Dialup X.25 (X.32) Configuration

Dial-up X.25 (X.32) Configuration

Most X.25 connections use a dedicated connection; either a line to an X.25 packet switch or a leased line that goes right through to the remote machine. However, X.25 can also operate over dial-up telephone lines using the CCITT-defined X.32 parameters for operation.

Establishing a Connection

The software only establishes a dial-up link to a remote system (usually a public packet-switched data network providing X.32 services) when a user process attempts to make a virtual circuit connection (tries to make a call). If a connection is not established with the remote system, the X.25 software stores the request and tries to make the telephone connection to the remote system again. The HDLC/LAPB protocol is started after the telephone connection, followed by the outgoing X.25 Call Request packet.

For V.25bis, you can configure alternate numbers to be dialed during dial failures. Alternate dial_defns should be entered in the order of desired execution in the link configuration file. Up to 20 dial_defns can be configured per link. When CTS is stuck low or the modem could not detect a dial tone, alternate dial_defns will not be attempted or a number will not be retried. For example,

 
dial_defn
{
    dial_string    5550001
    number_of_retries    1
    timeout_between_retry   20
}

dial_defn
{
    dial_string    5550002
    number_of_retries    0
    timeout_between_retry   20
}

In this example your system will attempt to dial the number 5550001 a maximum of two times and then, if necessary, attempt to dial 5550002. When all dial_defns have been attempted without success the diagnostic 0x9e is returned. Each new dial attempt will begin using the first dial_defn.

CTS and DSR are two EIA modem signals that have special meaning for dial-up connections. Most modems have configuration options for these two signals. Some typical settings for CTS are "Force high", "Dependent on interface" or "Follow RTS". CTS should typically be configured to follow RTS (request to send) which is forced high by the card. Dialing the modem is not possible until CTS is high, with a typical error return being:

 
CTS off - Unable to dial the call. Diagnostic: 0x9a hex.

DSR needs to be configured to be high only when the modem connection is established. Thus "Force high" is not a valid setting for DSR. If DSR is always high, the card will assume the modem connection is established and not attempt to dial the call. If DSR does not go high once the modem connection is established, the card will assume the call failed and report the following error.

 
DSR off - Unable to complete the call. Diagnostic: 0x9c hex.

Avoiding Premature Disconnection

If you are using X.32 to call other machines, it is suggested that you modify the X.25 T21 parameter, as the default value does not take the delay involved in connecting a telephone call into account. Depending on the telephone exchange to which you are connected, it may take up to 45 seconds before the modems are connected. T21 determines how long the local end will wait for an outgoing call request to be connected before abandoning the attempt in error.

To change the length of time that the card will wait before clearing a call, set:

T21 90

Avoiding Dial Out Hang Ups

The X.32 protocol defines that the machine acting as DCE should clear the telephone connection after a specified period (T14) with no X.25 SVC calls connected. It makes no such request for devices acting as DTE. If something goes wrong with the connection or the remote DCE, the card may not know it and leave a telephone call up that should be down. To avoid hanging the line, use the T3 timer. This will cause the card to poll the remote system, requiring a response within T1 seconds. This way, the card will always find out when there's a problem with the remote system, and disconnect the telephone call. To use this facility, set:

T32

This example has a poll of the remote system occurring at least every 2 seconds. Values from 1 to 100 are legal.

Dynamic DTE/DCE Selection

In a public data network environment, DTE/DCE selection is fixed - your end is the DTE, and the network vendor's switching equipment is DCE, as the CCITT indicates. When you are connecting a machine to a machine in a dedicated environment, you choose which will behave as DCE, and use the dte parameter to select how your X.25 software behaves.

In a telephone dial-up environment, this isn't so clear. What happens if the telephone network calls you? Should it act as DTE or DCE? What if you have a population of machines in the field that need to call each other?

The CCITT has a method for resolving this, defined in T.70. Briefly, it says that whoever establishes the telephone call acts as DTE. Therefore, the device which takes the telephone call must act as DCE. You select this mode of operation using DXEsupport. To have dynamic selection of DTE/DCE operation, set:

 
DXEsupport 1

For a complete list of parameters, see Dial Parameters.

Revision 6.1.0 (February 2003)