Ref: 99980086
Title: X.25 Questions
Date: 4/1/87

Copyright 3Com Corporation, 1991.  All rights reserved.

Q:  What is the difference between connection service and
interconnection service?

A:  These two names have created considerable confusion in the
past.  The concept is in fact quite simple:

*   Interconnection service is a mode in which in GS/1-X.25
acts as a router between two Ethernet networks across an
X.25 network.  XNS packets directed toward the remote
Ethernet network are encapsulated in X.25 packets and
carried across the X.25 network.

*   Connection service basically means everything that is not
interconnection service:

-   A PAD on a PDN connecting to a host on the LAN uses
connection service.

-   A terminal on a communications server connecting to
an X.25 host on the PDN uses connection service.

-   A terminal on a LAN connecting to an X.25 host on the
same LAN uses connection service.

Note that the number of sessions or virtual circuits is
not critical when the GS/1-X.25 is used in
interconnection service.  In this case a virtual circuit
is used as a pipe to route packets for possibly more than
one communications server session across the X.25
network.  The GS/1-X.25 could, for example, create only
one virtual circuit to route the traffic associated with
ten different communications server sessions.  In
connection service, however, there is a direct
correspondence between virtual circuits and
communications server sessions.


Q:  Can connection service and interconnection service be
performed on the same GS/1?

A:  Yes.  The sysgen program allows different lines of the GS/1
to be configured for different applications.  In fact,
connection service and interconnection service can even be
run on the same line if it is configured for interconnection
service.


Q:  Can the same GS/1 be configured to run as Data Communication
Equipment and as Data Terminal Equipment?

A:  Yes.  Though these two different applications have to run on
two different lines.


Q:  When should a GS/1 be used instead of a GS/1-X.25 to
interconnect two remote Ethernets?

A:  Whenever point-to-point line service is more economical than
Public Data Network service.
