Introduction to Tandem X.25 Capabilities

1 What is X.25?
065307 Tandem Computers Incorporated 1–1
This section provides an overview of X.25 concepts and related standards. If you are
already familiar with X.25 concepts and terminology, you may want to skip this
section.
Multi-Vendor
International
Communications
Protocol
One of the most common ways to connect computer systems that are in remote
locations —rather than in a single building or campus — is through a packet-switching
data network (PSDN). The protocol used for connecting computer systems to a PSDN
is X.25.
Benefits of X.25 The major benefits of using X.25 are its relatively low cost of communications, its high
degree of flexibility, its low capital cost for relatively high connectivity, and its fault
tolerance.
Low communications cost. The cost of X.25 connections may be less than
dedicated lines when use is intermittent. Cost may be further reduced if multiple
applications share the same X.25 link.
Flexibility. Virtually any point in the world can be reached by a PSDN. A PSDN
can easily accommodate changing communications requirements and network
expansion.
Low capital cost/high connectivity. X.25 provides a way to connect a large
number of systems through a single line between a computer system and a PSDN.
The need for fewer lines can lower communications capital cost by reducing the
number of modems and controller ports that must be purchased.
Fault tolerance. Reliability is inherent in the structure of an X.25 network. There
are usually several redundant connections between switching systems in an X.25
network; thus, if one transmission link fails, communications can be rerouted.
Need for a Common
Standard
When computers of the same type communicate with each other, they can follow their
own conventions (see Figure 1-1). For example, Tandem computers can communicate
with each other over an Expand network using Tandem proprietary networking
conventions. Likewise, IBM computers can communicate with each other over an
SNA (IBM Systems Network Architecture) network using IBM proprietary
conventions.
Different types of computers must adapt their propriety form of communications to
connect to different networks. Public data networks need a method to communicate
with all of the different types of computers without having to adapt to the proprietary
conventions of each computer type.