Ask the Wi-Fi Guru: December Edition

Wednesday Dec 24th 2008 by Aaron Weiss

In the December 2008 installment of our monthly Q&A series, the Wi-Fi Guru addresses range extension in a variety of deployment scenarios.

Seeing as this month of December finds us with a collection of questions related to extending wireless range, just imagine who would benefit most from a seamless global Wi-Fi network: Santa! You can imagine how frustrating it must be for him, zipping and zooming from one corner of the world to another, trying to manage his naughty or nice list via iPhone. Yes, Santa uses an iPhone, and yes, the naughty or nice list is stored on a server. Santa knows better than to rely on local storage. Data integrity is very important to Santa Claus.

Q: I have a situation where a location has six APs.  The location has three floors and I need two APs on each floor to get coverage.  I do get some overlap. One AP may cover a room 90-100 percent while another AP may cover it 20-40 percent. Sometimes, the weaker AP is connected to the client. My APs 802.11g, all have the same SSID, security settings, and channel. Should the channels on all the APs be the same? – Shannon

A: CliffsNotes answer: no.

You definitely have the right idea. Deploying multiple access points to cover a large or signal-challenged space is perfectly valid and quite common in schools, airports, libraries, and so on. Since your goal is to provide the broadest coverage possible, assigning each AP the same SSID and security settings is exactly the right thing to do. But when APs share a common coverage field (as you indicate through room coverage strength), it is important to set them to separate, non-overlapping channels. Right now, your APs are all yelling on the same frequency, effectively shouting over each other, making it difficult for your clients to "hear" one AP from another.

Although there are 11 channels to choose from in most Wi-Fi gear, there are really only three channels with zero overlap: 1, 6, and 11. (In fact, we covered this topic in more detail last month when looking at interference from cordless phones.) Since you have two access points on each floor, and because 6 is a default channel that may be in use by any other APs in the area, it might make sense to use channels 1 and 11.

When multiple APs are available, which share the same SSID, some clients will behave more intelligently than others. For example, in Windows XP the wireless client utility does not distinguish between multiple instances of the same SSID—meaning that clients will see just one AP—ostensibly the strongest. Other client utilities will actually display each AP with its strength, letting the user choose with which to associate.

Q: Using WDS, if I was looking to extend my range even further after router 2, how would I add router 3, which would talk to router 2, which talks to router 1? – Luke

A: When you need to extend wireless coverage to a space that is too large for one AP—but not so large that you need a cluster of APs like in the previous question—one answer is WDS, or the wireless distribution system. Luke's question refers to an earlier tutorial about configuring WDS with DD-WRT. In this article, we looked at how to use WDS to link two wireless routers. WDS itself is not limited to DD-WRT—you'll find WDS supported in stock firmware for some routers, though typically those priced toward the higher end of the market.

The idea behind setting up WDS is simple enough: one router is the "host" and the second router is the "client." The host router supplies the actual Internet connection, and also runs services for the whole network, such as DHCP, firewall, and port forwarding settings. The client router is essentially a repeater, bridging clients to the host router. To make all this work, you configure the WDS settings on the host router with the MAC ID of the client router; and configure the WDS settings on the client router with the MAC ID of the host router.

When adding a third router, there are two topographies you can create—a chain or a pyramid:


Host--Client 1--Client 2


Host--Client 1|Client 2

Which topography makes sense for you depends on how you're trying to extend the network—whether its reach needs to be extended from the second router (chain), or whether you want to extend the host router in an additional direction (pyramid).

Just like when you configured Client 1, you'll need to assign Client 2 a unique static IP on the LAN, disable the WAN/Internet connection, plus DHCP and firewall settings. Then, in the WDS settings for Client 2, you will enter the MAC ID for either Client 1 (chain configuration) or the Host router (pyramid configuration).

Likewise, you will configure the WDS settings for either Client 1 (chain) or Host (pyramid) with the MAC ID of Client 2.

It is important that Client 2 link back to only one router—in other words, you don't want to add MAC ID's for both Client 1 and Host. This would result in creating a loop rather than a chain or pyramid—and a loop is a bad thing that will mess up the network and require extra steps to solve.

Besides DD-WRT, you can also setup a WDS using the same principles with the Tomato firmware.

Q: We are deploying a Wi-Fi network in the Malaga (Spain) airport. It is a quite large network, consisting of 600 APs, approximately. In a terminal, my colleague is claiming he should install diversity antennas due to the metal ceiling tiles. This means we'll have to install four antennas per AP, which would increase significantly the expenses of project. I don't agree with this opinion, as I have previously installed APs above this type of ceilings and the attenuation observed wasn't significant enough to make us change the design. Obviously there is nothing like taking a few measurements, but I'd be very grateful if you shared with me your opinion and some bibliography that you might have, so I can support my arguments before my manager.—Antonio

A: First let's get our priorities straight—the only thing worse than a weak signal with airport Wi-Fi is having to pay $15 for the duration of a two-hour layover. Free airport Wi-Fi for all! Have some mercy, already—these days we have to pay extra just to wear socks on board the plane. Free the Wi-Fi!

With that out of the way, I have to admit that the metal ceiling tiles are of some concern. Of course, measurements are crucial here, since there are so many environmental factors that can affect signal propagation. Taking a look at this technical document published by Cisco, and you can see that in this kind of above-ceiling installation, their recommendation is to use remote antennas that can be mounted on the underside of the panels. This way, the AP body itself is above the metal, but the antenna below.

Considering the scale of this project, it might be worth running a cost estimate comparing the diversity antenna strategy versus single remote antennas mounted below the ceiling panels.

Between all three scenarios—diversity antennas above the metal panels, single antennas below the panels, and single antennas above the panels, the latter certainly seems like the most vulnerable. For one thing, wireless clients can vary considerably in power and sensitivity. This means that single antennas above the metal ceiling panels might provide a strong enough signal for a certain percentage of clients—let's guesstimate 80%—but the other 20% might not be able to associate. If the airport is planning to charge for access (boo! hiss!), this is potential lost revenue.

It is hard to say, of course, how relevant any of these concerns are without specific measurements. There is no doubt that metal can and will interfere with wireless signals, but the composition and thickness of the metal could make the difference between go and no-go.

There may be other ways to find a balance between cost and signal coverage. For example, perhaps it would be economical to swap out the metal ceiling panels beneath the AP's with a plastic panel. Or, split the difference and install single antennas on AP's above narrow areas, like walkway corridors, and install the more costly diversity antennas on AP's above gate areas where people are more likely to be sitting and congregating.

Once you get that worked out, let's talk about getting the planes to run on time!

Aaron Weiss is a freelance writer, author, and Wi-Fi enthusiast based in upstate New York. To submit your questions to the Wi-Fi Guru, simply click on Aaron's byline (above) and put "Wi-Fi Guru" in the subject line. Click here to read last month's column. For more by Aaron Weiss, read “ The Wi-Fi Planet Guide to Hotspot Safety for College Students.”

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