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WDS is an operating system deployment solution that allows new bare-metal or existing computers to boot up from a network-stored Windows PE boot image, run a WDS client, choose a Windows Imaging (WIM)-based installation image, and install it.

That installation image must be created. You can use any of the solutions that create a WIM file, such as WAIK, or event use the default installation images on the Windows 7 media. WDS, much like cloning solutions such as Ghost, will allow you to capture an image from a generalized template computer. The new WIM file will be captured and stored on the WDS server, available to be deployed to other computers.

WDS depends on a technology that was created by Intel called Pre-boot Execution Environment (PXE). PXE allows computers to boot up using their network cards instead of a hard disk, floppy drive, or USB device. The computer that is to be captured or deployed is booted up using PXE. This requires the following:

The hardware is capable of supporting a PXE boot --> The computer must have a network card that is Net PC/PC98 compliant. It is the network card that is providing the boot-from network functionality. Computers have provided this support for many years.

The BIOS is configured to allow a PXE boot --> Your computer may have the option for booting from PXE disabled. You will see a prompt to press a key, such as F12, to boot from the network during the machine’s POST, if it is enabled. You should enable the boot from network option in the BIOS of any machine that you will deploy operating system images to using WDS.



The computer will start the PXE client. This will broadcast on the network. It requires responses from two services:

• DHCP --> The computer is booting up using bare metal (it does not any operating systems installed). It will require a network configuration to communicate with the WDS server. This IP configuration will be provided by a DHCP server. This means that the broadcast domain (or subnet) that the client is booting up on must have a DHCP server that can respond to the client with a valid IP configuration for the current network.

• PXE Server --> The PXE is a component of the WDS server. It is made up of one (by default) or more (provided possibly by third parties) providers that will respond to PXE clients. This will initiate the next phase of the bootup process.

The PXE server is what is interesting here. The broadcast sent by the client is in the form of a DHCPDISCOVER packet with PXE options on UDP port 67.

Note: It should be noted that UDP 4011 will also be used if the PXE server is running on the same server as DHCP because both services cannot share UPD 67.



This packet will be ignored by DHCP servers. The PXE server will respond with a DHCPOFFER including the following:

• The IP address of a PXE boot server

• A PXE boot menu

• A timeout for the user to respond to the prompt

As said before, when a PXE client starts, it requires two things: an IP address and a Network Boot Program (NBP). The PXE client sends out a DHCP Discover; if the DHCP server and WDS server are on separate boxes, the PXE client receives two offers from the network: One for an IP address:



One for an NBP (the NBP offer would have no IP address):



The client accepts one of the IP address offers it received. Ideally, the PXE client accepts an offer that has both an IP address and a NBP, but if this is not offered, it instead accepts the IP address-only offer:





After it has the IP address, it sends out a new DHCP request for a NBP, which uses its new IP address and responds with a boot to use:

The PXE client then uses TFTP to download the specified NBP, and the client is prompted to press F12 to network boot (unless the NBP was changed to the non-F12 version).

The story is slightly different if the WDS server is also a DHCP server. In that case, the offer received by the client contains both an IP address and a NBP, which the client requests and is subsequently acknowledged by the server. At that point, the PXE client begins the NBP download.

The PXE boot menu, which is configured using the settings in the PXE server DHCPOFFER, will be presented to the user sitting at the client machine. A list of possible boot images will be presented if there is more than one suitable boot image. The process will continue when the user sitting at the client responds to the boot menu.

The PXE client will use TFTP to download the boot image that you have chosen from the WDS server. TFTP is the network protocol used for downloading all files during network boots, including the boot image.

TFTP is an inherently slow protocol because it requires one ACK (acknowledgment) packet for each block of data that is sent. The server will not send the next block in the sequence until the ACK packet for the previous block is received. As a result, on a slow network, the round-trip time can be very long. However, this is improved in Windows Server 2008 because of TFTP windowing. TFTP windowing enables you to define how many data blocks it takes to fill up a window. The data blocks are sent back to back until the window is filled, and then an ACK packet is sent. The result is fewer ACK packets and much faster download times for the client.

The boot image will download and boot up. The WDS client will now start up. This will allow the user to authenticate with the WDS server. If the WDS client is configured to deploy an image, then the user will select an installation image to be downloaded by the WDS client and installed.