rth American DS1 system. 2 It uses a 15-segment approximation of the logarithmic u-law curve (u = 255). The segments cutting the origin are collinear and are counted as one. So, again, we have a total of 16 segments. The coding process in PCM utilizes straightforward binary codes. Examples of such codes are illustrated in Figure 6.5 and are expanded in Figure 6.6 and Figure 6.7. The North American DS1 (T1) PCM system uses a 15-segment approximation of the logarithmic u-law (u = 255), shown in Figure 6.7. The segments cutting the origin are collinear and are counted as one. As can be seen in Figure 6.7, similar to Figure 6.5, the first code element (bit), whether a 1 or a 0, indicates to the distant end whether the sample voltage is positive or negative, above or below the horizontal axis. The next three elements (bits) identify the segment, and the last four elements (bits) identify the actual quantum level inside the segment. 6.2.3.1 Concept of Frame. As is illustrated in Figure 6.2, PCM multiplexing is car- ried out with the sampling process, sampling the analog sources sequentially. These sources may be the nominal 4-kHz voice channels or other information sources that have a 4-kHz bandwidth, such as data or freeze-frame video. The final result of the sampling and subsequent quantization and coding is a series of electrical pulses, a serial bit stream of 1s and Os that requires some identification or indication of the beginning of a sampling 11011111 11010100 11010000 Figure 6.6 The European E1 system, coding of segment 4 (positive). 2 More popularly referred to as T1. DIGITAL NETWORKS 1000XXXX 1001 X 1010XXXX 1011 X X 1100XXXX 1101XXX 1110XXXX 1111 X X 0111XXXX 0110XXXX 0101XXXX 0100XXXX 0011XXXX 0010XXXX 0001 X 0000XXXX Figure 6.7 Piecewise linear approximation of the u-law logarithmic curve used with the DS1 format. sequence. This identification is necessary SO that the far-end receiver knows exactly when the sampling sequence starts. Once the receiver receives the "indication," it knows a pri- ori (in the 