Transmission throughput is less than expected for a 3G WWAN data card in Windows Vista

You use a 3G wireless wide area network (WWAN) data card on a Windows Vista-based computer. The WWAN data card uses a connection that only receives data. However, you may find that the throughput is much less than the throughput of the same 3G WWAN data card when you use it on a Microsoft Windows XP-based computer.

This problem occurs because of the way that TCP receive window auto tuning is used in Windows Vista for connections that only receive data.

Windows Vista obtains a round-trip time (RTT) estimate at the time of connection setup and every time that a new segment of data that is transmitted. A connection that only receives data is limited to the single RTT sample that is obtained at the time of connection setup. Because the connection only receives data, the connection cannot "converge" to the actual RTT of the connection. For example, because the RTT data may fluctuate for various reasons, a "converge" operation is performed to estimate a meaningful RTT by using blending current and previous RTT data. However, if the connection cannot converge to the actual RTT of the connection, the bandwidth delay product (BDP) estimate that Vista obtains is also incorrect. Therefore, the connection's receive window is limited, and throughput of the connected transmission network is reduced.

Note Windows XP does not support the TCP receive window auto tuning feature. Instead, a static value may be configured in the registry to try to improve throughout. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Service pack information

To resolve this problem, obtain the latest service pack for Windows Vista. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Update information

The following files are available for download from the Microsoft Download Center:

Windows Vista, 32-bit versions

Download the 940646 package now. (http://www.microsoft.com/downloads/details.aspx?FamilyId=DD008AA1-4E44-427C-89B6-9E2D83D67D25)

Windows Vista, 64-bit versions

Download the 940646 package now. (http://www.microsoft.com/downloads/details.aspx?FamilyId=D45445D5-185F-4DC7-A457-74EB5419474C)

For more information about how to download Microsoft support files, click the following article number to view the article in the Microsoft Knowledge Base: Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help prevent any unauthorized changes to the file.

Prerequisites

No prerequisites are required.

Restart requirement

You must restart the computer after you apply this update.

Update replacement information

This update does not replace a previously released update.

File information

The English version of this update has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

Windows Vista, 32-bit versions

Windows Vista, 64-bit versions

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. This problem was first corrected in Windows Vista Service Pack 1.

For more information, click the following article number to view the article in the Microsoft Knowledge Base: