Northern UTA220/UTA220k ESTABLISHING AN X.25 SESSION, Initialize the PAD, ATDD Command, D-Channel

D-Channel X.25

ESTABLISHING AN X.25 SESSION

Before using the TA for X.25 communications on the D-channel, the user must subscribe to the ISDN D-packet service. If the user is subscribed to the D-packet service, a connection is automatically established between the TA and the ISDN packet handler when the TA is first powered up. This is known as the D-channel X.25 link connection. Once the TA is connected to the ISDN packet handler, the X.25 PAD is used to make an X.25 virtual connection to a specific remote host or device. The connection is called “virtual” because it is not a direct physical connection; a transmission path is created for each call and held until communications are completed and the connection is disconnected.

The following explains how an X.25 virtual connection (session) is established and disconnected, as well as some of the activities performed during the X.25 session.

Initialize the PAD

When the TA is first powered up, the TA is in the AT command mode. There are three AT commands that the TA supports to initialize the PAD:

ATDD

AT@P1 = P

AT@P1 = D

Once the PAD is initialized, the PAD prompt character * displays and the PAD is in the PAD command mode.

ATDD Command

The ATDD command is valid for initializing the PAD only when the D-channel X.25 link is established. If the ATDD command is entered when the D-channel X.25 link is not established, the TA responds with ERROR and remains in the AT command mode.

AT@P1=P or AT@P1=D Command

The AT@P1=P or AT@P1=D command initializes the PAD automatically when the TA is powered up. If the D-channel X.25 link is not established when the TA is powered up, the PAD is initialized to the PAD command mode. Likewise, if the D-channel link is connected when the TA is powered up, the PAD is initialized to the PAD command mode, just as though an ATDD had been executed.

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