PC Expansion and Interconnect Technologies

The entrants in this category are PCI, PCI-X and InfiniBand. The latter two specifications were devised in hopes of replacing the aging PCI architecture.

It's true that PCI was updated with a 66-MHz bus and 64-bit path for the server and workstation market in the past several years. However, the speeds of today's processors and peripheral components, with Gigabit Ethernet and Fibre Channel, have left us with some woefully inadequate I/O. PCI-X, the latest iteration of the PCI architecture, sports a 133-MHz bus and a 64-bit data path. This PCI technology and its generational updates are going to carry us for the next several years as updates to this kind of technology occur at a much slower pace than for processors. And that's actually a good thing. Changes to this kind of architecture can be tricky, and a slow, measured pace in this area lets processor technology proceed at its customary breakneck pace.

PCI-X DDR (Double Data Rate) will transmit twice on one clock cycle, similar to DDR SDRAM, and PCI-X QDR (Quad Data Rate) will transmit four times on one clock cycle. The bit width will remain 64 and the clock rate 133 MHz. The PCI-X QDR will be primarily for graphics cards and will replace the AGP (Accelerated Graphics Port) 4x standard. The entire point of PCI-X DDR and PCI-X QDR is to ensure a smooth transition to 3GIO (Third-Generation I/O).

Because of the component and manufacturing cost of implementing PCI-X on low-end, small-margin commodity machines, the same old boring PCI 2.3 standard, with its 33-MHz bus and 32-bit data path, will remain the standard on desktops until 3GIO arrives.

3GIO, the latest and greatest implementation of PCI, comes from the PCI-SIG (Special Interest Group). While current PCI designs are parallel, the main advantage of 3GIO is its serial interconnect. Originally known as Arapahoe, this design was started by the Intel-sponsored Arapahoe Working Group and came under criticism as being too closely tied to Intel. It was subsequently given to the PCI-SIG, which has referred to the technology by the code name 3GIO.

The next generation of PCI standard, PCI 3.0, will be a serial standard and will eventually replace both PCI-X and PCI 2.3. Although PCI 3.0 exists only on paper now, speculation is that 3GIO will be the PCI 3.0 standard. No confirmation of that yet, but it looks likely.

One of the vexing notions that has arisen over the years is that 3GIO will compete with InfiniBand, IEEE 1394 (FireWire) and USB. The standard is intended to provide expansion-bus interconnects to these interfaces, not replace them. Organizations, individuals and companies that suggest 3GIO will replace these standards have simply not done their homework. Is it technologically possible for 3GIO to do so? Yes. Will it? No.

The PCI-SIG does not intend to compete directly with that kind of peripheral interconnect, and the proliferation of FireWire and USB makes it unlikely that anyone would even try. On the InfiniBand front, 3GIO could be considered a competitor to the very low end. Again, however, that is not the intention of the standard. The PCI-SIG is putting quite an effort into making a clear road map for the next decade and making sure the backward-compatibility factors are maintained.

To that end, the PCI-SIG is continuing to support PCI 2.3 to help ease the transition to PCI 3.0. PCI 2.3 is a dual 5-volt/3.3-volt standard; the PCI-SIG intends PCI 3.0 to be all 3.3 volt.

The 3GIO specification has an impressive group of backers: Compaq Computer Corp., Dell Computer Corp., IBM, Intel and Microsoft. The group envisions the widespread adoption of PCI-X for servers and workstation-class machines in the near term, with conventional PCI still being used in desktop and consumer machines and 3GIO as the heir apparent. However, 3GIO will have a different pin-out and form factor, details of which are still in the works.

InfiniBand is often mistakenly touted as a replacement for I/O. It certainly gets lots of press. But the InfiniBand specification is intended, initially, to connect servers, networking equipment and storage. It will also be used to let processors talk to each other, creating parallel processing clusters or processing area networks. For the near term, that's as far as it will go: a switched point-to-point I/O fabric. It could, in theory, be used for everything from SANs (storage-area networks) to connecting legacy PCI card expansion buses. (For more on InfiniBand, see "To InfiniBand ... And Beyond", March 5, 2001).