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COMPARISON OF IEEE AVB AND AFDX Stefan Schneele, Fabien Geyer Innovation Works – Munich, Germany

Presenter: Stefan Schneele Tuesday, October 16 , Williamsburg, VA

Partly based on Airbus informations

COMPARISON OF IEEE AVB AND AFDX

Agenda Audio Video Bridging (AVB) Avionics Full Duplex Ethernet (AFDX) Comparison of key features Ethernet Compliance Bandwidth reservation End-to-End Latency & Determinism approach Redundancy & Clock Synchronization AVB SR Classes latency calculation AFDX latency calculation Performance Comparison Future of AVB

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AVB: What? Who? Why? For who? •

AVB means “Audio Video Bridging”

•

What? A set of networking protocols

•

Who? Developed by IEEE AVB Working Group

•

Why? “Provide the specifications that will allow time-synchronized low latency streaming services through IEEE 802 networks”

•

For who? Professional Audio/Video users, Automotive industry, home media-centers ĺ6HHhttp://www.avnu.org/about_us/our_members

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AVB: A set of protocols AVB is based on the following standards: • IEEE 802.1AS • Timing and Synchronization for TimeSensitive Applications in Bridged LAN • IEEE 802.1Qav • Forwarding and Queuing Enhancements for Time-Sensitive Streams • IEEE 802.1Qat • Stream Reservation Protocol • IEEE 1722 • Layer 2 Transport Protocol for Time Sensitive Applications in a Bridged LAN

Most of those protocols where published and approved by IEEE in 2010/2011. AVB Generation 2 is currently being investigated by IEEE AVB Task Group. Page 4


AVB Network example

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Source: Avnu.org


IEEE 802.1AS – Timing and Synchronization for TimeSensitive Applications in Bridged LAN AVB endpoints and bridges periodically exchange timing information in order to synchronize their clocks very precisely. Purpose: -

Allow synchronization of multiple streams

-

Provide a common time base for sampling and receiving data streams

Based on a PTP – Precise Timing Protocol

Source: Avnu.org Page 6


IEEE 802.1Qat - Stream Reservation Protocol

•

Network protocol used for reserving resources for the streams (on talker and listener side)

•

It is to layer 2, what RSVP (RFC 2205) is to layer 3

•

Flows are described with the following parameters (TSpec): • A maximum frame size • A maximum number of frames per “Class measurement interval” (125 us for Class A, 250 us for Class B)

Source: IEEE 802.1Qat Page 7


IEEE 802.1Qav – Forwarding and Queuing Enhancements for Time-Sensitive Streams

Enables a mixture of two types of traffic: -

Audio and video streams, or more generally time-sensitive streams,

-

Best-effort traffic.

To provide this, AVB uses: -

Credit-Based Shaper (CBS),

-

Strict Priority Queuing (SPQ).

Precedence of AVB traffic over best-effort traffic

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Summarizing the Interest Standard Ethernet becomes Real-time Optimized for Audio/ Video Streaming + Best effort (what about control data ?) Guaranteed latency bounds (e.g 2 ms for 7 hops with Class A) OP

Avionics

A/C Ops

Ethernet (AFDX + ARINC 429)

IFE

Ethernet

Ethernet CAN,…

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Cab Ops

CAN,…


AFDX: What? Who? Why? For who? •

AFDX means “Avionics Full-Duplex Switched Ethernet”

•

What? Aircraft Data Network (switch and end-system definition) defined in ARINC 664 Part 7

•

Who? Developed by Airbus for the A380 and standardized by ARINC (Aeronautical Radio, Incorporated)

•

Why? Provide deterministic Quality of Service for Ethernet flows inside an airplane

•

For who? Avionic industry (Airbus, Boeing, Bombardier, …)

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AFDX Principles • The AFDX network (aka ADCN : Avionics Data Communication Network) is a set of: – Switches – Avionics computers (LRU/LRM) are connected with the Switches through a duplex link at 10/100 Mbit/s Subscribers connected to the ADFX network are called AFDX E/S (End System)

– Harness (cables)

LRU LRM

SWITCH SWITCH

SWITCH SWITCH

SWITCH

LRM

SWITCH

SWITCH

SWITCH

LRU

LRU LRU

LRU LRU LRM

SWITCH

SWITCH SWITCH

SWITCH

SWITCH SWITCH

LRU LRM LRU

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The VL concept The VL is a communication channel between 1 single transmitter and one or several receivers : multicast channel

• Characteristics : A name

and an identifier 1 transmitter & the receiver(s) A guaranteed bandwidth : – Bandwidth Allocation Gap and Maximum Frame Size Bounded

latency time Static routes

LRU A

LRM D

VL2 VL1

LRU B

SWITCH

SWITCH

LRM C

LRU F

VL3

• Every communication in AFDX is done through a VL • Each application can have several VLs to transmit it’s data to one or several different receivers • One traffic class with two priorities Page 12

LRU E


The VL concept – more details • The VL is characterised by : • BAG (Bandwidth allocation Gap): minimum delay between to consecutive frames. BAG value: 1ms, 2ms, 4ms, 8ms, 16ms, 32ms, 64ms, 128ms • MVLS (Maximum VL Size): it represents the maximum size of the frame : min 17 octets ; max : 1471 octets (AFDX Payload ) BAG

MVLS

MVLS = Bandwidth BAG

• The frames are not sent to each BAG. The BAG limits only the bandwidth • The AFDX Switch supervises the VL to control the traffic (CRC, Frame size, respect of BAG) Page 13


The redundancy concept

Goal: High availability of the network Solution Redundant Network (A&B) / fully independent

sequence numbers added to frame Network BB Réseau

Réseau A Network A Par VL End System Tx

Par VL End System Rx

For each VL, 2 identical frames are sent on the 2 networks

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Comparison of key features between AFDX and AVB

Ethernet compliance AFDX

AVB

AFDX switches and end-systems are fully compliant with Commercial-OffThe-Shelf Ethernet switches

Part of the IEEE 802.1Q-2011 Standard

Physical Layer Adapted Specification for Wires (ARINC 664 Part 2) Additional Environmental Requirements Applied Connectors designed for Aeronautical Installations Network Nodes: Switches Minimum Performance Requirements Specified Maximum Frame Transition Time Fixed Supplemental Filtering and Forwarding Policies Page 15


Comparison of key features between AFDX and AVB Bandwidth reservation

AFDX

AVB

Each Virtual Link has a fixed bandwidth reservation defined by minimum and maximum frame size, and minimum inter-arrival time.

IEEE 802.1Qat defines a stream reservation protocol. Two classes are defined with a maximum target latency and jitter.

During design, a verification is done to certify that the network is able to support all the defined Virtual Links

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Class

CMI

A

125 ȝV

B

ȝV

Frame

Size < 1171 Bytes < 1500 Bytes

Latency target (7 hops)

Max Jitter

2 ms

125 µs

50 ms

1000 µs


Comparison of key features between AFDX and AVB Clock synchronization AFDX

AVB

No clock synchronization in the standard

Clock synchronization through the gPTP protocol (global Precision Time Protocol) with high accuracy (less than 1 µs difference over 7 hops)

Redundancy AFDX

AVB

Each packet sent by an end-system is duplicated and sent on two physically distinct networks

No redundancy defined (yet)

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Comparison of key features between AFDX and AVB End-to-End Latency & Determinism approach

AFDX

AVB

Network calculus approach to compute worst-cases and compare to requirements.

The AVB latency bounds are 2 ms over 7 hops for SR class “A” and 20 ms for SR class “B”

Hardware and software are developed accordingly to the highest (A) design assurance level. Iterative check of each configuration:

Envelope approach:

All communication requirements Safety requirements Optimization of priority and BAG

Bandwidth usage of class A/B < 75 % Not more then 13 streams Fixed packet distance

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AVB SR Classes latency calculation – Queuing delay Calculation described in IEEE 8021Qav

M0 Maximum sized frame for non-SR classes RA is the reserved data rate for SR Class A RB is the reserved data rate for SR Class B R0 is the transmission rate (portTransmitRate) maximum sized frame (M0 bits long) loCredit = maxFrameSize 㽢 (sendSlope/portTransmitRate) hiCredit = maxInterferenceSize 㽢 (idleSlope/portTransmitRate)

Worst-case for SR-Class B (source: IEEE 802.1Qav)

+Page Fan-In Delay, Permanent Buffer Contribtion 19


Aplying to a Avionic configuration The Traffic shaping function of the ES should be able to handle BAG values in range 1 ms to 128 ms. BAG values are limited to powers of 2 in order to simplify the ES design. See streams as Virtual Links 13 VL too less; adopt service classes to allow for more VL Links Two BAGs (observation times) too less: 2ms (Class A), 8ms (Class B), 32ms (Class C), 128 ms (Class D)

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Applying AVB latency calculation to AFDX The idea: Apply the AVB latency calculation approach to AFDX traffic, and see Virtual Links as AVB Streams. The BAG becomes the the observation time.

Nr of possible streams

100000

SR Class

BAG

Bandwidth

7 Hop delay

Nr of VLs

1000

A

2 ms

15%

3.68 ms

44

100

B

8 ms

15%

13.93 ms

178

10

C

32 ms

15%

57.37 ms

714

D

128 ms

15%

247.56 ms

2857

10000

1

Observation time (µs)

Relationship between observation time and number of possible streams

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Delay of AFDX traffic mapped to AVB SR classes


Future of AVB

•

AVB will be introduced in automotive industry

•

(Physical Layer driver/ enabler)

•

Second generation of AVB is in development: • IEEE 802.1Qbu – Frame Preemption for Ethernet • IEEE 802.1Qbv – Enhancements for Scheduled Traffic • IEEE 802.1ASbt – Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks Support for redundant paths Support for link aggregation

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Conclusion

•

Comprehensive overview about the basic principles behind IEEE AVB

•

Especially the credit based shaper algorithm (CBS) and the clock synchronization algorithm could be implemented accordingly to Aeronautic requirements

•

Multiple onboard applications dealing with video and audio in a non-safety critical environment could benefit with immediate effect of AVB

•

The set of AVB standards offer a complete framework and toolbox for sending real-time audio and video streams based on standard Ethernet

•

Combination of different traffic classes not sufficient solved (best effort, starvation due to SPQ)

•

Future work of our group will address the second generation of the AVB standard with focus on timing / latency analysis and failure modes

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Thank you

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