ACC X.25 Protocol User's Guide

ZX25D X.25 Protocol Driver
Implementation Notes
Chapter 3 59
acknowledgment. When an I-frame is received, timer T2 is started, and if
an I-frame is transmitted, the acknowledgment will be 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. The T2
timer value may be explicitly set by using the T2_timer statement. If this
statement has not been defined in the Link Term entry, T2 is set to half
of the value of T1, and for a 9600 baud link, T1 will usually be set to 400
milliseconds. N2 is usually set to 10.
Timer T4 is used at Level 2 (LAP-B) for DTEs. This is the maximum time
the DTE will allow without frames being exchanged on the data link. The
value of T4 is approximately 6*N2*T1. Note that for a DCE, this timer is
called T3.
Packet level timers are implemented according to the ITU-T/ISO
recommendation for T10, T11, T12, T13, T20, T21, T22, T23, T25, and
T26. A 180-second timeout is applied to the receipt of data packet
acknowledgments. At the end of the time-out, the virtual circuit will be
reset.
Summary of Timer Values
The following sections shows the mandatory and optional X.25 timer
values. Note that the DCE timer values are shown for completeness only.
NOTE The X.25 Protocol product supports some of the features of a DCE
interface, however these are intended to be used for loopback
configurations and direct computer to computer connections where the
other end of the connection is a DTE device.