【正文】 up”. Groups or supergroups are moved around as single units by the munications equipment and it is not necessary for the radios to know how many channels are involved. A radio can handle a supergroup provided sufficient bandwidth is available. The size of the groups is a promise as treating each channel individually involves far more equipment because separate filters, modulators and oscillators are required for every channel rather than for each group. However the failure of one module will lose all of the channels associated with a group. Time Diversion Multiplexing It is possible, with pulse modulation systems, to use the between samples to transmit signals from other circuits. The technique is known as time diversion multiplexing (TDM). To do this, it is necessary to employ synchronized switches at each end of the munication links to enable samples to be transmitted in turn, from each of several circuits. Thus several subscribers appear to use the link simultaneously. Although each user only has periodic short time slots, the original analog signals between samples can be reconstituted at the receiver. Pulse Code Modulation In analog modulation, the signal was used to modulate the amplitude or frequency of a carrier, directly. However, in digital modulation a stream of pulse, representing the original, is created. This stream is then used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation (PCM) is one of the two techniques monly used. All pulse systems depend on the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulation. It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal. It will be seen later that most of the advantages of digital modulation e from the transmitted pulses having two levels only, this being known as a binary system. In PCM the height of each sample is converted into a binary number. There are three step in the process of PCM: sampling, quantizing and coding. Optical Fiber Communications Communication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance a munication system is usually required. Within a munication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagic wave which acts as a carrier for the information signal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticated techniques have been developed for this process by using electromagic carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies. However,” munication” may also be achieved by using an electromagic carrier which is selected from the optical range of frequencies. In this case the information source provides an electrical signal to a transmitter prising an electrical stage which drives an optical source to give modulation of the lightwave carrier. The optical source which provides the electricaloptical conversion may be either a semiconductor laser or light emitting diode (LED). The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation optical carrier. Photodiodes (PN, PIN or avalanche) and , in some instances, phototransistor and photoconductors are utilized for the detection of the optical signal and the electricaloptical conversion. Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically. The optical carrier may be modulated by using either an analog or digital information signal. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (. onoff pulses). Although often simpler to implement,