3 km wi-fi link, how?

Hello,

I'm wondering if it's possible to set up a decent wi-fi link between
two points ca. 3 km (2 miles) distant from each other (I have line of
sight). Most important, I'd like to know what gear I'd need for a
project like this one. Which kind of (directional) antennae should I
choose? how much should their gain be? do I need special APs to drive
them, or external amplifiers, if anything like this is needed at all? I
hope that you can give me some suggestions, because I have no clue. I
tried to search the net, but these questions must be either too stupid
or too difficult to answer, as I found nothing (that is, nothing that
I'd be able to understand anyway).

Thanks in advance.

"Demetrio Girardi" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hello,
>
> I'm wondering if it's possible to set up a decent wi-fi link between
> two points ca. 3 km (2 miles) distant from each other (I have line of
> sight). Most important, I'd like to know what gear I'd need for a
> project like this one. Which kind of (directional) antennae should I
> choose? how much should their gain be? do I need special APs to drive
> them, or external amplifiers, if anything like this is needed at all? I
> hope that you can give me some suggestions, because I have no clue. I
> tried to search the net, but these questions must be either too stupid
> or too difficult to answer, as I found nothing (that is, nothing that
> I'd be able to understand anyway).
>
> Thanks in advance.

I would like to pursue projects like this where I live also, maybe we can
brainstorm together. I also know a very knowledgeable electronics wiz who
seems willing to give me free pointers.

Demetrio Girardi wrote:
> Hello,
>
> I'm wondering if it's possible to set up a decent wi-fi link between
> two points ca. 3 km (2 miles) distant from each other (I have line of
> sight). Most important, I'd like to know what gear I'd need for a
> project like this one. Which kind of (directional) antennae should I
> choose? how much should their gain be? do I need special APs to drive
> them, or external amplifiers, if anything like this is needed at all? I
> hope that you can give me some suggestions, because I have no clue. I
> tried to search the net, but these questions must be either too stupid
> or too difficult to answer, as I found nothing (that is, nothing that
> I'd be able to understand anyway).
>
> Thanks in advance.
>
Dead easy if its in the clear
thus might help .

http://www.terabeam.com/support/calculations/som.php

ok, here's something I don't get.

> http://www.terabeam.com/support/calculations/som.php

according to that calculator, using two 16dbI antennas with AP
transmission power of 10mW (don't know how feasible the last one is)
would lead to a 'Theoretical System Operating Margin' of 14.8 db. If I
got it right, this means that as long as real-world noise doesn't cause
more than 14 db power loss, it should work.

however, this other tool:

http://www.radiolabs.com/stations/wifi_calc.html

says that power output (for the same antenna gain and tx power) is 26
db. Power loss over 2 miles is 110 db (both tools agree on this). How
can it work then? what am I missing?

"Joseph Stewart" <(E-Mail Removed)> wrote:

>> I'm wondering if it's possible to set up a decent wi-fi link between
>> two points ca. 3 km (2 miles) distant from each other (I have line of
>> sight). Most important, I'd like to know what gear I'd need for a
>> project like this one. Which kind of (directional) antennae should I
>> choose? how much should their gain be? do I need special APs to drive
>> them, or external amplifiers, if anything like this is needed at all? I
>> hope that you can give me some suggestions, because I have no clue. I
>> tried to search the net, but these questions must be either too stupid
>> or too difficult to answer, as I found nothing (that is, nothing that
>> I'd be able to understand anyway).
>>
>> Thanks in advance.
>
>I would like to pursue projects like this where I live also, maybe we can
>brainstorm together. I also know a very knowledgeable electronics wiz who
>seems willing to give me free pointers.

You need to do a link analysis. There are any number of web sites that offer
online calculators. Here's just one:

http://my.athenet.net/~multiplx/cgi-...eless.main.cgi

Demetrio Girardi wrote:
> ok, here's something I don't get.
>
>
>>http://www.terabeam.com/support/calculations/som.php
>
>
> according to that calculator, using two 16dbI antennas with AP
> transmission power of 10mW (don't know how feasible the last one is)
> would lead to a 'Theoretical System Operating Margin' of 14.8 db. If I
> got it right, this means that as long as real-world noise doesn't cause
> more than 14 db power loss, it should work.
>
> however, this other tool:
>
> http://www.radiolabs.com/stations/wifi_calc.html
>
> says that power output (for the same antenna gain and tx power) is 26
> db. Power loss over 2 miles is 110 db (both tools agree on this). How
> can it work then? what am I missing?
>
I have used the terabeam figures with success many many times.
assuming its in the clear

On 27 Nov 2005 15:29:29 -0800, "Demetrio Girardi"
<(E-Mail Removed)> wrote:

>I'm wondering if it's possible to set up a decent wi-fi link between
>two points ca. 3 km (2 miles) distant from each other (I have line of
>sight).

You need MORE than line of sight. You need Fresnel zone clearance. At
2 miles, that's about 33ft radius from the center line at midpoint. Do
you have Fresnel zone clearance?
http://www.terabeam.com/support/calc...esnel-zone.php

>Most important, I'd like to know what gear I'd need for a
>project like this one.

Well, first you need to disclose what you plan to do with this link.
The details will determine the type of equipment. For example:
1. Do you need to pass only one MAC address or do you need a
transparent bridge (where you glue to networks together via wireless)?
2. How fast do you need to go? Mbits/sec thruput. If you're sharing
an internet connection, there's little need to go faster than the
broadband connection.
3. Need bandwidth management or QoS features? VoIP? Games?
4. How fanatical do you want to be about security? VPN? PPTP?
WPA2-AES?
5. Is there a cost limit?

>Which kind of (directional) antennae should I
>choose?

Never mind the type for now. It's the gain that's important. Once
the gain is established, then you can consider the type. For example,
if you need more than about 15dBi of antenna gain, you won't find a
yagi with that high gain. Similarly, panel antennas become huge above
14dBi. That leaves a dish which goes to 24dBi gain.

>how much should their gain be?

That needs to be calculated. See below. Incidentally, I've only done
this about a dozen times in this newsgroup but I guess it's difficult
to find with Google Groups or something. I guess I should add it to
the FAQ.

>do I need special APs to drive
>them, or external amplifiers, if anything like this is needed at all?

Nothing special. Depending on the answers to the aforementioned 5
questions, the hardware is fairly generic. For simplicity, I'll
assume a transparent bridge, where each end is identical, and passes
more than one MAC address.

>I hope that you can give me some suggestions, because I have no clue.

Hmmm... suggestion: Do the numbers first.

>I tried to search the net, but these questions must be either too stupid
>or too difficult to answer, as I found nothing (that is, nothing that
>I'd be able to understand anyway).

Yeah, that's the problem. If you don't have a vocabulary with the
necessary buzzwords, you can't find anything useful with Google.
Welcome to the wonderful world of wireless calculations.

Let's start with some assumptions.

Linksys WRT54G 802.11g with DD-WRT alternative firmware setup as a
bridge. The transmit output is about +17dBm but can be cranked up
(illegally) higher. I'll assume that you'll mount the WRT54G
somewhere near the antenna with a single 3ft cable (pigtail) using
LMR-240 coax. Loss will be about 4dB including connectors. Longer
cables will require larger LMR-400 coax plus a pigtail.

Now, I gotta do some guesswork. I'll assume that you can live with
about 6Mbits/sec thruput. That means your connection speed must be
about twice that or 12Mbits/sec. The receiver sensitivity varies with
the speed and modulation type.

The following was lifted from the DI-624 datasheet but is close enough
for most similar 802.11g radios:
* 54Mbps OFDM, 10% PER, -68dBm
* 48Mbps OFDM, 10% PER, -68dBm
* 36Mbps OFDM, 10% PER, -75dBm
* 24Mbps OFDM, 10% PER, -79dBm
* 18Mbps OFDM, 10% PER, -82dBm
* 12Mbps OFDM, 10% PER, -84dBm
* 11Mbps CCK, 8% PER, -82dBm
* 9Mbps OFDM, 10% PER, -87dBm
* 6Mbps OFDM, 10% PER, -88dBm
* 5.5Mbps CCK, 8% PER, -85dBm
* 2Mbps QPSK, 8% PER, -86dBm
* 1Mbps BPSK, 8% PER, -89dBm
12Mbits/sec sets the receiver sensitivity at -84dBm.

PER means Packet Error Rate which is similar to BER (Bit Error Rate)
but is easier to measure. You would not want to operate at a 10%
error rate. 1 in 10 packets trashed is useable, but no fun. This is
*ROUGHLY* the same as a BER of 1E10^5 which is one of the common
reference levels for measuring receiver sensitivity. Again, these are
measurement reference levels, not operating points.

Next I gotta guess the fade margin. That's how much stronger the
operating level is above the receiver reference level. 20dB is
considered to be a good minimum. In other words, your operating
signal level must be 100 times stronger than the receiver sensitivity
or the link is going to be flaky and unreliable. The relationship
between fade margin and reliability is:
Reliability Fade Margin
90% 8 dB
99% 18 dB
99.9% 28 dB
99.99% 38 dB
99.999% 48 dB
99% reliability might sound great, but that means your link will be
useless for 1% of the year, or dead 3.6 days per year. Don't go below
20dB fade margin.

Plugging the above guesswork into
http://www.terabeam.com/support/calculations/som.php
and playing with the antenna gains until I get 20dB fade margin:

TX power = +15dBm
TX coax loss = 4dB (3ft LMR-240 plus a mess of connectors)
TX ant gain = unknown
Distance = 2 miles
RX ant gain = unknown
RX coax loss = 4dB (same at other end)
RX sens = -84dBm (at 12Mbits/sec)
Fade margin = 20dB
yields a minimum antenna gain of +21dBi. That makes the antenna
required to be a dish. You can get away with a lower gain antenna by
running at a slower speed, shorter coax, or more tx power (at both
ends).

In general the numbers and assumptions above can only get worse, not
better. These calculations are for ideal environmental conditions,
without interference, Fresnel zone obstructions, reflections,
multipath, atmospherics, and terrain problems. If you have another
802.11b/g user along the line of sight, the high gain of the antenna
will create a substantial interference problem. I suggest you do a
site survey to see if there are any other users around.

I have some suggestions as to specific hardware and sources, but I
would like to see the answers to my questions, and corrections to my
assumptions first.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Wow, thanks for your extensive answers.

> Do you have Fresnel zone clearance?

The short answer is: I think so. However my setup would be
significantly different from the scheme terabeam proposes. One end of
the link would be approximately 1000ft higher (in altitude) than the
other. If this doesn't change the shape of the Fresnel zone too much,
it should be clear.

> 1. Do you need to pass only one MAC address or do you need a
> transparent bridge (where you glue to networks together via wireless)?

I'm not sure about what you mean by 'transparent' (no hops in between
the networks?), however I don't need anything special. I don't mind if
the two ends are on different subnets. So I'd settle for the cheapest /
easiest solution.

> 2. How fast do you need to go?

here the broadest-band internet connection avaliable goes 4Mbps, and
it's likely to remain such for a long time, so that would be enough.

> 3. Need bandwidth management or QoS features? VoIP? Games?

No need for QoS, but what has a router/AP to do with VoIP or games in
particular?
One end needs to connect to an ADSL line and do NAT, but an additional
router for that is no problem, they are fairly cheap.

> 4. How fanatical do you want to be about security? VPN? PPTP?
> WPA2-AES?

I don't know. As long as one cannot simply power up a laptop to steal
my bandwidth, I think I'm OK; but if slightly better security does not
mean a lot more money, I'd probably give it a thought.

> 5. Is there a cost limit?
Approximately €500 ($600). Above that it would be probably better to
pay to get a phone line. Anyway, I'm hoping to spend much less.

> Let's start with some assumptions.

I think you guessed right enough what I want to do. However I don't get
what fade margin is all about. Is 20db considered to be a good minimum
because of real-world interferences, or is it what I need in ideal
conditions?
Considering 9Mps throughput, two 18dbI antennas (the highest gain I can
get without having to buy stuff abroad) would mean a SOM of 19.8 db. Is
this too risky?

> The transmit output is about +17dBm but can be cranked up
> (illegally) higher

How much can it be boosted? I don't worry much about legality, since
here (in Italy) the government is vaguely aware of the existence of
wireless technologies, and I don't think anything is regulated. I'll
check, though.

> I have some suggestions as to specific hardware and sources

That would be wonderful, thanks.

On 28 Nov 2005 05:15:23 -0800, "Demetrio Girardi"
<(E-Mail Removed)> wrote:

>Wow, thanks for your extensive answers.

I'm not done yet. I threw together a collection of spreadsheets
stolen from various manufacturers, students, experts, and myself on
calculating the path. Some are a bit old and may have been updated.
http://802.11junk.com/jeffl/rf-calc/

>> Do you have Fresnel zone clearance?
>
>The short answer is: I think so. However my setup would be
>significantly different from the scheme terabeam proposes. One end of
>the link would be approximately 1000ft higher (in altitude) than the
>other. If this doesn't change the shape of the Fresnel zone too much,
>it should be clear.

The height at each end become signifigant to not hit the ground due to
the curvature of the earth. At 2 miles, that's not very important,
especially with your altitude. The drawing on the Terrabeam page is
clear enough. At midpoint (1 mile) do you have sufficient clearance?
You should be able to visually guess. If you cannot see the 33ft
radius at the midpoint, then some other objects are in the Fresnel
zone between your endpoint and the midpoint clearance area. The
sloppy way to do this is to take a photograph of path from some
location that is perpendicular to the path. Draw the line of sight
and Fresnel zone on the photograph using a straight line
approximation. That will show any problems.

>> 1. Do you need to pass only one MAC address or do you need a
>> transparent bridge (where you glue to networks together via wireless)?
>
>I'm not sure about what you mean by 'transparent' (no hops in between
>the networks?), however I don't need anything special. I don't mind if
>the two ends are on different subnets. So I'd settle for the cheapest /
>easiest solution.

802.11 wireless is ALL bridging. There are no subnets in a bridge
because there is nothing on Layer 3 (TCP/IP) involved. Everything is
done at the MAC layer (layer 2). The only thing the access ponit IP
address is used for is managing and configuring the box.

Transparent means that it looks to each end exactly as if it were
connected with a CAT5 cable. This implies that all broadcasts are
passed through the bridge, and that the wireless bridge can pass all
MAC addresses that it sees.

However, if you only have one computah at the remote end, and don't
need the bridge to be transparent, then a simple client radio at one
end, and a simple access point at the other, is sufficient and
cheaper. You decide.

>> 2. How fast do you need to go?
>
>here the broadest-band internet connection avaliable goes 4Mbps, and
>it's likely to remain such for a long time, so that would be enough.

To insure some headroom, 6Mbits/sec thruput is sufficient. Therefore,
you will probably operate at a 12Mbit/sec connection rate (as I
guessed). You will probably want to fix your connection speed in the
bridge setup instead of using "auto".

>> 3. Need bandwidth management or QoS features? VoIP? Games?
>
>No need for QoS, but what has a router/AP to do with VoIP or games in
>particular?

Lots. If you're bridging two busy networks, the wireless bridge needs
to decide how much junk will go across the bridge. It tends to also
become slowest thing on the network which constitutes a bottleneck.
Wireless is also a shared resource, where other users on the channel
fight for airtime or cause interference. Therefore, latency tends to
be erratic and longer than expected. If you have time critical
traffic (VoIP, video conferencing, streaming content server), then you
will need to prioritize the traffic through the bridge.

>One end needs to connect to an ADSL line and do NAT, but an additional
>router for that is no problem, they are fairly cheap.
>
>> 4. How fanatical do you want to be about security? VPN? PPTP?
>> WPA2-AES?
>
>I don't know. As long as one cannot simply power up a laptop to steal
>my bandwidth, I think I'm OK; but if slightly better security does not
>mean a lot more money, I'd probably give it a thought.

Minimum feature is therefore WPA encryption. No need for a RADIUS
server based 802.1x authentication as you will be the only user.

>> 5. Is there a cost limit?
>Approximately €500 ($600). Above that it would be probably better to
>pay to get a phone line. Anyway, I'm hoping to spend much less.

You pay for a phone line every month while the wireless solution is
only one payment. $600 is insufficient for a plug and play solution
such as:
| http://tranzeo.com/index.php?section..._section_id=44
Therefore, you'll probably end up doing it yourself. I still cannot
determine if you want a bridge-bridge solution, or an AP-client
solution. I'll go with the bridge-bridge because it's easier for me
to do as both ends are identical.

WRT54G or WAP54G radio $60
Coax pigtail $15
24dBi dish antenna $70
waterproof box $20
Misc wire, mounts, etc $20
=======
Total (one end) $185

$370 is the price if you build it yourself. Less if you scrounge the
parts or have existing hardware.

>> Let's start with some assumptions.
>
>I think you guessed right enough what I want to do. However I don't get
>what fade margin is all about. Is 20db considered to be a good minimum
>because of real-world interferences, or is it what I need in ideal
>conditions?

Unfortunately, 20dB under ideal conditions. Incidentally, fade margin
and SOM (system operating margin) are the same thing. It's how much
stronger the signal needs to be than the reference value used to
measure the receiver sensitivity. Very roughly, it's the amount that
you can REDUCE the signal and still have a functional (but error
prone) link. When installing a point to point link, I always install
attenuators in the coax line to measure my fade margin. If I
calculate for 20dB of fade margin, and the link dies when I add 20dB
of attenuation, then I did something wrong, somewhere.

Real world interference is difficult to quantify and calculate.
802.11g can handle considerable intereference and still function. What
you'll see is an increase in packet loss. Ping the other end of the
link continuously. Ideally, the latency should be less than 5msec for
a 2 mile wireles bridge. If you see <5msec most of the time, but
ocassional dramatic increases to much larger latency values, you are
getting interference. It's almost impossible to remove interference
from the system once it appears. Increasing the fade margin will
help, but that just creates a power war among the participants.

You're best defense is a highly directional antenna. With a 24dBi
dish, you only have about a 7 degree beamwidth. Only those sources of
interference within the 7 degree beamwidth will be a problem. This
relies mostly on luck instead of engineering, but it's the best I can
offer.

>Considering 9Mps throughput, two 18dbI antennas (the highest gain I can
>get without having to buy stuff abroad) would mean a SOM of 19.8 db. Is
>this too risky?

19.8dB is close enough. Note that it's not 9Mbits/sec thruput. It's
9Mbits/sec OFDM data rate. You'll get about half that in thruput
under ideal conditions.

>> The transmit output is about +17dBm but can be cranked up
>> (illegally) higher
>
>How much can it be boosted? I don't worry much about legality, since
>here (in Italy) the government is vaguely aware of the existence of
>wireless technologies, and I don't think anything is regulated. I'll
>check, though.

The Linksys WRT54G box is based on Linux and therefore is open source
firmware. Alternative firmware is available. One of the features is
the ability to adjust the transmit power. I don't approve much of
turning up the power from the usual +15dBm to +23dBm, but some value
in between is probably acceptable. For firmware, I use and recommend
DD-WRT:
: http://www.dd-wrt.com
If you purchase a WRT54G or WRT54GS, please avoid the WRT54G v5 as
this hardware mutation does not run Linux. See:
: http://www.linksysinfo.org/modules.p...howpage&pid=31
This is chart is a bit old but gives a fair idea of the features.

>> I have some suggestions as to specific hardware and sources
>
>That would be wonderful, thanks.

Not yet. You decide if it's going to be bridge-bridge or AP-client
type of system.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

>>The transmit output is about +17dBm but can be cranked up
>>(illegally) higher
>
>
> How much can it be boosted? I don't worry much about legality, since
> here (in Italy) the government is vaguely aware of the existence of
> wireless technologies, and I don't think anything is regulated. I'll
> check, though.

The tx limit in Italy (and in most European Countries) is 20db at the
antenna side. So, antenna + Access Point (or equivalent) do NOT have to
exceed 20db. Obviously you have to include cable/connector loss. So if
you have 30db antenna well you will have some problems in tx. No problem
with rx

By the way, you have also ask for an authorization to operate in outdoor
environments. So it's not exactly "free for use" in outdoor
environments, it's regulated.

Regarding the Italian regulation, it's very clear who can do what, and
it's easy to be issued of specific legal actions. So I would suggest to
double check before starting the transmission.

Have fun