Avionics Ethernet

The use of Ethernet is becoming the more and more common in avionics network applications. As a result there are several adaptions and profiles of Ethernet, specifically for avionics applications that are becoming standardized in order to increase interoperability. This post is intended to provide a brief overview of the most common Ethernet adaptions currently utilized in avionics systems.

ARINC 664 & AFDX^TM

ARINC 664 and AFDX^TM are probably the most well known Ethernet adaptations for avionics. ARINC 664 networks are "profiled" Ethernet networks which utilize Internet Protocol (IP) and User Datagram Protocol (UDP) as upper layer protocols.

Note: ARINC 664 is a standard which is specified by ARINC while AFDX^TM is an implementation of an ARINC 664 compliant network which is used by Airbus.

ARINC 664 adds a level of determinism when compared to a standard Ethernet network by providing a mechanism for bounding the latency through the network. All end nodes on the network are statically configured with pre-defined set of data routes. For each of these data paths (call Virtual Links) the transmitting end node is rate constrained, meaning it is only allowed to transmit to a maximum rate (specified in Ethernet Frames per second) on the data path. With all end nodes constrained to a static set of paths and a maximum data rate on each path, a worst case latency for the transmission of Ethernet frames from transmitter to receiver can be statically calculated (and guaranteed).

ARINC 664 also adds support for redundancy. Each end node on the network transmits all Ethernet traffic redundantly on two identical, but independent, Ethernet networks. The receiving end nodes use a redundancy management algorithm to identify and select redundant data so that the avionics application only receives on instance of each message.

ARINC 664 networks cannot be implemented using commonly available, standard Ethernet network interfaces and switches. Avionics specific end system interfaces and network switches are required to implement an ARINC 664 network because a standard network interface cannot implement the ARINC 664 rate constraints and redundancy management. Also, a standard Ethernet switch is not capable of policing the ARINC 664 rate constraint requirements of the end nodes.

For a more detailed overview ARINC 664 check out this video:

Time Triggered Ethernet (TTE)

Time Triggered Ethernet (TTE) is another deterministic Ethernet adaptation. TTE is formally defined in SAE specification AS6802. 

While ARINC 664 networks can be characterized as asynchronous, TTE networks can be characterized synchronous. TTE can be thought of as an extension of the ARINC 664 rate containing concept. While ARINC 664 limits the allowed rate of frame transmissions for data producers, it does not control the schedule or exact time at which end nodes can transmit. For this reason ARINC 664 can be thought of as asynchronous. TTE adds the use of a time division multiplexed network where each transmitter has a specific communications schedule which is globally administered across the entire TTE network. With this globally administered communications schedule, TTE guarantees that there will be no contention for network data links when a transmitting end node sends data.

Just like ARINC 664, TTE networks cannot be implemented with common standard Ethernet network interfaces and switches. TTE specific avionics switches and end system network interfaces are required.

For a more detailed overview TTE check out this video:

1 Gigabit and 10 Gigabit Ethernet

The primary goals of ARINC 664 and TTE are to provide some guarantees of maximum, worst case, latencies for data transmitted across the Ethernet networks. As explained above, this is accomplished by either rate containing communications (ARINC 664), or by administering a network wide transmission schedule (TTE). Both of these types of network controls are administered at the lower level of the communications protocol stack, that is in the layer 2 Ethernet MAC and as a result require special ARINC 664 or TTE specific hardware. 

An alternative approach, which is also used in avionics systems, is to simply over provision the network by using higher speed Ethernet options such as Gig-E and 10 Gigabit Ethernet. If the avionics applications are using a sufficiently small fraction of the overall network bandwidth available, then the worst case scenarios for network contention can become acceptable for the application. 

With this approach, no special (and expensive), ARINC 664 or TTE hardware is required to implement the network.