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.
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