Th e valu es of FSN and FIB in the MSU s from MTP2-A, and of BSN and BIB inSUs (MSU s, LSSUs, FISU s) from MTP2-B, are shown at both MTP s. This makes iteasi er to correl ate the figure and the text below.We assume that, prior to the transm ission of SUa, MTP2-B has been acceptingMSUs and has been sending SUs with BIB ¼ 0. MTP2-A has been sendingMSUs from its output buffer, with FIB ¼ 0.SUa positively acknowledges MSUs with FSN up through 24. MTP2-A keepssending MSUs. MSUb and MSUc pass the tests at MTP2-B and are accepted,because FIBb and FIBc are equal to BIBa, and the FSNs indicate the proper sequence.SUe positively acknowledges the MSUs in SUe (BIBe ¼ BIBa and BSNe ¼ 26).MTP2-A keeps transmitting MSUs from its output buffer. MSUd incurs a transmission error and is discarded by MTP2-B incoming processing. MSUf arriveswithout error, and with the proper FIB value, but fails the sequence test. It is therefore discarded and negatively acknowledged by SUh (BIBh = BIBe, BSNe ¼ 26).MSUg is discarded because its FIBg does not match BIBh. In this situation,MTP2-B does not send another negative acknowledgment, but signals a positiveacknowledgment in SUi (BIBi ¼ BIBh), in which BSNi indicates that MSUc (withFSN ¼ 26) is still the latest accepted MSU.MTP2-A receives the negative acknowledgment in SUh. Since BSNh ¼ 26,it starts a retransmission cycle of MSUs in its RB, beginning with MSUj
indicate
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