HDLC identifies a number of timers that may be used by an HDLC link (which is operating in the reliable ABM Mode) to provide reliable data transmission.
The T1 or acknowledgment timer is necessary for reliable operation of HDLC. The timer is started whenever an I-frame is sent and the timer is not already running. It is started afresh whenever a frame is received which actually acknowledges a transmitted I-frame which has not previously been acknowledged. If further frames remain unacknowledged, then the timer is started again by initialising it with the timer period. The timer therefore counts down all the time that I-frames have been sent but have yet to be acknowledged.
When the T1 timer counts down to zero, it indicates that one or more acknowledgements are missing. At this stage, the transmitter is unaware of the reason why the acknowledgment has not been received. It may be due one of three causes:
The transmitter must therefore take action to determine the status of the remote node. This is normally performed using a Checkpoint error recovery procedure.