As previously mentioned, the message transmission procedures used to access AFTN switches and to transfer messages between AFTN switches were developed from the original telex system. However, where the international telex service was supported by a dial up circuit switched network, the AFTN adopted the same procedures and used them over leased lines to connect the AFTN’s message switching centres.

Medium speed lines are used in most parts of the developed world. Low speed lines are used elsewhere. The HDLC link protocol is in use for example in the USA, and symmetric X.25 is used over leased lines in ASIA/Pacific regions.

The Message Transfer Protocol, including all of the service message definitions is described in ICAO Annex 10. Each message is transferred over a link with a serially allocated channel sequence number. This is used to detect missing or corrupted messages and manage flow control of messages between centres.

The AFTN procedures are not able to automatically detect messages that are circulating through a loop of switches. This can happen if routing tables are wrongly configured.

In spite of the widespread computer-supported generation of AFTN messages, which have been syntactically checked, there still exist many manual non-computerised user systems which are responsible for erroneous messages which must be repaired or rejected (causing a service message to be returned to the originator).

The AFTN’s service messages lead to problems when mapping AFTN procedures to and from X.400 (AMHS) system conventions.

An important feature of the AFTN is that of the requirement for Legal Recording of message traffic. A State processing a message has “legal responsibility” for it. The ongoing responsibility for a message when it leaves the State is further specified by routing tables. The requirement for ‘responsibility’ is due to the special legal requirements of the application environment.

“Legally recorded” messages must be held by each switching centre for a period of 30 days. A State having processed a message must be able to retrieve and show it together with its processing and an audit trail within that period if necessary.

At least the message (user’s and service message) headers have to be recorded. The recording functions are usually implemented together with other statistic gathering functions of the AFTN switch.

The security requirements are for authentication of origin and integrity. Here the AFTN is weak by modern day standards, since authentication is based on the fact that leased lines are used, and it would fail if some external party was able to access the leased line. There are no mechanisms for integrity.

AFTN messages may contain one or more addresses and the addressed users will often be attached to different AFTN switching centres. AFTN switches must make copies of the multiple addressed messages and transfer them to the various AFTN centres that serve the recipients. When so doing, the AFTN switch must ‘strip’ the appropriate addresses from the headers of message copies to ensure that only a single message copy will be routed to each recipient. The slide illustrates the forwarding, copying and address stripping procedures.

In order to distribute the incoming message to the addressed recipients, switch A must create two copies of it, one for switch B, and one for switch C. Switch A must remove ABCXYZZZ from the message copy it transfers to switch B. Switch A must also remove ABDXYZZZ and ABDYYZZZ from the copy sent to switch C.

This technique results in copies being made “as late as possible”, e.g. copies are made only when the paths to various destinations separate. The late copying strategy permits to save bandwidth on low throughput links as typically used on the AFTN.