PCI Express: I/O Moves Into the Fast Lane

Computerworld - As PC microprocessors pass the 3-GHz clock-speed mark and Gigabit Ethernet begins its move onto the corporate desktop, it's easy to forget that the typical expansion bus in desktops sold today still runs on 11-year-old technology.

The 32-bit, 33-MHz Peripheral Component Interconnect (PCI) bus specification, which debuted in 1992, remains the standard. PCI supports an aggregate bandwidth of just 133MB/sec. (1Gbit/sec.), and that bandwidth must be shared among all devices on its multidrop bus. "I/O has moved at an anemic pace," says Tom Bradicich, chief technology officer for IBM's xSeries server line.

An emerging I/O standard, PCI Express, is about to speed things up. The current specification, supported by the PCI-SIG vendor consortium and promoted heavily by Intel Corp., succeeds PCI's parallel bus design with a high-speed, point-to-point serial interconnect technology that Intel and others say will be both faster and more reliable. It will deliver an aggregate bandwidth ranging from 5Gbit/sec. (500MB/sec.) to 160Gbit/sec. (16GB/sec.). And because it's serial, the technology will work more efficiently with other emerging high-speed serial technologies such as Serial ATA and InfiniBand. It will also offer hot-plug and hot-swap support and enable direct peer-to-peer communication between attached devices without involving the processor chip set.

But PCI-SIG isn't just promoting PCI Express as a desktop PCI expansion bus replacement. The consortium is positioning it as a general-purpose I/O technology for desktops, workstations and servers that will also speed internal chip-to-chip and graphics adapters. Intel plans to release a PCI Express chip set for servers, code-named Lindenhurst, early next year, with a PC chip set to quickly follow.

On the desktop, the first PCI Express add-in devices are likely to be graphics chips because the Accelerated Graphics Port (AGP) standard has run out of gas, says Jim Pappas, director of technology initiatives for Intel's enterprise platform group. "All new graphics development [beyond AGP 8X] is being done on PCI Express," he says, noting that the first graphics products could be available by early next year.

Although vendors generally view a migration to serial I/O technology as inevitable, PCI Express' near-term prospects beyond high-end graphics are less clear. IT managers say desktops don't yet need the extra I/O bandwidth. "People are not taxing the equipment they have," says Scott Newton, director of IT at Otis Spunkmeyer Inc. in San Leandro, Calif.

PCI Express also represents a fundamental technology shift in that its physical interfaces, which include desktop expansion slots, a new PC Card format (code-named Newcard) and Mini PCI Express slots, won't accept existing PCI devices.

In the server world, where I/O bandwidth is more of an issue, PCI Express faces competition from enhanced versions of the PCI bus standard, including the emerging, 2GB/sec. PCI-X 2.0, due out later this year. But vendors are split on whether to stay on the PCI-X road for backward compatibility or go directly to PCI Express.