[lug] Local phone service with Comcast--any experience?

[Daniel Webb]

On Sun, Jun 26, 2005 at 02:18:32AM -0600, Nate Duehr wrote:

> If your SNR reported by the routers drops by 60dB, remember that dB is a 
> logrhythmic scale and 60 is a very large number on a log curve, compared 
> to a linear scale.  Your link is getting HAMMERED by something.  60dB is 
> a big hit.

I probably said it wrong.  The SNR drops by about 30dB, which is still getting
hammered.  Today I relocated one of the APs because the trees budding out have
gotten in the way of the line of sight (lesson: don't site APs in the winter
unless you don't mind moving them in the spring).  Now I get -65 from the
"rssi" reading from the wl util that Linksys released for their routers.  I
guess that is a negative dB rating, because it used to be -78.  I won't
complain at all since the APs are about 200 feet apart, and -65dB is just fine
for that distance based on my previous experience.  I'm hoping this new
improved signal will be enough to beat the noise.  I'm using directional
antennas on both ends of the link now (homemade 4 inch diameter, 10 inch long
cantennas).

> Also in the microwaves, "line of sight" isn't always all you need.  Path 
> losses in free air actually start to become important at 2 GHz and 
> above.  Especially at the low power levels of 802.11b unless you're 
> running illegal amplifiers and exceeding the FCC prescribed limits for 
> point to point or point to multipoint.  Most amp manufacturers are 
> counting on you using very lossy feedline to be legal.
> 
> To give an example of the path loss problem, a friend recently did a 31 
> mile shot with some gear in the 5.8 GHz band and the calculated path 
> loss was high enough that aiming the dishes was super-critical.  Nothing 
> less than each antenna directly pointed into the Fresnel zone of the 
> other antenna would create a workable link.

I'm guessing the path loss isn't much more than the distance-squared loss at
only 200 feet, is that right?  31 miles is damn impressive, that was
profession gear I assume?  I have read that many people have gotten several
miles with WRT54Gs and a decent antenna on both sides, but never anything
close to 30 miles.

> The 2.4 GHz band is almost literally a free-for-all.  I'm licensed to 
> use it an can transmit right on top of the 2.4 GHz 802.11b band with a 
> rather large signal compared to the allowable signal from an 802.11b device.
> 
> 802.11b is not a licensed user and operates under the FCC rules for 
> "ISM" (Industry, Science, and Medical) devices.  Part 15 devices are 
> also in that band.
> 
> It's a mess in any densely populated area of the country.  Most wireless 
> ISP's move away from it as soon as they can and buy more expensive gear 
> at 5.3 and 5.8 GHz.

Are either of those bands not ISM bands.  In other words, will they be just as
polluted as 2.4 GHz in a couple of years?  I'm assuming 5.8 GHz is ISM,
because I've seen 5.8 GHz phones.

> >[1] At least, I think that's how it works... radioheads correct me if 
> >that's
> >not how the carrier sense part of 802.11b works.
> 
> I can't remember ever looking to see if the spec says a client has to 
> hear the base in Managed mode before it's allowed to transmit or not.
> 
> Certainly they transmit constantly in Ad-Hoc mode when looking for a 
> link partner.
> 
> And unless broadcasts are turned off the bases transmit constantly 
> saying "I'm here"...every few seconds.

Let me rephrase what I was trying to find out...

802.11 uses "carrier sense" along with other things to avoid collisions.
In other words, if a 802.11 radio hears the carrier of another radio, it
doesn't transmit.  What I'm curious about, is what about non-802.11 radios,
like cordless phones.  If a 802.11 radio hears a cordless phone transmitting,
will it not transmit because of this, or are the cordless phone carrier and the
802.11 carrier different enough that the 802.11 radio just sees the cordless
phone as noise?

> Oh yeah... one more trick for 802.11b... the spec shows that when in 1-2 
> Mb/s mode, less "channels" are used (you know the channels overlap 
> right, and the only non-overlapping channels are 1, 6, and 11 in the 
> U.S.?).  One of the side-effects of using less channels is that more RF 
> energy is expended into that smaller bandwidth.  Sometimes when a 10Mb/s 
> link is flakey, forcing the routers to 1-2 Mb/s mode on each end can 
> gain you a considerable jump in SNR.

I've definitely seen this effect.  The link I've had running for 2 years now
is fine at 1 Mb mode, but almost unusable in "auto" mode.