Backplanes and Switched Fabrics
Adding Flexibility to ATCA for Non-Telecom Applications
Although developed for telecommunications, the PICMG 3.x, or ATCA, specification lends itself to a number of non-telecom applications. To enable these, however, flexibility in the packaging options is essential.
WALTER SCHINDLER, ELMA ELECTRONICS
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Design flexibility is one of the most important issues in electronic packaging. A packaging supplier must be an expert in the bus structure, the latest specifications, thermal management, electromagnetic compatibility, components in the marketplace and often other elements like shock/vibration, integration, etc. Still, nearly every customer wants the products a slightly different way. This is very true for ATCA—just about every solution is different. Therefore, packaging must offer solutions that are as flexible as possible to reach a wide range of applications. In providing flexibility for ATCA, some auxiliary benefits have arisen. One of them is the number of “non-telecom” opportunities that are emerging.
The PICMG 3 group of specifications was specifically geared for telecom central office (CO) applications. The features of ATCA were structured to be competitive with the wide range of custom solutions in the market for the CO. These include:
• 8U x 280 mm boards, with wider 6HP spacing
• Bulk -48 VDC distribution
• Use of serial interconnects for bandwidth
• Flexible user I/O
• Mandatory shelf management—IPMB-based
• High level of service reliability (99.999% and greater) through the integration of features for redundancy, serviceability and manageability
• Low-cost sheet metal solution (in high volumes)
Although developed for high-volume CO applications, the specification was not as conducive to prototyping. Therefore, development systems were created that offered more flexibility. One of the main results of this action was the incorporation of AC power supplies in ATCA. This allowed great flexibility in prototyping, development and demonstrations. The AC power supply is portable and can be plugged into a conventional wall outlet.
Suddenly, these standards-based system platforms were out on the market with tremendous bandwidth capability, geared for communications applications and had shelf management options. It didn’t take long for customers in the enterprise computing space to take notice.
Carrier-Grade vs. Enterprise
There are no set guidelines for which sets of features customers prefer in carrier-grade vs. enterprise applications. However, one can postulate the feature sets that are more logical from one to the other. Many of the enterprise applications demand a lower cost model for that market. For example, shelf management is required for the ATCA specification. But, in some enterprise applications, it is not seen as worth the extra cost.
Offering pluggable connectors on an ATCA backplane is one way to facilitate performance-enhancing features and lower-cost assembly. With pluggable shelf managers, fan trays and power entry modules, full hot swappability is a reality. Providing these items on a monolithic ATCA backplane helps reduce the number of components and the amount of cabling, thus helping reduce costs (Figure 1).
Conversely, a standard backplane with cabling for the shelf managers, etc., offers more flexibility in the design. A shelf manager interface board can allow the shelf manager to be placed nearly anywhere in the system where there is room and its presence does not affect the airflow. By having multiple versions of the backplanes, a developer gets the benefit of choosing which configuration is best for his application (whether carrier-grade or enterprise), often saving costs in the process.
There are other considerations for offering flexibility in ATCA backplanes for various applications. Some high-end applications may demand the super-reliable nature of a star topology for IPMB routing. A star design offers more reliability for shelf management vs. a bused configuration. However, this could be overkill for some applications. In fact, many non-telecom applications don’t use the shelf managers. So, offering various designs for the IPMB routing can be beneficial. The same reasoning goes for routing various topology options for backplanes.
A replicated mesh backplane can also facilitate implementations for dual-star or standard full-mesh topologies. Therefore, the backplane can be all-in-one, offering various configurations in one flexible unit. The drawback is between the mesh topology and the dual star. The mesh usually requires more layers, thus higher costs. In some applications it makes sense to develop the dual star separately, without routing the mesh configuration. This is particularly true for larger slot sizes, like the 14-slot backplane.
Chassis can also be made to offer flexibility for non-telecom applications. The most obvious change is providing the capability to plug into conventional AC wall outlets. Many of the chassis have ample spacing to provide an AC power supply(s) above the card cage. For example, a 2U chassis can have a 2-slot ATCA backplane and fit an AC power supply, shelf manager or both (Figure 2). A 3U AC unit has room for a 4-slot backplane, and a 5U AC unit can fit a 5-slot backplane. Many non-telecom customers desire the performance and standardization of ATCA, but need the power supply option.
Redundancy is often very important for CO applications. Full redundancy for the shelf managers, power entry modules and fan trays, as in the backplane shown in Figure 1, can be critical. But for many non-telecom applications, it is, again, overkill. A flexible design can provide the redundant options, but not make them a requirement. In order to achieve redundant cooling (single fan failure), the fan trays can be arranged in a push/pull configuration. If redundancy is not required, the pull fan trays can be omitted. Even the filter tray is not needed for certain applications. When cost is the deciding factor, a modular design can offer many possibilities.
Having a modular chassis design where building blocks can be added or taken off is probably the most effective way to cater to the widest range of applications. There are OEMs who need the AC feature and even require dual redundant power supplies. This is certainly a possibility, and with flexible configurations for the shelf manager, like the interface board, they can all fit within many of the chassis, without increasing the height of the chassis.
Of course, the size of the system platform is an important consideration as space is generally at a premium. It is important to have a choice of a wide range of standard chassis in various heights and with vertical and horizontal card orientations. The modular designs with the same building blocks offer the flexibility to react to any custom demand in the shortest period of time.
Finally, the shelf management connection can provide more flexibility. By offering the same pinout and connector type as the popular Intel Colorado Springs shelf manager, Elma is able to plug its IPM Sentry shelf manager into the same spot, even though the widths are different.
It has been interesting to see the different requirements and designs for various applications. On one side is the central office (CO), access and edge, and on the other side, enterprise, general communications and even some military interest.
One of note was an enterprise application using an ATCA chassis with an AC power supply. The design was achieved with a standard 2U ATCA AC platform and a shelf manager was not needed. Like many other applications, the company was not using the full 200W per slot. This provides more leeway in cooling configurations. Although they only had about 100W per slot, thermal simulations confirmed the cooling was more than adequate (Figure 3). The simulation allows the chassis designer to modify the location of air baffles, fan intake/exhaust configuration, CFM vs. static pressure, optimize use of the shelf manager and much more.
Another interesting design was an access edge application using a 2U ATCA AC platform with the power supply mounted on the rear panel. This saved space, and in this particular design, provided better airflow. RTMs were not required. A shelf manager was required and was cabled with a shelf manager interface board (Figure 4). Yet another application required a system platform to accept two front boards and two RTMs, redundant and pluggable fan trays, a filter tray, a 250W power supply, PEM at the rear and a shelf manager. With a flexible design, all of these elements can be packaged into a 2U-high chassis.
With a flexible and modular design, packaging companies can morph different and perhaps unusual designs into their standard product offering. This approach also helps the packaging company potentially stretch into new applications. Incorporating flexibility into ATCA systems has allowed the system platform designer to move into new arenas perhaps not originally conceived when the specification was first penned. Some of the interesting applications include enterprise computing, general commercial and military communications. New design requirements are emerging every day. Incorporating flexibility into designs will be a key differentiator in the future for AdvancedTCA.