(CRC-6) and E1 (2.048Mbit/s) (CRC- grade Medium grade According to CCITT recommendation M.550, system errors are classified as Anomaly Events (AE) such as frame errors, parity errors, code errors, and Defect Events (DE) such as loss of signal, loss of frame synchronisation etc. Table 61.10 shows the anomaly 61.3.5.3 Recommendation M.550 events detection processes available at the various telecommunica- Table 61.10 In service anomaly events (M.550) and CCITT recommendation M.550 is titled 'Performance limits for performance primitives (T1M1.3) bringing into service and maintenance of digital paths, sections and line sections'. This new recommendation is still under development BIP= Bit Interleaved Parity; FEBE = Far End Block Error (1991) by CCITT Study Group IV, although a partial version is *Only when operating in these frame formats printed in the 1988 Blue Books. More recent revisions can be found in Study Group IV documents. Bit rate interface Section/line M.550 defines practical performance criteria for digital circuits, T1, DS-1 BPVs (B8ZS, AMI) CRC-6 errors measured over shorter periods than the 1 month defined in G.821. 1.544Mbit/s FAS error G1-G6 bits Periods of 15 minutes, 24 hours and 3 or 4 days have been sug- Line-code errors (ESF data link) gested. It also recommends margins for ageing SO that maintenance intervals can be extended. BPVs (HDB3) CRC-4 errors Furthermore, M.550 defines Anomaly Events (AE) and Defect 2.048Mbit/s FAS error E bits Events (DE) for both in service and out of service testing, and Line-code errors indicates the number of events permissible in a measurement peri- BPVs (B3ZS) C-bit parity errors od. For in service testing, it suggests how events should be inter- 44.736Mbit/s FAS errors FEBE bits preted in terms of G.821 parameters. This parallels the North Line-code errors American ANSI T1M1.3 draft recommendation 'In service digital M13 parity errors transmission performance monitoring draft standard' (current ver- (P bits) sion T1M1.3/91-003R2) 34.368Mbit/s BPVs (HDB3) 139.26