By Roy Oishi, ARINC Industry Activities | October 1, 2008
The AEEC has been documenting methods for transferring digital information between avionics boxes since the 1960s. However, it wasn’t until the implementation of ARINC 429 as a reliable, high-integrity digital databus that provides deterministic message delivery that the concept of a federated architecture allowed digital information transfer among line replaceable units (LRU) to replace analog signals.
A federated architecture is one that has major functions (e.g., flight management, communications management, analog signal consolidation and conversion to digital data, etc.) implemented in LRUs that interchange information over digital databuses. The Boeing 767 and 757 were the first commercial aircraft to take advantage of the digital federated architecture. The Airbus A320 followed shortly and extended the idea to become the first fly-by-wire transport aircraft.
However, two generations of aircraft beyond the initial federated avionics aircraft, ARINC 429 has become the victim of its own success. The amount of digital information exchanged by LRUs has grown beyond the capability of ARINC 429 to carry it. The initial 1978 publication of the ARINC 429 standard included the definition of about a hundred (32-bit) data word definitions identified by unique "labels." By the early 1990s the number of such definitions had increased to the point that the ARINC 429 standard was divided into three parts. Part 2 contains the label and word format definitions and is approaching 200 pages in the latest supplement published in 2004. Part 3 defined a method for sending data files as bulk data transfer became important.
The ARINC 429 bus consists of a single transmitting LRU connected to one or more receiving LRUs transferring data at a rate of either 12-14.5 Kbps or 100 Kbps. The sending unit will place relevant 32-bit data words on the bus at a repetition rate appropriate for the data parameter(s) transmitted on the bus. The protocol for the receiving units is simple; take in the 32-bit word, read the label and if you need the data, take it. No source or destination addresses or time stamping is needed. This means two-way data transfer requires two 429 buses. Different data sets (i.e., different sets of unrelated labels) might require more than one 429 bus. This has led to a proliferation of 429 buses among the various LRUs in order to transfer needed information.
In 1987, the industry recognized the need for the ability to transfer data files as opposed to individually defined data "words" across ARINC 429. The Williamsburg protocol was documented for this purpose in ARINC 429 Part 3, which became part of Supplement 12. The ARINC 429 standard describes everything from the physical layer (e.g., signal levels, timing), the link layer (e.g., error checking), to the application layer (i.e., bit formats of the words and their meanings).
Beginning with the Boeing 777, the federated avionics architecture moved toward an Integrated Modular Architecture (IMA) with the Airplane Information Management System, or AIMS Cabinet. Several major functions (e.g., flight management, communications management, aircraft condition monitoring) previously implemented as independent LRUs were implemented using IMA.
Furthermore, the concentration of functions within the IMA Cabinet demanded increased bandwidth for communications with avionics implemented outside the Cabinet.
Basic Principles
ARINC Specification 664 Part 7 defines AFDX. AFDX takes existing Ethernet physical layer technology and, using switched Ethernet in full-duplex mode as a basis, adds deterministic packet delivery, and high-integrity and high-availability mechanisms.
In the Ethernet standard (IEEE 802.3) one physical layer option describes the use of two pairs of wires to be used for transmitting and receiving. One mode of operation, customarily called half-duplex, gave IEEE 802.3 its name: Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In this mode, each end system monitors its receive port for indications that something is being transmitted. Prior to transmitting, such indications are used to avoid interfering with the ongoing transmission (i.e., the carrier sense function). During a transmission, any indication of another transmission indicates a collision, which results in corrupted communication (i.e., the collision detection function). This was the original principle of arbitrating communication, adapted from voice radio communication procedures.
Full-duplex mode separates both communication directions in each end-system and can thus obviate the need for a CSMA/CD mechanism.
A given LRU may now communicate with many other LRUs over one set of AFDX wires as opposed to one ARINC 429 bus pair for each set of one-way data words and each set of recipient LRUs. Figure 1 illustrates this basic comparison.
The use of Ethernet permits the reuse of commercial off-the-shelf protocols, e.g. IP addressing and fragmentation, any Ethernet interface hardware and the switched Ethernet configuration.
But AFDX tailors these to the requirements of the avionics environment. For instance, Virtual Links, bandwidth allocation and redundant LANs add the necessary integrity, availability and deterministic performance needed for avionics applications. We will touch on each of these refinements to the underlying Ethernet.
The Aircraft Data Network defined in ARINC 664 has been developed in several parts and written with the view that commercially available information technology standards can be applied to aviation with minimal changes. ARINC 664 Parts 2 and 7 are based on IEEE 802.3 Ethernet. Further, where there are selections among the commercial standards or deviations for aviation requirements, there is provision to record and disclose those selections and deviations in the form of Protocol Implementation Conformance Statements (PICS) and Services Implementation Conformance Statements (SICS). The use of PICS and SICS increases interoperability, broadens supplier availability, and ultimately, reduces cost.
In Figure 2, the example AFDX connection between LRU A and LRUs F, G and H is expanded in detail. Each LRU has an AFDX end-system, which has both transmit and receive ports connecting it to the switch. The path from LRU A to the others in Figure 2 is a Virtual Link (VL).
In Figure 3, a single Virtual Link is illustrated. A Virtual Link is defined as one source (i.e., a Tx end-system port) and one or more destinations (i.e., Rx end-system port or ports). Virtual Links are defined for each unidirectional path through an AFDX LAN and are assigned a multicast MAC address. Each end-system (transmitter and receiver) is assigned a unicast MAC address. Each VL is assigned a maximum frame size and a minimum time between two transmissions of a frame, called the bandwidth allocation gap (BAG). In this manner, a Virtual Link is analogous to a single ARINC 429 bus in that it carries a unidirectional information stream.
The hardware that implements AFDX (e.g., Tx and Rx connections to the switch, the switch itself, etc.) replaces the many ARINC 429 buses needed for all of the LRUs on the AFDX LAN.
The deterministic aspect of AFDX is implemented by the architecture of a given aircraft LAN configuration. The controlled traffic that flows through the VLs plus the bounded transit times through end-systems and switches allows a determination of maximum latency between a sender and receiver. This also allows the bandwidth usage to be limited over any given small time interval, which then allows the deterministic properties of the LAN to be analyzed.
Traffic shaping is implemented in the end-system and a policing function is implemented in the switch in order to maintain the deterministic delivery of message frames across the LAN. The integrity of each packet sent across the VL is checked using a Cyclic Redundancy Check which is verified at the destination.
The high-availability aspects of AFDX are implemented by redundant switches and parallel connections to those switches as shown in Figure 4.
Each network has one or more AFDX switches. A failure, either hardware, protocol or software, will cause that network to be disabled. All messages are transmitted to both networks. Each receiving end-system implements a policy of accepting the first valid copy of any message. By implementing this policy in the end-system, i.e., the communication stack, the application software is relieved of any responsibility for dealing with the redundancy in the network.
The implementation of redundant networks also allows lost or corrupted frames on one network to be replaced by a copy from the other network. This work is performed at the end-system level.
AFDX Applications
AFDX has been implemented in the Airbus A380 and military A400M as well as the Boeing 787 next generation aircraft. In the A380, the AFDX backbone is connected to 23 major functions with about 120 subscribers. The advantages of using AFDX are weight savings by elimination of most of the ARINC 429 buses estimated at about 100 Kg, as well as providing for simpler configuration management.
The new A350XWB also will use this technology. Currently, 36 major functions with about 150 subscribers will be connected to the AFDX backbone.
The A400M provides an interesting case study, as it implements AFDX-to-1553 interfaces in order to take advantage of existing military LRUs which speak 1553, as well as new avionics which speak AFDX. The AFDX backbone connects 18 major functions with about 80 subscribers.
The Sukhoi Superjet 100 regional jet will be a modern fly-by-wire aircraft with Thales avionics incorporating an AFDX network. The expectation is that higher-speed Ethernet physical layers, e.g., fiber optic or Gigabit, plus faster switch technology will allow the basic AFDX concepts to serve the growing need for data transfer among modern avionics systems.
Jean-Paul Moreaux and Bruno Pasquier of Airbus contributed to this article.