Wireless Access

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Occasional Contributor II
Posts: 67
Registered: ‎06-04-2009

max. distance for bridging

Hi all,

i want to know what is the practical max. distance for bridging between 2 AP's

while calculating link budget when using 2 AP's with :
1- 16 dbm transmitted signal
2- 23 dbi antenna for transmitting with .9 db as cable loss
3- 23 dbi antenna for receiving with .9 db as cable loss
4- the frequency is 5470 Mhz

i got -67 dbm as a receiving signal strength at 9.9 Kilo meter distance,

considering that in the AP 85TX datasheet the receiving sensitivity is -66dbm at 54Mbps data rate

the question is could i get the 9.9 kilometer distance with 54 Mbps data rate?

what is the max. practical distance experience with 54Mbps data rate ?

what is the Max. actual throughput in the bridge link?

thanks in advance
Aruba
Posts: 760
Registered: ‎05-31-2007

max. distance for bridging

The Link Budget exercise that you have undertaken will give you the
distance you can transmit/receive, assuming the following:

1) Line of Sight -- No obstructions in-between transmitter and receiver
are assumed in the link budget calculations. This of course becomes
increasing difficult as distance goes up, and tends to require that the
transmit and receive antennas are placed higher and higher above the
ground. In addition to Link Budget you may consider calculating the
Fresnel zone to get an idea of ideal height of transmitter/receiver at
various distances if you are looking at long haul applications.
Example: http://www.afar.net/fresnel-zone-calculator/


2) Fade Margin -- I did not see you mention any fade margin in the link
budget calculation below. Always a good plan to build some margin or
safety factor into the equation to see what kind of link you will
actually get. RF will fluctuate and so building in the margin will
compensate and ensure you get your desired signal, or better at all
times during the day.


If you have clear line of sight, enough height to clear
obstructions/Earth, and some margin built into your planning, distances
like you have mentioned would be achievable. That beings said, for
every long haul application I have seen to date, there are many many
more that are much more straightforward to deploy (e.g. connecting
buildings across the street, across the parking lot, across the soccer
field).


Rds,

JF
Occasional Contributor II
Posts: 67
Registered: ‎06-04-2009

Re: max. distance for bridging

Hi all,
thnx jfernyc for your reply; would you please tell me what is the recommended fade margin to take into consideration?

i still need to know what is the maximum achievable actual throughput in link bridge?


also guys if some one came across a long distance Wi-Fi bridge deployment, it will be nice to share our practical experience, what is the main issues?, what is the max. distance and throughput etc.
New Contributor
Posts: 1
Registered: ‎10-01-2007

Try this free tool

http://cgi2.gbppr.org/wireless.super.main.cgi

I have used it with great success for several years.
It is the poor man's link analysis tool. Sometimes the server
gets busy and it may take a few tries to gain access.

The key to successful Point-to-Point terrestrial bridge links is planning.
The achievable distance for a given data rate is determined by transmitter output power, overall system gain/loss, terrain, and atmospheric conditions.

You need to select an antenna/feeder cable combination that will yield at least
15dB of Fade Margin at the desired data rate. Keep in mind that for every 3dBm increase in antenna gain, your EIRP radiated power effectively doubles.

Given the RSSI and radio sensitivity you state it sounds like you have a fade margin of around 1dB. This is not enough for you to have a stable link at 54Mbps.
The answer would be a higher gain antenna or lower loss feeder or both.

I do not know where you are located, but here in the U.S. the FCC permits up to
36dBm (4 watts) EIRP on unlicensed links. As long as you stay within your regulatory domain restrictions you can use a higher gain antenna.
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