t sig- nalling : value 1. Circuits that have long transmission lines are subject to attenu- Time (seconds) ation and distortion, although these can be rectified using re- value generation and amplification. 2. Connecting wires are needed for the whole of a telecommuni- cations circuit. A rotating alternator or an electronic oscillator causes the current to reverse direction at regular intervals to create a particular repeat- ing pattern or waveform. This is known as an alternating current. Figure 10.1 Sinusoidal alternating current waveform The advantages of alternating current signals are : 10/4 Signals and noise In Figure 10.1 between points A and B the current increases from Phase difference describes two waveforms that are identical ex- zero to a peak value in a positive direction. Between points B and C cept for their phase. Figure 10.3 illustrates two sinusoidal wave- the current gradually reduces to zero. Then between points C and D forms that have a phase difference. the current moves to a peak value in the opposite or negative Amplitude, frequency and phase represent the characteristics of a direction. Finally between points D and E the current gradually waveform, that may by varied SO that symbols may be transmitted returns to zero again. The whole sequence from point A to point E along the media, thus allowing a complete message to be sent. represents one complete rotation of the wire loop in the magnetic The energy of an alternating current waveform will travel along a field and is called one cycle of an alternating current waveform. transmission medium at a particular velocity, therefore a certain The periodic time, which is measured in seconds, is the time distance will be travelled in one cycle. The alternating current required for one complete cycle of the alternating current waveform waveform repeats complete cycles over equal distances. The dis- to be produced. tance representing each cycle is called the wavelength and is con- The frequency of the waveform is the number of complete cycles ventio