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It was inevitable. The rising clock speeds of processors and the falling voltages of integrated circuits have caught traditional system design architectures in a pinch. The practice of bolting peripherals onto a wide, multi-drop bus is facing a challenge. Increasing clock speed has brought with it an increase in electromagnetic noise on circuit board traces. At the same time, falling voltages have lowered noise margins on logic circuits.

The combination is proving deadly for the traditional multi-drop bus architecture. It is hard enough to get these systems to work at the 3.3V logic levels and 133-MHz speeds typically found in a PCI-based system. Imagine the difficulty of making such systems work at processor clock speed, now pushing 1-GHz, with supply voltages less than 2V. If you can't imagine it, just ask someone struggling with making a Rambus memory system manufacturable.

The industry's reluctant response to the mounting problems of higher speeds and lower voltages is to switch architectures. The emerging approach is the switched-fabric architecture, which allows high-speed buses to operate with a point-to-point connection from source to destination. That connection switches between pairs of units on the bus as needed, allowing a variety of devices to interconnect, one pair at a time. Two switched-fabric structures have already arisen and started to gain industry backing: RapidIO and InfiniBand. Both of them have trade organizations backing their growth and adoption.

At first glance, the switched-fabric approach may look like it would have an impact on system software. However, unless the system utilized the multi-drop bus' ability to move data from one source to many destinations simultaneously (multicast), there is no immediate impact on system software. A multi-drop bus only provides one data path at a time, after all, so the presence of a switch in the path makes no logical difference.

If a system did use a multicast transfer mode, its designers will need to be careful in their adoption of a switched-fabric architecture. One possible approach would to be to design the system so that all devices needing multicast signals reside on the same switch, allowing the switch device to provide the multicast behavior locally.

The first generation of switched-fabric designs will be able to run software just as if they were a multi-drop system. System designers shouldn't be content with that approach, however. A switched-fabric architecture offers system performance enhancement opportunities beyond their ability to handle high speed and low voltage. Designed properly, they can multiply the effective bandwidth of the bus.

Because the switched-fabric buses are point-to-point through a multi-port switch, the system architecture allows multiple data transactions to occur simultaneously. The trick is assigning the various processors, memory blocks, and I/O devices to switching nodes cleverly. That requires an understanding of your system's data-flow paths. A modem could easily stream data to a disk drive, for example, while a processor is accessing main memory, if you've designed your fabric so that those two paths don't overlap.

To achieve this type of multiple-path utilization will require system designers to consider both hardware architecture and application software needs simultaneously. That may be a new mindset for some. Multi-drop designs effectively decouple the two. So long as all the resources the software will need were on the system, it didn't matter much how they were interconnected. To gain maximum performance in a switched-fabric system, however, the allocation of resources and pathways to avoid pathway conflicts becomes critical.

Switched-fabric designs are far from mainstream, and multi-drop architectures are far from dead. Just look at the flurry of recent activity with PCI-X, which will extend the utility of the PCI bus for several more years, at least. Even so, the rise of the switched-fabric architecture for high-performance system design is inevitable. Equally inevitable, the migration of system performance will make tomorrow's mid-range systems look like the high-end systems of today. So, system designers—sooner or later—will have to embrace the new approach. Now is the time to start building your foundation-level understanding of it.

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Design How-To

Drop Multidrop and Switch to Switched-Fabric

6/8/2000 04:00 AM EDT
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