Figure 49 FillIn Signal Unit Fields

Fill-In Signal Unit Fields

FCS

Not Used

LI

FIB

FSN

BIB

BSN

Flag

The following list describes the FISU fields (these also are common to the LSSU and MSU):

• The frame check sequence (FCS) is the most important field in the FISU. You use this field to verify the integrity of the link between two adjacent signaling elements. MTP Layer 3 (MTP3) uses the bits in the FCS field to determine whether any errors occurred in the FISU, the LSSU, and the MSU. These bits perform error detection using the CRC-16 mechanism. The originating endpoint calculates FCS bit values using the CRC-16 equation. It applies the CRC-16 equation on the user data of the message and places the value in the FCS field. The receiving endpoint applies the CRC-16 equation on the received user data and compares the result with the value in the FCS field.

• The Length Indicator (LI) field identifies the type of signal unit. In the case of the FISU, the LI is set to a value of zero. The LI value is 1 or 2 for an LSSU and from 3 to 63 for an MSU.

• The Forward Indicator Bits (FIBs) and Backward Indicator Bits (BIBs) are used for retransmissions. Under normal conditions (no link errors), the FIB and BIB have the same value. As illustrated in Figure 4-9, the field length is 1 bit; therefore, only two values are possible: 0 or 1.

In the case of a rejected signal unit, the BIB value is toggled and an FISU is sent back. Toggling the BIB value causes the BIB and FIB to be unequal, which indicates a negative acknowledgment. The negative acknowledgment signifies a request for retransmission. When the originator retransmits the signal unit, the FIB is set to equal the BIB until the next retransmission is required.

• The Forward Sequence Number (FSN) and Backward Sequence Number (BSN) are used to acknowledge the link status and MSUs. Acknowledgments are accomplished by sending an FISU with the BSN value equal to the FSN value of the last signal unit. In the case of retransmissions, the BSN values are examined to determine which signaling units need to be retransmitted.

• The Flag field is used to indicate the beginning of a signal unit by implying the end of the previous signal unit. These signal units are separated on the signaling link by the binary value of the flag octet, which is set to 01111110.

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