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Wireless How To

Introduction

Too Many Wireless Networks

Wireless networks are wonderful things - when they work! But many consumers find that even if they manage to get their WLAN (wireless LAN) working, keeping it up and running reliably with good performance is often frustrating and a struggle. Though flaky equipment and improper setup can sometimes be the cause, all too often the growing popularity of WLAN equipment is itself the cause of wireless woes.

In this article, I'll explain the causes of problems in closely-spaced wireless LANs (and dispel some myths). I'll then give you fixes for common problems, and even tell you what not to waste your time trying.

So, how do you know that you have a problem from neighboring WLANs and not just something wrong with your own equipment? Take this little test:

  • Your "View Available Wireless Networks" window shows wireless networks other than your own - and lots of 'em
  • You keep losing connection to your AP, even when you're in the same room
  • Your wireless connection seems to crap out around the same time each day...usually in the late afternoon or early evening
  • You overhear your neighbor talking about the problems he's having with his wireless LAN
  • You live in a dorm, apartment building or neighborhood with large homes on small lots and broadband Internet service

If any of these sound familiar, then you probably should read on. If, on the other hand, you live out in the boonies where your cell phone doesn't even work and you have to drive over to see your neighbor, then this article probably isn't going to be much help!

NOTE: Please read references to access points (AP) or wireless routers as applicable to both kinds of products unless otherwise noted.

What's the problem?

The primary causes of wireless LAN problems in high-density areas are:

1) Too many users trying to use the same channel.

2) RF (Radio Frequency) interference from nearby WLANs

3) Improper wireless adapter settings

The first problem is a capacity issue, i.e. not enough bandwidth to go around. Simply put, there are too many radios trying to use the same channel (i.e. frequency) at the same time in the same area. "High density" is a relative term, but if you live in an apartment building or school dorm, you're definitely in this category. And even if you live in a single-family dwelling, if the distance between your and your neighbors' homes is 50 feet or so, and you know the names (SSIDs) of your neighbors' wireless networks, you're also in this category!

An 802.11g network has a best-case useable bandwidth of around 25Mbps. 802.11n can move this up to anywhere between 50 and 100 Mbps. But if any 11g clients are also actively using the same radio on an 802.11n router, throughput for both will be reduced by more than half. (See Add, Don't Replace When Upgrading to 802.11n for an explanation.)

Too Much Noise

The second cause falls into the category of RF-based interference. Though you might think of wireless LAN interference only in terms of 2.4GHz cordless phones and microwave ovens, WLAN equipment itself is becoming another growing - and perhaps dominant - category of RF "noise".

Every form of communication has to deal with two components: signal, which is the part that contains the desired information; and noise, which is everything else. Key attributes of radio receiver design are maximizing sensitivity to signal and minimizing sensitivity to noise.

As long as Wi-Fi products receive sufficient signal, the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism built into the protocol coordinates their communication. This access method - similar to CSMA/CD used in wired Ethernet - makes sure that only one product transmits at a time, so that the data is understood by all receivers.

But when the radio energy detected by a piece of WLAN gear can't be understood - even if that energy comes from valid WLAN equipment - it turns into noise. Wireless LAN gear does a remarkable job of differentiating between signal and noise, but not all products are created equal in this area.

If you're using 2.4 GHz band equipment, you probably know that your access point has eleven channels that it can be set to. You may not know, however, that only three of those channels should be used. The reason for this is illustrated in Figures 1 and 2.

2.4 GHz band adjacent channel overlap

Figure 1: 2.4 GHz band adjacent channel overlap

The yellow shaded area in Figure 1 represents the power from channel 2's signal that overlaps into channel 1's main lobe (the largest "hump" and also the frequency band that contains most of the signal's power). Since a significant amount of channel 2's main lobe overlaps into channel 1's main lobe (and vice versa), communication on both channels will suffer. (Note that this effect is the same for any two adjacent channels, not just Channels 1 and 2.)

Contrast this picture with the situation shown in Figure 2.

802.11b "non-overlapping" channel overlap

Figure 2: 2.4 GHz band "non-overlapping" channel overlap

Tip! TIP: See this section of our Atheros Super-G NeedToKnow - Part 1 for a full explanation of channel overlap

This figure has the same scale as Figure 1, but shows signals in the "non-overlapping" channels 1, 6 and 11. Since the power from each signal doesn't magically stop at the 22MHz 2.4 GHz band channel boundaries, there is still overlap between "non-overlapping" channels. But in this case, the yellow shaded area that represents channel 11's power that is overlapping into the main lobe of channel 6 is at least 30 dB lower (1/1000) than channel 11's peak power. For most well-designed radios, this 30dB difference between signal and "noise" is sufficient to ensure good rejection of the adjacent channels' signals (i.e. noise).

Use of overlapping channels isn't the only source of WLAN RF interference. As I'll explain later (What Doesn't Help), some of the techniques that your might be using to "ignore" other WLANs actually can hurt rather than help your own WLAN's performance!

The take-away from all this is that not only do you have to deal with possible problems from microwave ovens and 2.4GHz phones, but neighboring WLANs themselves can also become interference sources.

Tip Tip: Wireless networking management company Cirond argued that there are actually four channels (1, 4, 8, 11) that can be used for 802.11b and g with virtually no performance penalty. I recommend you stick with using Channels 1, 6, and 11 since they are more likely to be used by neighboring WLANs.

The third cause—incorrect settings—is just that. More on that topic later.

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