The Access Unit

The Access Unit

X.400 and the later recommendations describe a self-contained and complete messaging system, but other messaging system still exist and the acceptability of X.400 partly depends on how it can interface with pre- X.400 messaging mechanisns. This is important for two reasons:

  1. Potential users of X.400 have existing investments in pre-X.400 technology which they can not afford to ignore.
  2. With have initially a small user - base it is necessary provide interfacing to other technologies to encourage more users to adopt X.400 and make it widely available and popular.
A.U.s define how interpersonnel messaging users can communicate with users of pre-X.400 technologies, such as telex, teletex and even postal delivery services. AN AU is a device that interfaces with the MTS on one side and with a pre-X.400 service on the other side. It convertsformats, service elements, and protocols. An AU can provide non-X.400 services bidirectional flow of messages.

In the 1984 recommendations, only one AU was defined, the teletex AU. In the 1988 recommendations no AU is defined. In the related F.400 series recommendations, which are service descriptions intended to accompany the X.400 recommendations, the teletex, telex and postal delivery A.U.s are defined, though no Fax AU is defined. Since A.U.s seem only to be specified in the F recommendations means that no technical definitions is available.

Interfacing X.400 with pre-X.400 technology is one of the most important aspects of implmenting an X.400 environment. Service providers have felt pressure to incorporate X.400 with their traditional messaging systems. With no technical specifications available and no guidance from the CCITT, they have had to come up with their own solutions. As a result, there has been no standardization of these interfaces.

  • Back to X.400 Functional Model page
  • Back to to X.400 document page