21 specifications. Medium grade (two ends) 0.015% block allowance to each High grade 0.04% 61.3.5.1 CCIR Recommendation 594-1 Error performance of digital microwave radio systems is charac- terised using CCIR recommendations, notably Recommendation Allowable ES = 8%x0.4x or 0.000128%/km (61.3) 594-1, for 2500km link (64kbit/s unidirectional channel) as follows: 25000 1. BER worse than 1 X 10-6 for less than 0.4% of any month. Severely errored seconds are allocated on a block basis only. 0.1% 2. BER worse than 1 X 10-3 for less than 0.054% of any month. (of the total 0.2% G.821 specification) is allocated on a block basis 3. Errored seconds should not exceed 0.32% of any month. as shown in Table 61.8. The remaining 0.1% SES is allocated to 4. Residual BER should not exceed 5 X 10-9 (15 minute integra- medium and high grade sections to account for adverse operating tion). conditions such as propagation in microwave radio systems. For example, G.821 recommends that an additional 0.05% SES may be Recommendation 594-1 is compatible with the high grade portion allocated to a medium or high grade microwave radio section of of G.821. 2500km. An additional block allowance of 0.05% SES has been added for adverse propagation conditions and a specification for residual BER has been added. 61.3.4.2 Conversion of measurements Both the Residual BER (RBER) threshold and the % for G.821 parameters should be reduced in proportion for systems less than Error performance standards usually refer to measurements at 2500km long. 64kbit/s, whereas practical measurements on transmission systems As these recommendations are compatible with G.821, the same are invariably made at a higher multiplex rate. The 1988 (Blue measuring instruments and calculation are used to assess perfor- Error performance measurements 61/11 mance. The main consideration is that radio propagation is affected measurements rely on detecting errors in fixed or allowed bit pat- by weather and so results could be misleading unless measured over terns within the data s