cellular networks many different lower rate coders are defined and Excellent 5 Research goal Good 4 G.726 Hybrid coder ADPCM (e.g., GSM) 32 Kbps Speech quality Fair 3 (MOS) G.711 Poor 2 Vocoders Bad 1 Speech data rate, Kbps Figure 3.20 Comparison of speech-coding techniques. Signals Carried over the Network they operate at a fixed data rate from 3 to 13 Kbps. In the United States a variable-bit-rate coder is used in the code division multiple access (CDMA) cel- lular network. It varies the data rate between 1 and 9 Kbps depending on the speech characteristics. 3.8 Power Levels of Signals and Decibels In this final section on signals we explain the decibel, a measure of signal level and its change. We use this logarithmic measure or its variants in the tele- communications network for many purposes, for example, to express the voice level or the transmission and reception power of radio systems, such as mobile telephones, or an optical line system. 3.8.1 Decibel, Gain, and Loss Along the long-distance communication connection or channel, the power of the signal is reduced and amplified over and over again. The signal power needs to be rigidly controlled to keep it high enough in relation to back- ground noise and low enough to avoid system overload and resulting distortion. The reduction of signal strength, loss or attenuation, is expressed in terms of power loss. When the signal is regained, this is expressed in terms of power gain. Thus the absolute gain of ten corresponds to the loss of 1/10. Alexander Graham Bell was the first to use logarithmic power meas- ures. This was found to be handy and the unit for power gain was named in Bell's honor as decibel (dB). The gain in decibels is defined as follows: (3.11) If the output and input powers are the same, the absolute gain and loss both have values of 1 and the corresponding gain and loss in decibels are each 0 dB. If the gain is 10, the corresponding decibel value of gain is 10 dB. The loss is correspondingly 1/10, that is, equal to -10 dB. Thus if the power is