ACC X.25 Protocol User's Guide

X.25 Application Programming
X.25 Link Initialization
Chapter 472
X.25 Link Initialization
Before you can communicate across an X.25 link, it must be put into an
operational state. In the ZCOM subsystem this is initiated by enabling
the X.25 link ZLU using the zcntl() routine. There are three components
to X.25 which must all be functioning before you can communicate on a
Virtual Circuit.
There is X.25 at Level 1 which defines the physical layer (mechanical,
electrical, functional and procedural characteristics required to maintain
a physical connection between the DTE and DCE). If the physical layer is
properly connected, the next component of X.25, the Data Link layer
(level 2) will attempt to establish a connection.
The purpose of the Data Link layer is to provide error free transfer of
information units from one end of the user/network physical link to the
other. For X.25, the Data Link layer uses the HDLC/LAP-B protocol. This
is also known as the X.25 level 2 protocol or just Level 2 for short. When
the Data Link layer has been established, the ACC X.25 subsystem logs
a message similar to the following:
08:36:53 zx25d 00621 Link ZLU 001 HDLC/LAP-B established on
enable.
In addition, the ACC X.25 subsystem also generates an unsolicited
status message to indicate that X.25 at level 2 is up. Whenever Level 2
changes state, additional unsolicited status messages are generated. See
Appendix A, X.25 Status and Completion Messages, for more
information on how to read and interpret X.25 status messages. After
level 2 is up, X.25 attempts to establish its final component, the Packet
layer or X.25 at level 3, or simply Level 3. The Packet layer provides the
protocol governing the exchange of packets across the DTE/DCE
interface. When Level 3 has been established, the following message is
logged:
08:36:59 zx25d 00642 Link ZLU 001 X.25 Packet layer (lvl 3)
restart complete.