ith the received data. When the system has synchronised correctly, every bit error in the received data stream will be detected and recorded by the error counter. The error detector can then compute the bit error ratio for a given measurement period as in Equation 61.1 and can also analyse the results statistically to determine how BER varies as a function of time. Total number of bit errors (61.1) BER = Figure 61.10 A modern error performance analyser Total number of bits received The test patterns used at various bit rates are standardised by 64kbit/s channel of the hypothetical reference connection (HRX) of CCITT and are summarised in Table 61.5. In addition to p.r.b.s. test 27500km. patterns, repetitive word patterns with particular "ones" density can G.821 refers to the overall performance for the end-to-end be used to explore pattern dependency when trouble-shooting. 64kbit/s connection on a very long (27500km) international connec- tion. It's the starting point for error performance measurements, but needs interpretation before being applied to a practical transmission 61.3.4 CCITT Recommendation G.821 system G.821 defines how error performance parameters are calculated Error performance, or statistical analysis of error occurrence, is (Figure 61.11). The total measurement time is divided into 1 second usually evaluated in accordance with CCITT Recommendation periods, and unavailable time is subtracted to obtain the available G.821 (Table 61.6) which specifies the percentage of time that time on which the G.821 parameters are calculated. certain thresholds can be exceeded. It gives the error performance A period of unavailable time begins when the BER in each second of an international digital connection forming part of an ISDN, worse than 10-3 for a period of ten consecutive seconds. These ten measured over a period TL (e.g. one month) on a unidirectional seconds are then considered to be unavailable time. A new period of clock Shift register p.r.b.s. under Shift register clock Clock error Error Error det