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Determine Bandwidth Options to Use

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server Update Services

WSUS allows you to shape the deployment to fit your organization's bandwidth needs. The decisions you make about how to synchronize with Microsoft Update have a dramatic effect on the efficient use of bandwidth. Read the following sections to understand WSUS features for managing bandwidth.

WSUS enables you to download update metadata before downloading the update itself. With deferred download, updates are downloaded only after the update has been approved, which saves bandwidth and WSUS server disk space. You can test the files prior to deploying them on your network, and client computers download the updates from the intranet. Microsoft recommends deferring the download of updates (the default WSUS configuration), since it makes optimal use of network bandwidth and disk space.

Deferred Downloads of Updates

If you have a chain of WSUS servers, it is recommended that you do not chain them too deeply, for the following reasons:

  • In a chain of WSUS servers, WSUS automatically sets all downstream servers to use the deferred download option that is selected on the highest upstream server—in other words, the server that is directly connected to Microsoft Update. However, you may change this configuration (for example, to keep an upstream server doing full synchronization, while downstream servers defer their downloads).

  • If you have deferred downloads enabled and a downstream server requests an update that has not been approved on the upstream server, the downstream server’s request triggers a download on the upstream server. The downstream server then downloads the content on a subsequent synchronization, as shown in the "Deferred Downloads Using Multiple WSUS Servers" illustration. If you have a deep hierarchy of WSUS servers using deferred downloads, there is greater potential for delay as content is requested, downloaded, and then passed down the chain.

Deferred Downloads Using Multiple WSUS Servers

If you chose to store updates locally during the WSUS setup process, deferred downloads are enabled by default. You can change this option manually. See Advanced Synchronization Options for step-by-step procedures.

WSUS allows you to choose only the updates your organization requires during synchronizations. You can filter synchronizations by language, product, and classification of update.

In a chain of WSUS servers, WSUS automatically sets all downstream servers to use the update filtering options that are selected on the server directly connected to Microsoft Update. You can change this configuration to get a subset of languages on a downstream server, or you can defer the download of updates. Deferring downloads is described in Deferring the download of updates.

By default WSUS downloads Critical and Security Updates for all Windows products in every language, as well as Office updates and Windows Defender virus definitions. Microsoft recommends that you limit languages to the ones you actually use in order to conserve bandwidth and disk space. To change language options, or to change product and update classification options, see Using the WSUS 3.0 Configuration Wizard.

You can use express installation files to limit the bandwidth consumed on your local network, at the cost of bandwidth consumption on your Internet connection and disk space. By default WSUS does not use express installation files. To understand the tradeoff, you first have to understand how WSUS updates client computers.

Updates typically consist of new versions of files that already exist on the computer being updated. On a binary level these existing files might not differ very much from updated versions. The express installation files feature is a way of identifying the exact bytes that change between different versions of files, creating and distributing updates that include just these differences, and then merging the original file with the update on the client computer. Sometimes this is called delta delivery because it downloads only the difference, or delta, between two versions of a file.

When you distribute updates this way, there is an initial investment in bandwidth. Express installation files are larger than the updates they are meant to distribute. This is because the express installation file must contain all the possible variations of each file it is meant to update.

The upper part of the "Express Installation Files Feature" illustration shows an update being distributed with express installation files; the lower part of the illustration shows the same update being distributed without using express installation files. Notice that with express installation files enabled, you incur an initial download three times the size of the update. However, this cost is mitigated by the reduced amount of bandwidth required to update client computers on the corporate network. With express installation files disabled, your initial download of updates is smaller, but the full size of the download must then be distributed to each of the clients on your corporate network.

Express Installation Files Feature

The file sizes in the "Express Installation Files Feature" illustration are for illustrative purposes only. Each update and express installation file varies in size, depending on what files need to be updated. Further, the size of each file actually distributed to clients by using express installation files varies depending upon the state of the computer being updated.

ImportantImportant
Express installation files are often larger than the updates they are meant to distribute. On the other hand, it is always less expensive to distribute updates within a network using express installation files than to distribute full update files.

Not all updates are good candidates for distribution using express installation files. If you select this option, you obtain express installation files for any updates being distributed this way. If you are not storing updates locally, you cannot use the express installation files feature. By default, WSUS does not use express installation files. To enable this option, see Advanced Synchronization Options.

WSUS uses the Background Intelligent Transfer Service 2.0 (BITS) protocol for all its file-transfer tasks, including downloads to clients and server synchronizations. BITS is a Microsoft technology that allows programs to download files by using spare bandwidth. BITS maintains file transfers through network disconnections and computer restarts. For more information about BITS, see the BITS documentation on the MSDN site at http://go.microsoft.com/fwlink/?LinkId=79389.

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