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ARINC 818 Continues Success as Avionics Display Protocol

A decade since its launch, the robust ARINC 818 standard enjoys a rich ecosystems of tools and technologies have emerged and matured helping to feed the needs of aircraft system developers worldwide.

PAUL GRUNWALD, CHIEF SYSTEMS ARCHITECT GREAT RIVER TECHNOLOGY

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ARINC 818, titled Avionics Digital Video Bus (ADVB), was released as a standard ten years ago and was introduced to the world through the article "ARINC 818 Becomes New Protocol Standard for High-Performance Video Systems" published in the December 2006 issue of COTS Journal. Since then, the protocol has proliferated as the mission critical video bus in most of the new, large commercial and military aircraft programs, including the B787, KC-46A, 737MAX, A350XWB, COMAC C-919, and a dozen others around the world. In addition to being the bus connecting mission and video processors to a variety of displays (HDD, HUDs, HMDs), it has expanded into high-resolution radars and IR and optical sensors. ARINC 818-2 was released in 2013, which added features specifically to accommodate these new classes of high-speed, high-resolution sensors. In short, it has not just kept pace with rapidly developing demands and technologies, but kept ahead of them.

ARINC 818 Features

Here we examine he main technical features of the ARINC 818 protocol that have led to ever expanding adoption by the mil/aero community and discuss the broad array of development tools, test equipment, and embedded hardware now available. In 2005, Airbus and Boeing, in an effort to simplify systems design and reduce cost, initiated a new standardization effort through the Digital Video Subcommittee of ARINC, which went on to release ARINC 818. The desire was for a high-speed, low latency, mission critical video bus that was flexible enough to accommodate many different video formats and throughput requirements. The primary driver was to eliminate the proprietary video standards that existed in the avionics supply chain. For example, display manufacturers such as Honeywell, Rockwell Collins, and Thales each had their own video protocols, which were not compatible with one another.

ARINC 818 was built on the Fiber Channel Audio Video (FC-AV defined in ANSI INCITS 356-2002) protocol, but simplified the protocol by removing the need for handshaking. Since each packet includes a CRC, the integrity of each packet can be assured without the need for handshaking. FC-AV was used extensively as the video backbone in the F-18 and other programs. FC-AV is also used in MIL-STD-1760 Aircraft/Store Electrical Interconnection System for audio and video.

Technical Overview

The major aim of the ARINC 818 specification was to provide a robust protocol to handle the high bandwidth of modern avionics video systems. Fibre Channel remains the physical layer for the bus (FC0 and FC1) and also offers the advantages of routing and protocol capabilities found in modern networking protocols. ARINC 818 includes error detection through both ADVB packet and image CRC checks. 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, where most display applications us the line-synchronous class.

High Bandwidth

At the time ARINC 818 was ratified, the Fiber-Channel protocol supported link rates of 1.0625, 2.125, 4.25, and 8.5 Gbits/s. Since then, link rates of 14.025 and 28.05 Gbits/s have been released with even higher speeds planned as the market needs it. For example, a display at WQXGA resolution (2560 x 1600 pixels at 24-bit color) at 60 Hz would need throughput of 7,372 Mbits/s, which would fit on an ADVB 8X rate. ARINC 818-2 added 5.0, 6.375 (FC 6x), 12.75 (FC 12x), 14.025 (FC 16x), 21.0375 (FC 24x), and 28.05 (FC 32x) Gbit/s rates.

The 6x, 12x, and 24x speeds were added to accommodate the use of high-speed, bi-directional coax with power as a physical medium. The 5 Gbit/s rate was added to accommodate speeds supported by certain FPGAs. Also added to the specification was a provision to specify non-standard link rates for bi-directional return paths for applications such as camera control, where high-speed video links are not required.

Most cockpit display applications to date utilize rates of 1.0625 to 5.0 Gbits/s, it has been high-speed sensors that have pushed ARINC 818 rates. For example, a new infrared sensor required a channel-bonded 12.75 Gbit/s interface, which provides an effective throughput of over 25 Gbits/s. With ARINC 818 rates defined to 28 Gbits/s, there is really no practical limit to how much data can be transported in a single application when channel bonding (see below) is applied, so ARINC 818 will meet the throughput demands for displays and sensors for years come.

Low Latency

One of the most important features of ARINC 818 is the ability to deliver uncompressed video with very low latency, in many implementations, less than one frame. Low latency is important in real-time cockpit displays and especially in head-up displays (HUD) where differences in the HUD display images and real-world background can cause vertigo or motion sickness in the pilot.

The latency is generally determined by the implementation. In some cases, the image is streamed through FIFOs and can be almost real-time, that is, a latency of several video lines or less. Other implementations use two image buffers and display one while the other is filling ("ping pong") giving a latency of a single frame. Because most cockpit displays use line-synchronous timing and only a line FIFO receiver, this imposes strict timing characteristics on the transmitter, which will then require frame buffering of the transmitter to clock out precise line-sync timing. At 60 Hz the latency of a frame buffered transmitter is 16.6 milliseconds, which is typically sufficient, even for time-critical applications.

Channel Bonding and More

For higher bandwidth applications, it is possible to using multiple channels to carry a video stream. This is called channel bonding and is similar to Link or Port Aggregation using Ethernet. In most implementations, the input is split at the transmitter device into two or more ARINC 818 frames and the reassembled at the receiver for display or recording (Figure 1).

Figure 1
Higher bandwidth applications can use “channel bonding” where multiple channels are used to carry a video stream.

Because ARINC 818 uses Fibre Channel as the physical layer and the protocol has support for source and destination ID in the headers, simple networking is straightforward. Repeaters, routing, and fanout topologies are all possible. This allows a great deal of flexibility in the design of an overall avionics display system. However, ARINC 818 will not work on many Fibre Channel devices due to the bi-directional requirements of the base protocols, but dedicated ARINC 818 switches are available on the market.

 ARINC 818 allows for flexibility in the implementation of the video interface. This flexibility is desirable, because of the diverse resolutions, grayscales, pixel formats, and frame rates of avionics display systems. The system architecture must anticipate this flexibility and be interoperable with all video sources at a particular link rate.

The ARINC 818 specification intends that an interface control document (ICD) accompany particular ADVB implementations. The ICD will specify parameters such as link speed, image resolution, synchronization scheme, and frame rate. Avionics programs will always have an associated ICD. An ICD template is available from www.arinc818.com, which includes all pertinent parameters to assure compatibility between ARINC 818 systems. A particular piece of equipment that is compliant with ARINC 818 is not necessarily interoperable with another piece of equipment compliant with ARINC 818, unless they are both made to the same ICD.

ARINC 818 Packets

ARINC 818 has precise rules for constructing a packet. The ARINC 818 standard refers to the basic transport mechanism (packet) as an ADVB frame. It is important to refer to these packets as "ADVB frames" rather than simply "frames" to eliminate potential confusion with video frames. Figure 2 shows the basic structure of an ADVB frame.

Figure 2
This is basic structure of an ADVB frame. Each ADVB frame has a header comprised of six 32-bit words.

The start of an ADVB frame is signaled by an SOFx ordered set and terminated with an EOFx ordered set. Every ADVB frame has a header comprised of six 32-bit words. These header words pertain to such things as the ADVB frame origin and intended destination and the ADVB frame's position within the sequence. The payload can be video or associated data. The payload in one ADVB frame can vary in size but cannot be greater than 2112 bytes. Finally, all ADVB frames have a 32-bit CRC calculated for all data between the SOFx and the CRC word for built-in error checking. The CRC is the same 32-bit polynomial calculation defined for Fibre Channel. Figure 3 shows a detailed structure of an RGB XGA (1024 x 768) video frame within an ADVB container. An example of how ARINC 818 transmits color XGA provides a good overview and can be found in the article: "ARINC 818 Becomes New Protocol Standard for High-Performance Video Systems" in the December 2006 issue of COTS Journal.

Figure 3
Shown here is a detailed structure of an RGB XGA (1024 x 768) video frame within an ADVB container.

The majority of ARINC 818 implementations use optical fiber. Important additions to the ARINC 818-2 standard (over ARINC 818-1) paved the way for improved copper physical layers. Specifically envisioned was the use of newer active equalizer chips to greatly improve bandwidth and distance on coaxial cable. Included in ARINC 818-2 are methods to implement a return communication path (from video receiver to video source) on the same coaxial cable. Therefore, a complete camera or sensor interface can be achieved with a single high-bandwidth coaxial cable running at 3.1875 Gb/s or 6.375 Gb/s.

Programs and Applications

ARINC 818 has been adopted for use on both civilian and military programs: the B787, A400M, KC-46A, A350-XWB, B737 Max, B-777X, C 919, Korean LAH, and numerous other platforms. Aircraft adoptions are due in large part to the success of commercial cockpit suites such as the Rockwell Collins ProLine Fusion system (Figure 4) and the Thales TopDeck, which both use ARINC 818. ARINC 818 is also migrating into high-resolution sensor systems, where copper protocols like HOTLink no longer have sufficient bandwidth. In particular, high-speed IR sensors, radars, HUDs, HDDs, HMDs, high-resolution cameras, enhanced vision systems.

Figure 4
Aircraft adoptions are due in large part to the success of commercial cockpit suites such as the Rockwell Collins ProLine Fusion system, shown here in an B787 cockpit.

Implementing, testing, and validating ARINC 818 systems has become easier as the worldwide tool and embedded eco-system has expanded. The following is a list of the types of commercially available tools and hardware from Great River Technology, Avionics Interface Technologies, AIM GmbH and others:

Frame Grabbers - Capture and record ARINC 818, either in a production environment or on an aircraft.

Graphics Generators - Create ARINC 818 video streams from existing video or graphics files.

Format Converters - Convert from DVI, VGA, RS-170 to/from ARINC 818.

Protocol and Timing Analyzers - Capture and analyze line and frame timing as well as all the details of the protocol.

Universal ARINC 818 Generators - Quickly implement an ARINC 818 transmitter for any ICD through a simple GUI.

DO-254 Certifiable IP Cores - ARINC 818 IP that will drop into Xilinx or Altera FPGAs for flyable applications.

Switches - Four-channel or 10-channel switches for lab or flight-test use.

Video Concentrators - Time multiplex multiple video streams onto a single ARINC 818 link.

Embedded Hardware - XMC frame grabbers, XMC converters, VPX GP-GPUs, small-embedded converters.

Widespread Adoption Continues

ARINC 818 continues to be adopted throughout the military and aerospace world due to its flexibility, low latency, error checking, determinism, and high bandwidth characteristics. It is being used for all manner of cockpit displays, high-resolution cameras, and high-speed sensors. ARINC 818 has wide industry support from aircraft manufacturers and suppliers. With the addition of higher speeds, support for compression, and encryption, networking, and sophisticated display schemes, ARINC 818 adoption will continue to grow and expand the mission profiles within and beyond avionics.

The author Paul Grunwald was a member of the ARINC 818-2 standards committee.

 

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