tream, or on computing parity/checksums on a reasonable period (e.g. 1 month) blocks of data: BPVs and Line Code Violations are of limited use as they apply 61.3.5.2 Recommendation G.921 to a single transmission section. Equipment will not retransmit code violations to subsequent sections and SO this type of CCITT Recommendation G.921 defines the performance of a Hy- parameter cannot be used to indicate overall path performance. pothetical Reference Digital Section (HRDS) and is based on the (BPV stands for Bipolar Violations in ternary codes such as requirements of G.821. HDB3, B8ZS, B3ZS, 4B3T etc.) G.921 considers digital sections of 280km (or multiples of 2. Frame Alignment Signal (FAS) Errors are detected by checking 280km) and assigns percentage allocation of overall G.821 specifi- the bits in the repetitive frame alignment word. These form only cations. Shorter medium grade connections are also defined as a small part of the overall frame, and therefore provide only a shown in Table 61.9. sample or snapshot of error performance. Good for long term average error ratio (BER). Parity Errors are detected by computing odd or even parity for Table 61.9 Digital section quality classifications for error blocks of data. Has the advantage of checking all payload bits, performance but can be fooled by even numbers of bit errors in the data block. Effective at low error rates (e.g. less than 10 Used in DS-3 (44.736Mbit/s) transmission in the North America, in the Section HRDS length Allocation To be used in SONET/SDH frame standard, and in some line codes. quality circuit Cyclic Redundancy Checksum (CRC) is computed on blocks classification classification of data including payload bits and a CRC remainder sent to the 0.45% High grade receive end for comparison with the recalculated remainder. A Medium discrepancy indicates one or more errors have occurred in the grade data block. Provides very reliable detection, giving a good indication of % Errored Seconds as per G.821. Currently used Medium in T1 (1.544Mbit/s) ESF 