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

32 Chapter3

Using HDLC/LAP-B (ABM) Protocol

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 Comma nd

framemustbereceivedacrosstheHDLC/LAP-B(ABM)link.Shouldthis

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.

AnadditionaltimerreferredtoasT2isusedtocontrolthetransmission

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 t ransmitted, 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.