Available an improvement compared to ARINC 429

Available technologies

When selecting the Ethernet protocols, it is
important that they have deterministic behaviour, bounded latency and precise
synchronization with Quality of service.

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Avionics Full Duplex
Switched Ethernet (AFDX):

AFDX is a standard, patented by
international aircraft manufacturer Airbus,
for safety-critical applications that utilizes dedicated bandwidth while
providing deterministic quality of service(QoS). The AFDX is based on
ethernet data network and protocol specifications (IEEE 802.3 and ARINC 664,
Part 7) for the exchange of data between Avionics Subsystems . The primary features
of AFDX include full duplex, redundancy, determinism, high
speed performance, switched and profiled network. It supports for a deterministic
data network based on 10, 100 Mbps or 1 Gbps. The predecessor to AFDX, ARINC
429 is a point to multipoint bus system that supports one-to-one or one-to-many
connections. AFDX is an improvement compared to ARINC 429 due to the higher
data transfer rate (approximately one thousand times faster) and significant
reduce in wiring which reduces the weight .

Overview

AFDX uses the concept of Token
bucket (Asynchronous Transfer Mode – 
virtual connection / circuit is being established before the actual data
exchange happens between the endpoints). The possibility for collision of data is
eliminated using full duplex switched network – one for receiving and the other
for transmitting. The network is designed for the critical data being prioritized
using the QoS policies there by achieving latency, jitter and delivery within
the set parameters. A highly intelligent switch is used which is capable to
buffer packets for both reception and transmission. The messages are
encapsulated with in the UDP/IP and then the Ethernet Headers are placed.

The main components in AFDX system are

·       
AFDX
End system: It is the interface between the sub systems (global positioning system)
and the network

·       
AFDX
Interconnect: A full duplex switched Ethernet interconnect consists of switches
that forward frames to the appropriate destinations

·       
AFDX
Virtual Links: It is a unidirectional virtual connection from 1 to 1 or 1 to n
End systems

Virtual Links and Message
Flow

               In the traditional Ethernet, the
frames are routed based on the Ethernet destination address. In AFDX the frames
are routed using the 16-bit value called as Virtual Links. The Virtual links
shall partition the network into communication channels with predefined
scheduling time and link bandwidth. These are unidirectional and the switched
route the packets based on the virtual link ID. Each virtual link should at
least have one or even more predefined receiving end systems that the packets
are transmitted to.

 

When an application sends a message to a
communication port the source and destination end systems and the AFDX network
is configured to deliver the message successfully. For example, a message M is
transmitted to port S. Then the port connected to Endpoint S shall encapsulate
the message as per the AFDX protocol format and adds it on top UDP and sends it
to the AFDX switched network on VLID 50. The forwarding tables in the network
switch are configured to deliver the frame to the End system D1 and End system
D2. In the End systems, the message M is extracted from the frame and is
transmitted to the port D1 and D2.

Isolation and Scheduling

The virtual
link is assigned with two parameters

·       
Band
Allocation Gap (BAG) – it’s a value that range in the power of 2 from the
interval 1 to 128 ms. It represents the minimum interval that are transmitted
in milliseconds. Depending on the value configured for a virtual link the frame
can never be transmitted before that

·       
Lmax
– The largest frame in bytes that can be transmitted on the virtual link

For example, if a VLID 50 has the Lmax of 100
bytes and BAG is of 4ms, then the maximum bandwidth for VLID 50 is of (100*8*1000/4)
200 kbps. The choice of the BAG and Lmax depends on the requirement of the
application and the bandwidth capability.

AFDX Switch

               The AFDX switch forwards packets
according to a static MAC table. And Each MAC address in the table correspond
to a virtual link Identifier. The Rx and Tx buffers store packets in a FIFO and
the I/O processing unit in the switch will move the packets according to the
virtual link identifier. AFDX switch contain functions for filtering, policing
and monitoring. Filtering is done based on the frame integrity, frame length
and valid destination. Traffic policing is based on the token bucket algorithm
which keeps token for all the virtual links. When a frame is received, it
checks the account and if enough credits are available the packet is sent and
credits debited. The tokens are credited as time progress (depending on the BAG
and Lmax). Monitoring is used to log the switch operation and health of the network.
The traffic policing makes sure that no virtual links routed through the switch
that exceeds the bandwidth.

AFDX Message Structure