AFDX Tutorial

1
AFDX Tutorial

• Session One AFDX background

2
Introduction

• This Session One is built around four main
  topics
• General principle about "modern" communication
• The background of airborne data communication
• AFDX standard
• AFDX and A380

3
Part 1 Communication principles

• The key driver for the definition of the Network
  layering is the implementation of independance
  between application and communication means.

Application
Application
communication serives
communication serives
Network driver
Object Oriented paradigm
4
Usual communication layers
OSI reference Model
Application
Application services
Communication services
Presentation
Session
Transport
Network
DataLink/MAC
Physical connection
5
Part 2 Aircraft communication
Internal Aircraft communication
External Aircraft communication out of this
tutorial scope
6
Historical survey

• Until recently, there was never a strong need for
  networking inside an aircraft.
• When digital technologies were introduced, the
  communication was limited to digital data link.
• The introduction of digital technologies was done
  in the "control of platform" area not in the
  "information" area.

7
Digital transmission

• Two kind of digital usage on-board
• Processus Control
• based on sampling system techniques and data
  transmission
• data the digital value of an analogical
  parameter (e.g. speed heigth, attitude,....)
• transmission no response is expected
• Information systems
• based of information exchanges
• information a complex set of abtract values
  (e.g. digital map, flight plan, list of passenger
  duty free purchase, failure log book....)
• exchange a response is generaly expected, at
  least to indicated that the information is
  received.
• This "complex set of abstract values" gives
  usually a huge amount of bytes.... This is one
  reason that calls for higher speed data link

8
Processus control requirements

• As the transmitted data are involved in processus
  control , the transmission must be done with a
  minimum bounded delay.
• The stability of the flight relies on this
  transmission
• Time, integrity and availability are the key
  driver.
• Some principles
• no common shared resource (limited risk of common
  failure)
• one source, one ligne, several receivers
• the transmitter does not need to know who
  receives data
• no time synchronisation between transmitter and
  receiver
• common shared time is a kind of common resource
• Aeronautical response ARINC 429 Digital
  Information Transfer System

9
ARINC 429 reminder

• Each line has only one source and is connected to
  every equipment that need the data transmitted by
  the source
• Each data in individually identified (by a label)
  and sent

Application
Presentation
Session
Transport
Network
DataLink/MAC
Physical connection
10
Information System requirement

• In Information system, the major requirement is
  to insure that the information is transmitted
  without any error.
• Some principles
• the information should be acknowledged
• delay is not critical and messages can be sent
  again in case of error
• The former aircraft generation still used A429
  but added acknowledged data block

Transport
Network
DataLink/MAC
Physical connection
11
Avionics market evolution

• The evolution of the avionics market is exposed
  to a great pressure for reducing cost.
• In the same time, mature concepts arised
• Electronics Modularity
• Operating System
• Decision to re-use and share common resource

12
AFDX a real challenge

• The key driver for AFDX design choices must
  answer to lot of contradictory objectives
• To transmit data under strong time constraint
• To guarantee information exchange according to
  client/server model
• To reduce cost by using/reusing commercial
  component (COTS commercial off-the-shelf) under
  certification constraint

13
Technological choices

• Communication technologies from desktop computing
  market
• -gtBest candidate
• Ethernet for Physical layer
• Internet for upper protocol layer
• Communication technologies from multimedia
  telecom market
• -gtBest candidate
• ATM (backbone telecom and ADSL) and cell
  switching

14
Final choice

• Key drivers
• Heavy aeronautical background
• time constraint
• safety
• Arrival of Switched Ethernet (from ATM concept)
• Low cost, market size of desktop computing versus
  small telco market
• .... and the winner is...
• Switched full duplex Ethernet with some specific
  deviations to cope with real time/certification
  constraints
• AFDX Avionics Full DupleX switched Ethernet

15
Part 3 AFDX standard

• The standardisation body
• AFDX is undertaken by the civil aviation usual
  standardisation body ARINC/AEEC ADN working
  group
• ARINC Aeronautical Radio Inc. funded by
  airlines, in charge of the definiton of
  Aeronautical standards that ensure
  interchangeability and interoperability.
• AEEC Airlines Electronic Engineering Committee
• ADN Aircraft Data Network working group
• The standard
• AFDX is described as ARINC specification 664 part
  7
• The ARINC 664 covers in general, the usage of
  Ethernet as an airborne communication system,
  extended to the confidentiality issues and future
  IPv6 extensions.

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Key features of AFDX

• AFDX is the common communication system used for
  modular avionics architecture.
• It is compliant with the following design key
  features
• It is based on Open Standard
• as required by cost and commercial standard reuse
  objective
• It provides "Resource Sharing"
• as required by modularity, reuse, and cost
  objective
• It provides "Robust Partitioning"
• as required by resource sharing and safety,
  certification constraints
• It provides "Determinism" and "Availability"
• as required by safety, certification constraints
• The AFDX key features are mainly concentrated on
  the Data Link layer.

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AFDX an Open Standard
OSI reference Model
IEEE std
IETF Internet std
SNMP
TFTP
Application
Presentation
Session
TCP/UDP
Transport
IP
Network
IEEE 802.3 "Ethernet"
AFDX special features
DataLink/MAC
Physical connection
18
AFDX basic network architecture

• AFDX is based on the Ethernet switched network.
• It is built with
• Switches, network devices in charge of data
  forwarding
• End System, network devices in charge of data
  transmission/reception

modular avionocs
RDC
modular avionocs
LRU
ES
ES
ES
ES
SW
SW
SW
ES
SW
SW
SW
ES
ES
ES
ES
modular avionocs
modular avionocs
LRU
RDC
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AFDX key feature Resource Sharing

• The main resources shared by AFDX are
• the wiring and
• the attached network devices

IMA/IME module
IMA/IME module
LRU
RDC
ES
ES
ES
ES
SW
SW
SW
SW
SW
ES
ES
ES
ES
IMA/IME module
RDC
LRU
IMA/IME module
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AFDX key feature Virtual Link

• The robust partitioning for networking is applied
  on bandwidth allocated to "communication
  channel".
• The VL model is ARINC429 "single wire" and the
  ATM "Virtual Channel"
• one wire/channel for one data source, distributed
  to all who needed
• The AFDX response is
• one channel (named VL "Virtual Link") for one
  data source, distributed with multicast Ethernet
  addresschannels are merged together on one
  Ethernet data link

SW
ES
ES
ES
ES
AFDX Ethernet data link Virtual Link
ARINC 429 N/A twisted pair copper wire
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AFDX key feature "Firewalling"

• Another feature related to robust partition and
  safety is the integrated "firewall" provided by
  the AFDX.
• This firewall is implemented by Access Control
  List (ACL) mechanism.

Traffic filtering Restricted access for only
configured VL
Traffic filtering Restricted access for only
configured VL
ES
SW
ES
ES
Traffic filtering forwarding
ES
Ethernet data link Virtual Link
Traffic filtering
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AFDX key feature Redundancy

• In response to the "Availability" requirement
  AFDX network is basicaly redundant.
• Each End-System has the capability to send twice
  each message toward to independant set of
  switches.

network A
ES
SW
SW
ES
ES
network B
SW
SW
Key Feature Redundancy Management gt each
frames are sorted when received.
Key Feature Redundancy Management gt each
frames are numbered when transmitted.
23
AFDX key feature Latency management(1/3)

• The VL receive a "Bandwidth contract".
• This contract is expressed in terms of
• Maximum Frame Size (MFS)
• Minimum time between two frames
• named Bandwidth Allocation Gap (BAG)
• Max contractual bandwidthkbit/s
  MFSbit/BAGms
• Single VL max bandwidth c.a. 12Mb/s

determinism reason
Source Application
End System traffic shaping
BAG
BAG
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AFDX key feature Latency management(2/3)

• The robust partitioning relies on "Bandwidth
  contract" granted to each Virtual Link.
• The ES has Bandwidth Contract for each Virtual
  Link and must comply with this contract
• The Switches know the term of the contract for
  each Virtual Link and monitor the traffic to
  check if contract is respected.

Key Feature Traffic policing the traffic is
monitored according to bandwidth contract
Key feature Traffic shaping the traffic is
generated according to bandwidth contract
SW
ES
ES
ES
ES
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AFDX key feature Latency management (3/3)

• In AFDX context the determinism is defined as the
  control of maximum transmission delay through the
  network.
• The enabler of such control is precisely the
  bandwidth contract.
• Ethernet Switch provides better capability for
  determinism than usual Ethernet Hub because there
  is no collision and no transmission random retry.

Key feature Bounded latency The knowledge of
bandwidth contract allows to evaluated the worst
case filling level of shared output queue and,
hence to estimate the message delay
ES
SW
ES
ES
ES
Ethernet data link shared output message
queue Virtual Link
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AFDX "counterpart" Virtual Link

• Fit perfectly usual "non shared" aeronautical
  communication (ARINC429) like in "process
  control" where the bandwidth is continuously
  used.
• Difficult to manage bi-directional communication
  like in modern "information system"
• Leads to create large number of VL even if the VL
  is not used continuously

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AFDX "counterpart" Latency management

• The latency computation is based on the worst
  case that can happens.This is a certification
  concern not a performance concern!!
• As far as we can not state on the actual source
  traffic, the latency is systematically
  majored....
• This gives a certifiable network configuration
  that under-uses the true Ethernet capability

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Part 4 The AFDX and the A380

• Requested performance
• Airbus requirements impose a strong constraint on
  time and "proof of determinism"
• Computation of UDP message, IP fragmentation,
  traffic shaping, redundancy generation, Ethernet
  frame building lt 150µs
• Reception of continuous "back to back" Ethernet,
  traffic filtering, redundancy management, IP
  reassembly lt 150µs
• Frame forwarding, traffic policing, multicast
  management lt 100µs
• AFDX suppliers
• Two AFDX suppliers
• Rockwell-Collins Switches and End System
• Thales End System

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Open Standard benefits

• The use of "Open standard" such as Ethernet
  reduces the development cost in the following
  areas
• Laboratory Instrumentation.... Ethernet standard
  tools are used, no need to develop specific tools
• Design and development.... the definition of the
  standard relies on existing data and expertise
• However, this benefit should be mitigated because
  the use of equipment in an aircraft need to have
  trusted development that commercial components
  can not provide
• The result is that the material itself is still
  developped specifically for aeronautical market
  (...with the cost associated to certification
  compliance...)