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ms and must be a power of 2.
The jitter is an upper bounded transmit latency appearing as a frame time offset
within the BAG. The frame time offset is introduced by the contention of data during
VL scheduling, as well as by delays caused by the switch processing.
Figure 4 illustrates the scheduling of three frames with different jitter on the same VL.
Figure 4:
VL scheduling
with jitter
Jitter=0 0 < Jitter < Max Jitter Jitter = Max Jitter
Frame Frame Frame
BAG BAG BAG
Max Jitter Max Jitter Max Jitter
AFDX Communication Concept
16/30 􀁟 700008_TUT-AFDX-EN_1000
Sub Virtual Links
As the VL is a means for regulating the flow of frames onto the physical link, the
Sub-VL is a means for regulating the flow of frames onto the VL. Thereby the Sub-
VL makes it possible to optimize the VL's bandwidth utilization. E.g. Sub-VLs can be
used to reduce latency of short messages, such as sampling messages, against bulk
messages, such as queuing messages.
A VL must be capable of handling at least four Sub-VL FIFO queues. Each Sub-VL
queue is read in a round-robin fashion by the VL scheduler. This ensures that the
frames in the Sub-VL queues are evenly distributed onto the VL queue and later onto
the physical link.
Figure 5:
Sub-VL
Concept
1-1
1-2
1-n
2-1
2-2
2-n
Sub-VL FIFO
VL FIFO
Round-Robin
BAG
2-2 1-2 2-1 1-1 VL
BAG BAG
AFDX Port Types
AFDX® / ARINC 664 Tutorial 􀁠 17/30
AFDX Port Types
The AFDX ES provides two different port types for data transfers - the
communication port and the Service Access Point (SAP). The communication port
provides two types of services, namely the queuing and the sampling services which
are both based on UDP. The communication ports as well as the SAP ports can be
defined as either receive or transmit ports.
AFDX Communication Ports
Receive Sampling Ports
A receive sampling port places received messages in a buffer with storage capacity
for a single message. As a consequence all incoming messages on a sampling port
overwrite the previous message regardless of whether it was already retrieved by the
host application or not.
The host application can retrieve the sampling port's message buffer at any time and
will always retrieve the last message received by the sampling port. When no new
messages are received, the host application will get the last message received every
time it’s retrieving the message buffer.
Transmit Sampling Ports
A transmit sampling port continuously transmits the last message written to the port
by the host application. When the host application writes a new message to the
sampling port, the previous message on the sampling port is overwritten by the new
message which from then on is continuously transmitted according to the sampling
port's schedule.
Receive Queuing Ports
A receive queuing port appends received messages to the port's message queue. As
opposed to the sampling port it guarantees that no messages are overwritten. The host
application can retrieve messages from the message buffer at any time, and upon
doing so the respective message is removed from the message queue.
The message queue is a FIFO queue, thereby ensuring that the host application will
retrieve the messages in the same order they were received by the queuing port.
Transmit Queuing Ports
A transmit queuing port transmits messages on a FIFO based scheme. The messages
are transmitted as fast as possible and then removed from the message queue. New
transmit messages written by the host application to the queuing port are appended to
the message queue. An exhausted message buffer stops further transmission.
AFDX Communication Concept
18/30 􀁟 700008_TUT-AFDX-EN_1000
Service Access Ports (SAP)
SAPs are used for TFTP transfers and for communication between compliant
networks, e.g. other AFDX networks or LANs. When an ES receives a
communication request on a SAP, it is passed the IP source address (IP Src) as well as
the source UDP port number (UDP Src). When replying to the request, the IP Src is
specified as the IP destination address, and the UDP Src is specified as the destination
UDP port number.
Frame Fragmentation
AFDX frames transmitted on the wire are confined to be in the range 64 - 1518 bytes.
However, it is possible to define AFDX frames that exceed the maximum transmit
frame size. Frames conveying sampling port messages are not allowed to exceed the
1518 bytes limit, thus no fragmentation is required. However, frames conveying
queuing port messages are allowed to be up to 8KBytes large, thus requiring that
　
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