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Routing of Incoming Calls in an X.25 System

IP.com Disclosure Number: IPCOM000100473D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 138K

Publishing Venue

IBM

Related People

Bell, GT: AUTHOR [+4]

Abstract

This article describes a method for routing incoming X.25 calls to the correct application in a multi-user, multi-tasking environment. The X.25 Incoming Call packet has several elements which can be used to identify the incoming call. As there is no standard for the use of these fields, many different techniques have been used. A common technique is the use of the Call User Data field to identify a higher level protocol being used. This is not flexible enough for a complete system to be based upon it, although some systems (e.g. IBM 8100) have used this field alone. Such systems do not meet all requirements. As different manufacturers may have developed different internal standards, a method is required which can take account of information in not only the Call User Data, but also the Called and Calling Address fields.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 42% of the total text.

Routing of Incoming Calls in an X.25 System

       This article describes a method for routing incoming X.25
calls to the correct application in a multi-user, multi-tasking
environment.  The X.25 Incoming Call packet has several elements
which can be used to identify the incoming call.  As there is no
standard for the use of these fields, many different techniques have
been used.  A common technique is the use of the Call User Data field
to identify a higher level protocol being used.  This is not flexible
enough for a complete system to be based upon it, although some
systems (e.g. IBM 8100) have used this field alone. Such systems do
not meet all requirements.  As different manufacturers may have
developed different internal standards, a method is required which
can take account of information in not only the Call User Data, but
also the Called and Calling Address fields.

      The method allows applications to specify in advance which
calls they are able to receive, and the action to take if the
application is not running at the time of the call. It also
introduces some security measures to ensure that calls are not picked
up by an unauthorized application, which is essential in multi-user
systems.  A best match algorithm is used to compare incoming call
packets with a router table.

      Router Table: A router table is maintained which holds
information about potential incoming calls.
      The fields of this table are
           Calling Address
           Called Subaddress
           Call User Data
           Priority
           Action
           UserID

      Only the first three fields correspond to fields in the
Incoming Call packets.  The fields are grouped together and can be
referred to by a NAME.  An application then registers itself as
wishing to receive calls corresponding to a particular NAME.  This
process is described as LISTENing to a NAME.  It is possible to have
multiple entries in the table corresponding to a single name.  In
this case, one LISTEN command is taken to refer to all of the
corresponding entries in the table.

      The Called Address field given in packets is, in some networks,
longer than strictly necessary to identify the DTE.  This has lead to
use of the extra digits as a subaddress.  By only incorporating the
subaddress in the router table, the machine can be moved between
different links, and still use the same subaddresses.  These
subaddresses would typically be issued by a system administrator on
the basis of one subaddress per user.

      Calling Address - the table entry consists of digits up to the
length of an address, optionally followed by an asterisk.  For
example, if a network address consists of 15 digits, an entry in this
field may be '12345*'.

      Call User Data - the table entry consists of bytes specified as
a hexadecimal string, optionally followed by an asterisk.  For
exa...