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same SN), one received on network A and the other on network B.
If SkewMax is not exceeded, the RM applies a "first-valid-wins" policy on the two
frames, i.e. the first received frame is forwarded whereas the later received frame is
discarded. However, if SkewMax is exceeded, the RM considers the two frames to be
different from each other and hence forwards both.
In the case where the RM is disabled, both frame sequences are forwarded directly
from the IC to the upper layers.
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The AFDX Switch
The purpose of the AFDX switch is to physically interconnect the ESs and police that
the communication takes place according to the network configuration.
As depicted in Figure 9, the switch consists of various components each performing a
certain task of the switch.
The main component of the switch is the switching function which implements a
filtering and policing function to ensure that only valid incoming frames are
forwarded to the right physical ports.
The setup of the switching function is done using configuration data held in static
configuration tables.
The purpose of the monitoring function is to monitor and log all switch operations
and events such as frame arrivals and invalid frames. The Monitoring function
communicates with the network management function for operational and health
related information.
The purpose of the switch ES is to provide a means for functions that are external to
the network to communicate with the switch. For example dataloading and network
management functions communicate with the switch via the embedded ES.
Figure 9:
AFDX Switch
Architecture
Configuration
Tables
End System
Monitoring
Function
Switching Function
Filtering &
policing Function
The AFDX Switch
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Frame Filtering
The switch filtering functions examines all incoming frames and ensures that only
valid frames are forwarded to selected destinations. The filtering function verifies the
following:
􀂄 Ethernet destination address validity, i.e. the validity of the VL identifier
including the constant part
􀂄 The VL is received on an allowed destination port according to the configuration
table
􀂄 FCS Validity
􀂄 Ethernet frame size alignment, i.e. the frame size is a 1-byte integral
􀂄 Ethernet frame size range, i.e. the frame size must be in the range 64 -1518 bytes
􀂄 Adherence to the MTU on a per VL basis
Incoming frames not complying with these conditions are automatically dropped.
Traffic Policing
Traffic policing can be implemented using two different algorithms of which one uses
byte-based policing and the other frame-based policing. The byte based traffic
policing filters out the VL in terms of bandwidth usage expressed in bits per second.
The frame-based traffic policing filters out the VL in terms of bandwidth usage
expressed in frames per second. The switching function may implement one or both
of the two algorithms.
The implemented algorithm(s) operates on the basis of the VL identifier contained in
the MAC destination address. The VL defines a traffic flow and is characterized by
certain properties such as BAG, jitter and group of recipients. The properties of each
VL are contained in the configuration tables.
Traffic policing ensures containment of faults caused by ESs. Incoming frames that
do not comply with the configuration of the traffic flow (VL) to which they belong
are automatically discarded by the policing function.
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AFDX Network Configuration
Transmit Scheduling
For periodically transmitted messages on a VL it is essential to know whether the
given transmit intervals are possible to implement with the given VL. The time
constraints for a periodically transmitted message are defined by the properties of the
message itself and of the VL to which the message is assigned.
If the message is not fragmented, the complete message is transmitted in just one time
slot, defined by the BAG. However, if the message is fragmented, i.e. divided into
several pieces, each fragment will get transmitted in its own time slot.
Whether a message will get fragmented depends on its payload (PL) and on the
Maximum Transfer Unit (MTU) of the associated VL. In particular, the number of
message fragments is calculated as PLmsg/MTUVL rounded up to the next integer.
Formula 1 shows the inequation which all messages associated with the same VL
must obey. The left hand side of the inequation is a sum of the frequencies of all
　
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