DSS1 LAYER2 - Address field format

          MSB                         LSB
    bit:   8   7   6   5   4   3   2   1
          +----------------------+---+---+
    octet |    SAPI              |C/R|EA |
        2 |                      |   | 0 |
          +----------------------+---+---+
    octet |    TEI                   |EA |
        3 |                          | 1 |
          +--------------------------+---+

    The elements of the address field:

        EA   Address field extension bit

        C/R  Command/response field bit
                                           C/R
              Command  network -> user      1
              Command  user -> network      0
              Response network -> user      0
              Response user -> network      1

        SAPI Service access port identifier
              0          call control procedures
              1 - 15     reserved for future
              16         packet communication (X.25 3. layer)
              17- 31     reserved for future
              63         Layer 2 management procedures
              all others Not available

        TEI  Terminal endpoint identifier
              0 -63      non automatic TEI assignment user equipment
              64-126     automatic TEI assignment user equipment
              127        broadcast