ACC X.25 Protocol User's Guide

X.25 Application Programming
Sending and Receiving Data
Chapter 4 79
M-Bit Handling
If bit-4 of the tag byte is set by the application as it sends a message then
the last packet of that message will be transmitted with the M-bit set. It
is important, when making use of this facility, that the application send
exact multiples of the Level 3 packet size, as a partially complete packet
with the “More” bit set is illegal and will not be allowed by the network.
Note that the MUX card has a total of 22K bytes of outbound buffer
space for each port. The largest contiguous message that may be sent
without the M-bit set is approximately 4K bytes.
If the X.25 link option word bit (IRX) to allow fragmenting of received
messages has been enabled and the load on the Mux is such that the
received packets of an incomplete data sequence are sent to the
application, then bit-4 will be set in the tag byte received by the
application with the data. This indicates that the inbound data message
still has more data to come (e.g. this is a partial data message).
D-Bit Handling
1980 Mode
In this mode, the application can set the D-bit on all packets of an
outgoing message by setting bit 6 of the tag byte. Received packets with
the D-bit set are acknowledged by the X.25 firmware.
1984/1988 Mode
In this mode, the application can set the D-bit on all packets of an
outgoing message by setting bit 6 of the tag byte, provided that the use of
the D-bit has been agreed to at CALL setup. D-bit use negotiation can be
performed by the application issuing or acknowledging the CALL.
If an outgoing call is being made automatically on enable, then D-bit
usage will be negotiated if the parameters in the link option word and
the V. C. ‘poll’ word allow it.
Received packets with the D-bit set to 1 are acknowledged according to
the setting of the APD option in the virtual circuit option word.
The X.25 firmware is able to enforce negotiation of the D-bit by setting
the ND bit in the X.25 Link POLL word (see “Configuration Parameters
Definition” in Chapter 3, ZX25D X.25 Protocol Driver,). When this bit is
set any D-bit packets received will cause a RESET to be sent unless the
D-bit usage has been negotiated in the call setup.