3.3.2 Bandwidth The voice signal, which is the most common message in telecommunications network, does not look similar to a pure cosine wave in Figure 3.2. It Introduction to Telecommunications Network Engineering contains many cosine waves with different frequencies, amplitudes, and phases combined together. The range of frequencies that is needed for a good enough quality of voice, SO that the speaker can be recognized, was defined to be the range from 300 to 3,400 Hz. This means that the bandwidth of the telephone channel through the network is 3,400 - 300 Hz = 3.1 kHz, as shown in Figure 3.3. A human voice contains much higher frequencies, but this bandwidth was defined as a compromise between quality and cost. It is wide enough to recognize the speaker, which was one requirement for tele- phone channel. Bandwidth is not strictly limited in practice, but signal attenuation increases heavily at the lower and upper cutoff frequencies. For speech, channel cutoff frequencies are 300 and 3.4 Hz, as shown in Figure 3.3. The bandwidth is normally measured from the points where the signal power drops to half from its maximum power. Attenuation or loss of chan- nel is given as a logarithmic measure called a decibel (dB), and half power points correspond to a 3-dB loss. Decibels are discussed later in this chapter. Bandwidth, together with noise, is the major factor that determines the information-carrying capacity of a telecommunications channel. The term bandwidth is often used instead of data rate because they are closely related, as we will see in Chapter 4. Attenuation or loss of 3 dB decreases power to half and corresponds voltage drop Bandwidth B = 3.1 kHz from 1 to 0.707 (Half power) 3 dB 300 Hz 1 kHz 3.4 kHz Frequency (log. scale) T= 1 ms T = 3.3 ms T= 0.3 ms Sine wave oscillating Sine wave oscillating Sine wave oscillating 300 times a second 1000 times a second does 3400 times a second attenuates to half power not attenuate at all attenuates to half power Figure 3.3 Bandwidth of the telephone speech channel.