Reliability of wireless connections can be affected by a number of
factors, some of which you have no control over....


  The popular 2.4GHz band provides 11 channels.  But at full
power/bandwidth, a signal "spreads out" to adjacent channels -- two
channels to each side, for a total width of five.  
  This means that in a small area, you can only fully use three
channels:  1, 6, and 11.  Anything more, and you either have to reduce
power (reducing range and bandwidth), or deal with interference between
the devices.  So one of the first things to look at is how many of your
neighbors also have wireless routers.
  With the router on the first floor and the client on the second,
you're trying to pass signal through the floor (and maybe some walls). 
Even wood will cause some loss of signal strength, but this can be a lot
worse if there is a lot of steel in the structure.  And of course there
is electrical wiring and water and gas pipes....


  Even if your neighbors have their own WiFi routers, they may be
(accidentally or not...) connecting to yours.  It's estimated that a
router (or access point) can probably handle up to about 20 client
devices -- but only one can send or receive at a time!
  There may be a nearby non-WiFi source of interference.  Some microwave
ovens and older cordless phones can create interference in this band. 
About the only way to track down something like this is with a "spectrum
analyzer" -- most are expensive, but the "Wi-Spy" plugs into a USB port
and is only about $100...  And again, the source of the interference
might be inside a neighbor's condo.

  Many wireless clients, especially laptops, can also use their wireless
capability to share their Internet connection with others.  Even if you
haven't configured this -- there's a virus that turns on this
capability, advertising a name like "Free Public Internet"....  Most
people whose machines are infected with this never know it, unless it
leads to problems similar to what you describe.


  Your wireless gear is *probably* using the 2.4GHz band and the 802.11g
WiFi standard.  Two alternatives you might try are:
  - 802.11a, which uses the 5.0GHz band -- lots more channels, and it
gets you away from interference in the 2.4GHz band.  It tends to be a
bit more expensive, and you'd need to both replace your router and
provide USB or PCMCIA wireless cards for your laptop(s).
- 802.11n uses radio technology differently in order to handle
obstructions and reflections off of walls, etc -- this nay also allow it
to cope with interference more successfully.  Again, hew router and WiFi
cards....

  The "power line" networking gear that I've seen avoids wireless
altogether.  You router plugs into ComCast and "injects" a signal
modulated onto the household AC current that it is plugged into; the
client plugged into the same household current detects and demodulates
the signal (and reverses the process for traffic the other way).  This
should sound a bit funky -- both devices may need to be on the same
circuit for it to work at all well -- and if they're on different
floors, I wouldn't assume that.  I don't know of a good way to test
before you buy to make sure this option will really work.

  IP addresses have to be unique within a network for traffic to get
where it's supposed to go.  So the message you're seeing does indicate a
problem -- it's just hard to guess what's causing it.   In a dynamic
environment, you might see it briefly about once a year before it gets
automatically corrected -- any more often, and something almost
certainly needs to be corrected.

David Gillett, CISSP CCNP


  -------- Original Message --------
 From: Gordon Totty <[log in to unmask]>
 Date: Wed, October 10, 2012 5:02 am
 
> Now, D-Link is advertising a new PowerLine AV + Mini Adapter Starter Kit. I'm wondering if I should buy it
> to replace the Netgear range extender, or if it would be wiser to just find a far better wireless router. 
> Or...? 
... 
> Last question: what should I do when I get a message about an IP address conflict? I've ignored these
> messages because I have no idea what I should do. I get this message about once a month.
 

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