Introduction to Data Communications
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Supervisory (S-Format) LLC Control fields are used for Data Link supervisory control functions (handshaking). The S-Format Control fields are used for acknowledging I-Format PDUs, requesting retransmission, requesting a temporary suspension of transmission (buffers full - wait).

The last 2 bits are set to 0 1, to indicate that it is a S-Format Control Field

S - Supervisory function bits. Determines the purpose of the control field

The four 0s in a row are reserved bits and are always set to 0.

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

N(R) - PDU number received. Used with the Sliding Window and for acknowledging PDUs.

In the S-Format (Supervisory), there are 3 commands & responses:

	RR	- Receive Ready (awake & ready to receive)

	RNR 	- Receive Not Ready (got problems, hold off for awhile)

	REJ 	- Reject (received a bad PDU, send the PDU with this number again)

Introduction to Data Communications
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