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