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Driven by the proliferation of networked systems in today’s military aircraft, there’s a growing need for efficient and reliable high-capacity digital data links. Control systems using these data links have become more data-centric, and changing architectures are requiring greater amounts of data to be passed across multiple systems. The deployment of complex sensor arrays, video image processing, advanced data link communications, and other diverse aircraft-based applications are contributing to the increased demand for data processing.
Unfortunately, the growing number and sophistication of these systems has in many cases exceeded the capacities and capabilities of traditional aircraft data links, such as MIL-STD-1553 and ARINC 429. Although these in-place, proven technologies are low risk and work well for pure command control applications, they are severely bandwidth-limited by today’s standards.
Emerging command control digital data link technologies for aircraft applications fall into three broad categories: MIL-STD-1553 replacements, ARINC 429 replacements and multipurpose communications protocols. Many of these technologies, such as switched or fabric switch networks, are variations on or extensions of basic point-to-point architecture, but they each have unique strengths and limitations.
A widely used protocol for critical command control applications, mission systems and weapons, the venerable MIL-STD-1553 defined a simple time division command response multiplex drop databus. The drop bus architecture allows for multiple terminals to be connected onto the same set of wires, typically with no logical break in the lines (Figure 1).
Although most aircraft systems function fairly well at a 40-60 Hz refresh rate for flight data, only small packets of data can be transmitted (thirty-two 16-bit words). If more than simple commands need to be sent, 1553’s limited bandwidth of 32-word data packets and 1 Mbit/s transmission speed, fails miserably when compared to current technologies offering 256 32-bit words running at 1 Gigabit. Emerging 1553 alternatives include EBR-1553, HS-1553, Fibre Channel and IEEE1394b.
EBR-1553: Enhanced Bit Rate 1553 (EBR-1553) replaces the MIL-STD-1553 drop bus architecture with a 10 Mbit/s star topology implementation in which the RTs branch off from the bus controller (Figure 2). EBR-1553 is a new architecture, but its protocol is still that of 1553, a proven technology with significant industry confidence. However, EBR-1553 also retains some of the limitations inherent in the 1553 half-duplex, time division architecture. It can only achieve relatively slow data rates of 10 Mbits/s, over comparatively short runs of 40-100 feet. This technology is well-suited to applications requiring shorter line distance from the bus controller—typical implementations would be small diameter bombs (SDB) or Joint Common Missiles (JCM). EBR-1553 technical specifications are complete, and the technology should be an SAE-approved standard by mid-summer this year.
HS-1553: HS-1553, or High-Speed 1553 is currently being developed out of Wright Patterson AFB, Dayton, OH. It is intended for legacy 1553 installations in which more bandwidth is needed but where the wire plant cannot be changed-out—a common problem in many existing aircraft. Initial candidates may include the C130 and the F-16. Rewiring an aircraft is very expensive, and any new technology that utilizes the existing wire plant offers a cost-effective and practical way of introducing new terminals and new operation architectures. However, the only resemblance that this technology has to MIL-STD-1553 is that it will run on the same wire plant. Little detailed data on HS-1553 communications has been released to the industry.
Fibre Channel: Fibre Channel is already an avionics industry standard. Fibre Channel’s high transmission rates of 1.25, 2.5, 5 and 10 Gbits/s make it well-suited to a wide range of applications, including command control, mission systems, weapons, disk drives, instrumentation, simulation, signal processing and sensor/video data distribution. The point-to-point nature of Fibre Channel lends itself to supporting multiple transmission media—fiber, coax and so on—which allows the technology to meet many different installation and environmental requirements. Redundancy and multiple terminal communications can be achieved by implementing a switch topology. Numerous implementations—for the F-18, F-22 and F-35—of point-to-point architectures have been fully qualified for avionics use.
1394b: Like Fibre Channel, 1394b—also called Firewire—is an industry standard high-speed interface. It has gained wide support within the automotive industry for non-critical sensor and multimedia applications, and is also used extensively as a commercial component interface by Sony Corporation and by its creator, Apple Computer. A version of 1394b is being profiled within the SAE AS-1A3 group for command control applications. There are now dozens of avionics and space programs that have adopted 1394b as their command control data link, including flight controls for Joint Strike Fighter (JSF). The AS-1A3 profile has passed full environmental certification for F-35 applications. Like most of the commercial networks, 1394b can operate with determinism and low latency, and does so as drafted in the AS-1A3 document.
ARINC 429 Alternatives
Already in place and the historically preferred databus technology for commercial aircraft systems, ARINC 429 is a simple broadcast drop bus point-to-point architecture, as shown in Figure 4. ARINC 429 allows any connected terminal to pick-up data, provided it has the programming to receive the labels being transmitted. ARINC 429 only allows for one terminal to transmit on its twisted shielded pair media; any other terminals on the bus can only listen (up to 20). ARINC 429 is limited to a 12.5 to 100 Kbit/s data rate. Although the full bandwidth can theoretically be used to transmit single, 32-bit data word packets, the small packet size is an inherent limitation. Familiarity and ease of implementation are the principal strengths of ARINC 429, but its low data rates and single-word packet size make it unsuitable for complex networked applications. Alternatives to ARINC 429 include AFDX and ARINC 629.
AFDX: Specifically designed as a replacement for ARINC 429, Avionics Full-Duplex Switched Ethernet (AFDX) is currently under development for use in commercial aircraft command control. ADFX provides a full-duplex switched 100 Mbit/s Ethernet connection, offering an aggregate data rate 1000 times faster than ARINC 429 and larger packet sizes with message sizes up to nearly 8 Kbytes. The overall architecture is a full-duplex, dual-redundant switched Ethernet interconnect. The deterministic properties of AFDX are possible due to the use of switches and full-duplex links between components, thus eliminating any contention for the physical links. ARINC 664, Part 7, which is currently under development, contains the details regarding the AFDX protocol.
ARINC 629: A fully approved standard currently in use on Boeing’s 777 aircraft, ARINC 629 offers excellent command control functionality with its data rate of 2 Mbits/s, 20 times faster than ARINC 429 and 2 times faster than 1553. Its dual-simplex, arbitrated loop architecture is fully deterministic. However, since very few applications have employed ARINC 629 outside of the Boeing 777, parts are expensive and availability is limited.
In addition to technologies that directly replace MIL-STD-1553 and ARINC 429, other options exist in adapting all-purpose communications protocols for aircraft databus use. For more on these technologies, see the lead article in this section, “I/O Options Expand for Military Designers”, on p.18.
Choosing the Right Data Link Scheme
As our discussion has hopefully made clear, there is no single “best” high-capacity data link solution. Rather, the benefits of these various technologies are highly application-dependent. Selecting an appropriate data link entails careful, methodical consideration of complex factors. However, proceeding in a systematic fashion through some key decision points can help make the selection process easier.
Regardless of the bus, link or network of choice, there are some up-front implementation considerations common to any of the high-bandwidth data link alternatives. How is the network going to get its data into the system? Are there bridge chips, boards or intellectual property cores that provide sufficiently low latencies for the transmitted data to reach the memory, processor, backplane bus—such as PCI, PCI-Express, VME—or system to make the data useful? Will the technology work within environmental constraints, for example noise, EMI, temperature range, appropriate architecture, lightening and wiring considerations?
Other questions relate more to interoperability. Is the application a retrofit design? Does the system have to be compatible with others? What are the latency and determinism requirements? Is a handshake required? Are there other means of knowing that data has arrived? How much data needs to be transmitted and at what repeat rate?
Once those questions are answered, the choices can be reduced, based on the intended application, the data and the type of upper-level protocols that may need to be added. Some technologies are better suited than others for specific types of backplane, data links or network configurations and latencies. Compared in Figure 5 are the various data links viewed in relationship to their connectivity potentials. In most cases, the figure also represents the relative length of the data link connection.
Not Just a Technical Choice
Making a databus selection on purely technical grounds is not the best approach, however. When engineering a new system, it is also important to factor in all of the costs of developing and deploying the system. This includes costs for engineering labor, product integration, test, deployment and maintenance. What sort of development and test equipment is available? What is the maturity of these tools? Can or should the available tools be modified to meet project-specific needs? Can you count on third-party vendors to provide cost-effective products and support?
Like all trailblazers, system designers who choose to be among the first to implement data link technologies in new ways need to be prepared for complex challenges. However, by carefully matching current and emerging technologies to their intended applications, implementations and profiles, designers can ensure the best long-term architecture for future aircraft systems’ command control and data transfer needs.
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