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WHEN WIRELESS LANS COLLIDE...
Introduction
Wireless networks are wonderful things—when they work! But an
increasing number of consumers are finding that even if they manage to
get their WLAN (wireless LAN) working, keeping it up and running
reliably with good performance is often a frustrating and unsuccessful
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 ProblemSolver, 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 WinXP
"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
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 802.11b network has a best-case useable bandwidth of about 5Mbps.
This capacity can actually support a large number of users, if their
transmissions are short and intermittent—as they would be for
web-browsing, email, IM and the like. But with typical broadband
connection speeds of 1-2Mbps, you can see that it doesn't take too many
long downloads, video streams or webconferencing sessions running
simultaneously to exhaust this relatively small data "pipe".
Switching to 802.11g makes the "pipe" bigger, but nowhere near
the 54Mbps touted on product boxes. My testing shows that best-case real
(available) bandwidth for current-generation 11g products with clients
running WinXP is around 25Mbps. Using Win98 typically drops that to
closer to 20Mbps, and having any 802.11b stations (clients) associated
to an 11g WLAN will drop maximum throughput to around 12Mbps.
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 802.11b / g 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 802.11b or 802.11g 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.
Figure 1: 802.11b 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.
Figure 2: 802.11b "non-overlapping" channel overlap
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 802.11b 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: Wireless networking management company Cirond argues that there are
actually four channels (1, 4, 8, 11) that can be used for 802.11b and g
with virtually no performance penalty. See this FAQ for more info and a
link to the paper. I recommend you stick with using Channels 1, 6, and
11 since they are more likely to be used by neighboring WLANs.
Change Channels
Now that you understand the primary mechanisms at work, you're ready to
move on to ways to combat them. A simple first step is to change your
access point's channel. Since Windows XP's Wireless Zero Configuration
utility is no help in determining the channels that neighboring wireless
LANs are operating on, you'll need to fire up the wireless client
utility that came with your wireless adapter card.
Figure 3: ASUS WL-100g Site Monitor Search
Figure 3 shows a nice utility that comes with ASUS' WL-100g CardBus
client card [reviewed here]. It not only shows the SSID and channel of
each in-range AP, but its MAC address and signal strength—all very
handy info!
One you know the lay of the airwaves, the countermeasure is simple.
You'll just need to choose a channel—1, 6, or 11—that is used by the
fewest neighboring APs, has the lowest signal, is the least busy, or
hopefully all three!
Changing channels is easy, but you have to know how to access your
access point or wireless router's setup screens (this info is in your
product's setup guide and user manual). As an example, Figure 4 shows
the main Setup screen for the Linksys WAP54G [reviewed here], with the
yellow highlight indicating the Channel and SSID settings.
Figure 4: Linksys WAP54G access point setup screen
You'll probably want to change both settings—I'll explain the how and
why of the SSID setting later. Be sure to Apply, Save or whatever your
product has you do to make the settings stick after you change them.
By the way, while client utilities can help you count APs and determine
their operating channel, they won't show you how busy each of those AP's
are, i.e. how many clients are associated to them. For that, you need a
handy tool like my personal favorite—AirMagnet [reviewed here].
Figure 5: AirMagnet Handheld showing APs at CES2004
Figure 5 shows just one of the many views that AirMagnet can provide of
all in-range wireless equipment. This tree-type view shows access points
(the little towers) and their associated clients (the little laptops).
You can see that there are plenty of idle APs, and with a few stylus
taps, AirMagnet can show the channel they're operating on, too.
Unfortunately, AirMagnet and similar wireless LAN analysis tools are not
intended for consumers and are priced accordingly ($3000 and up). If
you're handy with Linux, you can try Kismet, but otherwise you'll have
to make do with counting APs and looking at signal strength to guide you
in your choice of new channel.
NOTE: Don't bother trying to change the operating channel of your
wireless client. The operating channel of Infrastructure-based wireless
LANs (those that use access points or wireless routers) is determined by
the AP, not the client. All you need to do is change the AP channel, and
its associated clients will follow.
One of these WLANs is not like the other
Finding unused airspace will solve most neighboring LAN problems. But if
that's not an option (or you've tried it and you still need help), you
might just need to tell your laptop to not go wandering and stay home!
In its zeal to make wireless networking as easy and automatic as
possible, the default behavior of WinXP's built-in Wireless Zero
Configuration utility is very, well, promiscuous. Once you use it to
successfully connect to a wireless LAN with a particular name (i.e.
SSID), it automatically considers that a "preferred network"
and will connect your wireless computer to it whenever it comes within
range.
This convenient feature becomes a problem, however, in areas where there
are multiple access points with the same name, but that are not part of
the same network! As far as your wireless laptop is concerned, APs with
the same name are part of the same network (this is how wireless LANs
with multiple APs are set up, actually). Since your laptop has no way of
knowing that those other APs with the same SSID as your AP are actually
belong to your neighbors' APs, it will at some point try to connect to
them, usually when it detects an AP with a stronger signal.
But if your neighbor's AP happens to have WEP or WPA encryption enabled,
or is using MAC address filtering (association control), your laptop
won't be able to connect. What you'll see is your wireless connection
dropping, then (maybe) reconnecting to your own AP (you may have to
rescan for networks and manually reconnect). You may think that your
wireless network has gone haywire, but in truth, your laptop's wireless
card is just trying to do its job and keep you connected to the best
signal available.
What makes this situation worse—by interfering with proper diagnosis
of the problem—is that the Available Networks part of XP Zero Config
doesn't show multiple instances of the same network name (SSID). So
unless you run the wireless client utility that came with your adapter
and it shows every AP that it detects, you can only guess at what your
wireless client is really seeing.
Turning again to the ASUS WL100g card as an example, Figure 6 shows that
its client utility accurately displays multiple APs, even if they have
the same SSID.
Figure 6: Multiple APs with same SSID
What this particular utility doesn't allow, however, is to force
connection to a specific AP. In the quick experiment that I ran, I could
only get a connection to the AP with the strongest measured signal (at
the time when the client scanned for APs). In most cases AP association
rules are baked into a wireless adapter's driver and unfortunately, most
manufacturers don't give users any control over it.
Making your client stay home
Fortunately there are two things you can do to keep your wireless
clients on their own network. The first is to change your SSID from its
default value and one not used by nearby WLANs. Choose something unique
and which doesn't divulge your name or location. Using only letters,
numbers, underscores and no spaces should give you plenty of options for
the SSID name.
TIP: Referring back to Figure 4, some APs allow you to set an "AP
Name", which has nothing to do with its SSID, but is used to tell
multiple APs apart for management purposes. So make sure you change your
APs SSID (sometimes called ESSID).
The second step—if you're using WinXP—is to clear the Preferred
Network list and make sure connection to non-preferred networks is
disabled. You'll find the icon for your wireless adapter in the Network
Connections window (Start > Settings > Network Connections).
Right-clicking on the icon and selecting Properties should bring up a
window similar to Figure 7.
Figure 7: Too many "Preferred" networks
The upper portion of the window lists Available, i.e. currently
detected, wireless networks while the lower section lists Preferred
networks. Simply delete every network except yours by selecting them and
clicking the Remove button. Then click the Advanced button to bring up
that window, and make yours look like Figure 8.
Figure 8: Advanced Wireless Network Properties
This will prevent your card from trying to connect to Ad-Hoc networks
(in the unlikely event that any are around), but more importantly
prevent automatic connection to any new wireless LANs that appear in
your neighborhood.
TIP: If your wireless client is moved to another location where wireless
LANs are present, you should repeat the Preferred Network clean-out when
you return to your normal location.
If you're not running WinXP or use your wireless adapter's client
utility instead, check it to see if has a similar "preferred
network" capability, and perform a similar clean-out if possible.
Some utilities use "connection profiles", which store all the
settings for connecting to a particular WLAN and require you to manually
switch among them. You shouldn't have to perform a "clean-out"
in this case, but you may need to delete unwanted profiles if your
client utility automatically creates them when it detects new networks
and automatically switches among them.
Survey the possibilities
If you can't find a clear channel and still have problems after locking
your clients down so that they don't go straying to other WLANs, it's
time to see if you can do something about all the unwanted signals
bombarding your poor little WLAN, i.e. reduce the RF noise. This is an
area where understanding the problem is especially important before
implementing a solution and that means you'll need something to measure
signal strength to help you perform a site survey.
A "site survey" is just a fancy term for walking around with
something that can measure wireless signal strength and recording what
you see. You can do this with your wireless laptop, provided that its
client application has the ability to show all in-range APs, their
channels and some indication of signal strength. It doesn't matter
whether the signal indicator reads in %, dBm, or no units at all, or
whether it measures signal strength, signal quality or both, since
you'll be looking mainly for changes in whatever indicator you use. It's
also helpful if the client utility has a fast-responding, real-time
signal indicator for the AP that it's connected to, and even better if
it can plot the signal strength over time.
If your client utility doesn't have any of these features, you have a
couple of choices, both of which will probably involve purchasing a new
client card. This isn't as bad as it sounds since the price of 11b cards
has dropped down to around $50. NetStumbler is a great free tool for
checking out your wireless neighborhood. It includes signal and noise
plotting capability and comes in versions that run on Windows and
PocketPCs. It works with cards using the Lucent (now Agere Systems)
Hermes chipset, which includes the ORiNOCO 802.11b card, but check the
release notes for the version you download for a list of other supported
cards.
If you'd rather go with a commercial solution, the client utility that
comes with ASUS WL-100 and WL-100g cards is excellent. Check my WL-100g
review for more details.
Once you have your signal measurement tool, take it to your problem
location(s) and see what it can see. Since you've already addressed the
SSID-related problems (right?), you're mainly looking for the signal
levels and channels of neighboring APs. The APs most likely to be
causing you grief are those on the same channel and with signal levels
greater than or equal to that of your own AP.
Once you understand the wireless environment your client is operating
in, you're ready to take steps to change it. Most of the techniques at
your disposal are described in the Wireless LAN Performance Improvement
NeedToKnow, but my general advice is to focus on solutions that reduce
interference from neighboring WLANs vs. boosting your own signal and
creating a problem for someone else. Many times a little aluminum
screening, intelligent use of directional antennas and just relocating
your AP can go a long way.
TIP: You can improve antenna performance even if your AP doesn't have
upgradeable antennas! See this FAQ for more info
802.11a
Sometimes, it's just time to move on. If your efforts at battling
802.11b / g interference are proving to be futile, consider changing to
802.11a. Contrary to what you've probably read, current-generation
802.11a products have performance equal to or better than many 802.11b
and g products. And since they operate in the relatively quiet (at least
for now) 5GHz band, all the neighboring 11b and g WLANs that are causing
you so much grief will, for all intents and purposes, disappear.
If you go this route, avoid single-band, i.e. 11a-only products. They
all have first-generation technology, which did have inferior range.
(See the Second-generation 802.11a NeedToKnow for more info.)
Shop only for dual-band (11a / 11g) or dual-band, tri-mode (11a/b/g)
products. Also be sure to hit the shopping search engines and check for
bargains since products containing 11a seem to move slowly and retailers
occasionally like to clear out inventory.
Social Engineering
As much as you may hate to admit it, you may not be able to solve your
wireless problems by yourself. Since the heart of the problem is caused
by lack of coordination (and communication) among users trying to run a
number of wireless LANs in too small an area, the most effective
solution would be to apply the design techniques used in large multi-AP
WLANs.
You may be surprised at the willingness of people to work together to
solve a common problem—especially if they don't really have to do
much. Put up a sign and call a meeting of your apartment building, dorm,
or neighborhood. If you've got the a WLAN problem, chances are others do
too.
Once you've got the interested parties together, the main order of
business is to see if you can work out a satisfactory channel assignment
scheme. If there are only three APs involved, the job is pretty simple.
But if you're dealing with more APs, you'll have to put in a more
effort.
Make a diagram of the APs as close to scale as you can get it. Once you
have the APs located, it's just a matter of juggling channel assignments
so that APs using the same channels have the lowest signal strength with
respect to each other. Since signal strength is primarily related to
distance, a practical approximation of this rule is to locate
same-channel APs as far apart as possible. In some cases, building
construction and other RF-unfriendly obstacles like trees, water,
screens, etc. may allow you to bend this "farthest-distance"
rule a bit. In multi-story situations, be sure to think in three
dimensions because radio waves travel in all directions!
Once you have your channels assigned, but sure to assign unique SSIDs to
each AP. Though you want to treat the APs as one big WLAN from a channel
assignment point of view, you still want multiple, separate WLANs from
an operational point of view. Unique SSIDs will keep clients from trying
to roam where they're unwanted.
Finally, if you find that your neighbors are unfamiliar with the wonders
of WEP / WPA, MAC address association control (filtering) and other WLAN
security features, help them get that set up too. With the problem of
WEP-related throughput essentially gone from current-generation WLAN
equipment, there's no reason to run wide-open WLANs if you don't want
to.
What Doesn't Help
When faced with a wireless LAN that won't behave, people will try most
anything in search of a cure. But some "solutions" don't
really help and can actually hurt your (and your neighbors') WLANs
performance. Let's look at some "solutions" commonly
suggested:
Turning on WEP / Using authentication
Encryption methods such as WEP and WPA or using one of the many 802.1x
authentication methods will keep unwanted clients from associating with
your wireless LAN, but not from trying! Preventing association keeps
clients from using significant amounts of your WLAN's bandwidth, but
association attempts—especially in areas with many wireless
clients—can still cause performance to degrade, particularly in slower
802.11b networks.
Put another way, use of encryption doesn't do anything to the radio
signal itself, but only to the information it is carrying. So while I
recommend enabling WPA (or WEP if WPA isn't available) for security
reasons, it doesn't do anything to reduce the effect of too many radios
in too small a space.
Turning off SSID broadcast
Although it's not the security precaution that some articles make it out
to be, and it won't really help keep your clients from trying to
associate with other wireless LANs, turning off the broadcast of your
APs SSID could help your neighbor's clients to stay on their own WLAN.
But again, change your default SSID, too, because if your neighbor's
laptop detected and saved it in its "Preferred network" list
at some point, it will still look for it during its association
attempts.
Selecting 11g-only mode
Owners of 802.11g equipment may have a few other knobs to fiddle with,
depending on the products they have. Some 11g APs allow disabling of the
802.11b "Protection" mechanism that enables slower 11b clients
to interoperate with faster 11g APs. Disabling "protection",
however, is similar to enabling WEP or WPA in that the radio
signal—and its negative effects—is still present. Shutting off
protection, however, can have a larger negative performance effect on
your WLAN than enabling WEP or WPA.
Figure 9: The effect of disabling 11b "protection"
(click on the image for a full-sized view)
Figure 9 shows throughput plots made with a Linksys WRT54G router
[reviewed here] and WPC54G CardBus card. Both traces were made with the
pair set to 11g-only mode, i.e. "protection" disabled. The top
trace shows throughput with no other clients in-range, while the bottom
trace shows the effect of a single 11b client card just trying to
associate with WRT54G—almost a 20% reduction in average throughput!
The amount of performance hit varies with many factors, but is so
pronounced because turning off protection essentially turns off the
coordination between 11b and 11g AP and clients. This raises the
occurrence of transmit collisions and increases the probability that
data will need to be retransmitted, lowering throughput.
TIP: For more on 11b protection, see the 802.11g NeedToKnow - Part 1.
Boosting your signal
As pointed out in the Wireless LAN Performance Improvement NTK, signal
boosting solves at best half the problem since it can only help a client
to "hear" an AP better and not vice-versa. Though I have to
admit that boosting could help keep your client from straying by
providing a stronger signal to latch onto, I would personally use this
only as a last resort. Solving a problem by passing it on to someone
else (your neighbor) isn't really a solution, in my opinion.
Using Super-G
Super-G's controversial "channel bonding" technique can
cause severe problems in nearby WLANs under certain conditions (see
the Atheros Super-G NeedToKnow - Part 1 for the details). But I put
using Super-G for this purpose in the same category as signal boosting,
i.e. not a real solution and something that may
cause more problems than it solves.
Closing Thoughts
The 802.11 protocol that modern wireless LANs are based on is amazingly
robust and capable of supporting dozens, if not hundreds of stations in
a given area. The key to successful operation, however, is cooperation,
not competition.
The old saw of "thinking globally, and acting locally" really
is the best approach to having multiple wireless LANs operate
successfully. Now that you know how, get out there and lead your
wireless neighborhood to wireless networking harmony!
_________________
wiresounds
http://www.minus-sounds.com
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