SS7 LSSU - Link Status Signal Unit

   Abbreviations:

   Flag: 01111110
   BSN:  Backward sequence number (is the sequence number of a SU being acknowledged)
   BIB:  Backward indicator bit
   FSN:  Forward sequence number (is the sequence number of the SU which it is carried)
   FIB:  Forward indicator bit
   LI:   Length indicator:  0 -> FISU, 1 or 2 -> LSSU, >2 -> MSU
   SF:   Status field
   CK:   Check bits (CRC)

                      message flow               first bit
                      --------->            LSB  bit1
    +-----+-------+---------+-+---+-+---+-+---+-----+
    |     |       |         | |   |F|   |B|   |     |
    | Flag|   CK  |   SF    | | LI|I|FSN|I|BSN| Flag|
    |     |       |         | |   |B|   |B|   |     |
    +--8--+---16--+- 8 v 16-+2+-6-+1+-7-+1+-7-+--8--+
             +----------+
         +-5-+ 3 ---+
       spare      C B A

                  0 0 0 - Status indication "O"  out of alingment
                  0 0 1 - Status indication "N"  normal alignment
                  0 1 0 - Status indication "E"  emergency alignment
                  0 1 1 - Status indication "OS" out of service, completion of "power on"
                  1 0 0 - Status indication "PO" processor outage
                  1 0 1 - Status indication "B"  busy (flow control, congestion)

(Technical Information)