Chapter 8. X.25 SNA Interconnection (XI) and Network Supervisory Function (NSF) 209
8.2 Functions and alternatives
The basic purpose of XI is to enable your SNA network to transport X.25 traffic
between compatible X.25 DTEs. Unlike NPSI, which provides protocol
conversion from X.25 to SNA that allows X.25 devices (such as terminals and
printers) to communicate with SNA applications, XI does not perform protocol
conversion. Thus, aside from accounting and management functions that are
ancillary to user function, there are no application considerations in XI migrations
unless you migrate from X.25. This is because the SNA network is transparent to
the X.25 devices and/or networks it connects. Some XI migration alternatives
follow.
8.2.1 Transporting X.25 traffic
The function of the XI product is a little different from most communication
controller functions because it is concerned only with transporting X.25 frames
irrespective of the devices or applications that are the endpoints of the
communication. There are various means of transporting X.25 traffic which,
depending upon your unique circumstances, may or may not be viable.
Migrating end devices from X.25
Migrating end devices to use a new networking technology, such as frame relay,
presents an opportunity to invest in newer and faster technologies, with
potentially increased function. This, of course depends upon the availability of
frame relay or some other suitable replacement service in your locations. It is
important, however, that you check your X.25 configurations to determine what
functions you are using. For example, X.25 contains a rich set of dial connection
control functions. If any users now have switched X.25 network connections, and
if they plan to use switched network connections in the new environment, you
should make sure that the new environment can support any call optimization,
accounting, and security functions currently implemented that will be required in
the new environment.
Migrating to an X.25 service
Since XI provides a fully standard X.25 DCE interface, you could replace the
SNA-based connections with circuits in an X.25 packet switched data network
(PSDN) service. As with the migration to a new networking service mentioned
above, the alternative of migrating to an X.25 service depends upon the
availability of an X.25 service in your location.
Migrating to router transport of X.25
It may be possible to migrate from your SNA network transport of X.25 traffic to
an IP router network transport of X.25 traffic. For example, according to the Cisco
210 IBM Communication Controller Migration Guide
Systems, Inc. Web site, their routers support transport of X.25 traffic over an IP
router network.
You will need to determine whether your specific DTE and DCE connection
requirements can be met, as well as whether performance, security, accounting,
management, and other required functions can be met by such a solution.
Recommendation
Regardless of which of the above alternatives are viable in your environment, you
should keep in mind that the list of parameters involved in XI customization is
extensive. You should examine all explicitly coded parameter values as well as all
default values to ensure that you understand the exact functionality of your
current implementation. You should then determine whether all currently
implemented functions are compatible with the capabilities of the new network. If
not, you need to make sure that the changes required for the new configuration
will meet your needs. For example, if you plan to replace the XI portion of the
network with a router-based IP network, you must make sure that the routers can
be configured for X.25 support in a manner that fulfills the requirements of your
organization.
8.2.2 NSF-based chargeback for X.25 transport service
Some organizations make use of the substantial accounting information provided
by NSF in order to facilitate chargeback for the network services utilized by
another organization. While it may be possible to obtain limited information by
querying router MIB variables, we have been unable to find any capability that
comes close to that offered by NSF.

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