ansmission into smaller pieces, send- ing pieces from each transmission and puts it back together again at the other end. A multiplexer, also known as a mux, is located at each end of the circuit. At the receiving end it is sometimes called the demux (demultiplexer). The multiplexer is used to maximize the use of a high cost dedicated line by enabling it to be used for multiple simultaneous transmissions. There are three types of multiplexing: Frequency division multiplexing is an analog technique, di- viding the bandwidth into smaller channels. This is used today for cable TV signals in what is known as broadband. The FM dial on a radio is an example of frequency division mul- tiplexing. This can take place in the air or on a coaxial cable. Time division multiplexing can operate in a digital (called pulse code modulation) or analog (called pulse amplitude modu- lation) manner. All modern PBXs use pulse code modulation also called PCM. Older PBXs used pulse amplitude modula- tion. Each device on the network is assigned a particular time slot on the high speed line whether it has something to trans- mit or not. The order in which the devices transmit always stays the same. The digital time division multiplexer operates by scanning the devices, sampling the first bit from each trans- mission and then repeating the process sampling the second bit. It continually transmits these interleaved bits down the tele- communications circuit. A limitation of time division multiplexing is that it cannot retransmit an error because the transmission is continuous. This is significant for data trans- mission, but insignificant for voice transmission. Statistical multiplexing is an improved form of time division multiplexing in the sense that it maximizes the use of the lines on your network, but can sometimes cause blocking if the sta- tistical expectations do not hold. The statistical multiplexers 242 a The Telecom Handbook CHAPTER 11 take advantage of the idle time of communications devices by dynamically allocating time slots 