Tips on Site Surveys and Installation of 802.11 Wireless LAN Access Points and Accessories
By Joe Mehaffey
Well, I wish deciding where to
put AP in buildings was an exact science and I could tell you exactly
how to proceed. But.. It is not that easy. Unfortunately
every building is different and while we can PLAN how things might work
inside a structure, a site survey after the building is built is
the only way to INSURE that all necessary areas are covered completely
by Wireless LAN signals..
Here are some things to consider:
1) Call backs are EXPENSIVE and my
experience with cheap APs is that there seems to always be
"something" wrong with them and just a few callbacks and you have
spent your profits. In your own home installation where you can
"mess with equipment" if it does not operate properly, the cheap units
are good buys and serve knowledgable users well and cost
effectively. With reference only to wireless 802.11 gear, I
have had good results with Cisco, SMC, and Senao while Dlink and
LinkSys brands have been less reliable.
2) The average PCMCIA card has an
output of about +17dbm (50mw) and so it does you no good to have a
500mw transmitter on one end (the access point) and a 50mw transmitter
on the client end (laptop). Transmit power should be balanced.
Since there are some good 100 and 200mw client cards out there, a good
design goal for your AP is around 200 to 300mw output BUT ONLY if you are using 200mw cards in your laptops. Gain
antennas are GOOD and a 6db higher gain antenna on just ONE end of a
circuit (say on the AP end) will double the range of the system since
it receives AND transmits with equal gain.
3) Stay away from the current WRT54g
Linksys AP as it periodically "locks up" on short power glitches.
It was worse until the midsummer firmware update and that helped it a
lot. However I have had one additional lockup on the new
firmware. The symptoms are that you can communicate with the unit
on the wired side just fine, but there is no RF output. A power
down/reset is necessary to get it going again. Linksys does not
seem to consider this a serious problem, but I do. The WPC54g
works fine but not if you want to do automatic drive mapping during
boot time as mentioned in my article.
4) The QUALITY equipment is Cisco but
I don't know if they have 54g equipment out yet. They sometimes
have problems but then FIX them!
5) I have tried out the SMC 2804WBR
and the PCMCIA card SMC2835W. Both work very well and are easy to
setup. In temperature testing with the SMC-2804WBR, I found
it shuts down (and does not self recover) if the temperature exceeds
120F. No other unit I have tested has failed to operate over its
specified temperature range. SMC techs said I was free to send it
in for exchange, but they doubted I would see any improvement. So
I put this particular unit to work in a client's basement where I am
sure it will live happily.
6) I like the Dlink 54g card but it
seems to have problems on any DELL brand computer unless it is running
XP and then it works fine. Works OK on Toshiba but other brands I
do not know about.
7) Senao has a 2511CD PLUS card that
has 200mw output (802.11b). If you use (say) a Cisco 240 AP
(50mw) with a 10db amplifier from RFLinx.com, you about double to triple the
range of the 50mw cards/AP combo. Senao is said to be "soon" going to have a 200mw
54g card and these folks seem to do a nice job. EnGenius 200mw is
a rebrand of the Senao product.
8) When you do a site survey think
about the fact that 802.11 signals are very low power and do not
readily penetrate many building materials. Consider the following
when designing a large 802.11 wireless LAN installation:
a) brick, metal
siding, metal screens in windows/doors, metal doors, some concrete
siding, some wood siding and cellulose insulation in walls all severely
attenuate signals at 2500mhz. Frame houses will generally have
more penetration of signals than brick houses but you won't know for
sure until you check. Frame "looking" houses might be aluminum siding
and aluminum foil backed insulation can sometimes be found inside
walls where you would never expect it.
50mw signals will pass thru about TWO normal sheet rock walls before
being excessively attenuated. After two walls, you will
find that the system will work some places and not others and I can't
predict without an actual site survey which will be good/bad.
c) Generally, try
and put the AP and its antenna inside the living space protruding from
a ceiling. Dropped ceilings are good. If you cannot find a
ceiling spot, you can put the AP in a closet or attic but
remember you are already signing up for attenuation thru the attic
floor. Also remember that when you look "sideways" at a shallow
angle (say 45 degrees) from an attic, in one direction perpendicular to
the ceiling joists, you are looking through a LOT of wood.
d) concrete floors
in multistory buildings are essentially not penetrated by 2500mhz
signals. At least, do not COUNT on it working on the other
side of a concrete wall, floor or ceiling.
e) If you use Cisco
350 series bridges (or other repeater/bridge techniques) you do not
have to string wires as these devices can "repeat" signals.
BUT.. If you do this, at each REPEAT, you only have HALF the
thruput left. IE.. A user operating directly on the main AP gets
11mbps. Then out of the first repeater he can get only
5.5mbps. If there is a second repeater and our user
operates through it to the first repeater and then back to the
AP, he will be down to about 2.75mbps maximum theoretical
9) If you do use gain antennas
you have to consider the antenna PATTERN. For
instance, if you use a high gain omni directional antenna on a
tower or high structure, the antenna may have very little signal
at ground level around the tower unless your "looking up at the tower"
elevation angle is less than maybe 20 to 30 degrees. If you buy a
high gain antenna, ask for a specification sheet and look carefully at
the antenna pattern specs and see if it meets your needs.
10) I suggest you not try and install RJ45
male connectors in the field but rather use jacks in the wall/on the
wall and premade patch cables to connect to devices like routers and
APs. The reason is that I find that field installed RJ45 plugs
are prone to go open and cause service calls. (If someone
knows product and technique for field installing RJ45 plugs in a
reliable manner, PLEASE let me in on it!)
11) I suggest you run multiple cat5 cables to
everywhere you MIGHT want to have an access point in new construction. Wire goes in
EASY while the walls and ceilings are open and HARD and time consuming later.
Have fun! Wireless is fun,
but it can be frustrating..
Comments? Additions? Suggestions? Error correction?
Please Email Joe Mehaffey
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