Avionics Data Buses Lecture 20 Spring 2020 Kesden Aeronautical Radio INC ARINC Aeronautical Services Company Standardization activities Commercial activities Publishes various standards ID: 1014688
Download Presentation The PPT/PDF document "14-760: Advanced Real-World Networks" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
1. 14-760:Advanced Real-World NetworksAvionics Data BusesLecture 20 * Spring 2020 * Kesden
2. Aeronautical Radio, INC (ARINC)Aeronautical Services CompanyStandardization activitiesCommercial activitiesPublishes various standardsBut standards committees are, in some ways, separate from commercial activities
3. ARINC 429Unless otherwise noted, slides based upon and using figures from: https://en.wikipedia.org/wiki/ARINC_429
4. ARINC 429: IntroductionDeveloped in 1977Common avionics data busAvionics = Aviation electronicsARINC 429 defines a data bus for avionicsPhysical standard, including mechanical and electronicLink layerData definitions
5. ARINC 429: Medium2-wire, twisted-pairOne transmitter, Up to 20 receiversStar or bus topology
6. ARINC 429: SignalingSelf-clockingSelf-synchronizingBalanced differential signaling12.5kbps or 100kbps
7. ARINC 429: BPRZ SignallingComplementary Differential bipolar return-to-zero (BPRZ)Bipolar Two poles, +10 and -10Differential Signal is driven between wiresReturn-to-zero Guaranteed transition between +10 and -10 or vice-versaComplementary 0 and 1 are “mirror opposites”
8. The signal has three states 'HI', 'NULL' and 'LOW' represented by the differential voltage between the two wires of the cable.A logical ‘1’ is signaled by transmission of a +10 ±1V pulse followed by a 0±0.5V null period.A logical ‘0’ is signaled by transmission of a –10 ±1V pulse also followed by a 0 ±0.5V null period.Slide credit: https://www.slideshare.net/yasir2761/avionics-buses-70416230ARINC 429: BPRZ Signaling
9. ARINC 429: Framing32-bit frame is known as a wordP Parity (odd parity is used)SSM Sign/Status Matrix. Indicates the sign of a number or a status, depending upon word’s LabelSDI Source Data Indicator. Indicates the source of the message or its destination, depending upon LabelOnly 2 bits = 4 identifiers (Not 20). Can represent subsystem vs stationLabel Some type of identifier associated with the message. Some are standard. Some are not. Thus some are interpreted the same way across aircraft, and some are not.
10. ARINC 429: DataBinary Coded Decimal (BCD) – SSM may also indicate the Sign (+/-) of the data or some information analogous to sign, like an orientation (North/South; East/West). When so indicating sign, the SSM is also considered to be indicating Normal Operation.Twos Compliment representation of signed binary numbers (BNR)Bit 29 represents the number's sign; that is, sign indication is delegated to Bit 29 in this case.Discrete data representation (e.g., bit-fields)The SSM has a different, signless encoding.
11. ARINC 429: SSM ExamplesNormal Operation (NO) - Indicates the data in this word is considered to be correct data.Functional Test (FT) - Indicates that the data is being provided by a test source.Failure Warning (FW) - Indicates a failure which causes the data to be suspect or missing.No Computed Data (NCD) - Indicates that the data is missing or inaccurate for some reason other than a failure. For example, autopilot commands will show as NCD when the autopilot is not turned on
12. ARINC 629Sources:https://pdfs.semanticscholar.org/84c1/c4d2e6a975446585d88cb7e0455b112df584.pdfhttps://web.archive.org/web/20160123042337/http://nafi.yolasite.com/resources/ARINC%20429_629_FINAL.pdfhttps://www.maxt.com/mxf/arinc_629_spec.html
13. ARINC 629: IntroductionIntroduced in May 1995Originally developed for Boeing 777Now also used in Airbus 330 and Airbus 340Designed as success for ARINC 429But was never really intended for use beyond Boeing 777
14. ARCINC 629: Key CharacteristicsBus topologyEach bus is a redundant, dual bus (“hot standby”)Multiple independent (dual redundant) buses are possible in same aircraftUnshielded twisted pairUp to 100M longCSMA/CA, TDMManchester encoding2 Mbps128 terminals
15. AFDXUnless otherwise noted, slides based upon and using figures from:https://www.xilinx.com/support/documentation/application_notes/xapp1130.pdf
16. AFDX: IntroductionAvionics Full Duplex Switched Ethernet (AFDX)Introduced in 2005Developed for Airbus 380Variations used on other aircraft, including Boeing 787Based upon Ethernet 802.3 protocolsCitation: Multiple sources, other than or in addition to xilinc document
17. AFDX: Key ConceptsVirtual LinkMimics single source, single drop or single source multi drop ARINC 424 connectionsAddressing and bandwidth requirements for each virtual link are defined in advanceKnown, predictable quality of serviceSwitch hierarchy is known in advance, so switch delays and capacities are known in advanceVirtual links have reserved bandwidth, so load isn’t a questionAll routes, addresses, etc, are known in advanceLatency, jitter, etc, can all be shaped and guaranteed
18. AFDX: RedundancyThe entire network is parallel, with data sent and received in parallel.Copies sent within 0.5ms of each otherEnd system manages redundancy, ensures ordered delivery, etcAvionics system gets single copy of data in order
19. AFDX: Topology And ROutingUp to 24 end systems per switchSwitches can be cascadedNo more than 4096 virtual linksRemember these are one wayBidirectional requires one for transmit and one to receiveVirtual links can be routed through switchesAll static based upon routing tablesNo routing protocol
20. AFDX: FrameIEEE 802.3 compliant frameNotice Sequence Number (SN) in what otherwise would be payloadUsed to ensure ordering, detect missing etcStarts at 0, but wraps around 255->10 indicates a reset of the transmitting end system
21. AFDX: MAC Source AddressNote: Source address32-bit Constant fieldAssigned by integratorSame for all devices in network16-bit VL identifier
22. AFDX: MAC Destination AddressNote: MAC Destination Addresses24-bit constant field, same as for source addresses16-bit user defined identifierController identifier3-bit interface identifierIdentified which of two networks001 for A, 010 for BNote: Two bit distances apart5-bits of 0s
23. AFDX: Virtual Links At SwitchesEmulate/Replace the point-to-point connections of ARINC 429Assigned bandwidth is defined by system and enforced by switchPorts are shared, ports have quotasEach VL also has a maximum frame sizeNeeded to ensure buffering capacityVLs are all predefined to ensure system has required capacity
24. AFDX: Sub VLs At SwitchesEach VL can have up to 4 sub-VLsIndividual bandwidth guarantees aren’t policed by switchHandled round-robin by switchIP layer has to reassemble fragmentation from round-robin Option by standardStandard does not define how sub-VLs are identifiedPossibly just by using VL identifiers
25. AFDX: Virtual Links at End SystemsEach end system is responsible for sending and receiving data via VLsMaximum of 128 VLs per end systemCan be any combination of send and receiverEach VL and sub-VL requires its own FIFO queueSub-VL FIFO ques fill VL FIFO queue round-robin
26. AFDX: Virtual Links at End SystemsTransmit responsibilities:Reading each VL queue.Incrementing the VL frame sequence number.Scheduling each frame for transmission to maintain the bandwidth guarantee within the allowed jitter.Transmitting redundant frames on both controllers A and BReceive responsibilities:Deleting redundant frames and policing ordinal integrity.Separating data by VL and writing received frames to the appropriate queuePass one or both copies of redundant data, as configured
27. AFDX: Bandwidth ControlEach VL can transmit at an assigned interval, within an assigned gapA VL can transmit a frame within a bandwidth allocation gapMinimum interval between beginning of consecutive frames from a VLFrames can be longer, within limitsFrames over predefined max length are dropped as errorsIf nothing to send, no need to send1ms to 128 ms2k, where k is from 1-7
28. AFDX: Bandwidth ControlJitter is time padding at the beginning of the bandwidth allocation gapIt allows the end system some time if juggling VLsMaximum jitter is limited in advanceUp to 40ms due to transmission technologyPlus more depending upon bandwidth of medium, to keep bounds proportional
29. ARDX: Latency LimitsLess than 150 uS at an end systemSwitch less than 100uS
30. AFDX: Redundancy ManagemntEnd system checks integrity of each frameChecksum + expected sequence number based upon prior sequence numberDiscard and send error message on errorResets are importantSequence numbers are counted even for discarded framesResets can force recovery0 frame indicates resetAnything after 0 is a valid starting pointWhen passed up, compared to make sure same (not just internally consistent as by MAC)One discarded, one sent alongFrames with same sequence numberDuplicates within skew amount of timeSkew set according to topology, max 5msConsidered new frames with bogus sequence number after that
31. AFDX: Frame Filtering At SwitchesSpecial feature of AFDX switches vs EthernetVerify:Frame size is within limitsInteger number of bytes (alignment check)Checksum verifiedIncoming switch port VL is verifiedDesitination MAC reachable
32. AFDX: TrAffic Policing At SwitchesAfter frame filtering discards invalid framesDiscarded frames don’t count against bandwidthAny frame in excess of VLs bandwidth quota is discardedByte-based or frame-based policingToken bucket accounting
33. Aside: Token Bucket AlgorithmA mechanism for enforcing bandwidth quotasThe token bucket algorithm can be conceptually understood as follows:A token is added to the bucket every 1/r seconds.The bucket can hold at the most b tokens. If a token arrives when the bucket is full, it is discarded.When a packet (network layer PDU) of n bytes arrives,if at least n tokens are in the bucket, n tokens are removed from the bucket, and the packet is sent to the network.if fewer than n tokens are available, no tokens are removed from the bucket, and the packet is considered to be non-conformant.Algorithm description taken directly from: https://en.wikipedia.org/wiki/Token_bucket