Aerospace

AFDX Tutorial

Session One : AFDX background

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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

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Part 1 : Communication principles

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

Network driver

Specializationcommunication layers

Factorization

Network driver

Application Application

Communication services

communication serives

Application

communication serives

Application

Object Oriented paradigm

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Usual communication layers

OSIreference

Model

Physical connection

DataLink/MAC

Network

Transport

Session

Presentation

Application services

IETF Internetstd

IP

TCP/UDP

SNMP

IEEEstd

IEEE 802.3"Ethernet"

FTP

IEEE 802.11"WI-FI"

HTTP

POP SMTP

ITUstd

...

G992.1"ADSL"

ATM

Application

Communication services

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Part 2 : Aircraft communication

Internal Aircraft communication

External Aircraft communication : out of this tutorial scope

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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 platformcontrol of platform" area not in the "informationinformation" area.

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Digital transmission

Two kind of digital usage on-board: Processus ControlProcessus 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 systemsInformation 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

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Processus control requirements

As the transmitted data are involved in processus controlprocessus control , the transmission must be done with a minimum bounded delay. The stability of the flight relies on this transmissionTime, 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

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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

Physical connection

DataLink/MAC

Network

Transport

Session

Presentation

Application

Application

label data

A429

label data parity32 bit

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Information System requirement

In Information systemInformation 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

errorThe former aircraft generation still used A429 but added acknowledged data block

Physical connection

DataLink/MAC

Network

Transport

Application

A429 williamsburg

A429

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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

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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

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Technological choices

Communication technologies from desktop computing market

->Best candidate : Ethernet for Physical layer Internet for upper protocol layer

Communication technologies from multimedia telecom market

->Best candidate : ATM (backbone telecom and ADSL) and cell switchingIEEE

std

IEEE 802.3"Ethernet"

IEEE 802.11"WI-FI"

IETF Internetstd

IP

TCP/UDP

SNMP FTP HTTP

POP SMTP ...

ITUstd

G992.1"ADSL"

ATM

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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

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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

OSIreference

Model

IETF Internetstd

IP

TCP/UDP

SNMP

IEEEstd

IEEE 802.3"Ethernet"

TFTP

Physical connection

DataLink/MAC

Network

Transport

Session

Presentation

Application

ARINC 664 Part 7 : AFDX

IEEE 802.3 Ethernet MAC + PHY

IP

UDP TCPoptional

SNMP TFTP

AFDX special features

ARINC 600

ARINC 653

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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

AFDX : basic network architecture

ESES

SWSW

modular avionocs

ESES

modularavionocs

ESES

LRU

ESES

RDC

ESES

SWSW

SWSW

SWSW

modularavionocs

ESES

SWSW SWSW

modularavionocs

ESES

LRU

ESES

RDC

ESES

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IMA/IMEmodule

RDC

AFDX key feature : Resource Sharing

The main resources shared by AFDX are

the wiring and

the attached network devices

IMA/IMEmodule

ESES

IMA/IMEmodule

ESES

LRU

ESES

RDC

ESES

SWSW

SWSW

SWSW

ESES

SWSW

SWSW

IMA/IMEmodule

ESES

LRU

ESES ESES

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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

ESESESES

ESES

ESESAFDX

Ethernet data link

Virtual Link

ARINC 429

N/A

twisted pair copper wire

SWSW

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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.

SWSW

ESES

Ethernet data link

Virtual Link

ESES

ESES

ESES

Traffic filtering : Restricted access for only configured VL

Traffic filtering : Restricted access for only configured VL

Tra

ffic

filt

erin

g+

fo

rwa

rdin

g

Tra

ffic

filt

erin

g

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network B

network A

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.

ESES

SWSW SWSW

SWSW SWSW

ESES

ESES

Key Feature : Redundancy Management=> each frames are sorted when received.

Key Feature : Redundancy Management=> each frames are numbered when transmitted.

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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 bandwidth[kbit/s] = MFS[bit]/BAG[ms]Single VL max bandwidth = c.a. 12Mb/s

Source Application

End System traffic shaping

sent fralme

delayed frame

BAG BAG

delayed frame

determinism reason

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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.

SWSW

ESES

ESES

ESES

ESES

Key feature : Traffic shapingthe traffic is generated according to bandwidth contract

Key Feature : Traffic policingthe traffic is monitored according to bandwidth contract

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SWSW

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.

ESES

Ethernet data link

shared output message queue

Virtual Link

ESES

ESES

ESES

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

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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 systeminformation 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 < 150µs

Reception of continuous "back to back" Ethernet, traffic filtering, redundancy management, IP reassembly < 150µs

Frame forwarding, traffic policing, multicast management < 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...)