SUS is the server-side component of the update process. On client systems, related components include the Automatic Update Client (AU) and the Background Intelligent Transfer Service (BITS).
Automatic Update Client
What is the Automatic Update Client?
Automatic Update Client enables the download and installation of Windows updates. If this service is disabled, a computer will not be able to use the Automatic Updates feature of Windows. The AU client component is installed by default along with operating systems “above” Windows 2000 SP3, Windows XP SP1, and Windows Server 2003.
When used in an SUS environment, the AU client will talk to the local SUS server and check for approved updates. If there are updates that have been approved and are needed by the client, then based on its AU options are configured (more on those options shortly) the client will install those patches.
If the AU client system is configured via Group Policy settings, all the options will be grayed out on the client system, as you see in the figure above. Normally, the AU client takes anywhere between 17 and 22 hours to check for updates on SUS SERVER. That said, this time is random and you can’t define it in the current version of SUS (SUS SP1). Just in case you’re curious, AU stores patches in C:\WUTEMP or C:\ProgramFiles\WindowsUpdate\wuaudnld.tmp while waiting for installation processes to complete.
Background Intelligent Transfer Service
What is the Background Intelligent Transfer Service?
The Background Intelligent Transfer Service (BITS) transfers files in the background using idle network bandwidth. If the service is stopped, features such as Windows Update will be unable to automatically download programs and other information. If this service is disabled, any services that explicitly depend on it may fail to transfer files. This is especially true if these clients do not have a fail-safe mechanism to transfer files (such as directly via Internet Explorer).
If an AU client detects that patches are available to be downloaded, it hooks up BITS to download those patches. The BITS service is configured for manual startup, and is started only when required.
If you want to dig a little deeper, use the Bitsadmin tool (it’s on WinXP CD – \Support\Tools\Support.cab) to see a list of BITS-related jobs and what they are reporting. Specifically, run Bitsadmin /list /allusers /verbose to see what is in the queue. Here are few Bitsadmin Examples. This tool really helps in troubleshooting AU Clients and any problems that may arise.
As discussed, SUS consist of both server and client components. The follow sections outline how these elements work together.