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226 Chapter 8: Establishing an X.25 Connection
Foundation Topics
X.25 Basics
In the 1970s, a suite of protocols was needed to provide WAN connectivity across public data
networks (PDNs), which had already been deployed with great success.
Standardization in the deployment of PDNs was lacking. Industry participants at the time
thought that standardization would increase the number of clients who would subscribe to PDN
services, if a way could be found to decrease cost while increasing functionality. The result of
this development effort was the X.25 specification, which includes X.3, X.75 and X.29. These,
however, are a bit beyond our discussion at this time.
X.25 was developed primarily by telco service providers. The specifications were designed to
work well in a multivendor environment. End-users can order lines from these telcos and make
use of the public packet-switched networks (PSNs).
Fees charged for this service are usually fixed recurring charges; however, the billing can also
be based on negotiated usage-based rates. In modern deployments, however, a number of
package offerings exist, each with differing feature sets. Billing is structured in a manner
similar to the billing of leased lines and the number of packets sent across those lines, as well
as the duration of the connection.
X.25 and its related protocols are administered by an agency of the United Nations called the
International Telecommunications Union (ITU-T). The ITU Telecommunication Standardization
Sector is the committee responsible for voice and data communications.
The ITU-T has created and evolved the standards for X.25 over the last 30 years or so. The
ITU-T specifications detail connections between DTE and DCE for remote terminal access and
computer communications. These X.25 standards include the creation of a layered model for
X.25.
The X.25 layered model defines Link Access Procedure, Balanced (LAPB) as the Layer 2
protocol and Packet Layer Protocol (PLP) as the Layer 3 protocol. Many representations of the
X.25 model show X.25 as the Layer 3 protocol and we will do the same. However, you should
understand that technically, PLP is the Layer 3 protocol.
The specifications for X.25 are updated regularly. Normally the update cycle is every four years.
The specifications dated 1980 and 1984 are the most commonly deployed versions.
The ISO has produced ISO 7776:1986 as a comparable standard to the LAPB standard. Also,
ISO 8208:1989 was created to be the equivalent to the ITU-T 1984 X.25 recommendation.
Cisco's X.25 software follows (but not completely) the ITU-T 1984 X.25 recommendation.
Cisco has created its own implementation by combining the high points of both the ITU-T
recommendations and the ISO recommendation. The implementation details are as follows: