Introduction to Tandem X.25 Capabilities

2 Tandem X.25 Features and
Capabilities
065307 Tandem Computers Incorporated 2–1
This section describes the features and capabilities provided by the X.25 Access
Method (X25AM), the Tandem product for X.25 connectivity. If you are not already
familiar with X.25 concepts and terminology, please refer to Section One, “What is
X.25?”
X25AM X25AM connects Tandem computer systems to public and private data networks
using the X.25 standard set of networking conventions.
Scope of Support X25AM works with any public or private packet network that supports the 1980 or
1984 CCITT X.25 Recommendations. X25AM has been certified for use with many
public networks around the world, including all those operated by European countries
that require certification of access software. A list of supported networks is provided
in Appendix A.
X25AM supports LAPB (Link Access Procedure Balanced), which is the data link
procedure generally used for X.25 connections. LAPB allows a point-to-point
connection between two stations, where each station can establish and disconnect the
link, and initiate data transfers. (X25AM software releases prior to C30.08 supported
both LAPB and LAP; X25AM C30.08 and later software releases support only LAPB.)
X25AM provides more capabilities than those required by the bottom three layers of
X.25 functionality indicated by the OSI model. These additional capabilities make it
easier for the lower layers to communicate with different types of higher-layer
applications on different types of systems. The application interface to X25AM is
achieved through Guardian 90 procedure calls or Pathway — the same file interface
used by other Tandem products.
X25AM can be configured for either DCE or DTE operation, allowing Tandem systems
to be used as either end of a switched or leased-line X.25 connection.
Subsystem Components The X25AM subsystem consists of the following components:
X25AM process — The X25AM process is a program running on a Tandem system.
The X25AM process controls the X25AM line and subdevices. A user application
communicates with the X25AM process through Guardian 90 file-system
procedures. Other processes (including other Tandem products) can also
communicate with the X25AM process for different types of connectivity. One
X25AM process is required per one X25AM line. However, multiple applications
or other processes can communicate with one X25AM process.
For example, an Expand process, a SNAX process, and multiple user applications
can all communicate through a single X25AM process and a single X25AM line.