e merely introduce the connection between the time-and frequency-domain descriptions with a couple of exam- ples. The reader is referred to [1] for mathematical treatment of the transfor- mation between the time and frequency domains. In Figure 4.2, two examples of time-domain signals and corresponding spectrums are presented. In the first example we see an ordinary rectangular digital pulse with duration of T seconds and its corresponding spectrum. If, for example, the pulse a duration T = 1 ms, the strongest spectral content lies below 1 kHz (1/T = 1/1 ms = 1,000 1/s = 1 kHz), as shown in Figure 4.2. From this we get a thumb rule that we can send 1,000 pulses of this kind in a second through a channel with a bandwidth of 1 kHz, which corresponds to a 1-Kbps binary data rate. To increase the data rate, we should decrease T and the spectral width and the required bandwidth is increased correspondingly. For example, for a Introduction to Telecommunications Network Engineering A pulse and its spectrum A pulse Amplitudes of in the time frequency components domain in the frequency domain Time/sec Frequency/Hz 1/T 1/T Radio frequency burst and its spectrum Amplitudes of Radio frequency frequency burst with carrier components frequency f Time/sec fc 1/T Figure 4.2 Signals in the time domain and the spectrum. 10 times higher data rate, we must use a 10 times shorter pulse, which would require a 10 times wider bandwidth. In the other example in Figure 4.2, a digital pulse is sent as a radio- frequency burst. This is one example of digital amplitude modulation (AM) known as amplitude shift keying (ASK). Now the spectrum is concentrated around the carrier frequency, fo instead of zero frequency. Note that the spectral width around carrier frequency depends only on the pulse duration T, as in the previous example. If we now increase the data rate (decrease pulse duration), we make the spectrum wider, and a wider radio-frequency band is required. Note that if we let T increase without limit, the spectral width decreases and 