|This article needs additional citations for verification. (June 2009)|
ARINC 818: Avionics Digital Video Bus (ADVB) is a video interface and protocol standard developed for high bandwidth, low latency, uncompressed digital video transmission in avionics systems. The standard, which was released in January 2007, has been advanced by ARINC and the aerospace community to meet the stringent needs of high performance digital video. The specification was updated and ARINC 818-2 was released in December of 2013, adding a number of new features.
In aircraft, an ever-increasing amount of information is supplied in the form of images, this information passes through a complex video system before reaching cockpit displays. Video systems include: infrared and other wavelength sensors, optical cameras, radar, flight recorders, map/chart systems, synthetic vision, image fusion systems, heads-up displays (HUD) and heads-down primary flight and multifunction displays, video concentrators, and other subsystems. Video systems are used for taxi and take-off assist, cargo loading, navigation, target tracking, collision avoidance, and other critical functions.
ARINC 818/ADVB is a Fibre Channel (FC) protocol which builds on FC-AV (Fibre Channel Audio Video, defined in ANSI INCITS 356-2002), which was used extensively on video systems in the F-18 and the C-130AMP. Although FC-AV has been used on numerous programs, each implementation has been unique. ARINC 818 provides an opportunity to standardize high-speed video systems.
Overview of ARINC 818 Protocol
ARINC 818 is a point-to-point, 8B/10B encoded serial protocol for transmission of video, audio, and data. The protocol is packetized, but is video-centric and very flexible, supporting an array of complex video functions including the multiplexing of multiple video streams on a single link or the transmission of a single stream over a dual link. Four different classes of video are defined, from simple asynchronous to stringent pixel synchronous systems.
ADVB Packet Structure
The ADVB frame is the basic transport mechanism for ARINC 818. It is important to refer to these packets as “ADVB frames” rather than simply “frames” to eliminate potential confusion with video frames.
The start of an ADVB frame is signaled by a SOFx 4-byte Ordered Set and terminated with an EOFx Ordered Set. Every ADVB frame has a standard Fibre Channel header composed of six 32-bit words. These header words pertain to such things as the ADVB frame origin and intended destination and the ADVB frames position within the sequence. The Source ID field (SID) in the ADVB frame header allows video from each sensor to be distinguished from the other sensors.
The “payload” contains either video, video parameters or ancillary data. The payload can vary in size, but is limited to 2112 bytes per ADVB frame. To insure data integrity, all ADVB frames have a 32-bit CRC calculated for data between the SOFx and the CRC word. The CRC is the same 32-bit polynomial calculation defined for Fibre Channel.
ADVB Container Structure
The ARINC 818 specification defines a “container” as a set of ADVB frames used to transport video. In other words, a video image and data is encapsulated into a “container” that spans many ADVB frames. The “payload” of each ADVB frame contains either data or video. Within a container, ARINC 818 defines Objects that contain certain types of data. That is, certain ADVB frames within the container are part of an Object.
An example of how ARINC 818 transmits color XGA provides a good overview. XGA RGB requires ~141M bytes/s of data transfer (1024 pixels x 3 bytes per pixel x 768 lines x 60 Hz). Adding the protocol overhead and blanking time, a standard link rate of 2.125Gbit/s is required. ARINC 818 “packetizes” video images into Fibre Channel frames. Each FC frame begins with a 4 byte ordered set, called an SOF (Start of Frame), and ends with an EOF (End of Frame), additionally, a 4 byte CRC is included for data integrity. The payload of the first FC frame in a sequence contains embedded header data that accompanies each video image.
Each XGA video line requires 3072 bytes, which exceeds the maximum FC payload length, so each line is divided into two FC frames. Transporting an XGA image requires a “payload” of 1536 FC frames. Additionally, an ADVB header frame is added, making a total of 1537 FC frames. Idle characters are required between FC frames because they are used for synchronization between transmitters and receivers.
Although ARINC 818 was developed specifically for avionics applications, the protocol is already being used in sensor fusion applications where multiple sensor outputs are multiplexed onto a single high-speed link.
Low-speed implementations of ARINC 818 (1.0625Gbit/s) can use copper (twinax or STP) or fiber, and high-speed implementations (2 Gbit/s+) can use either 850 nm MM fiber (<500m) or 1310 nm SM fiber (up to 10 km). ARINC 818 lends itself to applications that require few conductors (slip rings, turrets), low weight (aerospace), EMI resistance, or long distance transmission (aerospace, ships).
Flexibility vs. Interoperability
ARINC 818 is flexible and can accommodate many types of video and data applications. It is the intention of the standard that all implementation be accompanied by a small interface control document (ICD) that defines key parameters of the header such as: link speed, video resolution, color scheme, size of ancillary data, timing classification, or bit-packing schemes. Interoperability is only guaranteed among equipment built to the same ICD.
ARINC 818 transmitters must assemble valid FC frames, including starting and ending ordered sets, headers, and CRC. This can easily be done with VHDL state machines, and many PLD SerDes include built in CRC calculations.
The flexibility of ARINC 818 allows for receiver implementations using either full image buffers or just display-line buffers. For either, synchronization issues must be considered at the pixel, line, and frame level.
Line buffer or FIFO-based receivers will require that the transmitter adhere to strict line timing requirements of the display. Since the display horizontal scanning must be precise, the arrival time of lines will also need to be precise. ARINC 818 intends that timing parameters such as these be captured in an ICD specific to the video system.
The authors of ARINC 818 built upon many years of combined experience of using FC to transport different video formats, and key implementation details are included in the specification, including examples of common analog formats.
- Aircraft flight control systems
- Fibre Channel 8B/10B encoding
- Fibre Channel network protocols
- Integrated Modular Avionics
- 818-1 Avionics Digital Video Bus (ADVB) High Data Rate, published by ARINC 2007
- ARINC 818 Becomes New Protocol Standard for High-Performance Video Systems, COTS Journal, Dec 2006
- Explaining ARINC 818, Avionics Magazine March 1, 2008