On Thu, 18 Oct 2007 04:04:01 +0000 (UTC), (Geoffrey
S. Mendelson) wrote:

wrote:
Just to chime in on this topic, I think some incorrect information was
passed along regarding legality of transmission power - the FCC does
in fact limit unlicensed maximum transmission wattage on the 2.4Ghz
range, however the use of high gain antennas is NOT illegal. You can
use extremely high gain directional antennas to achieve links over a
20+ mile LOS link, and still be within legal limits for unlicensed
operation.

Running a 32 km LOS link is problematic in practice. At least the
first Fresnel zone should be clear of obstacles. At midpoint you
should have about 30 m free space _below_ the direct optical path
(i.e. you would have to add 30 m to the antenna towers), but still you
should have 10-20 dB fade margins for reliable operation.

Think of it as using as talking thru a funnel - all it is
doing is using that same power more efficiently and more focused. Now
if you are talking about running the signal thru and actual signal
amplifier, then yes, that would be illegal.

Unfortunately, this is also incorrect. The limit is for EIRP. The more focused
the beam, the higher the EIRP.

The exact limit the U.S. is 1 watt EIRP for mobile devices and 4 watts
EIRP for fixed (point to point) links. That's a combination of transmiter
power plus antenna gain plus feed line loss.

Assuming ordinary +20 dBm cards with 16 dBi antennas at both ends, you
would end up to the +36 dBm EiRP (4 W) transmit power. The free space
loss at 32 km and 2.45 GHz is 130 dB, so the receiver would get -94
dBm, which is very little for broadband traffic, especially that
figure does not contain the fade margin. In fact using +10 dBm cards
and 26 dBi antennas would give 10 dB more receiving power.

Ham radio is limited to transmitter output power, which is quite different.

It could be worse, the limit here is 100mW EIRP.

If you can find a WLAN card with separate Rx and Tx port, put an
omnidirectional antenna on the Tx port and a 2 m paraboloid (30-33 dBi
gain) on the Rx port and quite long point to point systems could still
be built. Of course any interference within the beam would also be
picked up.

Paul OH3LWR

Paul Keinanen wrote:
Assuming ordinary +20 dBm cards with 16 dBi antennas at both ends, you
would end up to the +36 dBm EiRP (4 W) transmit power.

Which puts you right at the legal limit for the U.S. If your transmitter
is slightly higher power, or the antenna is more efficent you go over
the limit.

The free space
loss at 32 km and 2.45 GHz is 130 dB, so the receiver would get -94
dBm, which is very little for broadband traffic, especially that
figure does not contain the fade margin. In fact using +10 dBm cards
and 26 dBi antennas would give 10 dB more receiving power.

If I understand you correctly the EIRP would remain the same?

Around 1980 I worked a place that used IR beams for high speed (4800bps),
data and it worked until someone built a hotel in the way. :-)

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

In article ,
Paul Keinanen wrote:

Running a 32 km LOS link is problematic in practice. At least the
first Fresnel zone should be clear of obstacles. At midpoint you
should have about 30 m free space _below_ the direct optical path
(i.e. you would have to add 30 m to the antenna towers), but still you
should have 10-20 dB fade margins for reliable operation.

I understand that there's another issue with long links which can
cause some problems. 802.11 cards are designed to automatically
acknowledge packets received, and retransmit them if they aren't
acknowledged promptly. This retransmission protocol is done by the
card firmware, and occurs below the level of any operating-system
packet acknowledgement or retransmission (e.g. TCP).

The automatic-retransmission timeouts in the cards are set to a fairly
short time-period, in order to improve throughput under typical usage
conditions.

A 32 km link is going to have a round-trip time of around 200
microseconds. If the transmitting card's timeout value is set to less
than this, it'll start retransmitting the "lost" packet before a
successful acknowledgement can come back. Lather, rinse, repeat.

Traffic will actually get through, I gather... the successfully-
received packets will be forwarded to their destination, even though
the sending card thinks that they haven't been received (and logs them
as "discarded, excessive retransmits"). Throughput suffers badly,
though.

Some cards and/or access points apparently allow the retransmission
timeout value to be changed, to allow for sufficient speed-of-light
travel time.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Thu, 18 Oct 2007 11:22:20 -0700, (Dave Platt)
wrote:

In article ,
Paul Keinanen wrote:

Running a 32 km LOS link is problematic in practice. At least the
first Fresnel zone should be clear of obstacles. At midpoint you
should have about 30 m free space _below_ the direct optical path
(i.e. you would have to add 30 m to the antenna towers), but still you
should have 10-20 dB fade margins for reliable operation.

I understand that there's another issue with long links which can
cause some problems. 802.11 cards are designed to automatically
acknowledge packets received, and retransmit them if they aren't
acknowledged promptly. This retransmission protocol is done by the
card firmware, and occurs below the level of any operating-system
packet acknowledgement or retransmission (e.g. TCP).

Fade rates typically vary between a few times a second to several
days, thus resending a frame one millisecond after the initial frame
is not usually going to make a big difference, if the initial frame
was, say 10 dB below the threshold (i.e. deep in the noise).

While COFDM and various spread spectrum systems will help combating
(fast) frequency selective fading, but these do not help in slow fade
due to varying vertical atmospheric temperature and humidity profiles.

You still need the fade margin for slow fading, if reliable
communication is required.

The situation is different in typical amateur radio usage, in which
you are most interested in rare situations, in which the signal
strength increase above the average signal level, thus increasing the
communication range in rare occasions.

Paul OH3LWR

You can try the Motorola Canopy system. We, at work, use both the 5.7 GHz
and the 900 MHz systems and get a good 20 to 30 KM range. The web site is
http://motorola.canopywireless.com/solutions/

Norman
wrote in message
oups.com...
I am trying to establish a wireless network for Internet Access, Video
Conferencing and Intranet Applications, I though to use either a
2.4GHz Sector or Omni Directional Antenna would do the job but I can
find any product that goes as far the distance I am trying to cover (a
radius of 20Km). My idea is to use a powerful radio with the Antenna
monted on a tall mast at the location of the internet feed (via VSAT),
this location is idea for LOS at the other locations.

Does anybody has experience/advise with this ?

An ISP in my area has a rather large wireless network established, probably 30 miles from main office to my end of their network. I am at the far end of the network, about 12 access points "deep", meaning my traffic hops through 12 bridges-access points (repeaters) before it reaches the ISP's backbone. It works fairly well for internet browsing, better than I would have thought, but I've never tried internet telephony or video conferencing. That ISP is using 802.11b radios throughout the network, so the network certainly can't handle a lot of internet traffic, but it works.