ACC HDLC/LAP-B (ABM) User's Guide

Using HDLC/LAP-B (ABM) Protocol
Timeout Processing
Chapter 332
Timeout Processing
Frame timeouts are set in the Poll and Select configuration words. The
T1 timer defines the period within which a response to a Command
frame must be received across the HDLC/LAP-B (ABM) link. Should this
timer expire before the expected response is received, then the Command
frame will be re-transmitted up to a pre-determined maximum number
of times (N2). The maximum number of re-transmissions is also set by
the Poll and Select configuration words. If the timer expires on that
number of successive occasions, then the LAP-B link will be reset.
An additional timer referred to as T2 is used to control the transmission
of acknowledgments to received I-frames. As the acknowledgment may
be ‘piggy-backed’ onto a transmitted I-frame, it may not always be
necessary to transmit an explicit flow control frame to send the
acknowledgment. When an I-frame is received, timer T2 is started, and if
an I-frame is transmitted, the acknowledgment is sent with it, and timer
T2 will be cancelled. Only if T2 expires (no I-frame having been
transmitted) will an explicit flow control frame be transmitted. T2 is
always set to half of the value of the T1 timer.