Ref: 22340006
Title: X.25, X.28, and X.29 Interfaces to GS/X.25-IP, Version 3.0
Date: 12/27/90

Copyright 3Com Corporation, 1991.  All rights reserved.

X.28 Interface:

The X.28 interface (hardware connection between the GS and the PDN) is
fully compatible with the 1984 version of the CCITT's X.28 recommendation.
It supports all 22 PAD parameters.  The following commands to check
connection status between the GS and the PDN are supported:

  STAT:   Request for status information of virtual circuit.  X.28
          responds with the message "FREE" or "ENGAGED."

  CLR:    Clear a virtual cricuit.  X.28 responds with the message
          "CLR CONF" or "CLR ERR."

  PAR?<PARMS>:  Request for current values of the specified parameters.
          If no PARMS are specified, it lists all the parameters and their
          values; otherwise, it lists the values of the specified parameters.

  SET<PARMS>:   Modify current values of the specified parameters.

  SET?<PARMS>:  Initialize PAD parameters to a set of standard PAD
          parameters values.

  RESET:  Reset the virtual circuit.

  INT:    Interrupt the virtual circuit.

When a connection is made from a CS user, the X.28 module sends a welcome
message in return.  An X.28 PAD selection command can then be entered in
order to make a call.  The selection command format is

   GS/X.25> R,Ggg - nnnnnnnnnnnnnnDdddddddddddd

where   GS/X.25> = Gateway command prompt
        R        = Reverse Charging Request
        ,        = Delimiter
        Ggg      = Closed User Group Access Request
        -        = Facility field separator
        n...n    = Called Address Block (mandatory)
        D        = Call User data field identifier
        d...d    = Call User data field

Note:  The Call User data field identifier can be the letter P, signalling
that a password is to be placed in the Call User data field.  In that case,
echo will be turned off.

For Telenet, use the selection command format c xxxyy, where xxx = area code
(3 digits) and yy = local address (1 to 4 digits).

For more information refer to the CCITT X.28 recommendation.


X.29 Interface:

The X.29 Interface (to download PAD parameters from host to PAD) is fully
compatible with the 1984 version of the X.29 recommendation.  It supports
the following message types:  Set PAD Parameters, Read PAD parameters, Set
and Read PAD Parameters, Parameter Indication, Invitation to Clear,
Indication of Break, and Error.  For more information, refer to the CCITT
X.29 recommendation.


X.25 Interface:

The X.25 Interface (used for Call Set-up, Data Transfer, and Call Clearing
phases of the virtual circuit) supports some CCITT 1980 facilities and
some CCITT 1984 facilities.

Establishing a connection:  The connection between the CS user and the X.25
host is built by the gateway as follows.  Once the X.28 module receives a
connection from the CS user, it sends a welcome message to the user
and waits for the PAD selection command signal from the CS user.  At this
stage, the module accepts X.28 commands.  In response to the selection
command, X.28 builds and sends an X.25 Call Request packet, which has
the PAD protocol information encoded in the user data area, to remote
host via the X.25 network.  Once the host accepts the call, the X.25
layer will notify the X.28 module and cause the X.28 module to send a
"COM" message back to the CS user.

Transferring Data:  The gateway is totally transparent to the user while
in the data transfer mode.

Ending the Connection:  There are two ways to terminate gateway
connections--host-initiated and user-initiated.  The host-initiated
termination occurs when the CS user is logging out from the host.  Depending
on the implementation, the host may send an Invitation to Clear X.29 command
to the PAD and the PAD will send a Clear Request.  Or, the host may Clear a
virtual circuit.  In either case, the gateway sends a "CLR CONF" message to
the CS user.  In the single-step connection, after sending the "CLR CONF"
message, the gateway breaks the connection to the CS user.  In the two-step
connection, the CS user is still connected to the gateway.

The CS user can initiate a disconnect either with the Telnet "disconnect"
command (Ctrl D) or the Telnet "close" command, or with the X.28 command CLR.


Telnet Command/Options Handling in LAN-to-WAN Mode

Mapping of Telnet commands to X.29 commands is not necessary in the LAN
to WAN connection.  Whenever an X.3 parameter is changed, either by an
X.28 command, an X.29 Set command, Set and Read command sent from the
remote host to the gateway, or by the gateway Network Manager using SET
of SETD commands, no Telnet options negotiation is necessary because the
X.3 parameters are local to the gateway.  The gateway provides the
transparent connection.