erworking between PCM codecs. The simplified block diagram (Figure 61.1) shows the connection Either noise or tone stimulus can be used, with levels at the digital from one end user to another via various network sections. Meas- interface being interpreted from the digital 8 bit codes present in the urements will be used to characterise the overall end-to-end trans- selected 64kbit/s timeslot. The companding algorithm is usually mission performance of the system, known as the digital path. checked by a gain versus level plot (Figure 61.2), or quantising Measurements may also be made on individual network sections, and on the electrical parameters at network interfaces, including PCM codec measurements. The types of measurement and the associated standards are shown in Table 61.1. Measurements, in common with other telecommuni- cations standards, follow either the CCITT recommendations or the North American ANSI T1 standards. Note that many of these standards undergo continuous revision to include new technology Input level and operating conditions. It is important to obtain the latest version (dBmO) of documentation from the appropriate standards organisation. The following sections cover firstly interface measurements, and secondly transmission measurements, particularly error perfor- mance criteria. Interface measurements 61.2.1 PCM measurements Figure 61.2 CCITT Recommendation G.712: Variation of PCM measurements characterise the analog to digital and digital to gain with input level (nise) analog conversions that take place at the interface between an analog telephone line and the digital network. In order to provide distortion versus level (Figure 61.3). A typical set of PCM codec the best signal to noise ratio and dynamic range within the digitised measurements is shown in Table 61.2. 64kbit/s channel, companding (compression and expansion) of sig- nals is used. 61.2.2 Electrical interface specifications This involves non-linear conversion between analog amplitude and digital code, which must be exactly mat