wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

24-8 Industrial Communication Systems
connectivity. This reduces the possible collision domain from the entire network to the single link between
two elements. The second assumption consisted in using full-duplex links: each avionics subsystem is
connected to a switch via a full-duplex link comprised of two twisted pairs (one pair for transmission and
one pair for reception). In fact, full-duplex switched Ethernet eliminates the possibility of transmission
collisions on links: the CSMA-CD medium access control protocol is no longer necessary. This eliminates
the inherent indeterminism of vintage Ethernet and the collision frame loss, and shifts the problem to the
switch level where various flows enter into competition for the use of switch resources. The switch implements
the classic IEEE 802.1d bridging algorithm [IEEE98]: Reception and transmission buffers are used
in the switch for storing multiple incoming and outgoing Ethernet frames. The role of the switch is to
filter and to retransmit frames from the incoming buffers to the outgoing buffers. The store and forward
bridging mechanism reads the destination addresses of each received frame and retransmits them according
to the port ID stored in the forwarding table of the switch. If a temporary congestion appears on the
output port of a switch, it can significantly increase end-to-end delays of frames and even lead to frame
loss through buffer overflow. This is why dedicated mechanisms have been added to the classic full-duplex
Ethernet in order to guarantee the determinism of an AFDX network.
24.6.2 the ARINC 664 Standard
The AFDX has been initiated by Airbus for the evolution of the A380 aircraft toward IMA as represented
in Figure 24.6. AFDX concepts have been standardized as in ARINC 664 with the help of many avionics
manufacturers: Airbus, Boeing, Rockwell Collins, Honeywell, etc. [ARI02,ARI03]. This standard adapts
existing Ethernet standards, describes the global communication system of the aircraft, and focuses on
the interconnection of domains with different safety levels. In particular, it explains how the critical
avionics domain, responsible for aircraft control, can be connected to the open-world domain.
More precisely, the standard defines two kinds of embedded networks: compliant networks and profiled
networks. Compliant networks conform exactly to existing standards: IEEE 802.3, 802.1d [IEEE98]
and can be used in the lowest safety-level domains, such as the open-world domain. Profiled networks
deviate from existing standards, when specific deviations are needed to achieve required levels of
LRU environment
Ethernet
Ethernet
LRU environment
429
429
GWM
IOM
CPM
CPM
IOM
GWM
Avionics world
Switch
Switch
AFDX network
Open world environment
FIGURE 24.6
AFDX IMA avionics architecture.
© 2011 by Taylor and Francis Group, LLC