hat this in-service error detection scheme does not indicate BER unless one assumes a certain error distribution (random or burst errors), to predict the average errors per block. Rather, it provides a block error measurement. This is very useful for estimating percentage of errored seconds (%ES) that is usually considered the best indication of quality for data transmission-itself a block or frame transmission process. CRC-4 error checking is fairly reliable with the ability of detecting 94% of errored blocks even under poor BER conditions (see ITU-T Rec. G.706, Ref. 5). Sub-multiframe Bits 1 to 8 of the frame in timeslot 0 Frame number (SMF) Multiframe Figure 21.2 E1 multiframe, CRC-4 structure. Notes: E = CRC-4 error indication bits; Sa4 to Sa8 are spare bits. These bits may be used for a network management (maintenance) link. C1 to C4 are CRC-4 bits. A stands for remote alarm indication. (From Table 4b/G.704, p. 81, CCITT Rec. G.704, Fascicle III.4 IXth Plenary Assembly, Melbourne, 1988.) 21.5 SYSTEM DEPTH NETWORK MANAGEMENT PROBLEM Another powerful feature of E1 channel 0 (when equipped) is the provision of local indication of alarms and errors detected at the far end. When an errored SMF is detected at the far end, one of the E-bits (see Figure 21.2) is changed from a 1 to a 0 in the return path multiframe (TSO). The local end, therefore, has exactly the same block error information as the far-end CRC-4 checker. Counting E-bit changes is equivalent to counting CRC-4 block errors. Thus the local end can monitor the performance of both the go and return paths. This can be carried out by the network equipment itself, or by a test set such as the HP 37722A monitoring the E1 2.048-Mbps data bit stream. In the same way, the A-bits return alarm signals for loss of frame or loss of signal from the remote end. Loopback testing is a fine old workhorse in our toolbox of digital data troubleshooting aids. There are two approaches for DS1 (T1) and E1 systems: intrusive and nonintrusive. Intrusive, of course, means that 