How is Mil-Std-1553 viewed by the primary user of the avionics bus? For an answer, Avionics Magazine interviews Bill Wilson, an avionics architecture technical expert with the Aeronautical Systems Center (ASC/ENAS) at the Engineering Directorate of Wright-Patterson Air Force Base, Ohio. An electrical engineer with 29 years of experience in databus and backplane issues, Wilson is the point person at ASC for questions regarding Mil-Std-1553, the workhorse avionics bus, not only for the Air Force, but for all of the military services.
Bill discussed with Avionics Magazine the evolution of the 1553 standard, what it has meant for military platforms, and current research and development activity regarding how to increase 1553 data throughput without rewiring the thousands of aircraft in the U.S. arsenal today.
Avionics Magazine: The Mil-Std-1553 databus is more than 20 years old and slow, at 1 megabit per second (Mbit/sec). But it’s fundamental, isn’t it?
Wilson: Very nearly every U.S. Defense Department aircraft and most NATO [North Atlantic Treaty Organization] aircraft have 1553 in them. 1553 is installed on Air Force bombers and fighters, as well as Navy and Army aircraft. The aircraft include F-15s, F-16s, F-22s, B-1s, B-52s, and B-2s. 1553 is even installed in a number of army tanks, ships, missiles, satellites and the International Space Station. 1553 is planned for use on the Joint Strike Fighter (JSF), as well.
The current standard is 20 years old, but at the time it was introduced, our need for bandwidth was fairly low. Mil-Std-1553 was released in 1973, Mil-Std-1553A was released in 1975, and Mil-Std-1553B was released in 1978. In fact, at the time it was introduced, 1553 was at the forefront of technology. The original thrust of the standard was to provide a command-and-control bus that would reduce the wiring count on aircraft by replacing multiple sets of wires with a single bus. These dedicated wires were combined together on some of our aircraft in bundles that were as much as 6 inches (15.2 cm) thick. The Air Force saved about 1,200 pounds (544 kg) in wire bundles with the introduction of 1553 on the B-52, for example. By moving to a bus concept, where a single piece of media interconnects multiple devices, we probably saved 300 to 500 pounds (136 to 227 kg) of weight for fighter aircraft and at least 1,000 pounds (436 kg) for transports and bombers.
1553 has become a key interface standard for all the avionics on our aircraft. The standard must be included in all design decisions whenever upgrades are planned. 1553 is a key ingredient of flight-critical systems such as those that interface with the wing control surfaces on military aircraft.
Avionics Magazine: Can 1553 evolve to meet the need for higher data rates?
Wilson: The Air Force has been looking at how to get greater bus speed without having to change out any of the hardware. That doesn’t mean there won’t be modifications. It simply means that, given the cost to replace wiring, we would prefer developing some solutions that would allow us to increase performance without changing much of the existing hardware or software.
Avionics Magazine: How much would it cost to replace existing 1553 wiring?
Wilson: It’s very difficult to modify the wiring in the wing of a fighter aircraft because you have to maneuver around wing structures and fuel bladders. Taking the aircraft skin off to get at the wiring is expensive, but it gets even more costly when you have to requalify the installation for safety. We estimate that it would cost about $1 million per aircraft to replace the 1553 wiring in the wings and recertify that it’s safe.
We have a lot more room in a transport aircraft or a bomber, but even here the replacement costs are fairly high. We estimated that a single wire running between two boxes on a tanker aircraft–excluding the cost of any documentation or safety certification–would be about $80,000.
You can imagine what it would cost to replace existing 1553 cards in the avionics boxes on a 1553 network with interface cards for a different bus technology. (A 1553 network could include 15 to 30 or more avionics boxes.) Our experience says that changing the interface cards would require major software modifications, as well, and this is something we have to avoid. The cost for software alone could exceed the cost of the hardware additions to upgrade the network.
Avionics Magazine: What is the Air Force considering as an alternative?
Wilson: One possibility could be the development of a high-bandwidth signaling approach that could co-exist with the existing 1553 signals on the same bus. An upgrade could be limited to replacement of the bus controller with a dual-speed bus controller, using existing wiring. This controller could talk to existing 1553 terminals at the current, low data rate and talk to newer terminals at a higher rate.
Avionics Magazine: Is there any specific research the service is sponsoring?
Wilson: The Air Force’s Wright Laboratory currently has a small contract with Edgewater Computer Systems to identify what the tradeoffs are in actually doing this. From a system point of view, we want to identify the merits of each of a number of approaches and then develop selection criteria to help guide future efforts.
Some of the areas that will be assessed are various physical signaling approaches (such as Carrier Amplitude Phase, or CAP, and Discrete Multi-Tone, or DMT), as well as methods for achieving even higher performance (such as using repeaters or bridges, and higher-layer protocols). A repeater is simply a way of refreshing the signal so that the effects of attenuation and noise can be filtered out. A bridge is similar to a repeater except that it also allows concurrent operation (i.e. in parallel) operation on separate segments of the bus.
We don’t want to re-invent anything, so we may borrow protocol techniques from commercial bus technologies such as fibre channel, Scalable Coherent Interface (SCI), or Asynchronous Transfer Mode (ATM), and from telecommunications technologies such as Digital Subscriber Line (DSL).
We need to use only those features that will maintain the high real-time standards of 1553. The selection of protocol techniques and physical signaling methods must be supported with the proper analysis and simulation to make sure that we understand all the interactions within the 1553 network. Analysis and simulation will highlight the merits of each approach and allow us to demonstrate feasibility.
Avionics Magazine: How fast do you think you could go with existing wiring?
Wilson: Our goal for the future is to provide a set of solutions along with tradeoff data for System Program Offices (SPOs) so that they can pick and choose an approach to increase the bandwidth and meet their program objectives. Raw speed is often not a good measure of performance, but we estimate that we should easily achieve 100 Mbits/sec and maintain the same robustness that 1553 is famous for. Additional increases above this depend partly on how efficient the commercial signaling techniques will be and on other techniques that haven’t been evaluated as yet.
One technique that some users may like but hasn’t been evaluated could be in shortening the cable length by installing a bridge or gateway where the cable is accessible within the weapons system. This would allow increased data rates by reducing the effect of noise and attenuation on the signal.
Another method to be assessed would be to tailor the bandwidth between nodes (receivers/transmitters) to obtain higher bandwidths between R/Ts or nodes that are close together. The performance characteristics for these various approaches will be identified by analysis and simulation, and documented as part of an architecture document that hasn’t been built yet. If the analysis and simulation confirm our preliminary estimates, we will begin looking for some additional funding to build prototypes and actually demonstrate this. A very tough aspect of this is to provide increased capability without major hardware or software modifications, but we believe that we can do it.