SS7 FISU - Fill In Signal Unit

                      message flow
                      --------->   LSB    bit1
    +-----+--------+-+---+-+---+-+---+-----+
    |     |        | |   |F|   |B|   |     |
    | Flag|   CK   | | LI|I|FSN|I|BSN| Flag|
    |     |        | |   |B|   |B|   |     |
    +--8--+---16---+2+-6-+1+-7-+1+-7-+--8--+

   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

   CK:   Check bits (CRC)