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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. | 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. 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. Length Indicator MTP2 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 the number of octets between the LI and the CRC fields. Using telecommunications conventions MTP2 measures the size of SUs in octets. An octet is simply another term 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 for LSSUs 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. NOTE Layers above the
