Editing High-Level Data Link Control (section)

=== P/F bit ===
Poll/Final is a single bit with two names. It is called Poll when part of a command (set by the primary station to obtain a response from a secondary station), and Final when part of a response (set by the secondary station to indicate a response or the end of transmission).  In all other cases, the bit is clear.

The bit is used as a [[Token passing|token]] that is passed back and forth between the stations.  Only one token should exist at a time.  The secondary only sends a Final when it has received a Poll from the primary. The primary only sends a Poll when it has received a Final back from the secondary, or after a timeout indicating that the bit has been lost.
* In NRM, possession of the poll token also grants the addressed secondary permission to transmit.  The secondary sets the F-bit in its last response frame to give up permission to transmit.  (It is equivalent to the word "Over" in radio [[voice procedure]].)
* In ARM and ABM, the P bit forces a response.  In these modes, the secondary need not wait for a poll to transmit, so the final bit may be included in the ''first'' response after the poll.
* If no response is received to a P bit in a reasonable period of time, the primary station times out and sends P again.
* The P/F bit is at the heart of the basic '''checkpoint retransmission''' scheme that is required to implement HDLC; all other variants (such as the REJ S-frame) are optional and only serve to increase efficiency.  Whenever a station receives a P/F bit, it may assume that any frames that it sent before it last transmitted the P/F bit and not yet acknowledged will never arrive, and so should be retransmitted.

When operating as a combined station, it is important to maintain the distinction between P and F bits, because there may be two checkpoint cycles operating simultaneously.  A P bit arriving in a command from the remote station is not in response to our P bit; only an F bit arriving in a response is.