Re: Wireless for RV campground

On Sun, 25 Jan 2009 17:37:18 GMT, Stephen <stephen_hope@xxxxxxxxxxxx>
wrote:

anyhow - Jeff does this for a living so has answers based on more
experience than i do.

Yeah, something like that. Actually, I get most of answers from many
years of experience doing everything from RF design to supporting
installed systems. The problem is that I only see the systems that do
NOT work. I'm sure there are wireless systems out there which are
installed, work out of the box, do everything expected, never needs an
upgrade, and never does anything unusual. These may exist, but I
never get to see them. Everything I see is presumed to be broken or
defective.

Time for a rant. My opinion of 802.11n is far from complimentary.
I'll skim the highlights and leave out the politics (for now):

1. The standard mixes spatial diversity and beam forming
technologies, which are two radically different technologies. In many
cases, the customer doesn't have a clue which they need or are using.
Some even do both:
<http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps10092/white_paper_c11-516389.html>
<http://www.theruckusroom.net/2009/01/cisco-validates-smart-wifi.html>
The problem is that spatial diversity requires a compatible 802.11n
client radio. Beam forming will work with anything.

2. Most of the customers that have 802.11n router eventually have me
disable the spatial diversity feature and just use it for 802.11g.
That's because at any distance beyond a room or office, the 802.11n
error rate is so high, that the router reverts back to 802.11g speeds.
You can watch it happen by monitor the client connection speed.

3. 802.11n is all about speed, not range. However, that won't stop
companies from publishing irreproducible test results and graphs
showing the 802.11n does increase range. For example:
<http://www.xirrus.com/pdfs/Tutorial_802.11n.pdf>
on Pg 8 shows a graph of range versus rate for a/b/g/n. It would
appear that at long range, "n" still works. However, that's not the
way it works or is shipped. What happens is that when the error rate
drops below the threshold where the retransmissions slow the
connection down to below 802.11g speeds, the access point just
switches to 802.11g and disables "n". The graph shows the connection
speed, not the actual thrupt. The lack of speed units of measure
should be an obvious clue. Xirrus even dumped the graph in their
tutorial on wireless range at:
<http://www.xirrus.com/pdfs/Tutorial_Range.pdf>
<http://www.xirrus.com/library/>
Despite this issue, the Xirrus tutorials, wall charges, and webinars
are well worth reading. The product is also very interesting, but is
far too expensive for my typical customers.

4. 802.11n is a win in one respect. It minimizes the air time used
to move a given amount of data. Therefore, more users can share the
same air time. The faster the traffic moves through the air, the more
users a system can handle. Visualize a coffee shop with one user
stuck on 1Mbits/sec 802.11b. In the same time that this user takes to
download some amount of data, a 54Mbit/sec 802.11g connection can
download 54 times as much. With 802.11n, possibly 300 times as much.

5. Outdoor use of the spatial diversity type of 802.11n is a waste of
time. That's because the distances involved make the possibility of
using reflections to improve speed somewhat dubious. Spatial
diversity bonds multiple streams, with different propagation times.
You'll find those in the typical highly reflective indoor environment,
but not so much outdoors.

However, beam forming type of 802.11n should work nicely outdoors. If
there's a nearby source of interference, the access point will put a
big hole in the antenna pattern and effectively prevent the
interference from becoming a problem. At the same time, it increases
the gain in the direction of desired client radios. For an
interference infested RF environment, beam forming is a good thing.
Too bad some can't tell the difference between an incident and
reflected signal to eliminate multipath. Also, too bad the antenna
has to be a PCB phased or switched array inside the access point. One
could install the entire access point on top of a pole, but few
commodity routers a made for this. In addition, since external
antennas are both forbidden and don't work anyway, it's not possible
to customize the antenna pattern by using sector or directional
antennas. However, there are beam steering outdoor routers and
antenna systems (who's names I can't seem to find).

6. The lack of an external antenna connector in all 802.11n is what
kills 802.11n for me in many applications. I often have to install a
panel or sector antenna in an office up high to customize the
coverage.

Enough ranting for now.... lunch time.

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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