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170 SS7 MESSAGE TRANSFER PARTDigital cross-connect systems are very similar to the digital switchblocks inexchanges (Section 1.7.2). They have a number of ports to which first-orderdigital multiplexes are attached, and the paths in cross-connect systems are bidirectional64-kb/s paths that connect a channel (time slot) of one multiplex to a channelof another multiplex. However, unlike the paths in exchange switchblocks, thesepaths are semipermanent and are established and released under command oftelecom personnel.In Fig. 8.2-1, each channel in the multiplexes (b) corresponds to a channel in oneof the digital transmission systems (a). The cross-connect system segregates thechannels attached to the exchange and the channels attached to the signal transferpoint. This is done by setting up paths such that the channels in the multiplexes(c) are associated with the digital trunks and SS7 signaling data links of theexchange, and the channels in multiplexes (d) carry the signaling data links of thesignal transfer point.Transfer Rates of Signaling Data Links. The channels of North Americanfirst-order digital multiplex transmission systems (T 1 systems) nominally transferbits at a rate of 64 kb/s—see Fig. 1.5-4(a). However, there is a limitation on bit patterns:at least one bit in every time slot should be a “1” (pulse). This is a minorproblem for time slots that carry the speech samples of PCM trunks and is solvedby substituting the bit pattern 0000 0000 by the pattern 0000 0001. This causes asmall—but acceptable—distortion in the decoded analog waveform.The sequences of 1’s and 0’s on a SS7 signaling data link are unpredictable.Therefore, in the time slots associated with signaling data links, bit 8 is permanentlyset to “1.” This reduces the transfer rate of SS7 signaling links carried by these transmissionsystems to 8000 7 ¼ 56 kb/s.The first-order digital multiplexed transmission systems defined by ETSI andused in most countries outside North America, use a form of transmission thatcan handle any sequence of 1’s and 0’s. SS7 signaling links on these transmissionsystems can therefore operate at 64 kb/s.8.3 OVERVIEW OF MTP LEVEL 2In a signaling point, each signaling data link is connected to a MTP2. The MTP2 functionsare similar to those of the signaling terminals in SS6 signaling. The primaryMTP2 responsibility is to transfer MSUs across the signaling link, including errordetection and correction. MTP2 also monitors and controls the status of the link [5,7].The reliability objectives for SS7 signaling links are as follows [8]:1. The probability that errors in a received MSU are not detected should be lessthan one in 10 10 .2. Failures should not cause the loss of more than one MSU in 10 7 .3. Less than one in 10 10 messages should be delivered out-of-sequence to theuser parts.