Signaling System No.7 Protocol Architecture And Sevices part 15

Signal Unit Delimitation A flag octet that is coded as 01111110 separates consecutive signal units on a signaling data link. The flag octet indicates the beginning or end of an SU.
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Signaling System No.7 Protocol Architecture And Sevices part 15Signal Unit DelimitationA flag octet that is coded as 01111110 separates consecutive signal units on asignaling data link. The flag octet indicates the beginning or end of an SU.NOTEIt is optional whether a single flag is used to mark both the beginning and end of anSU, or whether a common flag is used for both. The latter is the most commonimplementation.Because the 01111110 flag pattern can also occur in an SU, the SU is scannedbefore a flag is attached, and a 0 is inserted after every sequence of fiveconsecutive 1s. This method is called bit stuffing (or 0 bit insertion). It solves theproblem of false flags, because it prevents the pattern 01111110 from occurringinside an SU. The receiving MTP2 carries out the reverse process, which is calledbit removal (or 0 bit deletion).After flag detection and removal, each 0 that directlyfollows a sequence of five consecutive 1s is deleted. Figure 6-5 shows how thesending node adds a 0 following five 1s while the receiving node removes a 0following five 1s. Figure 6-5. Zero Bit Insertion and DeletionAs another example, if the pattern 01111100LSB appears in the SU, the pattern ischanged to 001111100LSB and then is changed back at the receiving end.This method continuously processes the stream of data on the link, inserting a 0after five contiguous 1s without examining the value of the next bit. < Day Day Up > < Day Day Up >Length IndicatorMTP2 must be able to determine the SU type to process it. The length indicator(LI) provides an easy way for MTP2 to recognize the SU type. The LI indicates thenumber of octets between the LI and the CRC fields. Using telecommunicationsconventions, MTP2 measures the size of SUs in octets. An octet is simply anotherterm for a byte; all SUs have an integral number of octets.The LI field implies the type of signal unit. LI = 0 for FISUs, LI = 1 or 2 forLSSUs, and LI >2 for MSUs. Because MSUs contain the actual signaling content,their size is relatively large compared to the two other types of SUs.NOTELayers above the MTP can handle larger data streams than the MTP; however,these streams must be segmented into MSUs at MTP2 for transmission over thesignaling link.The signaling payload is placed in the SIF, which is found in an MSU. The SIF canbe up to 272 octets in size, rendering the maximum length for an MSU as 279octets. If the MSU size is greater than 62 octets, the LI is set to the value of 63;therefore, an LI of 63 means that the SIF length is between 63 and 272 octets. Thissituation arises from backward-compatibility issues. The Red Book specified themaximum number of octets in the SIF as 62, and the Blue Book increased it to 272(which was previously allowed only as a national option). (See the section ITU-T(Formerly CCITT) International Standards in Chapter 2, Standards, forinformation about the meaning of the Red Book and Blue Book.)MTP2 uses the LI information to determine the type of SU with minimumprocessing overhead; therefore, the inaccuracy of the indicator above 62 octets isnot an issue. MTP2 adds an overhead of six octets along with one additional octetfor the MTP3 SIO when creating each MSU. This brings the total maximum size ofa transmitted SU to 279 octets (272 maximum SIF size plus seven for MTP2overhead and the SIO).NOTEIn ANSI networks, when 1.536-Mbps links are used, a 9-bit length indicator isused, and the actual SU length is checked against the LI value. < Day Day Up > < Day Day Up >Error DetectionThe error detection method is performed by a 16-bit CRC on each signal unit.These 16 bits are called check bits (CK bits).NOTEThe process uses the Recommendation V.41 [ITU-T Recommendation V.41:CODE-INDEPENDENT ERROR-CONTROL SYSTEM, November 1988]generator polynomial X16 + X12 + X5 + 1. The transmitters 16-bit remainder valueis initialized to all 1s before a signal unit is transmitted. The transmissions binaryvalue is multiplied by X16 and then divided by the generator polynomial. Integerquotient values are ignored, and the transmitter sends the complement of theresulting remainder value, high-order bit first, as the CRC field. At the receiver, theinitial remainder is preset to all 1s, and the same process is applied to the serialincoming bits. In the absence of transmission errors, the final remainder is1111000010111000 (X0 + X15).The polynomial that is used is optimized to detect error bursts. The check bits arecalculated using all fields between the flags and ignoring any inserted 0s. The SPthen appends the calculation to the SU before transmission as a two-octet field (CKfield). The receiving SP performs the same calculation in an ...