Control (1 byte) (LAPB)

Protocol: LAPB LAPB PCI

Field: control

Length: 1 byte

Contents: frame format / sequence number.

Format Command Response Bit Values

Supervisory RR RR N(R) P/F 0001

RNR RNR N(R) P/F 0101

REJ REJ N(R) P/F 1001

Unnumbered SARM DM 000 P/F 1111

SABM 001 P 1111

SABME 011 P 1111

DISC 010 P 0011

UA 011 F 0011

CMDR 100 F 0111

FRMR 100 F 0111

Information I N(R) P N(S)0

Information format is indicated by the first bit (LSB) that is set to zero.

I frames are used for the sequenced transfer of data between DTE and DCE.

S frames are used to supervise the exchange of I frames by acknowledging good frames and rejecting bad ones. The two LSBs of the S frame Control field are set to 01, and the remaining bits carry a code indicating the frame type and the acknowledgment number N(R).

U frames are used to establish and disconnect the DTE/DCE link. U frames are indicated when the two LSBs of the Control field are set to 11. The remaining bits are encoded to indicate the type of frame being sent. Common to all frame types is the Poll/Final bit (P/F) that indicates the urgency of the command or response.

N(r) = receive sequence number

N(s) = send sequence number