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.

The signaling payload is placed in the SIF, which is found in an MSU. The SIF can be up to 272 octets in size, rendering the maximum length for an MSU as 279 octets. 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. This situation arises from backward-compatibility issues. The Red Book specified the maximum 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," for information about the meaning of the Red Book and Blue Book.)

MTP2 uses the LI information to determine the type of SU with minimum processing overhead; therefore, the inaccuracy of the indicator above 62 octets is not an issue. MTP2 adds an overhead of six octets along with one additional octet for the MTP3 SIO when creating each MSU. This brings the total maximum size of a transmitted SU to 279 octets (272 maximum SIF size plus seven for MTP2 overhead and the SIO).