Understanding SOSA and Its Importance for RF Designers

The Sensor Open Standards Architecture (SOSA)^TM consortium was started to provide more commonality across platforms for the US Air Force, Navy, and Army. Part of the effort is to bring together previously disparate efforts such as HOST, CMOSS, VICTORY, etc, into more compatible forms. The benefits to the military are the ease of integration, less training time, a more plug-n-play ecosystem, less confusion, less vendor lock, lower costs, and more. It also gives them the ability to have a more manageable innovation process and help lead design requirements at the front end. In short, the Department of Defense is heavily pushing the implementation/adoption of the standard across the branches of the military. As the sensors commonly utilize RF signaling, it is important for those in the radio frequency device community to have an understanding of this significant undertaking.

From an embedded computing perspective, SOSA marries the sensor end of a system to the processing end. While embedded computing standards in the past focused more on mechanical and electrical interoperability, SOSA is the culmination of the customer’s (Tri-service community) input and their application requirements to the hardware, software and other vendors/OEMs/contractors' innovative drive coming together. SOSA leverages the standards of the VMEbus International Trade Association (VITA) of 3U and 6U OpenVPX and their associated specifications that support the technology. Often the customer’s desires are not (yet) practical or the supplier community’s creative designs are not the right fit. But in the end, the ebb/flow of this process is timelier, fluid, and the community is more efficient and effective as a result. An important element of SOSA is the optical and RF sensor relaying input to the computer (or vice versa).   

SOSA is leading the push for a combination of various slot profiles of plug-in boards that go into a backplane. From a chassis platform perspective, SOSA’s efforts are driving the backplane speed requirements, hotter plug-in cards that require more advanced cooling solutions, the RF and fiber interfaces that lead to new I/O solutions, and chassis management.

So, what does SOSA entail from a chassis platform vendor’s point of view? In short, SOSA is a collection of SOSA-aligned slot profiles. We say “aligned” because a SOSA system platform will need to go through conformance standards to become official (once the process for conformance is fully defined and available). A slot profile has a certain pinout describing what various types of signals/pins do and where they go. This includes the data plane, control plane, expansion plane, system management, utility, power, and more. Other factors include the data rate, RF and optical pin positioning, etc.

Figure #1 shows a common slot profile (with a 14.6.11 suffix) highlighting the pin allocation along with the optical/RF connector type. This tells the backplane designer how the slot needs to be routed. That said, there are often specific routing instructions on how backplane pins are mapped. Figure #2 shows a group of SOSA-aligned backplanes with various configurations and RF/optical aperture types. There is an important caveat when it comes to whether a system is SOSA-aligned. At this phase in SOSA’s evolution, not all slots in the system or areas of a system need to meet the standard to be SOSA aligned. While the specification uses 12 V power along with some 3.3 V AUX, you can have some slots in the system that are not SOSA and use, say, 5 V power as well. Also, other slots may have a profile that is not used in SOSA.

Figure 1:  The slot profiles tell the board manufacturer what pins on the module should be allocated for which type of signal and it tells the backplane manufacturer what the slot interface entails.

Figure 2:  The various options for RF and optical interfaces bring a wealth of OpenVPX/SOSA-aligned backplane configurations that are possible for 3U boards, 6U boards, and 3U/6U hybrid options.

Power, Fans, & Chassis Hardware Management

Broader elements of the SOSA-aligned enclosure include the power supply, (optional) fans, chassis manager, and I/O interfaces. A SOSA-aligned system should have a chassis manager. The chassis manager can be used to monitor the power supply and fans (if the system has fans). It can also talk to SOSA boards in the system via the system management bus on the backplanes. Depending on the features of the chassis manager, it can perform sequential power-ups of the cards, graceful shutdowns, etc. Utilizing a RESTful interface, the newer generation web-based API is definitely preferred to RMCP, which is still optional.  While a plug-in type for the chassis manager is possible, obviously this takes up one of the backplane slots. Figure 3a. Another popular option is a mezzanine-based chassis manager that does not consume a slot. It fits behind the backplane and securely mounts to it. (See Figure 3b.)

Figure 3a:  Chassis management is a requirement for most SOSA-aligned enclosure systems. A chassis manager can come in a pluggable form, but this consumes a slot.

Figure 3b: A mezzanine-based approach can reside beneath the backplane and be mounted to it, this saves slot space inside the enclosure.

RF Interfaces

A key element of the chassis is the I/O interface. SOSA has optional connector interfaces that meet the “customer” end of the performance demand. There are various options for RF, optical and hybrid interfaces from the backplane to the plug-in card.  See Figure 4 for an example. On the front (or rear) panel end, there are also a variety of MIL 38999 connectors for the RF channels with contacts supporting up to 70 GHz and are designed to terminate to standard .047” and .086" semi-rigid and flexible cables (in the NanoRF version for example). There are additionally several other position counts and styles for these RF interfaces. Other times, discrete SMA interfaces will be placed on the front (or rear) panel of the enclosure. As SOSA encourages technology advancement, the types and interfaces (and of course performance capability) will likely morph over time. That said, VITA is the arbiter of maintaining and upgrading many elements of the core standard on which SOSA is based. VITA is always mindful as an organization, where possible, to maximize backward compatibility and longevity of its standards. In fact, the VME specification is still being used in a decent number of military systems today even though the original specification was ratified nearly 40 years ago.  

Higher Backplane Speeds, Higher Performance

Another element of the SOSA initiative is to maximize performance. For the backplane/chassis supplier, this means faster speeds across the backplane and more advanced cooling requirements. A few years back PCIe Gen3 speeds (8 Gbaud/s) across the backplane were commonplace and 40GbE (4x lanes of ~ 10Gbaud/s) was the higher speed exception. With SOSA, 40GbE is often the lower speed option with PCIe Gen4 (16 Gbaud/s) and 100GbE (4x lanes of ~ 25Gbaud/s). These speeds require advanced routing techniques, high-grade PCB material that is more expensive, higher-speed connector options of the VPX connector, back-drilling of vias, and more. The routing challenges are exacerbated by the optical and RF connector module that may be installed in the backplane, particularly when the P2 connector (and that routing space) is consumed in a 3U OpenVPX backplane with these connectors.  

While the higher speeds bring backplane routing challenges, the enclosure cooling capability is also being taxed. Many SOSA-aligned boards exceed 125 W in a VITA 48.2 conduction-cooled board format. Cooling these modules often requires creative solutions such as having airflow over the fins of a conduction-cooled card mat for a 19” MIL rugged rackmount enclosure as shown in Figure 4 or in a rugged ATR chassis. For more advanced cooling, there are options for airflow through the module (AFT), airflow over fins on the module itself (Air Flow By or AFB) and liquid cooling either through the module or through the sidewalls of the enclosure.

Figure 4: A card mat that employs “fins” to dissipate heat can be employed in a chassis, keeping the airflow on the outside of the card cage, but providing a way to cool the hotter modules in a system.

Power and I/O Affects

Another change was to make the power simpler and more consistent.  While there are various power input options, the power output for SOSA power supply units (PSUs) is limited to 12 V, with some 3.3 V AUX. In order for the PSU to meet SOSA requirements, it should have the capability to “speak” with the chassis manager via an Intelligent Platform Management Controller (IPMC).  

While we have discussed the speed, cooling, and chassis management of these systems, there is a more elusive concern that can sneak up on you. With the higher performance systems and RF/optical interfaces in the chassis, the I/O space on the front (or rear) panel can often be constrained. For the MIL 38999 interfaces, there are standard spacing guidelines that should be followed so that a gloved technician can access the cabling. With SWaP (Size, Weight, and Power) restrictions for most systems, there is often only so much space available for the size of the enclosure. Therefore, the I/O requirements and spacing required should be closely reviewed for any design.  

Making Your Solution SOSA Aligned

SOSA is helping make the military computing system more efficient and effective by simultaneously advancing technology, but keeping a smaller subset of core options. The result is a powerful standard that maximizes the re-use of a core set of modules and minimizes integration, training, and support time. It would be advisable to check out the VITA specifications that were mentioned herein and joining the SOSA consortium is open to US companies.