ACC X.25 Protocol User's Guide

Chapter 3 43
ZX25D X.25 Protocol Driver
Configuration
X.25 Link Device Option Word Definition
The option word for the X.25 link (LAP-B) terminal defines:
• The default D-bit enable setting for automatic call generation.
• Whether to use the T25 timer (on outstanding DATA packets).
• Whether to complete transmitted messages immediately or wait for
acknowledgment.
• Whether incomplete received data packet sequences can be sent up to
the receiving application.
• Whether to hold acknowledgments for t he received packets until the
window is full.
• The type of level 3 RR (acknowledgment) policy to follow.
The format of the option word for an X.25 link (LAP-B) terminal ZLU is
as follows.
UFR The UFR bit, if set, causes the X.25 link to be reset
whenever a Supervisory Response frame is received
with F = 1 (an unsolicited final response). Otherwise
theFbitistreatedasitissettozeroandthelinkisnot
reset.
DBT The D-bit default applies only if no overriding value is
specified in the definition of the virtual circuit. Refer to
the section on “D-Bit Handling” in Chapter 4 , “X.25
Application Programming,” for more information.
T25 The “T25 Disable” option is used to disable timeouts on
transmitted DATA packets. The T25 timer is an
optional feature of X25.
NTX The “No Tx Wait” option for transmitted messages is
used to reduce the number of transmit buffers used on
the Mux to hold unacknowledged messages. This helps
to avoid running out of buffer space on the Mux in a
system with a combination of many l ogical channels,
many transmitted messages, delayed responses from
the network, and/or a large window size. It is
recommended that this bit always be set.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Not Used (must be 0) UFR DBT T25 NTX IRX HLD RRP 0