 the gathering of G.821 statistics. Signal (FAS) and the Not Frame Alignment Signal (NFAS). Bits 2 These relate to the long term quality performance of the connection, to 8 of the FAS, and bit 2 of the NFAS, are fixed and define frame and may be displayed in the form of histograms to assist in identif- synchronisation. Bit 3 of the NFAS is used to indicate remote end ying periods of degraded service. loss of synchronisation, and bits 4 to 8 are used for application By setting up two simultaneous test calls, an assessment of cross- specific signalling functions. Bit 1 of the FAS and NFAS are talk interference between channels can be made, either by using a disregarded for the purposes of frame synchronisation, but can be voice call in one channel which may be degraded by a data call in used for CRC4 operation. Frame synchronisation occurs when 3 the other, or by using two data calls which may mutually interfere. consecutive correct FAS (bits 2 to 8), and 3 corresponding correct In the case of voice calls set up, analogue measurements may be NFAS (bit 1), are received, and is considered lost when 3 consecu- made, by sending calibrated tones and measuring received level and tive errored FAS are received. 61/20 Telecommunication system measurements The integrity of the data contained in the frames can be checked Functional test of terminal equipment using a CRC4 in bit 1 of the FAS and NFAS. Multiframes are formed of two sub-multiframes, each of 8 frames. Bit 1 of each NFAS in the first sub-multiframe is set to the sequence 0010, and in the second sub-multiframe to 11XX, where X bits are 'don't care' as far as multiframe alignment is concerned, and are used to signal remote CRC errors. The four FAS bit 1's of each sub-multiframe form a four bit word corresponding to the CRC of the data in the preceding sub-multiframe. An error is detected when the received Loop test through network CRC word does not correspond to one locally calculated on the received data. 61.5.6.2 Out of service Primary Rate tests End to end test t