ACC X.25 Protocol User's Guide

ZX25D X.25 Protocol Driver

Configuration

Chapter 3 43

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 the 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

the F bit is treated as it is set to zero and the link is not

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 logical channels,

9 8 7 6 5 4 3 2 1 0

Not Used (must be 0) UF

T25 NT