Ask the Wi-Fi Guru About Wi-Fi Repeaters and Windows XP

By Aaron Weiss

July 21, 2010

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru explains why a reader's Wi-Fi repeater performance isn't so bad after all, points the way to relief for Windows XP Wi-Fi woes, and describes how to get more than just a stale bagel from the lobbies of finer hotels.

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru explains why a reader's Wi-Fi repeater performance isn't so bad after all, points the way to relief for Windows XP Wi-Fi woes, and describes how to get more than just a stale bagel from the lobbies of finer hotels.

Would you like to ask the guru a question? Write the editor.

In my previous column I may have said a few unkind words about the "better" hotels who nickel-and-dime guests over wireless Internet fees. On a recent trip I discovered a loophole in one major hotel that may (or may not) be found at other properties, through which free wireless can be enjoyed. Because this workaround does not involve any nefarious means, it seems like fair game to be discussed publicly.

It is not uncommon for hotels to offer free wireless in the lobby area even when only fee-based wireless is provided in rooms. At the unnamed hotel where I recently stayed, when you connect to the wireless network your Web browser is redirected to a login portal ­- fairly typical. When you connect in the lobby, the login portal says that the fee is complimentary, and then you click through to authorize your machine. When connecting in-room, the same portal lists a real fee, such as $9.95 for 24 hours (also known as "highway robbery").

These login portals typically work by storing the unique MAC address for your wireless adapter in a table. When you connect, the portal checks to see whether your MAC address is already in the authorization table and, if so, whether its timestamp is still valid (has your 24 hours expired?). Once expired, your MAC is flushed from the authorization table and, of course, you are sent back to the login portal.

While in this particular hotel, I wondered what the odds were that the free portal and the fee portal actually connected to the same backend authorization table. In other words, if you authorized your machine in the lobby for free, could you then take it back up to your room, where it would remain authorized (for the 24 hour period)?

Happily, the answer was yes­--whoever put together this hotel's wireless infrastructure did not choose or bother to actually build two separate authorization systems. You could avoid the daily fee by simply popping down to the lobby, logging into the portal, and heading back to your room. Plus, you could pick up complimentary coffee and a stale bagel on the way.

How much bandwidth can I expect to lose with a repeater/bridge?

Q: You know, I tried DD-WRT firmware in my WRT54G, and after a few false starts, I got Repeater/Bridge to work. Unlike what many articles said, I did not lose 50 percent of my bandwidth. I lost a little (10 to 25 percent) but not the predicted 50 percent. I know that the common feeling is that a R/B has to slow the network down by repeating everything, but they must have gotten around some of the overhead. My setup is a stock WRT54g in my house, and a DD-WRT V24 (standard build) in my barn. - David

A: We've certainly talked about the 50 percent bandwidth loss when using a repeater, and with good reason -- because it's true. However, let's look at how to interpret this figure by putting it in context.

When a wireless router is configured as a repeater ­-something you typically need special firmware to accomplish such as DD-WRT or Tomato -- it acts as both a client and an access point (AP). As a client, it receives the wireless signal from an upstream AP, using whatever security credentials are (or are not) required for that link. As an AP, it broadcasts that signal using whatever network name (SSID) and security credentials you choose, independent of the source AP.

Outside of some enterprise-grade routers, your average wireless router has only one radio. It cannot receive and broadcast simultaneously. Therefore, it must receive the signal first, and then switch to broadcasting, and then switch back to receiving, and then switch back to broadcasting, and so on. Consequently, the one radio can devote only half its time to either role.

In practice, this does not mean that your connection to the Internet from the repeater will be halved. Remember that your Internet connection speed is likely a fraction of your wireless link speed. For example, suppose you have router A and router B configured in a repeater-bridge scenario. You have two computers, one connected to router A and one connected to router B. Finally, you have a broadband connection connected to router A, with a max speed of 10Mbps (common for U.S. broadband).

The computer connected to router A may be linked at 54Mbps and the computer connected to router B may also be linked at 54Mbps. If you tried copying a large file from computer A to computer B, you would expect to see only half this speed because the repeater has to work double duty as receiver and broadcaster.

But if you tested your Internet speed from either computer, both should be nearly the same. This is because your broadband connection is slower than even the half-duplex link from your repeater.

Another way to think about this is that the repeater does not automatically halve the speed of any data you push through it -- it halves the maximum available speed on that link. If your source can't reach that speed anyway, you may not see any reduction at all.

Finally, note that any machines wired to the repeater are not subject to this issue anyway. The repeater is not using its radio to broadcast data to wired clients, and so these machines continue to enjoy a full-duplex link to the primary AP.

Why is my XP/SP2 laptop giving me Wi-Fi fits?

Q: I'm having this problem and don't know what to do with it. I recently reformatted my laptop and installed Windows XP SP2. Everything works just fine, except the wireless connection. The laptop connects to the network (at home or at the libraries) just fine, works for a while and then stops. It says the laptop is connected, there is connection but it won't do anything, keeps showing the message "Try Again" or "check the network connection". I turn off the Wi-Fi (of the laptop) and turn it back on, it works for a bit and then same problem. - Anonymous

A: Without more details this is something of a guess, but the word that stood out to me in your question is "XP". Actually, those are letters. Either way, XP has become notorious for somewhat flaky wireless behavior.

In many cases, what appears as flakiness is actually Windows trying to be helpful, through the XP "Wireless Zero Configuration" service. Ostensibly this service is designed to manage wireless connections but it can be overzealous, causing connections to drop as it attempts to link up with alternatives when you haven't asked it to. Often the simplest solution is to disable WZC, a subject you can read more about here.

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