Tips from the Windows Pros: Squeezing Top Speed from Your High-Speed Connection

If you're using a high-speed connection such as DSL or cable, you can make an adjustment to your computer's networking software to get the best speed from your new connection.

Internet TCP/IP software sends data in chunks called packets, which are sent in a stream from, say, a Web server to your computer. The receiving computer sends acknowledgments every so often to indicate that the data has been received correctly or to indicate that something was lost or garbled during transmission.

A server sends only a limited amount of data before it expects to receive an acknowledgment. If the limit is reached before an acknowledgment is received, the sender has to stop and wait for one.

Ideally, for the fastest possible transfers, the sender should never have to stop sending because acknowledgments for data sent earlier should arrive before this limit has been reached. Then the sender can go on sending, again hoping that the data will be acknowledged before the limit is reached.

For cable and DSL modems, the data rate is so high and the cross-country travel times so long that a considerable amount of data can be "in flight" before an acknowledgment could possibly be returned. So, to get the maximum use of your DSL or cable connection, you must tell Windows to make the limit larger than normal for a LAN connection.

This limit, called the receive window, should be larger than the data rate times the roundtrip time for data traveling back and forth between the two computers. This number is the maximum amount of data "in flight." A typical round trip time is .100 seconds for interstate Internet traffic, so for various data rates, the receive window should be at least

At 100Kbps * 0.100 sec /bits per byte	= 1.2KB
500Kbps	6KB
1.5Mbps	19KB
4Mbps	75KB

The default value used by Windows 9x, NT, and 2000 is only about 4KB! This means that Windows sends or lets a remote server send only 4KB and then sits and waits while your high-speed connection sits idle.

Windows 2000 and XP slowly boost the window size all by themselves during long file transfers, but when you're browsing Web pages with lots of small graphic images, they never get a chance to boost the window size enough for you to realize the full potential of your fast connection.

The maximum window size that you can specify in Windows 2000 and XP is greater than 64KB, but 64KB is a practical maximum for DSL or cable service. To set the receive window, you must use the Registry Editor, which is described in Chapter 32, "The Registry." You must add a Registry value to a key that contains TCP/IP software parameters.

CAUTION

Before adding a Registry value, be sure to read Chapter 32, with its dire warnings about the risks of editing the Registry and its urgings for you to back up the Registry before making a change of this sort.

Setting the Receive Window in Windows XP and 2000

To set the receive window in Windows XP and 2000, do the following:

1.	Open Registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters.
2.	Select Edit, New, DWORD value, and name it GlobalMaxTCPWindowSize. Set its value to 20000 (hexadecimal). See Figure 19.23 to see this entry in the Registry Editor. Figure 19.23. You set the GlobalMaxTCPWindowSize value in this dialog box. Note that Hexadecimal must be checked.

You must restart your computer for the setting to take effect.