The Aeronautical Fixed Service (AFS) is central to operational efficiency in the air traffic management (ATM) domain as it ensures the efficient and timely exchange of information and data that are critical to guaranteeing the continuing safety, regularity and efficiency of international air navigation. It is also a crucial element for delivering on the responsibility that all Contracting States of ICAO incur under Article 28 to the Convention on International Civil Aviation, otherwise referred to as the Chicago Convention of 1944, in relation to the provision of necessary facilities and services to facilitate international air navigation.

Traditionally, AFS has been provided using the X.25 protocol based Aeronautical Fixed Telecommunication Network (AFTN), which is specified in Volume II of ICAO Annex 10, and complemented in Europe by the X.25-based CIDIN (Common ICAO Data Interchange Network). Although the X.25 protocol used for AFTN is compatible with the lower layers of the modern and largely Internet protocol-driven Aeronautical Telecommunication Network (ATN), AFTN presents a number of limitations, including the fact that AFTN operates on only character-oriented procedures, which renders it inadequate for delivering contemporary aeronautical messaging services. There are also obvious limitations in the areas of capacity, protocols, speeds, throughput, functionality, versatility and interoperability.

AFTN provides point-to-point communication based on the legacy X.25 or asynchronous protocols with a communication model that is based on the store-and-forward messaging principle, which allows for the forwarding of only the relevant addresses in a process known as “address stripping”. There are also AFTN systems that support message exchange over a variety of other physical layers and standards such as HDLC (High-level Data Link Control), TCP (Transmission Control Protocol) including MEP (Message Exchange Protocol), TP4 (ISO Transport Protocol class 4), and CIDIN. There are also other relatively low speed AFTN variants that feature the long-outdated telegraph technology.

The AFTN addressing scheme is built upon a fixed hierarchical address format composed of 8 upper case letters designating: ICAO Region, ICAO State Code, Location, Organisation, and Organisational Unit or the Communication Terminal. With this, the AFTN is capable of handling multiple addressing of just up to a maximum of 21 with one of five message priority levels, namely SS, DD, FF, GG, and KK. The AFTN message is largely byte-based consisting of character strings. Generally, the system supports two message formats (ITA-2 and IA5) with the character set consisting of only characters that are available for typewriters. What this means, in essence, is that information handled on AFTN are mainly static and textual. For both ITA-2 and IA5 formats, a maximum message text length of 1800 characters or a maximum of 2100 characters, if inclusive of heading, is allowed. However, messages with a text part longer than 1800 are usually segmented in parts, in accordance with ICAO Annex 10, Volume II (Attachment C).

In recent times, the incremental paces of global air traffic and the increasing complexities of the operational, technical, and regulatory aspects of the global air navigation system have been forcing industry stakeholders, particularly air navigation service providers (ANSPs), to turn to new and emerging technologies in an attempt to keep pace with contemporary and emerging techno-operational realities.

Although the industry has been weighing a number of technological options from digital remote tower offerings to the emerging L-Band Digital Aeronautical Communication System (LDACS), one obvious technology that has been attracting the industry’s attention as a viable option for delivering efficient aeronautical messaging services is the Air Traffic Service (ATS) Message Handling System or AMHS.

AMHS ARCHITECTURE

The AMHS is intended as a replacement for AFTN/CIDIN and is fully compliant with ISO/IEC 10021 standards. It is also based essentially on the X.400 messaging standards introduced in 1984 by ITU-T with a Directory Service that is based on the X.500 standards as well as routing and transmission that are largely based on TCP/IP (Transmission Control Protocol/Internet Protocol) as specified in ICAO Doc. 9896. Specifications relating to AMHS architecture and operation are contained in ICAO Doc. 9880, Part II.

As identified in ICAO Doc 9880 Part II, the functional objects of an AMHS End System include: 1) a Message Transfer System (MTS), which also integrates ATS Message Servers containing X.500 Directory Servers and X.400 Message Transfer Agents (MTAs), which serve as message switches using the store-and-forward principles; 2) ATS Message User Agents (UAs) for accessing MTAs and AMHS users. An UA may also incorporate a Directory User Agent (DUA) for access to a Directory; 3) a Message Store (MS), which takes delivery of messages for the UA as well as store messages for an AMHS user; and 4) an Access Unit (AU), which provides access to external communication centres. An ATS Message Handling System may also include an AFTN/AMHS gateway, known as an MTCU (Message Transfer and Conversion Unit) for conversion of AFTN/CIDIN messages to AMHS messages and vice versa.