Republished with permission from WatchGuard Technologies, Inc.

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Expanding Your WLAN Reach

By David Piscitello, President, Core Competence

My previous LiveSecurity article, "Expanding Your Small Business Wireless LAN," explains what you do when your office outgrows your one and only access point. But what about those situations where wireless coverage is good in most areas, but less so for some unfortunate users on the outskirts of your access point's current reach? Before you yield to the temptation of building a DIY Yagi antenna for your access point, consider these off-the-shelf solutions that won't cost an arm and a leg, and will keep you on the friendly side of the FCC.

What's sapping your signal strength?

When wireless users complain about poor service, they're actually helping you identify locations where the strength of the radio signal from your access point is weak. Signal strength is a measure of reception, and it's influenced by (a) the power output of the transmitting source (e.g., an access point); (b) physical and electromagnetic interference along the path between the transmitting source and receiver that causes attenuation or path loss; and (c) the ability of the equipment at the receiving end to differentiate between signal from noise and process it (known as receiver sensitivity, discussed in this PDF). Several techniques boost signal strength, including:

  • Antenna alignment, repositioning, or replacement
  • Improving receiver sensitivity
  • Amplifying or repeating signal, and
  • Adjusting antennae power output.

You can use these techniques individually or in combination, so let's examine how each improves signal strength.

Antenna replacement: friend or foe?

Pringles can and homemade parabolic antennae are part of the wireless pop culture. Yes, you can build an antenna that increases signal output, but be forewarned: the U.S. Federal Communications Commission (and similar organizations worldwide) frown upon unapproved modifications to equipment used in the Industrial, Scientific, Medical (ISM) frequency band. Firebox X Edge Wireless owners should be aware that antennae modification and replacement invalidates the FCC license of your Firebox X Edge Wireless (so don't say I didn't warn you!).

You can purchase FCC-approved replacement antenna for many access points (APs), even consumer-grade varieties (e.g., Linksys, D-Link, and Buffalo Technologies). Also called high gain, these antennae increase signal strength in one or more directions by reducing strength in other directions. Replacement antenna can cost as little as US $20.00 per pair or as much as several hundred dollars, depending on the gain you require. You'll get what you pay for. Access points often come "stock" with omni-directional, dipole antennae. When you swap a dipole antenna with an inexpensive replacement, what you'll get is a longer dipole antenna that flattens coverage: instead of a ball, the shape of your coverage will be more like a hockey puck.

Antenna replacement for directing signal is a different proposition. Whereas a stock dipole antenna covers large round (or square) areas from an access point, a directional antenna covers a rectangular area radiating in one direction from the AP. Before you swap out a dipole for a directional antenna -- for example, a Yagi, panel, or parabolic -- think about the impact on all areas covered by this access point.

Receiver Sensitivity

Antenna replacement at an access point can improve service and prove cost effective. Before you get overly enthused about antenna replacement for individual stations, consider other possibilities that might be more cost effective, less obtrusive and less cumbersome than external antenna. A better solution might be to choose wireless adapter cards that have superior receiver sensitivity.

Receiver sensitivity is measured in decibels of radio (signal) power per milliwatt of power (dBm). The technical specifications of most Wi-Fi adapters usually identify the minimum signal that must be present to maintain an association with an access point, and many identify the dBm required to maintain specific data rates. As an example, consider the Receiver Sensitivity of an 802.11g network adapter I randomly selected off the Web. Remember, the lower the dBm required to maintain signal, the better the adapter will be able to lock onto your AP's signal:

Access Rate

Receiver Sensitivity

54 Mbps

-68 dBm

48 Mbps

-68 dBm

36 Mbps

-75 dBm

24 Mbps

-79 dBm

18 Mbps

-82 dBm

12 Mbps

-84 dBm

11 Mbps

-82 dBm

9 Mbps

-87 dBm

6 Mbps

-88 dBm

5.5 Mbps

-85 dBm

2 Mbps

-86 dBm

1 Mbps

-89 dBm


You can avoid the inconvenience of trying to assemble individual product spec sheets for receiver sensitivity by visiting the Receive Sensitivity page at FreeNetworks.org.

WLAN Range Extenders

Suppose your access point serves most of your users nicely, but you need to stretch the coverage in one direction for a few relatively distant users. A $60-100 investment in a WLAN range extender (WRE) or signal repeater could be just the ticket. A WRE is a device that associates with an access point, and provides Wi-Fi service to stations beyond the reach of that access point by "repeating" signal (and hence transmitted traffic) for both the AP and its associated stations. A WRE has its own antenna and power source; in fact, some simply plug into a wall outlet. It associates with an AP as if it were a station, so it doesn't need a wired Ethernet connection.

One drawback to WREs is that they typically employ proprietary methods of associating with, and synchronizing service sets with, APs. That means f you use a Linksys AP, you'll probably need to buy a Linksys WRE. WREs also cut throughput in half. Every frame you send must be re-sent by the repeater, which doubles the number of frames transmitted over the channel.

Scotty, we need more power!

When you move to NASCAR country as I have, you quickly learn "y'all need to increase yer horsepower" answers eight out of ten automotive questions, especially ones having to do with speed. With WLANs, if you want more speed, the same answer applies: if you want to operate at higher data rates over distance, you'll need to add more power. Several options are available to folks who want to turbocharg-- um, amplify radio signal. And some are even legal!

With certain access points, you can increase the power output through the management interface. Seattle Wireless hosts an extensive hardware comparisons site: here, you'll find the transmit power or range of power for access points, wireless routers and bridges, and network adapters. Table entries containing a power range identify access points with adjustable power.

What if your access point's standard firmware doesn't allow power adjustment? Investigate third party and OEM firmware developers like SveaSoft, who offer replacement firmware for APs. Replacement firmware can boost power as far as you want to take it, even beyond what is legal. SveaSoft offers firmware for certain Linksys APs, and will release a firmware (Talisman) that is compatible with Belkin, Buffalo Technologies, and other manufacturers. License for this software is dirt cheap at US $20.00.

A third alternative is to interpose signal booster hardware between your access point and antenna. Companies like Hawking Technologies sell units in the US $100 range that allow power adjustment from 100w-500w and are compatible with major AP products.

Another "add hardware" solution is to investigate pre-802.11n MIMO (Multiple-Input, Multiple-Output) technology. MIMO APs use essentially the same power as today's access points. By transmitting and receiving radio signals over multiple antennae, MIMO APs can sustain higher data rates in coverage areas where single antenna APs would fall victim to multipath distortion. For more information about MIMO, read "Going the Distance with MIMO."

Before you juice up your engine, remember to consider the overall impact on coverage. By increasing coverage, you may extend your wireless reach beyond your own facility and cause interference with neighboring WLANs. Or you may make it easier for wardrivers to eavesdrop on your signal. A prudent course of action is to increase power gradually. For each increase, measure overall coverage, and repeat until you have improved coverage in weak areas, or you find that this solution alone will not suffice. Lastly, remember that the FCC regulates power output in the ISM band.

Conclusions

Most organizations choose wireless LANs to eliminate boundaries, but in some cases, you need to eliminate boundaries before you can harness the full benefits of WLANs. The good news is you have a wide and varied set of technologies available to solve the wireless problem that is uniquely yours.

Resources

Understanding WLAN signal strength.

WLANAntennas.com is a good resource for WLAN and radio terminology

Understanding WLAN Antenna (SearchNetworking free registration required)

Beagle-ears.com's helpful summary of FCC Rules for ISM Band Wireless Equipment

Find a power output calculator at RadioLabs, Inc.

Wildpackets offers a good white paper (PDF) on Converting Signal Strength

Extending WLAN Range with Repeaters

Wireless fun with expensive mobile antennae at Defcon12:
High-tech Hijinks in Sin City


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