Poor Wi-Fi performance? Wi-Fi interference can slow network speeds to a crawl. Here are tips for dealing with crowded airwaves.

IT'S A BRIGHT AND EARLY SATURDAY MORNING and the friendly folks from the San Diego Wireless Users Group (SDWUG) are out in force, building a free wireless network across the communities of Golden Hill and Sherman Heights near downtown San Diego. Some people simply see Wi-Fi as a client delivery solution, but SDWUG members have been trying to scale the technology beyond a simple Ethernet replacement and into a pseudo last-mile solution. (I should note that SDWUG only works with ISPs that allow this kind of bandwidth sharing in their Terms of Service, making these projects completely legal!) You can check out the details at http://www.socalfreenet.org.

In this article, I review some of the "secrets" of deploying large-scale Wi-Fi networks in urban areas and other crowded environments such as office parks and neighborhoods. Although this topic could fill an entire book, I'll focus on the specific areas of power management, modulation techniques, and antenna tricks you can use to improve your wireless network performance in densely populated areas.

The little Wi-Fi that could

Who knew? Wi-Fi has gained a popularity unlike any other technology in recent times. Looking back, it's simple. Wi-Fi is easy, inexpensive and when it comes right down to it ... Wi-Fi is fun. It lets you do things you couldn't ever do before. The cable is gone and your imagination is free to roam around the room (living room or board room).

People are using Wi-Fi in ways never imagined by its "founding fathers" who designed 802.11 as a wireless Ethernet replacement. The maximum reliable segment length for wired Ethernet is 100 meters. The most common implementation of a Wi-Fi network is an access point providing a coverage zone (often called a hotspot) with a footprint of a couple hundred feet. Wi-Fi is great for providing wireless coverage in a home or across a large space in an enterprise environment. Unfortunately, with so many people discovering the benefits of wireless networking, Wi-Fi is at risk of becoming a victim of its own success.

Air traffic

In 1985, the FCC waved its magic wand and declared that frequencies in the 900MHz, 2.4GHz and 5.725-5.850GHz range were free for public use. No need to purchase expensive licenses like a traditional radio or TV broadcaster (which is why Wi-Fi frequencies are called unlicensed). This act created the Industrial, Scientific & Medical (ISM) band of frequencies. As long as individual users play nice by keeping their transmission systems (called "intentional radiators") under 1 Watt of output power, the users are free to deploy their wireless networks without a license or intervention from the FCC. Note that the 5GHz frequencies used by 802.11a fall under another FCC mandate called Unlicensed-National Information Infrastructure (U-NII), released for unlicensed public use in 1997.

The 2.4GHz range of frequencies (2.4GHz-2.4835GHz) is typically referred to as the "junk band" due to the overwhelmingly large number of devices found in this space. Everything from cordless phones, baby monitors, microwave ovens and more all compete for precious "airtime". But now, with millions of users deploying their own 802.11 networks, the battle for frequencies has reached an all-time high. Too many Wi-Fi networks means high error rates, slow speeds, and perhaps even a total inability to connect. Deploying a large-scale network across a major metropolitan area has become nearly impossible. According to Frank Keeney, co-founder of socalwug.org (a wireless user group based in Pasadena) and owner of wlanparts.com, "In LA, we've had to make all kinds of adjustments to the network in response to the increasingly congested unlicensed spectrum. Links that worked six months ago are now starting to become unreliable."

Of course, using Wi-Fi to build a large-scale metropolitan area network was never part of the design requirements for this technology. Founders envisioned users sitting on their couches and checking e-mail, not deploying Wi-Fi to bring free Internet access to entire neighborhoods. Yet groups everywhere are trying to do just that.

To combat the crowded airwaves, there are a number of steps you can take:

* Increase power

* Avoid omni-directional antennas when possible

* Modify antenna polarization

* Use different modulation techniques

* Use out-of-band frequencies.

Increasing power

Adding amplifiers to your 802.11 system is a poor choice for a number of reasons. It's the equivalent of talking louder when you're in a crowded restaurant and you can't hear the person next to you. This might seem like a good short-term solution, but think about what happens next. The people at the table next to you can't hear each other either, so they start talking louder. Then, the next table gets even more rowdy. Before you know it, everybody is shouting and nobody can hear anything through the noise. Now, imagine the same thing happening with RF waves in the air around you. As a courtesy to your community, "cranking up the volume" should always be an option of last resort because it eventually just compounds the problem.

Avoiding omni-directional antennas

Most people choose omni-directional antennas because they radiate power in all directions in the shape of a donut. At first glance, this makes sense because the wireless clients are mobile and require wireless coverage from a variety of locations.

The factor that often gets overlooked is that antennas serve two functions: They transmit and they receive. Because the receiving also occurs in the same donut-shaped pattern, an omni-directional antenna actually collects much more ambient noise from the surrounding area then a directional antenna. In many cases, you might be able to architect your network using a series of point-to-point backhaul links, instead of using large omni-directional antennas in a hub-and-spoke design. While you probably won't be able to avoid omni-directional antennas altogether, with some clever engineering, you might be able to reduce the number of them in your system.

Keep in mind that you can achieve 360-degree coverage by using three separate directional antennas, each with 120-degree beam spreads. This costs a little more (and you will need three radios), but it also effectively triples your output power. Some antenna manufacturers even build ready-to-go units that include three separate elements for this very purpose! You still have to deal with ambient noise, but each radio picks up only one third as much.

Modifying antenna polarization

RF waves are composed of two components: An electric field and a magnetic field (hence, the term "electromagnetic wave"). These two waves are transverse to each other. This means that one of the waves is on the horizontal axis and the other wave is on the vertical axis. The term "antenna polarization" refers to the orientation of the electric field. So, if the electric wave is oscillating in the vertical plane, you have a vertically polarized antenna.

For the purposes of this discussion, you can simply think of the antenna polarization as a special marker on an antenna that indicates "this way up". The idea is to make sure that each antenna is identically polarized. Matching the polarity on each side of the RF system ensures the strongest possible signal strength. In a situation where one antenna is vertically polarized and the other antenna is horizontally polarized, the signal strength suffers tremendously. This condition, which you want to avoid, is called "cross-polarization."

The neat part about polarization is that you can have one pair of antennas that are vertically polarized right next to another pair of antennas that are horizontally polarized and the systems only minimally interfere with each other. Because most point-to-multipoint 802.11 systems use vertical polarization; this means that you could potentially operate another system using horizontally polarized antennas (or even diagonally polarized antennas) and reduce the amount of interference between the systems to an acceptably low level.

In the case of a panel or sector antenna, changing the polarization is as simple as physically rotating the antenna 90 degrees. With omni-directional antennas, however, it's a bit more complicated. Because the internal design of the antenna is different (and thus, fewer units are produced), it's usually much more expensive to purchase horizontally polarized omni-directional antennas than the more commonly available vertically polarized variety.

Using different modulation techniques

802.11b utilizes a modulation technique known as Direct Sequence Spread Spectrum (DSSS). This technique utilizes a group of frequencies 22MHz wide, known as a "channel." There are devices that utilize other modulation techniques, such as Frequency Hopping Spread Spectrum (FHSS). FHSS devices use frequencies that are only 1MHz wide, but "hop" according to a predefined sequence across 79 different channels.

It isn't a perfect solution, but co-mingling a DSSS system with a FHSS system generally produces less interference than two DSSS systems in the same physical space (even in the same frequency range). If possible, you could architect your network to use FHSS as a backhaul solution and DSSS (802.11b) to provide client access. Even if both systems utilized the 2.4GHz spectrum, your performance would be better then using DSSS with overlapping channels.

Using out-of-band frequencies

If your client access radios operate at 2.4GHz (as is the case for 802.11b), a simple solution to avoid interference is to use different frequencies for your backhaul solution. For example, you could use 802.11a (in 5 GHz) as a backhaul and this wouldn't interfere at all with the 2.4GHz client access radios. There are also some backhaul solutions that operate in the 900 MHz space.

Using devices in the 5GHz or 900MHz range might cost a little more, but it provides an extremely effective and elegant solution. In SDWUG's project, we use 802.11a as a backhaul, in conjunction with 802.11b client access radios.

Life in an unlicensed world

While there are many challenges involved with using unlicensed spectrum, let's not forget--it sure beats the alternative! Purchasing licenses from the FCC call cost millions of dollars. The unlicensed rules established by the FCC are designed to minimize the amount of interference caused by adjacent RF systems. However, we can't expect the FCC to step in and mediate disputes every time two people want to operate 802.11 systems near each other. As users of the RF spectrum, we must all take steps to be "good neighbors" and respect the FCC rules in the spirit of sharing this national treasure. In this article, I've outlined some steps on how you can survive a noisy Wi-Fi environment and operate wireless equipment in any dense urban environment. If they don't work, you can always try baking some cookies, meeting your neighbors and trying to work out a frequency-sharing arrangement.

Terms

Last-mile solution--The way bandwidth is delivered to users in the last mile of the distribution path.

Backhaul--A wireless link between an access point and a source of bandwidth.

Lee Barken, CISSP, CCNA, MCP, CPA, is the co-director of the Strategic Technologies And Research (STAR) Center at San Diego State University. He has worked as an IT consultant and network security specialist for Ernst & Young's Information Technology Risk Management (ITRM) practice and KPMG's Risk and Advisory Services (RAS) practice. Lee is the president of the San Diego Wireless Users Group and writes and speaks on the topic of wireless LAN technology and security. He is the technical editor for MOBILE BUSINESS ADVISOR magazine, and the author of How Secure Is Your Wireless Network? Safeguarding Your Wi-Fi LAN (ISBN 0-13-140206-4). [e-mail], http://www.sandiegowirelesstraining.com.