frames containing the frame alignment signal, bit 1 is used original equipment. to transmit the CRC-4 bits. There are four CRC-4 bits, referred to Loss of frame alignment is detected simply by monitoring the as C1, C2, C3, and C4 in each SMF. In the frames not containing the frame alignment signal. PCM-30 differentiates between loss of frame alignment signal, bit 1 is used to to transmit the CRC-4 frame alignment, being a failure to synchronise on the frame align- multiframe alignment signal and two CRC-4 error indication bits ment signal FAS (0011011), and loss of multiframe alignment, (E). The CRC-4 multiframe alignment signal is a 7-bit sequence which is caused by a failure to synchronise on the multiframe having the form 0011011. The E-bits are used to indicate received alignment signal (0000) contained in bits 1 to 4 of time slot 16 of T1 and PCM-30 alarms and error conditions 21/11 Table 21.3 CRC-4 multiframe structure. E= CRC-4 error indication bits; S4 to S8 = Spare bits; C1 to C4 = CRC-4 bits; A = Alarm indication signal (AIS) Sub-multiframe Frame (SMF) number SMF I SMF II frame 0) The former is the red alarm, the latter is the multiframe red n-bit code is compared with the code that was calculated by the alarm. Table 21.4 illustrates how these alarm conditions are trans- transmitting equipment. Any discrepancies between the two codes mitted. T1 multiplexers can also signal a blue alarm. This is a are counted as CRC errors. CRC errors are by far the best in service continuous ones pattern across all 24 channels (the F-bits, however, performance measurements because the CRC scheme allows detec- remain unchanged) to indicate an upstream failure. tion of errors on all of the data bits within each group of frames, with an accuracy of about 98 percent. 21.6.2 Error conditions Slips are yet another impairment. Slips are the most common impairments caused by frequency deviations and timing problems. The most basic impairment that T1 or E1 equipment can suffer from A slip is the insertion or deletion of data bit