ACC X.25 Protocol User's Guide

72 Chapter4

X.25 Application Programming

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 b y 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 D ata 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,theData 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. S ee

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 b e used.