How to setup internet access on a ferry traveling 8km sea.

Discussion in 'Networking Issues' started by Toro22, Aug 8, 2005.

  1. Toro22

    Toro22 Guest

    Hi - problem as follows

    A ferrey with a local network of several PC's is traveling in a straigt line over 8km of water.

    The ferry is visible from the ticket office on both sides.

    My first thougt was to use a parabolic 18dBi antenna mounted 15m up to cover the stretch. Then installing an AP like the WET11G on the ferry with an omnidirectional antenna to connect to shore.

    But from what I can tell.. there is a chance that the whole stretch will not be covered properly.

    Using two antennaes one at each end solved the range problem, but introduces the problem of the two transitters overlapping. I expect then I would have to use different cannals/ SSID's...

    Good ideas ?
    Increase power?
    Any way for the mobile point to autoswitch between the two routers ?
  2. littlewhoo

    littlewhoo Network Guru Member

    I think there is no need to install APs at both ticket booths. Setup one AP with a directional antenna (pointing exactly along the line, the ferry is travelling) at one of the ticket booths and another AP on the ferry, also with a directional antenna.

    If you have good antennas, there should be no problem, establishing a WLAN link over a distance of 8 km. You just have to ensure, that the antennas are mounted high enough, so that you not only have a clear line of sight, but also a free fresnel zone. This is much easier with a WLAN connection crossing water, because there are no trees, buildings or other fixed obstacles in the fresnel zone.

    To get a free fresnel zone for a 8 km link, both antennas should be mounted at the same hight and at least 16m above ground (or water in this case).
    You can check the calculations for youself on this page:

    Having APs/WLAN Routers, with adjustable tx power (like the Linksys WRT54G) may help. But I would rather go for better antennas, than for more tx power.

    The only problem I can see in this setup is, that all kind of water in the air (snow, rain and especially fog) will absorb the energy of the WLAN signal an therefore increase the free space loss. So if it's often very foggy over the water in the area where you want to use this WLAN connection, this might be a problem.
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