attractive multidrop capability for simple low-speed asynchronous terminals without imposing any require- ment for addressing or special communication protocols. Since the TDM is a digita