ATN and OSI - Data Communications Support

AMHS is one of several several different communications applications used to support Air Traffic Control Service operations. Others are:

• Ground to ground  - including AMHS, The ATN Directory, Controller-Controller communications, Radar ..

• Air to ground  - including controller to pilot dialogues and other aircraft.

In earlier times there was always the temptation to develop a different underlying data network specialised to each application. This was an expensive way of going about building a communications infrastructure. In recognition of this, the International Standards communities (ISO and the ITU-T), developed a standard reference model together with data communications services and protocols that is now known as 'Open Systems Interconnection'. It can be used to support widely differing types of applications, and ensures that compliant systems from different suppliers can actually interchange data.

More recently, ICAO Standards have been developed on the foundation of OSI to ensure that the data of different  Air Traffic Control communications applications can be carried by a single, common underlying data network.This is known as the Aeronautical Telecommunications Network (ATN). Its intent is to avoid having to build a separate and different data communications network to support each different ATC application.

The ATN is based on the International Standards Organisation (ISO) OSI Standards. So, AMHS and the ATN Directory have been defined formally by ICAO.

The ATN can handle both ground-to-ground traffic (where the 'end systems' are statically bound to the ATN), and for air-to-ground traffic, where one of the 'end systems - e.g. an aircraft',  is only dynamically bound to the ATN at different points - the application is 'mobile'. These mobile aircraft end systems appear at different access points on the ATN as a flight progresses. This latter 'mobility' introduces a high complexity and cost to the ATN technology, and some ICAO regions seem to be reluctant to implement the full ATN until a concrete air-ground requirement is identified within their region.

Some of these ICAO regions have identified the Internet's TCP/IP data communications technology as a low-cost alternative to the full ATN which is to be used in the interim for ground-ground applications until a full ATN can be economically justified - i.e. until it is required to support air-ground traffic.

The co-existence of ATN  and TCP/IP data networks is not a problem for AMHS, since most of the global ground-to-ground applications are based on Application Layer Relays, at which the mappings between ATN and TCP/IP can easily be performed using 'dual protocol stack' implementations.

It is interesting to note that currently, the ICAO Standards bodies are working on adapting TCP/IP to support the ATC requirements for air-to-ground data communications.

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