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.

In addition, the ACC X.25 subsystem also generates an unsolicited status message to indicate that X.25 at level 3 is up. At this point the X.25 link is in a normal operational state and can be used to establish calls and transfer data across virtual circuits. However, note that VC ZLUs must also be enabled and activated before they can be used.