Packet Switching - Digi TransPort User Manual

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.
Digi TransPort User Guide 558