Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book) |
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Signal Unit DelimitationA 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. NOTE It is optional whether a single flag is used to mark both the beginning and end of an SU, or whether a common flag is used for both. The latter is the most common implementation. Because the 01111110 flag pattern can also occur in an SU, the SU is scanned before a flag is attached, and a 0 is inserted after every sequence of five consecutive 1s. This method is called bit stuffing (or 0 bit insertion). It solves the problem of false flags, because it prevents the pattern 01111110 from occurring inside an SU. The receiving MTP2 carries out the reverse process, which is called bit removal (or 0 bit deletion).After flag detection and removal, each 0 that directly follows a sequence of five consecutive 1s is deleted. Figure 6-5 shows how the sending node adds a 0 following five 1s while the receiving node removes a 0 following five 1s. Figure 6-5. Zero Bit Insertion and Deletion
As another example, if the pattern 01111100LSB appears in the SU, the pattern is changed to 001111100LSB and then is changed back at the receiving end. This method continuously processes the stream of data on the link, inserting a 0 after five contiguous 1s without examining the value of the next bit. |
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