SS7 MSU - ISUP messages

   SS7 - ISUP signalling information field (SIF):

                                            message flow ----->  |
    +- - - - - - - - - - - - +-------------+-----+---+-----+-----+- - -
    |    Parameters / Fields | Message type| CIC |SLS| OPC | DPC | SIO
    |              Components+-------8-----+12+4-+-4-+--14-+--14-+- - -
    +- - - - - - - - - - - - +            spare|
                                                 |-routing label-|
                                             |-----label---------|

The elements of the ISUP messages are:
 - routing label: (Q.704/2)
     DPC - destination point code (cod: binary)
     OPC - originating point code (cod: binary)
     SLS - signalling link selection (load sharing)
 - CIC - circuit identification code (Q.763/2)
 - message type code

      mandatory parameters fixed part
           mandatory parameter A
           .
           .
           mandatory parameter F
      mandatory parameters variable part
      +--- pointer to parameter M
      |    .
      |    .
      |+-- pointer to parameter P
      ||   pointer to optional part ------->---+
      ||   ---------------------------------   |
      +|->- lengt indicator of parameter M     |
       |    Parameter M                        |
       |     .                                 |
       |     .                                 |
       +->- lengt indicator of parameter P     |
            Parameter P                        |
            ------------------------------     |
       optional part        -------------------+
            ---
            Parameter name X
            Lengt indicator of parameter X
            Parameter X
            ---
             .
             .
            ---
            Parameter name Z
            Lengt indicator of parameter Z
            Parameter Z
            ---
            End of optional parameters