MULTIPLEXING

Figure 1-10.—A time-division multiplexer (TDM) system. the decoder. The decoder reassembles the data into a be designated in a single modulation change. If two form compatible with the receiving system. In the receiver section, the incoming signal is often fed to the receiver timing logic to control the receiver timing circuitry. MULTIPLEXING One requirement of a data communications system is for it to transmit as many intelligent signals as possible in a fixed period using a single- communications channel. The rate of data transmission is measured in the number of bits per second (bps) transmitted. The bps rate is often confused with the baud rate. Baud refers to the rate at which a modulated signal between two devices changes in 1 second. For example, if the signal between two modems changes frequency or phase at a rate of 2,400 times per second, the baud is 2,400. If you are using a modulation method in which a single modulation change carries one bit, the 2,400 baud is also 2,400 bits per second. Using more sophisticated modulation methods, several bits of information can bits of data are transmitted with each modulation change, the data transfer rate is 4,800 bits per second at 2,400 baud. The data signals being transmitted are normally multiplexed to increase the transmission rate of data over the communications channel or to increase the efficiency of the channel by allowing multiple users of the same channel. The two methods commonly used to multiplex communications channels are time-division multiplexing and frequency-division multiplexing. Time-Division Multiplexing Time-division multiplexing (TDM) grants each user full channel capacity, but assigns time slots to each user. Each user is connected to a time-division multiplexer. Data signals from the user are fed to the time-division multiplexer buffer, and the time slots are rotated among the users and scanned for data. Figure 1-10 illustrates the typical construction of a 1-9