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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).
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Although WfM has been superseded by newer management standards, such as Intel’s Active Management Technology, the capabilities that PXE enabled are still valuable tools for network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 automated, unattended management 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.
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预引导执行环境
过去，对用户的计算机进行配置或查错，就意味着IT部门或公司帮助台实际要派人带着安装软件、诊断工具和驱动程序盘上门，一个一个地解决问题。这样的安排对于非常小的机构是可行的，但对于必须支持数以千计计算机的IT管理员来说，这无疑是一场恶梦。
随着那些数以千计的计算机通过企业网络互连起来，设计师就有了答案。在上世纪90年代中期，英特尔公司的研究人员与众多软硬件厂商一起，开始促进一项叫连线管理（WfM）的开放标准规范。
为了减少总体拥有成本，WfM允许IT管理人员利用标准的通信软件和远程管理应用程序与PC机进行远程交互，以便对PC进行监视、升级和配置。从WfM到PXE（预引导执行环境）实现的WfM，都要求用户的PC机使用标准化的硬件（包括电路、BIOS、内存、电源和网卡）。WfM覆盖了大多数的PC联网技术，包括桌面管理接口、远程唤醒（也叫局域网唤醒或服务引导）以及预引导执行环境（PXE）。
PXE让你回溯到所有的计算机都有内藏式硬盘机之前的年代。最常见的PXE是从固件、即只读式存储器或可编程ROM芯片，而不是从磁介质引导计算机。从固件引导计算机不用依赖电磁设备（物理的磁盘机），这就增强了可靠性、消除了磁盘机的读出错误以及加快了启动的过程。PXE也用于通过网络启动计算机。
虽然WfM已被更新的管理标准，如英特尔公司的主动管理技术所取代，但对网络管理员而言PXE实现的功能仍是有价值的工具。
PXE适合用户PC机和工作站自动的、无人值守的管理。它是基于行业标准的因特网协议，包括TCP/IP和动态主机配置协议（DHCP）。支持PXE的PC机通常有一网卡，即便计算机本身是关机的时候该网卡仍是活跃的并与局域网相连。它监听着局域网的流量，看是否有一特殊的数据序列，即该PC机惟一的介质访问控制地址重复6次。当网卡收到此“魔幻包”时，就开启PC。为使此能发生，必须在该PC的固件BIOS中专门实现局域网唤醒功能。对某些非常陈旧的PC来说，局域网唤醒可能成问题，因为它需要网卡和主板之间有电源连接，这个必不可少的硬件要求不是能用BIOS升级而蒙混过关的。但对于多数企业PC机而言，这不是个问题。
一旦本地的PC唤醒，它就启动网卡和进行配置。使用PXE需要合适的服务器基础实施。当PXE客户机启动时，它必须做两件事。首先，从DHCP服务器获得IP地址。不幸的是，PXE要求的某些选择不是在所有的DHCP服务器上都行得通，因此多数支持PXE的软件还包含一个代理DHCP服务。这个代理服务不直接提供IP地址，但允许所有的DHCP操作。
与DHCP服务器相连之后，系统确定PXE启动服务器的位置，从要启动的PC给该服务器发送相应的文件。DHCP服务器提供引导文件名，然后PC从简单文件传送协议（TFTP）服务器上下载。
一旦支持PXE，在任何时候PC启动后，屏幕上就会显示信息，供用户选择是引导到PXE还是从本地磁盘或光盘序列引导。PXE提供的引导选择菜单包括各种诊断和维护工具，进行诸如病毒扫描、检查硬盘机完整性、清点已安装的软件、更新驱动程序或在PC机上安装全新的操作系统。所有这些都是远程地、而且大部分是自动地完成，几乎或者就是不需要人工干预。
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