Ask the Wi-Fi Guru About Long-Range Bridges

By Aaron Weiss

March 17, 2011

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru explains the workings of wireless handshakes and describes what it takes to build a Wi-Fi bridge with a two-mile range.

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru explains the workings of wireless handshakes and describes what it takes to build a Wi-Fi bridge with a two-mile range.

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


Sometimes free Wi-Fi isn't all it promises to be. These days, the allure of free wireless Internet has become so powerful that, in a recent survey, 80 percent of respondents "prefer to shop at a retail store that offers Wi-Fi vs. a location without." But in practice, sometimes the promise of free Wi-Fi can be a frustrating letdown, if not outright misleading.

For example, on a recent trip I was in a major airport that advertised free Wi-Fi. The catch? You could only visit the airport's own website. Sure, you could look up flight status or other basic travel information, but to surf the "real" Web you still had to pony up. Besides, $8 for 24 hours access is a little steep when you only have a 90 minute layover.

At a café, I found "free" wireless, but in this case, the catch was that it was so slow as to be virtually useless. In another coffee shop, the free Wi-Fi included a litany of constraints about how long you could stay, where to sit, which customers have priority seating, and so many other rules that it was clear the manager did not actually want anyone using the "free" wireless at all. Anyone can say they offer free Wi-Fi, but actually doing it right seems to be another story.

When does an AP get the address of a passive station?

Q: I was wondering when is the first time that an AP gets the MAC address of a passive station? - PK

A: Everyone is familiar with selecting an available wireless network from a list. But how does your computer know which wireless networks are available? Wireless adapters can operate in one of two modes: active or passive.

Most of the time, we are using active mode. In this state, the wireless adapter sends out a request, called a probe, which can be "heard" by any access points (AP) in the area. The AP's can respond to the probe with information, such as their SSID (network name). In some cases, an AP might be configured not to provide its SSID, which is sometimes used as a weak form of security.

In this mode, the wireless station (or client) is engaged in a formal conversation with the AP, and so both sides can know each other's MAC address.

Some wireless adapters support passive mode. In this case, the client does not send out a probe. Instead, it tunes into each Wi-Fi channel and "listens" for traffic. Like looking through a one-way mirror, the passive mode client can see all traffic on that channel but is not seen by the AP or other clients.

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