Cyclic Redundancy Check (CRC). The CRC value is used to detect and correct data transmission errors. The error check field is immediately before the closing flag. It contains 16 bits generated as a CRC code.
Length Indicator (LI). The length indicator indicates the number of octets (8 bit bytes) between itself and the CRC check sum. It serves both as a check on the integrity of the SU and as a means of discrimination between different types of SUS at level 2. Accroding to the protocol only 6 of the 8 bits in the length indicator field are actually used to store this length. Thus the largest value that can be accumodated in the length indicator is 63. For MSUs with more than 63 octets following the length indicator, the value of 63 is used. LI gives the length of the signal unit. A value of LI greater than two indicates that the SU is a message signal unit.
The 6 bit LI can store values between zero and 63. If the number of octets which follow the LI and precode the CRC is less than 63, the LI contains this number. Otherwise, the LI is set to 63. An LI of 63 indicates that the message length is equal to or greater than 63 octets (up to a maximum of 273 octets). The maximum length of a signal unit is 279 octets: 273 octets (data) + 1 octet (flag) + 1 octet (BSN + BIB) + 1 octet (FSN + FIB) + 1 octet (LI + 2 bits spare) + 2 octets (CRC).
The remaining two important fields service information octet (SIO) and signalling information field (SIF) used in MSU are described below :
Service information octet (SIO). It indicates the user part according to the message (e.g., telephone, data or ISDN). The SIO field in an MSU contains the 4 bit subservice field followed by the 4 bit service indicator. FISU and LSSUs do not contain an SIO. The subservice field contains the network indicator (i.e., national or international) and the message priority. The service indicator specifies the MTP user, thereby allowing the decoding of the information contained in the SIF. Table 7.4 shows the service indicator and MTP user.
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