Control - Supervisory Information (LLC Type 2)

a

If LLC is used connection-oriented (COLS - connection-oriented link services), the control field is 2 bytes in length (S-format LLC).

The first to bits (value: 10) indicate supervisory information.

The S bits (supervisory function bits) indicate the current function.

The P/F bit (poll/final) indicates the flow direction.

The seven N(r) bits contain the transmitter receive sequence number.

SS = 00 = RR / Receive Ready

SS = 01 = RNR / Receive Not Ready

SS = 11 = REJ / Reject

LLC Type 2 - S-Format - RR

RR-LPDUs (Command/Response) are used by LLC stations to indicate that they are ready to receive another I-format LPDU.

All outstanding I-format LPDUs may be acknowledged by the N(r) value, too. LLC stations which had receiver contestion may indicate that they are ready to receive again.

S-format LPDUs don't have any information field because they only deal with data flow control; they don't convey user data.

LLC Type 2 - S-Format - RNR

RNR-LPDUs (Receive Not Ready) are used by LLC stations to indicate that they are not ready to receive more I-format LPDUs. All error-free received I-format LPDUs are acknowledged by the N(r) value.

Any further incoming frames can't be processed by the station that sends the RNR-LPDU und must be repeated by the sending station after receipt of a RR-LPDU.

S-format LPDUs don't have any information field because they only deal with data flow control; they don't convey user data.

LLC Type 2 - S-Format - REJ

REJ-LPDUs (Reject) are used by LLC stations to indicate that some I-format LPDUs could not be received properly.

All error-free received I-format LPDUs are acknowledged by the N(r) value. All frames with higher N(r) value must be repeated.

There will be only one REJ-LPDU sent at a time. Each REJ-LPDU has to be replied with an I-format LPDU.

S-format LPDUs don't have any information field because they only deal with data flow control; they don't convey user data.