Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
     
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Signal Unit Alignment

Loss of alignment occurs when a nonpermissible bit pattern is received or when an SU greater than the maximum SU size is received.

MTP2 constantly processes the data stream, searching for flags that delineate the SUs. The maximum number of consecutive 1s that should be found in the bit stream is six (as part of the flag), because the transmitting end performs 0 bit insertion. If seven or more consecutive 1s are detected, this signifies a loss of alignment.

The SU length should be in multiples of octets (8 bits). The minimum size of an SU is six octets (FISU), and the maximum size is 279 octets (MSU). If an SU is outside these parameters, this is considered a loss of alignment, and the SU is discarded.

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