ACC X.25 Protocol User's Guide

Chapter 4 73

X.25 Application Programming

Sending and Receiving Data

Before you can send to or receive data from an X.25 Virtual Circuit, the

X.25 Link must be up, the VC ZLU must be enabled and activated, and

in the case of an SVC, a call must be established. The zcntl() routine i s

used to enable and activate the VC ZLU and the ACC X.25 API routines

are used to establish a call on an SVC. Once these conditions are met,

you may send data using the zsend() routine and read data that has

arrived on t he application’s program ZLU using the zread() routine. Note

that before inbound data from a VC ZLU is delivered to an application’s

program ZLU, you must cal l the zset_rcvr() routine for each and every VC

ZLU whose inbound data is to be delivered to the application.

All application data is transmitted (or received) as the data field of a

data packet contained within t he I-field of an information frame. All

level 2 and level 3 p rotocol h eaders and/or trailers are transparently

suppliedbytheprotocolforoutgoingmessagesandremovedbythe

protocol for incoming messages. Only the actual data to be transferred is

dealt with by the application program.

When reading inbound data from the application’ s program ZLU queue,

care must be taken to examine the type of data that has arrived. The

zread() routine returns the ZCOM message header and the message data

buffer. The message header contains information on which VC ZLU the

data came from and what type of data the message represents. Note that

the message could contain inbound data from t he VC ZLU, unsolicited

status information, or buffer completion status from a previous zsend()

call. The following example shows how to enable a VC, s etup the VC’s

receiver for inbound data, read a data message and interpret its

meaning: