Wireless Access

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Access network design for branch, remote, outdoor and campus locations with Aruba access points, and mobility controllers.
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max. distance for bridging

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  • 1.  max. distance for bridging

    Posted Dec 13, 2009 05:00 AM
    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

  • 2.  RE: max. distance for bridging

    Posted Dec 13, 2009 10:45 AM
    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



  • 3.  RE: max. distance for bridging

    Posted Dec 13, 2009 01:23 PM
    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.

  • 4.  RE: max. distance for bridging
    Best Answer

    Posted Dec 14, 2009 03:23 PM

    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.