Introduction to Tandem X.25 Capabilities

Flexible Call-Setup
Tandem X.25 Features and Capabilities
2–4 065307 Tandem Computers Incorporated
Flexible Call-Setup X25AM provides flexible call setup options to meet your application needs through a
combination of permanent and switched virtual circuits, subaddressing, and optional
user facilities.
Permanent and Switched
Virtual Circuits
Up to 254 circuits can be configured for each X25AM line. (For D-series systems, up to
511 circuits can be configured.) The maximum number of circuits determines the
maximum number of subdevices that can be added to the line.
Permanent virtual circuits — A permanent virtual circuit provides a permanently
assigned logical channel, connecting the local DTE to a specific remote DTE.
Call-establishment packets are not required. PVCs are used for applications that
require constant and immediate availability.
Switched virtual circuits — A switched virtual circuit is appropriate for
applications that do not need constant connectivity and do not need immediate
data transfer. Virtual circuits carry some extra overhead and time delay, because
additional packets must be sent to establish and clear the communications session.
Subaddressing Subaddressing directs calls to specific devices or applications by assigning different
subaddresses (or port numbers) for each device or application on an X.25 link.
Note Subaddresses or port numbers refer to a logical connection, not a physical port.
You can assign a single port number to multiple subdevices: if you have two or more
terminals connected to an X25AM line and it does not matter which terminal receives
the call, both terminals can share a single subaddress.
The availability and implementation of subaddressing depends upon the network’s
addressing scheme. Some networks use the final 1 or 2 digits of the NTN portion of
the address, while other networks insert the subaddress within the CUD field.
Figure 2-2 illustrates the relationship between addresses, port numbers, and
subdevices. Port #1 is shared by two terminals. A call to this port number would be
directed to the first available terminal. Port #2 connects through a spooler to a printer
subdevice. The printer has different characteristics than the terminals have, so it could
not share the same port.