On Monday, September 22, 2003, at 10:05 PM, [EMAIL PROTECTED] wrote:
1 - '11a cards don't emit as much power as 802.11b - they're designed for very
close distances (50m) in an office where 100 Mbit e-net is being replaced.
(at least that was the design concept) - power output on 11a is generally
speaking lower than b, especially at same price points.
This isn't the reason for lower power on OFDM. The issue(s) is (are) PAPR and cost.
2 - Ok - so you're going to now fork over the $ and buy an amp for 802.11a.
Shopping for 802.11b amps (we've all drooled over the 10W "export only"
amps :) you get accustomed to 500mW, 1W, 10W, etc. 802.11a - lucky to find
a 1W amp - and it compares with those 10W "export amps" for 11b. So now
I'm starting to re-think this - if the amp is needed, I'm in the poor house.
And you'll need to consider how you're going to control same. Amps for 11b take advantage of the fact that the preamble is relatively long. With 802.11a/802.11g, there is a much shorter time before the (short) training symbols have to be received at the other end of the link. If your amp ain't on by then, you're SOL.
(Jim knows the number of symbols that can be passed by popular OFDM 802.11x chipsets, but isn't going to reveal the answer.)
3 - cards. With the advent of 802.11g, 802.11a cards may go the way of
fiber in offices. Why spend more for a non-compatible card, when you can
spend less for a compatible card that's almost as fast. Then I called a
"friend of a friend" who works at intel (pre-centrino days) - he says forget
11a. Go with b/g. Intel is backing b and then g on the motherboards of
both desktops and laptops under a super secret name soon to be anounced. 11a
will die / fade into oblivion.
Its almost zero cost to add 802.11a, and there are severe issues with 802.11g attempting to leverage the existing 3 channels in 2.4.
The air is cleaner @ 5GHz. 802.11a will survive.
4 - power - but but but - 5.8 is SOOOO quiet when I go up there on my
spectrum analyzer - and I even bought dual band antennas that do both 2.4
and 5.6/5.8 so I'm ready to go to 5.8 any time!... But then I did the
path loss at the higher frequency. Power faster at 5.8 than 2.4.
Alot faster. Add relative humidity and God forbid rain and path loss
is really nasty at 7-10 miles. How much were those amps again???
You're forgetting antenna gain. If you can manage to keep the same physical size antenna, then the additional gain (the antenna is electrically 'larger" at 5.7GHz than at 2.4GHz) will over come the path loss as long as your path-loss exponent is 2.0.
Finally - since 802.11b was running with 47 dB of link margin (!) even
with the summation of all the silicon valley noise in 2.4 and ricochet's
crappy 1W 2.4 GHz radios on every poletop (we had 4500-5000 of those to
contend with) -- there was no reason to migrate to 11a, even though
funds had been set aside. 11b became the link. Even though I had planned
for 11a to bd
Eventually you'll be interference limited.
If you _really_ want to be forward thinking then 802.11g will work with peoplenever - 8Mbit is the max we'll probably ever install. I want to consider
who just have b cards, and still offer high speeds later. Realistically,
when do you think you'll pull an OC3 into your complex?
the connection rates at the low signal areas. I don't know if it's
better to have no signal in some areas or variable signal in all areas
(or if a vs b would do that)
802.11a and 802.11g can both train down to 6Mbps, which has about the same SNR requirements as 802.11b's 5.5Mbps. As an additional bonus, some 802.11g/802.11a chipsets have superior rx sensitivity to any existing 802.11b chipset.
So you might just find that 12Mbps OFDM goes 'farther' than 11Mbps 802.11b from your favorite vendor.
As for the antenna - it's not so much were you put the antenna as it is
where the ANTENNA puts the signal.sure but give that I don't want any wierd lobes i expect we'll use a pretty standard omni ...
Rooftop omnis with electrical downtilt aren't bad - without it, they shoot out
to the hroizon. I have a professional (REAL professional - uses a 3"
diameter pole mount) 12 dB (conservative) 2.4 G omni that is 4' long.
This is a short, slippery slope to being interference limited.
If you really need power - buy an amp. 100mw in - 1000 mw out (+10 dB) -
and many amps have a GaAs FET pre-amp - 10 dB boost on the receive side -
for a link margin theoretical improvement of 20 dB, versus the 200 mW
card's theoretical 3dB, and often actual -7 dB or worse depending on how
they built their receiver's front end.
You do realize that with 1000mW of tx power, you're limited to 6dBi of antenna gain, right?
-- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
