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

Embedded Networks in Civilian Aircraft Avionics Systems 24-7
TI
AT
Level 1 Level 2 Level 3 Level 3
ASG
ASG ASG PSG
M2 M1
M3
TG 2 TG 1 TG 3
FIGURE 24.5
ARINC 629 combined protocol.
a fixed cycle duration (needed for level 1 periodic transmissions), an aperiodic time-out (AT) is used to avoid
transmission of aperiodic data that could make the cycle overflow. Level 3 aperiodic messages are allowed to
span multiple cycles (backlog messages have a higher priority than current cycle level three messages). Level
2 aperiodic messages must be transmitted in the current cycle otherwise they are lost; the transmit interval
must be calculated so as to give enough space for both levels 1 and 2 messages.
The assignment of timer values is similar to BP mode: ASG is equivalent to SG (greater than each TGi)
and PSG must be greater than ASG (the standard defines PSG as five times the selected ASG).
In conclusion, implementing an ARINC 629 data bus requires the development of a global avionics
system communication scheme in order to guarantee deterministic transmission ordering for critical
data. BP delivers true real-time behavior for systems requiring periodic and fixed length messages (periodic
mode). Variable length periodic messages or aperiodic messages result in variable length cycles
(aperiodic mode). CP protocol offers stable periodic response time for systems, which require some
combination of periodic and aperiodic transmissions (with no interference of aperiodic messages on
periodic messages). However, taking into account sporadic or aperiodic messages can lead to an underutilization
of the data bus. In fact, the main drawback of the ARINC 629 has been the cost of interfacing
an efficient but complex avionics-specific communication protocol.
24.6 arINC 664: Avionics Full-Duplex Ethernet
Avionics full-duplex Ethernet (AFDX) is part of the ARINC 664 standard and is based on classic switched
Ethernet technology but introduces a virtual link (VL) paradigm, which is an important concept for
characterizing the incoming traffic of the network.
24.6.1 Full-Duplex Switched Ethernet
Reusing Ethernet technology for avionics presents many advantages: high throughput offered to the
connected units (100.Mbps compared to a few Mbps with the ARINC 629 standard), high connectivity
given by the network structure, a mature industrial standard, and a significantly lower connection cost
than with a proprietary or aeronautical specific protocol. Despite these advantages, Ethernet was not
used for critical systems in previous aircraft because of its random physical medium access protocol:
CSMA/CD [IEEE98]. An embedded network must have determinism properties such as the bounded
transmission delay of any data. The CSMA/CD protocol fails to offer such a guarantee because of potential
collisions on the physical medium during the transmission and because of random (binary
exponential back-off [BEB]) retransmission algorithms.
To solve this problem, the first assumption was to adopt a switched Ethernet technology; all units are
directly connected by a point-to-point link to an Ethernet switch since cascading switches offer the desired
© 2011 by Taylor and Francis Group, LLC