SS7 - SCCP messages

    Signalling Information Field (SIF):

                                                 message flow ----->  |
    +-E + - - - - - Parameters ----------+------------+---+-----+-----+- - -
    | O |User message data|Message header|Message type|SLS| OPC | DPC | SIO
    +-P-+- - - - - - - - -+--------------+------------+-4-+--14-+--14-+- - -
        |                 |
        |                 |                           |-routing label-|
        |                 |                           |-----label-----|
        |                 +----------------------+
        |                                        |
        +------------------+---------------------+
  TCAP  |Component Portion | Transaction Portion |
        +------------------+---------------------+

    DPC - destination point code (cod: binary)
    OPC - originating point code (cod: binary)
    SLS - signalling link selection

    The parts of the SCCP messages:

    1    routing label  (Q.704/2)
    2    message type code
    3    mandatory fixed part
    4    mandatory variable part
    5    Optional part

    The  Message header includes called and calling-party addresses
    and local references