AFDX® / ARINC 664 Tutorial 23/30
Network Reliability
The inherent AFDX features such as traffic shaping (bandwidth management and
bandwidth policing) and limited transmit latency and jitter, make AFDX a
deterministic and reliable network. However, in order to improve reliability even
further, the AFDX network has a double, i.e. a redundant network transmitting the
exact same data. The two AFDX networks are called "network A" (also "red
network") and "network B" (also "blue network").
The purpose of the redundant network is to mitigate the consequences of potential
network failures caused by e.g. damaged cables and connectors or devices (e.g.
switches) generating babbling data.
Since ultimately only one valid data stream is required by the ES application, a
handling of the redundant streams is required.
As depicted in Figure 8, the ES implements Integrity Checking (IC) and Redundancy
Management (RM) to ensure data integrity and that only one data stream is forwarded
to the upper protocol layers and from there to the application. The IC and RM
services are provided by the ES without the involvement and knowledge of the
application.
Figure 8: AFDX Integrity Checking and Redundancy Management
Integrity Checking (IC)
The first step for handling the redundant data streams is the IC, which is done
separately for each network and on a per VL basis. The IC is always enabled and is
done independently of the RM, also if the RM is turned off and both networks are
used independently of each other.
The IC is applied on the MAC layer, i.e. on the Ethernet frame which contains a one
byte Sequence Number (SN) as the last byte of the payload as illustrated in Figure 7.
MAC
Layer
MAC
Layer
Integrity Checking (IC)
Detection and elimination
of invalid frames
Integrity Checking (IC)
Detection and elimination
of invalid frames
Redundancy
Management (RM)
Elimination of redundant
frames
IP & UDP Layers
Network A
Network B
Application
Network
Management
End System
Network Reliability
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The SN is the basis for the IC algorithm and is used differently in transmitting and
receiving mode.
SN Usage in Transmitting Mode
The SN is a value in the range 0 - 255 and is handled separately for each VL on each
of network A and B. Prior to transmission, the SN is incremented by one for each
consecutive frame (whether fragmented or not) on the same VL. With SN = 255 in the
last transmitted frame, the SN is wrapped around to 1 in the following frame. Upon a
reset or start-up of the transmitting ES, the SN is set to 0 in the first transmitted frame.
SN Usage in Receiving Mode
In receiving mode, the IC uses the SN to determine if frames have been lost or
whether a babbling switch is causing the same frame (with the same SN) to be
transmitted over and over again.
The IC algorithm accepts all frames that comply with one of the following criteria:
SN = 0 (The transmitting ES is started or reset)
SN = Previous SN + 1
SN = Previous SN + 2
All frames not complying with these criteria are discarded.
Redundancy Management (RM)
The purpose of the Redundancy Management (RM) is to evaluate the two frame
sequences delivered by the IC (see Figure 7), discard possible duplicate frames, and
forward only one copy of each frame to the upper protocol layers.
The RM makes use of the configurable SkewMax parameter which is given in ms and
must be specified for each receive-VL defined in the ES. SkewMax defines the
maximum allowed time between the reception of two redundant frames (i.e. with the
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