14-760: ADVANCED REAL-WORLD NETWORKS AVIONICS DATA BUSES LECTURE 20 * SPRING 2020 * KESDEN AERONAUTICAL RADIO, INC (ARINC) • Aeronautical Services Company • Standardization activities • Commercial activities • Publishes various standards • But standards committees are, in some ways, separate from commercial activities ARINC 429 Unless otherwise noted, slides based upon and using figures from: https://en.wikipedia.org/wiki/ARINC_429 ARINC 429: INTRODUCTION • Developed in 1977 • Common avionics data bus • Avionics = Aviation electronics • ARINC 429 defines a data bus for avionics • Physical standard, including mechanical and electronic • Link layer • Data definitions ARINC 429: MEDIUM • 2-wire, twisted-pair • One transmitter, Up to 20 receivers • Star or bus topology ARINC 429: SIGNALING • Self-clocking • Self-synchronizing • Balanced differential signaling • 12.5kbps or 100kbps ARINC 429: BPRZ SIGNALLING • Complementary Differential bipolar return-to-zero (BPRZ) • Bipolar Two poles, +10 and -10 • Differential Signal is driven between wires • Return-to-zero Guaranteed transition between +10 and -10 or vice-versa • Complementary 0 and 1 are “mirror opposites” ARINC 429: BPRZ SIGNALING • 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-70416230 ARINC 429: FRAMING • 32-bit frame is known as a word • P Parity (odd parity is used) • SSM Sign/Status Matrix. Indicates the sign of a number or a status, depending upon word’s Label • SDI Source Data Indicator. Indicates the source of the message or its destination, depending upon Label • Only 2 bits = 4 identifiers (Not 20). Can represent subsystem vs station • Label 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. ARINC 429: DATA • Binary 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. ARINC 429: SSM EXAMPLES • Normal 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 ARINC 629 Sources: • https://pdfs.semanticscholar.org/84c1/c4d2e6a975446585d88cb7e0455b112df584.pdf • https://web.archive.org/web/20160123042337/http://nafi.yolasite.com/resources/ARINC %20429_629_FINAL.pdf • https://www.maxt.com/mxf/arinc_629_spec.html ARINC 629: INTRODUCTION • Introduced in May 1995 • Originally developed for Boeing 777 • Now also used in Airbus 330 and Airbus 340 • Designed as success for ARINC 429 • But was never really intended for use beyond Boeing 777 ARCINC 629: KEY CHARACTERISTICS • Bus topology • Each bus is a redundant, dual bus (“hot standby”) • Multiple independent (dual redundant) buses are possible in same aircraft • Unshielded twisted pair • Up to 100M long • CSMA/CA, TDM • Manchester encoding • 2 Mbps • 128 terminals AFDX Unless otherwise noted, slides based upon and using figures from: • https://www.xilinx.com/support/documentation/application_notes/xapp1130.pdf AFDX: INTRODUCTION • Avionics Full Duplex Switched Ethernet (AFDX) • Introduced in 2005 • Developed for Airbus 380 • Variations used on other aircraft, including Boeing 787 • Based upon Ethernet 802.3 protocols Citation: Multiple sources, other than or in addition to xilinc document AFDX: KEY CONCEPTS • Virtual Link • Mimics single source, single drop or single source multi drop ARINC 424 connections • Addressing and bandwidth requirements for each virtual link are defined in advance • Known, predictable quality of service • Switch hierarchy is known in advance, so switch delays and capacities are known in advance • Virtual links have reserved bandwidth, so load isn’t a question • All routes, addresses, etc, are known in advance • Latency, jitter, etc, can all be shaped and guaranteed AFDX: REDUNDANCY • The entire network is parallel, with data sent and received in parallel. • Copies sent within 0.5ms of each other • End system manages redundancy, ensures ordered delivery, etc • Avionics system gets single copy of data in order AFDX: TOPOLOGY AND ROUTING • Up to 24 end systems per switch • Switches can be cascaded • No more than 4096 virtual links • Remember these are one way • Bidirectional requires one for transmit and one to receive • Virtual links can be routed through switches • All static based upon routing tables • No routing protocol AFDX: FRAME • IEEE 802.3 compliant frame • Notice Sequence Number (SN) in what otherwise would be payload • Used to ensure ordering, detect missing etc • Starts at 0, but wraps around 255->1 • 0 indicates a reset of the transmitting end system AFDX: MAC SOURCE ADDRESS • Note: Source address • 32-bit Constant field • Assigned by integrator • Same for all devices in network • 16-bit VL identifier AFDX: MAC DESTINATION ADDRESS • Note: MAC Destination Addresses • 24-bit constant field, same as for source addresses • 16-bit user defined identifier • Controller identifier • 3-bit interface identifier • Identified which of two networks • 001 for A, 010 for B • Note: Two bit distances apart • 5-bits of 0s AFDX: VIRTUAL LINKS AT SWITCHES • Emulate/Replace the point-to-point connections of ARINC 429 • Assigned bandwidth is defined by system and enforced by switch • Ports are shared, ports have quotas • Each VL also has a maximum frame size • Needed to ensure buffering capacity • VLs are all predefined to ensure system has required capacity AFDX: SUB VLS AT SWITCHES • Each VL can have up to 4 sub-VLs • Individual bandwidth guarantees aren’t policed by switch • Handled round-robin by switch • IP layer has to reassemble fragmentation from round-robin • Option by standard • Standard does not define how sub-VLs are identified • Possibly just by using VL identifiers AFDX: VIRTUAL LINKS AT END SYSTEMS • Each end system is responsible for sending and receiving data via VLs • Maximum of 128 VLs per end system • Can be any combination of send and receiver • Each VL and sub-VL requires its own FIFO queue • Sub-VL FIFO ques fill VL FIFO queue round-robin AFDX: VIRTUAL LINKS AT END SYSTEMS • Transmit 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 B • Receive responsibilities: • Deleting redundant frames and policing ordinal integrity. • Separating data by VL and writing received frames to the appropriate queue • Pass one or both copies of redundant data, as configured AFDX: BANDWIDTH CONTROL • Each VL can transmit at an assigned interval, within an assigned gap • A VL can transmit a frame within a bandwidth allocation gap • Minimum interval between beginning of consecutive frames from a VL • Frames can be longer, within limits • Frames over predefined max length are dropped as errors • If nothing to send, no need to send • 1ms to 128 ms • 2k, where k is from 1-7 AFDX: BANDWIDTH CONTROL • Jitter is time padding at the beginning of the bandwidth allocation gap • It allows the end system some time if juggling VLs • Maximum jitter is limited in advance • Up to 40ms due to transmission technology • Plus more depending upon bandwidth of medium, to keep bounds proportional ARDX: LATENCY LIMITS • Less than 150 uS at an end system • Switch less than 100uS AFDX: REDUNDANCY MANAGEMNT • End system checks integrity of each frame • Checksum + expected sequence number based upon prior sequence number • Discard and send error message on error • Resets are important • Sequence numbers are counted even for discarded frames • Resets can force recovery • 0 frame indicates reset • Anything after 0 is a valid starting point • When passed up, compared to make sure same (not just internally consistent as by MAC) • One discarded, one sent along • Frames with same sequence number • Duplicates within skew amount of time • Skew set according to topology, max 5ms • Considered new frames with bogus sequence number after that AFDX: FRAME FILTERING AT SWITCHES • Special feature of AFDX switches vs Ethernet • Verify: • Frame size is within limits • Integer number of bytes (alignment check) • Checksum verified • Incoming switch port VL is verified • Desitination MAC reachable AFDX: TRAFFIC POLICING AT SWITCHES • After frame filtering discards invalid frames • Discarded frames don’t count against bandwidth • Any frame in excess of VLs bandwidth quota is discarded • Byte-based or frame-based policing • Token bucket accounting ASIDE: TOKEN BUCKET ALGORITHM • A mechanism for enforcing bandwidth quotas • The 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.