QuickStudy: Preboot Execution Environment

Computerworld - Listen to the Computerworld TechCast: Preboot Execution Environment

Once upon a time, configuring or troubleshooting a user's computer meant that someone from IT or the corporate help desk had to actually visit that computer—with installation software, diagnostics and driver disks in hand—and deal with its problems one-on-one. That arrangement was practical for very small organizations, but for the IT administrator who had to support thousands of computers, it was a nightmare.

As those thousands of computers were increasingly interconnected via enterprise networks, designers came up with an answer. During the mid-1990s, researchers at Intel Corp., along with a wide range of hardware and software vendors, began promoting an open-standards specification called Wired for Management (WfM).

PXE harkens back to the era before all computers had internal hard disk drives. PXE most commonly involves booting a computer from firmware—i.e., a read-only memory or programmable ROM chip—rather than from magnetic media. Booting from firmware removes dependence on an electromechanical device (the physical disk drive), which enhances reliability, eliminates drive read errors and speeds up the boot process. PXE can also be used to boot a computer over a network.

Although WfM has been superseded by newer management standards, such as Intel's Active Management Technology, the capabilities that PXE enabled are still valuable tools for network administrators.

PXE is geared toward auto-mated, unattended man-age-ment of user PCs and workstations. It is based on industry-standard Internet protocols, including TCP/IP and Dynamic Host Configuration Protocol (DHCP). A PXE-enabled PC typically has a network interface card (NIC) that remains active and connected to the LAN even when the computer itself is powered down. It listens to LAN traffic for a special data sequence—the PC's unique media access control address repeated six times. When the NIC receives this "magic packet," it powers up the PC. For this to occur, wake-on-LAN must be specifically enabled in the PC's firmware BIOS. Wake-on-LAN can be problematic for some very old PCs, because it requires a power connection between the NIC and the motherboard—a definite hardware requirement that can't be fudged with just a BIOS update. This isn't a problem for most enterprise PCs, however.