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

24-12 Industrial Communication Systems
Shapers
Multiplexer
Unregulated
flows
VL3
VL2
VL1
BAG BAG BAG
Jitter = 0
Jitter ≠ 0
FIGURE 24.10
AFDX VLs scheduling and jitter.
24.6.5 Network Redundancy for Safety and Fault Tolerance
As on any Ethernet network, bit errors may occur. In this case, a frame will be deleted thanks to detection
by an invalid Ethernet frame check sequence (FCS), and it will not arrive at the destination. Likewise,
faulty equipment (switch or end-system) may send rubbish on the network or simply exceed its VL contracts.
In this case, the next switch on the path will reject the extra communication (thanks to policing
mechanisms in the switch input ports), and prevent the faulty equipment from polluting the rest of the
network. Again some data will be lost in the process.
In order to provide fault tolerance for both types of errors, the AFDX is actually doubled as depicted
in Figure 24.11. End systems have two Ethernet network interfaces and the whole topology of switches
and links is doubled, leading to a so called “red” network and a so called “blue” network. An additional
parameter in the VL definition states/defines whether the VL needs redundancy, i.e., whether frames of
this VL are sent on both the red and the blue networks. Nearly all VLs are duplicated on the two networks.
Of course, only one frame is returned by the protocol stack on a receiver: the first valid frame to
arrive is returned and the second one (if it ever arrives) is deleted. However, this scheme requires proper
matching of red and blue frames. For this reason, the standard Ethernet frame format has been modified
to include an 8 bits sequence number (protected by the FCS).
FIGURE 24.11
AFDX redundant networks.
© 2011 by Taylor and Francis Group, LLC