Error second estimation is given by Equation 61.4 where, n is the number of errors in the ith second, N is the higher bit rate divided Local Medium Medium Local by 64kbit/s, and j is the total number of seconds. grade grade grade grade grade (61.4) 64kbit/s = Figure 61.12 Hypothetical reference connection Y% DM measured at the line rate can be compared directly to Y% DM at 64kbit/s. Y% SES measured at the line rate can be converted Table 61.7 Allocation of degraded minutes and errored directly to Y% SES at 64kbit/s, but if during the test a loss of frame seconds objectives alignment is detected (or a slip), this time as a percentage should be added. Circuit classification Allocation of degraded minutes and The conversion for errored seconds is more complicated. Since errored seconds objectives the higher multiplexed bit rate contains many 64kbit/s channels, we need to know how many errors are contained in each errored second Local grade (two ends) 15% block allowance to each end at the higher rate in order to estimate how many 64kbit/s channels Medium grade (two 15% block allowance to each end have been errored. Assuming errors are distributed evenly within ends) the frame (the worst case condition), then Equation 61.4 is repre- sentative. High grade 40% (equivalent to conceptual However, the validity of all these conversion algorithms is being quality of 0.0016%/km.) debated in standards committees. Bursts of errors in a high rate transmission stream may cause loss of synchronisation in a sub- sequent demultiplexer, creating an extended error burst on a tribu- Table 61.8 Allocation of severely errored seconds tary output. Some practical measurements seem to support these conclusions. Refer to the latest version of recommendation G.821 for guidance on conversion algorithms. Circuit classification Allocation of severely errored seconds objectives 61.3.5 Related error performance standards Local grade (two ends) 0.015% block allowance to each Several other standards exist, however generally these are related to G.8