DSS1 LAYER2 - S-frame

S-frame - supervisory frame (it has no information field) for the error free information transfer and for the traffic control.

    Control field formats:   (codes)

           8   7   6   5   4   3   2   1
          +--------------+-------+-------+
    octet |              |       |       |  S - format
        4 |X   X   X   X | S   S | 0   1 |  supervisory frame
          +--------------+-------+---+---+
    octet |                          |   |
        5 |    N(R)                  |P/F|
          +--------------------------+---+

          +--+---+---+---+---+---+---+---+
       S  |0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |  RR -receive ready
          +--+---+---+---+---+---+---+---+
          |    N(R)                  |P/F|
          +--------------------------+---+
          +--+---+---+---+---+---+---+---+
       S  |0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 |  RNR -receive not ready
          +--+---+---+---+---+---+---+---+
          |    N(R)                  |P/F|
          +--------------------------+---+
          +--+---+---+---+---+---+---+---+
       S  |0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 |  REJ -reject
          +--+---+---+---+---+---+---+---+  request retransmission from N(R)
          |    N(R)                  |P/F|
          +--------------------------+---+

           N(R)  Receive sequence number
           P/F   Poll/final bit