l measures used in the engineering of a telecommunications network. Many others are in use, but they will be easy to understand if the need arises if the reader is familiar with the most important ones just discussed: decibel and dBm. Our goal was merely to introduce the reader to the decibel measure. 3.8.3 Digital Milliwatt As we have seen, PCM systems have a strictly limited operational range. The upper limit is defined by the code word representing the maximum sample value. If an analog signal has a higher amplitude, it is severely distorted because of clipping. The other limiting factor is quantizing noise, which reduces performance as the signal level decreases. In a digital international connection, the PCM equipment at both ends has to be compatible and it has to convert digital information into the same analog signal level and vice versa. Therefore, control of the power level at the PCM encoder input is extremely important. For this purpose, the ITU-T has defined a digital sequence of code words. By decoding this sequence, a 1-kHz sine wave is produced at the 0-dBm power level (Figure 3.23). A digital milliwatt can be understood to be the reference signal for the analog signal levels in the network. The actual measured signal power level is written as a dBm0 value when it is compared with the reference level gener- ated by the digital milliwatt, which then generates a 0-dBm0 level to all ana- log signal points. If the nominal signal level at a measurement point is designed to be lower, the measured dBm value is lower as seen in Figure 3.23. There a PCM decoder contains a 4-dB attenuator and the actual signal level is -4 dBm when the digital milliwatt is applied. However, the measured decoder output level is the same as the level generated by the digital milliwatt and thus it can also be written as 0 dBm0. Figure 3.13 showed the SQR of a PCM codec, and the measurement signal level was compared to the reference level generated by a digital milliwatt and thus written as a dBm0 value. The overload threshold o