Any modern avionics communications bus like AFDX builds on the success of previous iterations. In some cases, older data transfer models are still widely used.

While ethernet connections have recently caught on in the field of avionics, their methods of operation were once considered far too random and impossible to determine at a given moment in time. With the development of Avionics Full-Duplex Switched Ethernet (AFDX), virtual channels with certain properties were used to offer increased determinism to the extent that ethernet connections could be used.


Boosting Determinism in Data Transfer

Virtual links are added on top of the ethernet layer and work in much the same manner as those in the Arinc 429 standard, except that they exist as logical links rather than physical ones. Each channel of this kind has three important properties aside from its ID that make the information sent much more reliably received.

  • The bandwidth allocation gap sets a delay between the sending of individual virtual link frames, avoiding the congestion of information that often occurs when physical links are used.
  • The maximum L2 frame size (LMAX/Smax) determines the largest common packet size so that fragmentation and other approaches are no longer required.
  • Bandwidth limits help to reduce the amount of data sent at once, thus improving the reliability of the information received.

 

A Proven Fall-back for AFDX

The capabilities of different data transfer methods are varied and the specific option chosen depends on what the purpose of the data is. Although more advanced technologies like switched ethernet networks have been developed, older approaches are still maintained for a number of reasons.

Primarily, the Arinc 429 standard is the unidirectional bus of choice when it comes to commercial uses in which safety is of paramount importance. Other than in situations where latency is critically significant, this standard is utilized when a single transmitter for simple data is enough to get the job done.

These factors and its inherent reliability, having been in use since 1977, make this standard a commonly relied-upon fall-back for more advanced data transfer methods. For example, it plays such a role most commonly in the Airbus A380.


Enforcing the Rules Governing Virtual Links

The Arinc 664 standard introduces a number of virtual link parameters that can be enforced as rules by specific hardware. Ultimately, AFDX switches may be used to make sure that virtual links behave properly, as well as to manage the parameters which control latency and bandwidth. The overall result is a much more reliable and determinable set-up that can be expected to remain constant throughout its use.      


Using a Simulation Board to Test Avionics Suites

Naturally, when integrating avionics suites and running various hypothetical scenarios, it’s important to have a specially designed test and simulation board suitable for use according to Arinc 664 part 7.

The ideal situation is to have a test and simulation board that has full dual redundant support to receive and transmit data. Compatibility with multiple channels from other avionics protocols, including MIL-STD-1553 among others, allows for the widest range of testing to take place. 

Given the importance of virtual links in the latest avionics standards, any testing board used should be able to define these virtual links for the transmission of data. Simulation of data receipt is rather flexible and two methods are commonly used in test scenarios: sequential monitoring and VL-based monitoring.

The advances made by AFDX should always be supported by redundancy mechanisms and simulation practices.