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Chapter 9 - X.25 Pad Support

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An X.25 network uses a packet-switching protocol to transmit data. This protocol relies on an elaborate worldwide network of packet-forwarding nodes (Data Communications Equipment [DCEs]) that participate in delivering an X.25 packet to its designated address.

Dial-up Asynchronous Packet Assemblers/Disassemblers (PADs) constitute a practical choice for Remote Access clients because they don't require an X.25 line to be plugged into the back of the computer. Their only requirement is the telephone number of the PAD service for the carrier.

Note This chapter is specific to X.25 PADs. X.25 cards can also be supported through WAN miniport drivers.

The Remote Access Service lets you access the X.25 network in two general ways:


Method of access

Client (for the Windows™ or Windows NT operating systems)

Asynchronous Packet Assemblers/Disassemblers (PADs)

Server and client (for Windows NT systems only)

Direct connections

The next section tells how to access the X.25 network in both ways for specific configurations.

X.25 Configurations

The Remote Access Service for X.25 networks offers two configurations for the client and one for the server:




Dial-up PAD


Direct connection to the X.25 network through X.25 smart card


Direct connection to the X.25 network through X.25 smart card

Pad.inf Format

Similar in format to Modem.inf (which contains script information used to talk to the modem), Pad.inf contains conversations between the client software and the PAD. For details, see Appendix C, "Understanding Modem.inf." Pad.inf is located in the \systemroot\SYSTEM32\RAS folder.

The macros in the following list are reserved words, which you cannot define in Pad.inf to create a new entry. Reserved words are case insensitive.

  • x25address 

  • diagnostics 

  • userdata 

  • facilities 

Caution Using reserved words as macro names in Pad.inf could result in unpredictable behavior of the Remote Access software.

Sample Pad.inf

The following sample Pad.inf file will help you create a section within Pad.inf for your X.25 network. This example shows an entry for Sprintnet:


;The following three lines are temporary.

; The next line will give a delay of 2 secs -
; allowing the PAD to initialize



; The @ character sets the SPRINTNET PAD for 8 databit communication.


; The D character requests a 9600 speed.
; We don't care for the response so we ignore it.

; A carriage return line feed again to initialize
; the PAD read/write buffers


; Set X.3 settings on the PAD which make it work well with RAS.
; Broken into two parts since the line is too long.
COMMAND=SET 1:0,2:0,3:0,4:1,5:0,6:1,7:0,8:0,9:0,10:0,11:0<cr>

; Set the other half of X.3 parameters
COMMAND=SET 12:0,13:0,14:0,15:0,16:0,17:0,18:0,19:0,20:0,21:0,22:0<cr>

; Finally try to call RAS X25 server
COMMAND=C <x25address><cr><lf>


; CONNECT response means that the connection completed fine.
; X25ERROR response means connection attempt failed - the X25 CAUSE and
; DIAGNOSTIC information will be extracted from the response and
; sent to the user.
; ERROR responses are for generic failures.

After this sample conversation for SPRINTNET is completed (with the correct responses), the X.25 connection is established. If errors are detected during the PAD conversation, no connection is made.

Note The Remote Access Service currently works with PADs set to 8 data bits, 1 stop bit, and no parity. Consult the documentation for the PAD to see how to install these settings.

In Pad.inf, you can use the COMMAND_ series of commands (COMMAND_INIT, COMMAND_DIAL, and COMMAND_LISTEN) or the generic COMMAND. But do not mix the two families of commands. For more information on the COMMAND_ series, see Appendix C, "Understanding Modem.inf."


For troubleshooting information, see the Remote Access online Help.

Accessing X.25 Through Dial-Up PADs

Operating between the client and the Remote Access server, an asynchronous PAD converts serially-transmitted data into X.25 packets. When the PAD receives a packet from an X.25 network, it puts the packet out on a serial line, making communication possible between the client and the X.25 network.

Remote Access clients can connect with Remote Access servers through dial-up PAD services supplied by X.25 carriers, such as Sprintnet and Infonet. After the client's modem (modem A in Figure 9.1) connects to the PAD's modem (modem B), the client software must converse with the dial-up PAD. When their conversation is successfully completed, a connection is established between client and server. The conversation (command/response scripts) for the PADs supported by an X.25 carrier is stored in the Pad.inf file. Remote Access software supplies one example. To customize your PAD, see "Pad.inf Format," in this chapter.

For example, Pad.inf contains two Sprintnet entries: Sprintnet Standard and Sprintnet Alternate. Generally, if you are calling through 9600 bits-per-second (bps) or faster dial-up PADs, try Sprintnet Standard. If you are calling through 2400 bps or slower dial-up PADs, try Sprintnet Alternate.

If one Sprintnet entry fails to connect reliably, try the other one. Sprintnet dial-up PADs should work with both.

Note For dial-up PADs, you must use the COMMAND= format, not the COMMAND_INIT, COMMAND_DIAL, and COMMAND_LISTEN format.

Figure 9.1 shows how a client connects to the Remote Access server through a dial-up PAD and the X.25 network.


Figure 9.1 Connecting to the Server Through a Dial-Up PAD 

Note For best results when using a dial-up PAD, use a modem that matches the one used by the PAD carrier (or at least matches the V. protocol supported by the carrier's modem).

The following table compares connecting through dial-up PADs and connecting directly to the X.25 network:

Dial-Up PAD

Direct connection

Saves the expense of a dedicated leased line (direct connection).

Requires expensive leased line.

Allows connections from hotels, airports, homes—anywhere a phone line is available.

Requires users to dial in from a fixed location.

Requires two steps to connect.

Connects conveniently in one step.

Limits maximum communication speed to whichever speed is slower, the modem's or the PAD's.

Lets communication take place up to the speed of a leased line, 56 kilobytes (K).

Allows less control in configuring PADs.

Offers greater reliability.

Only a client can connect through a dial-up PAD.

Both servers and clients can connect directly.

PAD and Serial Configuration

To configure your PAD correctly, set the X.3 parameters according to the information shown in Table 9.3 later in this chapter.

The configuration of the dial-up PAD should be as follows:

  • 8 data bits 

  • 1 stop bit 

  • No parity serial communication 

For dial-up PADs, make sure your vendor supports this configuration. The PADs might already be set to the correct configuration for connecting directly through an internal X.25 smart card. If they are, do not change the configuration.

Connecting to the X.25 Network Directly

RAS also supports connecting directly from the remote computer to the X.25 network through a smart card, which acts like a modem. An X.25 smart card is a hardware card with a PAD embedded in it. To the personal computer, a smart card looks like several communication ports attached to PADs. To access the X.25 network through a direct connection, you must have

  • a direct line connection to an X.25 network (clients only) 

  • a smart card 

Note The server side always requires an X.25 smart card, but the client side requires one only when connecting to the X.25 network directly.

Note For connecting to the 12network directly, you must use the COMMAND_INIT, COMMAND_DIAL, and COMMAND_LISTEN format.

Figure 9.2 shows how the server and a Windows NT client (both equipped with smart cards) connect to the X.25 network directly.


Figure 9.2 Connecting to X.25 Directly 


The Remote Access server does not support callback on X.25 networks.

Setting Up the Remote Access Server for an X.25 Network

After installing Windows NT Server and adding the Remote Access Service, follow these steps:

To set up the Remote Access server for an X.25 network 

  1. Install the X.25 smart card (according to the manufacturer's instructions). 

    A communications driver for the X.25 smart card that emulates communication ports is supplied by the hardware manufacturer or by a third party. 

  2. Make sure your X.25 smart card is configured with the X.3-parameter values shown in Table 9.3. 

  3. From the list of devices on the Remote Access Setup dialog box, select an entry corresponding to the X.25 smart card. 

  4. In setting up the Remote Access server, make sure that the ports selected are configured for dial-in. 

Note Make sure that the speed of the leased line can support all the serial communication (COM) ports at all speeds at which clients will dial in. For example, 4 clients connecting at 9600 bps (through dial-up PADs) will require a 38,400-bps (4 times 9600) leased line on the server end. If the leased line does not have adequate bandwidth, it can cause time-outs and can cause the performance for connected clients to degrade.

Parameter number

X.3 parameter



PAD Recall






Data Fwd. Char



Idle Timer



Device Ctrl



PAD Service Signals



Break Signal



Discard Output



Padding after CR



Line Folding



Not Set



Flow Control



Linefeed Insertion



Padding after LF






Character Delete



Line Delete



Line Display



Editing PAD Srv Signals



Echo Mask



Parity Treatment



Page Wait


Caution Failure to set these values as shown prevents the Remote Access Service from functioning properly. For information on setting these values, see the instructions with your X.25 smart card.

Setting Up Remote Access Clients

This section tells how to set up Remote Access clients for connecting to the X.25 network through PAD services and for connecting to the X.25 network directly.

Connecting Through Dial-Up PADs

Following these steps to connect a client to an X.25 network:

  1. Dial from the client's modem to a PAD (modem-to-modem). 

  2. Establish a connection over the X.25 network between the PAD and the server-side X.25 smart card. 

After you've established a connection, communicate as you would through any asynchronous connection. For a complete description of connecting through dial-up PADs, see "Accessing X.25 Through Dial-Up PADs," earlier in this chapter.

Configuring Client PADs

The client PAD, through which a remote computer connects to the X.25 network, might have previously been set to X.3-parameter values that are incompatible with the Remote Access Service. Therefore, it is important to configure the X.25 smart card on the Remote Access server so that it changes the client PAD's X.3 settings to the values in Table 9.3 as soon as a connection is established through X.29 commands. To configure an X.25 smart card to make these changes, see the configuration manual for your specific card.

Note If the X.25 smart card on the server end does not support commands for the X.29 language, the client PAD script must set the X.3 parameters locally. If you have problems, contact the support site for your X.25 smart card vendor.

Connecting Directly

To set up the client for connecting directly to the X.25 network, follow the procedures used in setting up the Remote Access server. See "Setting Up the Remote Access Server for an X.25 Network," earlier in this chapter. Make sure the communication ports are selected as dial-out.

Configuring Remote Access Software for X.25

Connecting to a server through an X.25 network is similar to connecting through a phone line. The only difference is that the phone book entry must specify an X.25 PAD type and an X.121 address.

To add a phone book entry with X.25 or to add X.25 to an existing entry 

  • See RAS online Help. This online Help also provides troubleshooting information. 


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