Wireless Home Networking - Index of

Chapter 18: Ten Ways to Troubleshoot Wireless LAN Performance
Sometimes, what’s causing you trouble is something
simple — which you can fix simply. For
example, one of us (and we won’t say who —
Pat) was surprised that his access point just
stopped working one day. The culprit was his
beagle, Opie, who had pulled the plug out of the
wall. As obvious as this sounds, it took the
unnamed person (Pat) an hour to figure it out.
Now, if someone told you, “Hey, the AP just
stopped working,” you would probably say “Is it
plugged in?” The moral: Think of the obvious and
check it first.
Here are a few more “obvious” things to check:
Problem: The power goes out and then comes
back on. Different equipment takes different
lengths of time to reset and restart, which causes
the loss of connectivity and logical configurations
on your network.
Solution: Sometimes, you need to turn off all your
devices. Leave them all off for a minute or two,
and then turn them all back on, working your way
from the Internet connection to your computer —
from the wide area network (WAN) connection
(your broadband modem, for example) back to
your machine. This process allows each device
to start up with everything upstream properly in
place and turned on.
Problem: Your access point is working fine, with
great throughput and a strong signal footprint,
until one day it all just drops off substantially. No
hardware problem. No new interferers installed
at home. No new obstructions. No changes of
software. Nothing. The cause: Your next-door
neighbor got an access point and is using his on
the same channel as yours.
Solution: This problem is hard to debug in the first
place. How the heck do you find out who is causing
invisible interference — by going door to
door? “Uh, pardon me; I’m going door to door to
try to debug interferers on my access point. Are
Check the obvious
you suddenly emitting any extraneous radio
waves? No, I’m not wearing an aluminum foil hat.
Why?” Often, when debugging performance
issues, you need to try many of the one-step solutions,
such as changing channels, to see whether
they have an effect. If you can find the solution,
you have a great deal of insight into what the
problem was. (If changing channels solved the
problem, someone nearby was probably using
the same channel, and you can then start tracking
down who that is!)
The wireless utility for the adapter may have a
tab, called a site survey or station list, that lists the
APs in range. The tab may show your neighbor’s
access point and the channel it’s on.
APs that follow the 802.11n draft 2.0 standard
dynamically switch channels when there’s too
much interference. The 802.11n equipment we
have seen does not even give you an option to
choose a channel because of this dynamic
switching capability. Keep in mind that the higher
speed of these APs is achieved by combining
channels so they can send and receive data on
more than one channel at a time and can use
more than one antenna to send and receive data.
To take full advantage of the dynamic nature of
802.11n, you need an 802.11n AP or router as well
as an 802.11n network adapter in your computer.
Before you chase a performance issue, make
sure that you have one. The advertised rates for
throughput for the various wireless standards are
misleading. What starts out at 54 Mbps for
802.11g is really more like a maximum of 36 Mbps
in practice (and less as distance increases). For
802.11n, it’s more like 125 Mbps at best, rather
than the 248 Mbps you hear bandied about. You
occasionally see high levels (like when you’re
within a few yards of the access point), but that’s
rare. The moral: If you think that you should be
getting 248 Mbps but you’re getting only 100
Mbps, consider yourself lucky — very lucky.
329