essages indicate the service. which may be in the retransmission store to be retransmitted. The signal messages are sequence numbered. Idle (fill-in) SUs are transmitted when no message SUs are avail- 43.3.1.2 No.7 formatting principles able for transmission. Idle SUs, not deposited in the re-transmission store and not retransmitted on error detected, are CRC error The signal messages are HDLC octet byte variable bit length signal checked; this being a form of preventative maintenance. units (SUs) with opening and closing flag delimiters coded A variant of the above basic error control is adopted for long 01111110 (CCITT, 1988d). The circuit label includes the originating propagation time signalling links (e.g. satellite). In this case the and destination exchanges point codes, plus the circuit identifica- whole content of the retransmission store is retransmitted repeatedly tion of the traffic circuit directly connecting the originating and destination points. when there are no new signal messages to be sent (so called preven- HO codings identify the various signal groups (address signals, tive cyclic retransmission). This is a form of forward error correc- tion. call supervision signals, etc.). H1 codings identify particular signal messages in a relevant HO group; e.g. answer signal (H1 coding) in Receive end monitors measure the error rate, automatic change- the call supervision group (HO coding). over to the backup signalling facility being initiated on unaccept- able performance. The backup may be associated or quasi-associated signalling. Figure 43.10 shows a typical associated 43.3.1.3 No.7 error control signalling backup. The up-to-four signalling links, but at least two, are signalling load shared. Assuming a four link signalling module, The basic error control is error detection by redundant coding in normal operation each signalling link serves, typically, 1000 (CRC) and error correction by re-transmission (CCITT, 1988e). traffic circuits. In extreme error conditions the one remaining C.C.S. This requi