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124 INTRODUCTION TO COMMON-CHANNEL SIGNALINGSince M 1 is “older,” its initial transmission takes place before the initial transmissionof M 2 . Assuming that both messages are received error-free and acceptedby ST B ,M 1 is entered in buffer IB of ST B before M 2 . Buffer IB is also operated“first-in, first out” for each application priority, and processor P B retrieves and processesM 1 before M 2 —Fig. 5.2-5(a).We speak here of “in-sequence” delivery of messages. In-sequence delivery is ofimportance when messages M 1 and M 2 pertain to the same call.Now suppose that message M 1 is received with errors by ST B and therefore is discarded—Fig.5.2-5(b). ST B informs ST A by sending a negative acknowledgment of M 1 .In SS6, ST A retransmits message M 1 only, and processor P B therefore receivesM 1 after M 2 —Fig. 5.2-5(a). When these messages pertain to the same call, thisout-of-sequence (O-S) delivery causes a problem for P B in the processing of the call.In SS7, when signaling terminal ST B receives message M 1 with errors, it discardsthe message—and all following messages, until it receives an error-free retransmissionof M 1 .ST A , after receiving a negative acknowledgment of message M 1 , retransmitsall sent messages (starting with M 1 ) before sending out any new message. Inthis way, SS7 eliminates the major cause of O-S message deliveries.We shall see later that certain failures of signaling links can also result in O-Sdeliveries. SS7 thus reduces the probability of such deliveries but does not eliminatethem. Therefore, the call-control procedures in both SS6 and SS7 have to take intoaccount the possibility that messages are not delivered in their proper sequence.5.2.6 Cyclic Redundancy CheckingError detection of SUs in SS6 and SS7 is done by cyclic redundancy checking(CRC), a technique used in many data communication systems. The CRC procedurefor SUs from ST A to ST B is described below (Fig. 5.2-6).Figure 5.2-6. Cyclic redundancy checking.