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　　Although WfM has been superseded by newer management standards, such as Intel’s Active Management Technology, the capabilities that PXE enabled are still valuable tools for network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

　　過去，對(duì)用戶的計(jì)算機(jī)進(jìn)行配置或查錯(cuò)，就意味著IT部門或公司幫助臺(tái)實(shí)際要派人帶著安裝軟件、診斷工具和驅(qū)動(dòng)程序盤上門，一個(gè)一個(gè)地解決問題。這樣的安排對(duì)于非常小的機(jī)構(gòu)是可行的，但對(duì)于必須支持?jǐn)?shù)以千計(jì)計(jì)算機(jī)的IT管理員來說，這無疑是一場(chǎng)惡夢(mèng)。
　　隨著那些數(shù)以千計(jì)的計(jì)算機(jī)通過企業(yè)網(wǎng)絡(luò)互連起來，設(shè)計(jì)師就有了答案。在上世紀(jì)90年代中期，英特爾公司的研究人員與眾多軟硬件廠商一起，開始促進(jìn)一項(xiàng)叫連線管理（WfM）的開放標(biāo)準(zhǔn)規(guī)范。
　　為了減少總體擁有成本，WfM允許IT管理人員利用標(biāo)準(zhǔn)的通信軟件和遠(yuǎn)程管理應(yīng)用程序與PC機(jī)進(jìn)行遠(yuǎn)程交互，以便對(duì)PC進(jìn)行監(jiān)視、升級(jí)和配置。從WfM到PXE（預(yù)引導(dǎo)執(zhí)行環(huán)境）實(shí)現(xiàn)的WfM，都要求用戶的PC機(jī)使用標(biāo)準(zhǔn)化的硬件（包括電路、BIOS、內(nèi)存、電源和網(wǎng)卡）。WfM覆蓋了大多數(shù)的PC聯(lián)網(wǎng)技術(shù)，包括桌面管理接口、遠(yuǎn)程喚醒（也叫局域網(wǎng)喚醒或服務(wù)引導(dǎo)）以及預(yù)引導(dǎo)執(zhí)行環(huán)境（PXE）。
　　PXE讓你回溯到所有的計(jì)算機(jī)都有內(nèi)藏式硬盤機(jī)之前的年代。最常見的PXE是從固件、即只讀式存儲(chǔ)器或可編程ROM芯片，而不是從磁介質(zhì)引導(dǎo)計(jì)算機(jī)。從固件引導(dǎo)計(jì)算機(jī)不用依賴電磁設(shè)備（物理的磁盤機(jī)），這就增強(qiáng)了可靠性、消除了磁盤機(jī)的讀出錯(cuò)誤以及加快了啟動(dòng)的過程。PXE也用于通過網(wǎng)絡(luò)啟動(dòng)計(jì)算機(jī)。 　　
　　雖然WfM已被更新的管理標(biāo)準(zhǔn)，如英特爾公司的主動(dòng)管理技術(shù)所取代，但對(duì)網(wǎng)絡(luò)管理員而言PXE實(shí)現(xiàn)的功能仍是有價(jià)值的工具。
　　PXE適合用戶PC機(jī)和工作站自動(dòng)的、無人值守的管理。它是基于行業(yè)標(biāo)準(zhǔn)的因特網(wǎng)協(xié)議，包括TCP/IP和動(dòng)態(tài)主機(jī)配置協(xié)議（DHCP）。支持PXE的PC機(jī)通常有一網(wǎng)卡，即便計(jì)算機(jī)本身是關(guān)機(jī)的時(shí)候該網(wǎng)卡仍是活躍的并與局域網(wǎng)相連。它監(jiān)聽著局域網(wǎng)的流量，看是否有一特殊的數(shù)據(jù)序列，即該P(yáng)C機(jī)惟一的介質(zhì)訪問控制地址重復(fù)6次。當(dāng)網(wǎng)卡收到此“魔幻包”時(shí)，就開啟PC。為使此能發(fā)生，必須在該P(yáng)C的固件BIOS中專門實(shí)現(xiàn)局域網(wǎng)喚醒功能。對(duì)某些非常陳舊的PC來說，局域網(wǎng)喚醒可能成問題，因?yàn)樗枰W(wǎng)卡和主板之間有電源連接，這個(gè)必不可少的硬件要求不是能用BIOS升級(jí)而蒙混過關(guān)的。但對(duì)于多數(shù)企業(yè)PC機(jī)而言，這不是個(gè)問題。
　　一旦本地的PC喚醒，它就啟動(dòng)網(wǎng)卡和進(jìn)行配置。使用PXE需要合適的服務(wù)器基礎(chǔ)實(shí)施。當(dāng)PXE客戶機(jī)啟動(dòng)時(shí)，它必須做兩件事。首先，從DHCP服務(wù)器獲得IP地址。不幸的是，PXE要求的某些選擇不是在所有的DHCP服務(wù)器上都行得通，因此多數(shù)支持PXE的軟件還包含一個(gè)代理DHCP服務(wù)。這個(gè)代理服務(wù)不直接提供IP地址，但允許所有的DHCP操作。
　　與DHCP服務(wù)器相連之后，系統(tǒng)確定PXE啟動(dòng)服務(wù)器的位置，從要啟動(dòng)的PC給該服務(wù)器發(fā)送相應(yīng)的文件。DHCP服務(wù)器提供引導(dǎo)文件名，然后PC從簡(jiǎn)單文件傳送協(xié)議（TFTP）服務(wù)器上下載。
　　一旦支持PXE，在任何時(shí)候PC啟動(dòng)后，屏幕上就會(huì)顯示信息，供用戶選擇是引導(dǎo)到PXE還是從本地磁盤或光盤序列引導(dǎo)。PXE提供的引導(dǎo)選擇菜單包括各種診斷和維護(hù)工具，進(jìn)行諸如病毒掃描、檢查硬盤機(jī)完整性、清點(diǎn)已安裝的軟件、更新驅(qū)動(dòng)程序或在PC機(jī)上安裝全新的操作系統(tǒng)。所有這些都是遠(yuǎn)程地、而且大部分是自動(dòng)地完成，幾乎或者就是不需要人工干預(yù)。