fcd-2 provides statistics of line bipolar violations and out-of-service events. in addition, when the crc-4 option is used, the quality of transmission over the e1 main link can also be evaluated.

    when the crc-4 option is enabled, frames are arbitrarily grouped in groups of 16 (these groups are called crc-4 multiframes, but they do not bear any relationship to the 16-frame multiframe organization used with 256 superframe explained before). a crc-4 multiframe always starts with a frame that carries the frame alignment signal. the crc-4 multiframe structure is identified by a six-bit crc-4 multiframe alignment signal, which is multiplexed into bit 1 of timeslot 0 of each odd-numbered frame of the multiframe (up to frame 11 of the crc-4 multiframe). each crc-4 multiframe is divided into two submultiframes of 8 frames (2048 bits) each. the detection of errors is achieved by calculating a four-bit checksum on each 2048-bit block (submultiframe). the four checksum bits calculated on a given submultiframe are multiplexed, bit by bit, in bit 1 of timeslot 0 of each even-numbered frame of the next submultiframe.

    at the receiving end, the checksum is calculated again on each submultiframe and then compared against the original checksum (sent by the transmitting end in the next submultiframe). the results are reported by two bits multiplexed in bit 1 of timeslot 0 in frames 13, 15 of the crc-4 multiframe, respectively. errors are counted and reported as crc-4 error statistics.