Preboot Execution Environment

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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).



Aimed at reducing the total cost of ownership, WfM allowed IT managers to interact remotely with PCs for monitoring, updating and configuring, using standardized communications software and remote management applications. From WfM to PXE Implementing WfM called for standardized hardware (including circuitry, BIOS, memory, power supplies and network interface cards) in user PCs. WfM covered a range of PC networking technologies, including the Desktop Management Interface, remote wake-up (also called wake-on-LAN or service boot) and the Preboot Execution Environment (PXE).

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Although WfM has been superseded by newer management standards, such as Intel's Active Management Technology, the capabilities that PXE enabled are still valuable tools for network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

Once the local PC wakes up, it starts up the network card and configures itself. Making use of PXE requires an appropriate server infrastructure. When a PXE client boots, it must do two things. First, it obtains an IP address from a DHCP server. Unfortunately, PXE calls for some options that may not work on all DHCP servers, so most software that supports PXE also includes a proxy DHCP service. This proxy service doesn't offer IP addresses directly but does allow DHCP operation.

After connecting to the DHCP server, the system locates a PXE boot server that will send it the appropriate files from which to boot. The DHCP server provides a boot file name, and the PC then downloads it from a Trivial File Transfer Protocol (TFTP) server.

Once PXE is enabled—anytime the PC boots up—an on-screen message appears, offering the user the choice of booting to PXE or continuing with the normal boot sequence from the local hard disk or optical media. PXE offers a menu of boot options that can include a variety of maintenance and diagnostic tools that do things like scan for viruses, check the integrity of hard drives, inventory installed software, update drivers or even install an entirely new operating system on the PC. All this can be done remotely and largely in an automated fashion, with little or no hands-on intervention required.


PXE Boot Process

PXE Boot Process

Kay is a Computerworld contributing writer in Worcester, Mass. You can contact him at

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