Westermo MR Series - Manual (Page 376)

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Web Interface and Command Line Reference Guide www.westermo.com
X.25 Packet Switching 11
Introduction 11.1
X.25 is a data communications protocol that is used throughout the world for wide area network-
ing 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 con-
trol and flow control mechanisms. Layer 3 is the Network layer. This defines the data and control
packet struc ture 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.
Westermo 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 configura-
tion 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 informa-
tion 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 11.2
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 com-
mand 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 connec-
tion to the X.25 network has been established.
D-channel X.25 11.3
The unit can transfer data to/from X.25 networks over the ISDN D-channel if your ISDN service
pro vider supports this facility. The speed at which data can be transferred varies depending on the
service provider but is generally 9600bps or less.