Real-time Data Bus in Project CRIAQ
Moving information between avionics subsystems on aircraft has never been more
crucial, and it is here that electronic data transfer is playing a greater role than ever
before. Since its entry into commercial airplane service on the Airbus A320 in 1988,
the all-electronic fly-by-wire system has gained such popularity that it is becoming
the only control system used on new airliners.
This project aims at establishing, developing, advancing, analyzing, testing and
simulating the new emerging ARINC 664, also known as AFDX for new Boeing and
Airbus airplane types.
What is AFDX? ARINC 664 is defined as the next-generation aircraft data network
(ADN). It is based upon IEEE 802.3 Ethernet and utilizes commercial off-the-shelf
hardware thereby reducing costs and development time. AFDX builds off of this
standard, as is formally defined in Part 7 of the ARINC 664 specification. AFDX was
developed by Airbus Industries for the A380. It has since been accepted by Boeing
and is used on the Boeing 787 Dreamliner. AFDX bridges the gap on reliability of
guaranteed bandwidth from the original ARINC 664 standard. It utilizes a star
topology network of up to 24 end systems that are tied to a switch, where each switch
can be bridged together to other switches on the network. By utilizing this form of
network structure, AFDX is able to significantly reduce wire runs thus reducing
overall aircraft weight. Additionally, AFDX provides dual link redundancy and
Quality of Service (QoS).
History
Avionics buses are traditionally slow to evolve, partly because requirements change
so slowly and partly because of the costs of development, certification, and
sustainment. It is with the development of new airplanes that the demand for new bus
architectures evolves. This can be seen in the adoption of Fibre Channel for JSF and.
Prior to AFDX, Aircraft Data Networks utilized primarily the ARINC 429 standard.
This standard, developed over thirty years ago and still widely used today, has proven
to be highly reliable in safety critical applications. This ADN can be found on a
variety of aircraft from both Boeing and Airbus, including the Boeing 737, 747, 757,
767 and Airbus A330 and A340. ARINC 429 utilizes a unidirectional bus with a
single transmitter and up to twenty receivers. A data word consists of 32 bits
communicated over a twisted pair cable using the Bipolar Return-to-Zero Modulation.
There are two speeds of transmission: high speed operates at 100 kbit/s and low speed
operates at 12.5 kbit/s. ARINC 429 operates in such a way that its single transmitter
communicates in a point-to-point connection, thus requiring a significant amount of
wiring which amounts to added weight.
Another standard, ARINC 629, introduced by Boeing for the 777 provides increased
data speeds of up to 2 Mbit/s and allowing a maximum of 120 data terminals. This
ADN operates without the use of a bus controller thereby increasing the reliability of
the network architecture. The drawback of this system is that it requires custom
hardware which can add significant cost to the aircraft. Because of this, other
manufactures did not openly accept the ARINC 629 standard.
Other safety-critical bus technologies provide the same capability. For example,
TTP is designed to provide time domain separation of groups of participants (such as
nodes) on a bus. TTP can support a single fixed group or multiple groups; each
participant within each group and each group are allowed time slots on the bus that
are scheduled to ensure that every participant always has time to complete its data
transfers. TTP can also detect whether scheduled participants are present and working
correctly; it can detect transmission errors and tolerates faulty nodes. Time Triggered
Protocol (TTP), have been sluggish to be adopted and might only find use in niche
applications. However, although the rate of change might be slow, the nature of the
market still leaves room for much innovation in packaging, soft cores, and test
equipment by embedded computing vendors.
MIL-STD-1553B is similarly deterministic in its scheduling of bus traffic, but TTP is
much more flexible and capable. However, MIL-STD-1553B is firmly entrenched in
military avionics and mission systems. Because of this widespread use, it still remains
the medium of choice for many upgrade and improvement programs.
Figure 1: MIL-STD-1553B
Advantages of AFDX
As mentioned previously the AFDX is based on IEEE 802.3 Ethernet and
TCP(UDP)/IP general principles, which is based on commercial 10/100Mbit switched
Ethernet. However it uses specific concept and features to provide secure data transfer
with real time constraints, to provide the guarantee of services and determinism
required for civil aerospace requirements. The AFDX specific features and concepts,
as part of the protocol itself must provide: specific addressing strategy, transmission
timing constraints and redundancy management. Although, it uses Ethernet media to
reduce cost, it is derived from the ATM specifications which, as opposed to a standard
Ethernet network can address the requirements summarized below.
BANDWIDTH GUARANTEE
Bandwidth control is achieved with advanced queue management and multiple
bandwidth use strategies. It introduces the notions of BAGs (Bandwidth
Allocation Gap) and of maximum frame size to allocate bandwidth as in ATM
CBR (Constant Bit Rate), and UBR (User Bit Rate). This guarantee applies to the
key notions of AFDX, the virtual links.
REAL TIME CONTROL
Real-time performance is achieved with latency control in the form of the
maximum network transit delay control (end-to-end latency) and also includes an
accurate time-stamping logic (specific to CES End Systems).
SERVICE GUARANTEE
Guarantee of services is the feature which made ATM the telecom standard
selected against IP for voice, video and binary transmission in the 3G world. The
user can select a variety of services and constantly monitor the payload for each
of the services, which are concurrently executed. AFDX will be the first avionics
standard to combine the simplicity of Ethernet connections with the richness and
security of the ATM protocol and all of this packaged in an avionics
environment.
The AFDX network main components are:
 Avionics specific physical layer
 COTS components for MAC layer
 AFDX switches:
Active elements addressing 3 functions:
- to establish a point to point connection between a sender and several receivers
via twisted pairs cable
- to establish communication between subscribers on the network
- to check frame integrity and bandwidth

AFDX end systems:
The AFDX End system is the subsystem which must be embedded in each
avionics systems equipment connected to the network.
Communication among subsystems
Avionics applications communicate with each other by sending messages using
communication ports. The specification of an operating system API for writing
portable avionics applications can be found in ARINC 653. In particular, ARINC 653
defines two types of communications ports–sampling and queuing ports. Accordingly,
it is necessary that End Systems provide a suitable communications interface for
supporting sampling and queuing ports. The AFDX ports, defined in ARINC 664, Part
7, include sampling, queuing and SAP ports. The AFDX sampling and queuing ports
correspond to ARINC 653 sampling and queuing ports, respectively. AFDX
introduces a third port type called a Service Access Point (SAP) port. SAP ports are
used for communications between AFDX system components and non-AFDX
systems.
Architecture of the Data bus Design
Avionics architectures typically separate the flight safety-critical elements such as
primary flight control, cockpit, landing gear, and so on from less critical elements
such as cabin environment, entertainment, and, in the case of military aircraft, the
mission systems. This separation offers less onerous initial certification and allows
incremental addition, as is often required for regulatory reasons, without the need for
complete recertification. Significant savings in weight and power could be made with
an integrated systems approach, using centralized computing supporting individual
applications running in secure partitions with critical and non-critical data sharing the
same bus. The most widely adopted of these is ARINC 664.
Integration of aircraft communication backbone
The switch will integrate the entertainment applications and safety-critical application
by DiffServ-alike QoS management mechanism. That means a fixed priority
scheduling method is employed to serve the application with in differentiate manner,
and the entertainment can only served when no safety-critical control message exists.
The hard real-time layer takes charge of the flight control, cockpit, landing gear, and
so on, the safety-critical applications as well as the cabinet pressure and thermal
sensor, in which no deadline misses is permitted.
The second soft real-time layer takes charge the real-time transmission with
traditional and versatile on-ground amusement applications. Note that this layer
serves a larger data rate (throughput) but less stringent real-time requirements in
comparison with hard real-time layer. These flows will never go into the hard
real-time layer so as to avoid disturbing the applications working on hard real-time
layer, but the hard real-time layer may send the control data to soft real-time layer,
such as the interruption of multimedia for the pilot commander’s broadcasting.
In order to implant the sensor network, multimedia server into Ethernet based airplane
communication; ARINC 828 provides an extension capacity from aircraft control data
backbone.
ARINC 828 "Electronic Flight Bag (EFB) Interface" is a document containing the
description of a series of physical connectors based on MIL Spec 38999 which can be
used to connect EFBs of all hardware classes with aircraft. ARINC 828 combines
classical aircraft interfaces with PC technology like USB, DVI, LVDS and Ethernet.
ARINC 828 was adopted in September 2007. A successor, called ARINC 828
Supplement 1, is supposed to appear in late 2008. It should include data formats and
protocols allowing to access services like cockpit installed terminals, printers or
air-ground networks.
Upon the extension connection from backbone data bus, the soft real-time layer aims
at the multimedia transmissions for entertainment applications. The on-ground
versatile networking and technologies, such as WiFi, Peer-to-Peer and Web services,
can decrease the significantly the wiring cost of aircraft and enrich entertainment
activities for the client on-board.
No Real-Time Layer
Soft Real-Time for less critical
elements such as cabin environment,
entertainment,
Hard Real-time for the flight
safety-critical elements
Figure 2 : Real-time Aviation Data Bus Architecture
State-of-the-Art Development and Simulation Enviorment
CES is one of the AFDX manufacturer and developer who has developed a unique
concept by following the principles of modular electronics, which offers a significant
improvement over the COTS method. CES aerospace systems provide all hardware
elements (processor boards, aerospace interfaces, real-time networks, high-speed
system couplers, storage devices, chassis), software elements (operating systems,
libraries, examples and drivers for the required interfaces) and services, so that the
user receives an application-ready configuration.
An example of the AFDX development is shown in Figure 3.
Figure 3: CES development environment
CES is the A380 data bus provider and has undertaken a project cooperating with
Thales for a AFDX testbed.
Brajou, F.; Ricco, P., "The Airbus A380-an AFDX-based flight test computer concept,"
AUTOTESTCON 2004. Proceedings , vol., no., pp. 460-463, 20-23 Sept. 2004
Future work and state-of-the-art
In fact, more advanced projects are also in progress. Fibre optic technologies have the
potential to advance the current state-of-the-art, but in the past, fibre has primarily
been used in problem areas to address issues such as electromagnetic interference and
security requirements. The Boeing 777 Fibre Distributed Data Interface (FDDI) is an
important landmark in establishing the role of fibre in aircraft data networks.
Notwithstanding this important landmark, FDDI already suffers from technology
obsolescence and this highlights the need for greater industry cooperation.