User`s guide

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X.25 PACKET SWITCHING
Introduction
X.25 is a data communications protocol that is used throughout the world for wide area
networking across Packet Switched Data Networks (PSDNs). The X.25 standard defines the
way in which terminal equipment establishes, maintains and clears Switched Virtual Circuits
(SVCs), across X.25 networks to other devices operating in packet mode on these networks.
The protocols used in X.25 operate at the lower three layers of the ISO model. At the lowest
level the Physical layer defines the electrical and physical interfaces between the DTE and
DCE. Layer 2 is the Data Link Layer that defines the unit of data transfer as a “frame” and
includes the error control and flow control mechanisms. Layer 3 is the Network layer. This
defines the data and control packet structure and the procedures used to access services
that are available on PSDNs.
A further standard, X.31 defines the procedures used to access X.25 networks via the ISDN
B and D-channels.
Digi ISDN products include support for allowing connected terminals to access X.25 over
ISDN B channels, the ISDN D-channel or over TCP. They can also be configured so that if
there is a network failure it will automatically switch to using an alternative service. The
Packet Assembler/Disassembler (PAD) interface conforms to the X.3, X.28 and X.29
standards.
Up to six PAD instances (from an available pool of 8), can be created and dynamically
assigned to the asynchronous serial ports or the REM pseudo-port.
Each application that uses the unit to access an X.25 network will have its own particular
configuration requirements. For example, you may need to program your Network User
Address (NUA) and specify which Logical Channel Numbers (LCNs) should be used on your
X.25 service. This information will be available from your X.25 service provider. You will also
need to decide whether your application will use B or D-channel X.25.
Once you have this information, the PAD configuration pages can be used to set up the
appropriate parameters.
B-channel X.25
The unit can transfer data to/from X.25 networks over either of the ISDN B-channels.
Once the unit has been configured appropriately, the ISDN call to the X.25 network can be
made using an ATD command or by executing a pre-defined macro. The format of the ATD
command allows you to combine the ISDN call and the subsequent X.25 call in a single
command. Alternatively, the X.25 call may be made separately from the PAD> prompt once
the ISDN connection to the X.25 network has been established.
D-channel X.25
The unit can transfer data to/from X.25 networks over the ISDN D-channel if your ISDN
service provider supports this facility. The speed at which data can be transferred varies
depending on the service provider but is generally 9600bps or less.