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these frames (when exceeding the limit) must be divided into fragments transmitted
one after the other.
In addition, the AFDX protocol allows limiting the maximum size of a transmitted
frame even further by means of the parameter Maximum Transfer Unit (MTU). Each
VL has a MTU associated with it, and the frames exceeding the MTU will be
fragmented prior to transmission and subsequently reassembled by the receiving ES.
Fragmentation and re-assembly is inherent in the AFDX implementation and is thus
not managed by the host application.
AFDX® / ARINC 664 Tutorial 19/30
AFDX Network Protocol
AFDX Protocol Stack
As a protocol derived from Ethernet, the AFDX Media Access Control (MAC) data
link layer is almost identical to the Ethernet MAC layer. The only difference is the socalled
Sequence Number (SN) which can optionally be inserted as the last byte of the
Ethernet payload. The function of the SN is explained in later chapters.
On top of the Ethernet layer, AFDX implements the Internet Protocol (IP) layer which
manages frame fragmentation and re-assembly as well as packet forwarding. The
latter, however, is not being used in AFDX since the routing is carried out on a VL
basis.
The last protocol layer of the AFDX protocol stack is the User Datagram Protocol
(UDP) which is connectionless with no transmission error control. UDP was chosen
since it is more efficient than the alternative Transmission Control Protocol (TCP).
Although TCP is a connection-oriented protocol providing transmission error control,
this is not required since the AFDX bandwidth policing and redundancy management
ensures a very low frame loss probability. The UDP payload holds the ADFX user
data.
Figure 6 depicts the AFDX protocol stack with its different layers.
Figure 6: AFDX protocol stack
MAC
Hdr SN FCS
Payload
IP
Hdr
UDP
Hdr
AFDX Network Protocol
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AFDX Frame Structure
Ethernet Preamble
To signal the transmission of a new message on the network, the transmitting ES
sends out a stream of bytes, called the preamble, prior to transmission of the actual
frame. The preamble consists of alternating 0 and 1 bits that give the receiving ESs
time for synchronization and otherwise prepare for reception of the actual frame. At
the end of the preamble, the transmitting ES sends out the Start Frame Delimiter
(SFD) to break this pattern and signal the beginning of the actual frame immediately
after the SFD (see Figure 7).
Ethernet Protocol
The first part of the Ethernet frame is the MAC destination address where AFDX
encodes the VL identifier in the last two bytes. Following the destination address is
the MAC source address, where the ES can encode information such as the network
ID, the equipment ID and the Interface ID. After the MAC addresses follows the
EtherType field which is used to indicate which protocol type is transported in the
Ethernet frame. In AFDX this 2 byte field always has the value 0x0800 meaning
Internet Protocol, Version 4 (IPv4).
Ethernet Payload
Following the EtherType field is the Ethernet payload which contains the IP structure,
the UDP structure as well as the AFDX payload followed by the Sequence Number
(SN). The IP and UDP structures are 20 and 8 bytes long, respectively and the SN is 1
byte long. Since the Ethernet frame is specified to be in the range of 64 to 1518 bytes,
the ADFX payload must consequently be in the range 17 to 1471 bytes. This
calculation is done by simply subtracting the protocol overhead (6 + 6 + 2 +20 + 8 + 1
+ 4 = 47) from the max. and min. frame sizes.
Furthermore, by using padding it's possible to specify the AFDX payload down to 0
bytes.
Ethernet Error Control
The last field of the Ethernet frame is the Frame Check Sequence (FCS) which is 4
bytes long. The transmitting ES uses the Cyclic Redundancy Checksum (CRC)
algorithm to calculate a checksum over the entire frame which is then appended as
trailing data in the FCS field. The receiving ES uses the same algorithm to calculate
the checksum and compare it with the received checksum. If the two checksums are
not identical the receiving ES discards the frame.
AFDX Frame Structure
AFDX® / ARINC 664 Tutorial 21/30
Ethernet Postamble
Ethernet specifies a minimum idle period between transmissions of frames called the
Interframe Gap (IFG), which is not strictly required by AFDX. However, for reasons
of compatibility, the IFG also applies to AFDX. The IFG is specified to be 96 bit
times, i.e. the time it takes to transmit 96 bits on the network. On a 10 Mbit/s network,
the IFG idle time is thus 9.6 us. On a 100 Mbit/s network, the IFG idle time is 960 ns.
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