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8.3 OVERVIEW OF MTP LEVEL 2 171In earlier CCS systems, error correction consisted of retransmitting a messagesignal unit that has incurred transmission errors. The retransmitted MSU arrivedafter MSUs that were sent later and did not incur errors (out-of-sequence delivery).Since the bit error rate on typical signaling links is in the range of 10 24 to 10 26 ,objective 3 dictates an error-correction procedure that does not cause outof-sequencemessage delivery. This is accomplished as follows. When a MTP2receives a mutilated message signal unit, say, MSU m , it discards that MSU andall subsequently received MSUs, until it receives an error-free retransmitted copyof MSU m . Conversely, when the distant MTP2 receives the negative acknowledgmentof MSU m , it retransmits that MSU and all subsequently sent MSUs.SS7 error correction is discussed in Sections 8.4 and 8.5. By eliminating themajor cause of out-of-service MSU delivery, SS7 allows simpler (and moredrastic) procedures to cope with the receipt of an out-of-sequence MSU.8.3.1 MTP2 StructureThe main parts of MTP2 are shown in Fig. 8.3-1. Link control (LC) controls theother functional units of MTP2. In the first place, it coordinates the transfer ofsignal units. LC also monitors the operation of the signaling link. It communicatesFigure 8.3-1. Structure of MTP2.