Introduction to Tandem X.25 Capabilities

Comparing X.25 with other Networking Standards
What is X.25?
1–4 065307 Tandem Computers Incorporated
is an asynchronous terminal, it must be equipped with an additional hardware or
software interface that is capable of sending and receiving packets.
The DCE can be a modem, a piece of cable, or a network processor. Note that when
two host computers are connected through a PSDN, the connection involves two
DTE/DCE interfaces.
Comparing X.25 with
other Networking
Standards
X.25 is not the only networking standard, and it is actually only a partial model of data
communications. A complete model of data communications services specifies
everything from the electrical interface that connects the DTE to the DCE, to the
interface for the end-user application. X.25 does not prescribe any standards relating
to the end-user application.
The first complete model of data communications to be widely implemented was the
IBM Systems Network Architecture (SNA). However, SNA is a vendor-dependent
model, developed only for IBM computer systems. On the other hand, most data
communications environments require the ability to communicate with multi-vendor
equipment.
A complete model of data communications that is not vendor dependent is the Open
Systems Interconnection (OSI) model. When this model is fully implemented, user
applications do not have to adapt to the idiosyncrasies of different computer systems
(for mapping character sets, handling line feeds, or whatever). However, the OSI
model is still in its early stages of implementation. Currently, the OSI model is used
more in Europe and Japan than it is used in the United States.
Both the OSI and the SNA models include seven functional layers of communications.
X.25 specifies only three layers of communication, which are equivalent to the first
three functional layers of communications described by OSI, and which are roughly
equivalent to the first three layers of communications described by SNA. OSI
incorporates the X.25 standard as its own method of handling the lower layers of
communication. SNA has its own IBM standards for the lower layers, but it can be
adapted to run over the X.25 lower layers for connecting with PSDNs.
Figure 1-3 compares X.25 to the OSI and SNA models.