) Recognition Signating Error network Link state Congestion detection Transmitted Signaling functions control part control delimination and received data link (layer 3) (layer 1) alignment Transmission Retrieved MSU MSU Message signal unit Signaling message flows Signal unit Controls and indications LSSU Link status signal unit Figure 14.4 Interactions of functional specification blocks for signaling link control. Note: The MSUs, LSSUs, and SUs do not include error-control information. [From ITU-T Rec. Q.703, Figure 1/Q.703, page 2 (Ref. 7).] 14.6.2 Error Detection The error detection function is performed by means of the 16 check bits provided at the end of each signal unit. The check bits are generated by the transmitting signaling link terminal by operating on the preceding bits of the signal unit following a specified algorithm. At the receiving signaling link terminal, the received check bits are operated by using specified rules which correspond to that algorithm. If consistency is not found between the received check bits and the preceding bits of the signal unit according to the algorithm, then the presence of errors is indicated and the signal unit is discarded. 14.6.3 Error Correction Two forms of error correction are provided: the basic method and the preventive cyclic retransmission method. The basic method applies to (a) signaling links using noninter- continental terrestrial transmission means and (b) intercontinental signaling links where one-way propagation is less than 15 msec. The preventive cycle retransmission method applies to (a) intercontinental signaling links where the one-way delay is equal to or greater than 15 msec and (b) signaling links established via satellite. In cases where one signaling link with an intercontinental link set is established via satellite, the preventive cycle retransmission method is used for all signaling links of that set. The basic method is a noncompelled, positive/negative acknowledgment, retransmis- sion error correction system. A signal unit that has been 