f the PCM coder Signals Carried over the Network Calibration of decoder: Calibration of encoder: 0 dBm0 Coder 0 dBm Coder Analog Analog Digital Digital Data word sequence, digital milliwatt Adjust to Adjust to 0 dBm 0 dBm0, 0 dBm Internal adjustable amplifiers and attenuators 0 dBm0 Coder -11 dBm Coder Analog Analog 11 dB Digital Data word sequence, Digital -4 dB 0 dBm0 digital milliwatt 0 dBm0 -4 dB -4 dBm -4 dBm Figure 3.23 Digital milliwatt. is +3.14 dBm0 (1-kHz sine wave) and the higher level signal is distorted. Note that 0 dBm0 is only a reference level for testing and measurement pur- poses and the actual average level of speech in an analog speech channel is of the order of -15 dBm. Measuring systems that generate a bit sequence of the digital milliwatt are used for decoder calibration as shown in Figure 3.23. When this is done, we can loop a digital signal back from the encoder to the decoder and adjust the encoder SO that 0 dBm at the input of the encoder produces 0 dBm at the output of the decoder. The digital milliwatt for European PCM is defined by the 8-word data sequence shown in Table 3.2. Note that before decoding we have to invert every other bit, namely, bits 2, 4, 6, and 8. The PCM decoder produces a 1-kHz - sine wave at the 0-dBm power level when this sequence is inserted into the digital input of the decoder. We often find it necessary to adjust the power level at the interface of PCM equipment SO that the following system will not be overloaded. For this purpose PCM equipment contains adjustable amplifiers and attenuators. Figure 3.23 shows an example in which an analog input signal is amplified by 11 dB before encoding and attenuated by 4 dB after decoding. Problems and Review Questions Problem 3.1 Explain how the characteristics of digital data and voice communications differ. Introduction to Telecommunications Network Engineering Table 3.2 Data Sequence for Digital Milliwatt Word Bit Number 12345678 00110100 00100001 00100001 00110100 10110100 10100001 10100001 10110100 Problem 3.2 What 