[Technology Report]
Hot-Swap Hardware And Software Hurdles Continue To Fall
CompactPCI hot-swap standards are making steady progress, but new interfaces loom on the horizon.
After years of design-engineer frustration, hot-swap and live-insertion technologies are gradually evolving from expensive, special-purpose solutions to mainstream design alternatives. Thanks to standardization efforts from board vendors on both the hardware and software sides, hot-swapping designs, to at least some extent, are becoming easier and more reliable.
But obstacles still exist for full and transparent card swapping. Vendors have to surrender proven proprietary solutions for new and sometimes less-functional standardized ones. Adding features to standards becomes more difficult. It's not enough to have an operable hardware interface. System software and device drivers have to be written to dynamically reconfigure for the removal and insertion of cards.
How important is hot swapping in embedded design today? It depends on the industry. In telecommunications, the expectation of 100% uptime is the rule. So almost half of the system designs require hot swapping, according to a survey conducted by Venture Development Corp. (VDC), Natick, Mass., and presented at the 2000 Bus and Board Conference held in San Jose, Calif., this past February. In other fields, it's less critical but still quite important.
More importantly, this study expects hot-swap needs to be significantly higher in the future. Almost all industries predict that at least 25% of their applications will employ hot-swap technologies (Fig. 1).
High availability and fault tolerance are the primary reasons behind system requirements for hot swapping. As society becomes more dependent upon computers and automation, systems need as close to 100% uptime as possible. This is particularly critical in telecommunications and factory-floor operations. It's also a growing demand for systems handling data, transportation, and medical networks.
Plus, more complex systems tend to have lower reliability. Individual components are actually growing more dependable. But having more components in systems tends to reduce overall reliability. Hot swapping compensates for what can be a lower mean-time-between-failure rate for the whole system.
There are two fundamental purposes for providing hot-swap features on a bus interface. The obvious reason is to repair a faulty or damaged card without having to shut down the system. Supporting hot swapping and live insertion for repair assumes that when a card or module goes bad, the system can detect the failure and take the affected module offline. It can then notify the system administrator or repair facility that the failure occurred.
Thus, it's assumed that either the card wasn't a critical component or that some redundancy enables the system to perform at least some of its functions without it. If the component is so critical that its failure is automatically going to crash the system or render it unable to perform its primary tasks, then hot swapping may not make sense.
When the card or module is replaced, the system must be able to detect the replacement, prepare the new module for system operation, and then bring it smoothly back online. This process can be either driven by the system operator or done automatically.
Hot Swap For Reconfiguration Using hot swapping for system reconfiguration, on the other hand, can be done as a replacement or an enhancement. It also can be some combination of the two. The system software must be able to recognize new features that have been added by the card and take advantage of them. At the very least, this process demands updated device drivers in order to access those features.
Hot-swap capability has been a cornerstone of CompactPCI thinking since the beginning of its standards effort. That goes back to late 1996, when a formal hot-swap subcommittee of the CompactPCI group was formed. To come up with a successful standard, the group had to factor in interoperability between platform vendors and suppliers of operating systems (OSes)and other system software, as well as adapter-board manufacturers.
The connector interface would stay the same. But the subcommittee had to decide whether it should use a passive or active CompactPCI backplane for the cards (Fig. 2). This decision would determine all other design aspects of the hot-swap specification.
The passive-backplane approach could have fragmented the technology. There would be hot-swap cards and non-hot-swap cards of the same design, because bus-isolation and power-management circuitry would have to reside on the adapter cards themselves. Using an active-backplane approach would move those functions from the adapter card to the backplane. Adapter cards would then be universal and plug into either a standard or a different hot-swap-enabled backplane. The outcome was a nod in favor of an active-backplane design.
The resulting specification defines six classes of hot-swap-compliant software. It allows initial deployments with minimal requirements for live insertion and extraction. But these will evolve over time to reach increased complexity and sophistication. Hot swapping accomplishes this goal with a two-level software hierarchy. At the top level, software is classified as being either for general or specific use. The bottom layer has three hot-swap performance grades. They're defined when applied in the specific- or general-use categories.
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