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5.2 SIGNALING LINKS AND SIGNAL UNITS 123Retransmission of a Message in RB. A copy of the message is entered into OP,where the SU is formed as described above. The message itself remains in RB.Receiving a Signal Unit. All received SUs are checked for errors by input processing(IP). SUs with errors are discarded. Error-free SUs are processed further. Ifthe SU is a message SU, its message parameters, which constitute the processormessage, are entered into buffer IB. In SS7, the link parameters of a message SUare passed to TC. In SS6 and SS7, the parameters in link SUs are passed to TC.Some link parameters acknowledge received message SUs. When TC receives apositive acknowledgment of a message in RB, the message is deleted. When TCreceives a negative acknowledgment of a message in RB, the message is markedas “to be retransmitted.”5.2.5 Message SequencingWe have seen that the retransmission of messages in buffer RB has priority over theinitial transmission of messages in OB. We now examine the selection for transmissionof a message in a buffer.CCS messages belong to several signaling applications, such as call-control ormanagement of trunks. Each application has a priority class. Buffers OB and RBare operated “first-in, first out” for each application: the message selected for transmissionis the “oldest” message with the highest priority class.Let us focus two call-control messages, M 1 and M 2 , in buffer OB of signalingterminal ST A (Fig. 5.2-5). M 1 was entered in buffer OB by processor P A beforemessage M 2 .Figure 5.2-5. Transfer of call-control messages M 1 and M 2 .