Machipongo Revisited

Part 1: A New Access Point

On July 23, 2002 we returned to Machipongo Station with a new access point, and Orinoco AP-500, with an Orinoco "Silver" 802.11b card. As mentioned in Diary 46, the Zcomax XI 1000 access point did not work in the field with any of the amplifiers we tried. It was apparently just too weak (or possibly defective). The Zcomax radio's nominal transmit strength is 13 dBm, or 20 milliwatts (mW). The Orinoco Silver card has a transmit strength of 15 dBm, or 32 mW -- a 50% increase in signal output. This was apparently sufficient to do the trick, as everything worked fine after the substitution of the Orinoco unit. Either that extra 50% was enough to get the amplifier into Transmit mode, or perhaps the Orinoco is simply a better unit.

A few spot checks of reception, both inside the building and outside,
confirmed that we indeed had a working rig.

From Top: Battery, inverter, outlet strip, charging unit,
Access point (white), DC Injector (for amplifier), power-over-ethernet
adapter for Teletronics CPE, and an Ethernet hub to connect 'em all
up.

Part 2: Wiring a Boat for the Internet

On August 9 we set up one of the LTER's boats for mobile high-speed
Internet access. Anticipated uses include:

• Instantaneous tide information
• Weather reports
• Correspondence/reporting back to the lab
• Videoconferencing with the lab or with others;
• Entertainment while stuck at low tide (see Diary 45)
  

So we set up a fairly simple arrangement with an amplifier and
antenna. Since different laptop users will need different Adaptors
("pigtails") to connect to an external amplifier and antenna, we went
with a standard N connector for the user's connection. As of this
writing, we had not obtained a reasonably weatherproof N connector to mount on the boat's console, so we settled for one we could hide away in the hold between uses.

Since this is intended for use with 802.11b-equipped laptops, the
amplifier and antenna work in the 2.4 GHz spectrum.

We chose a Young Designs (YDI) amplifier, an indoor version -- which
simply means that it has no separate DC injector for power. The cable
lengths here were so short that we didn't need to mount the amplifier
on the antenna mast. We were able to use the boat's native 12 volt
batteries to power the amp, so there is no inverter in the picture.

Small YDI Amplifier in the Hold

We used LMR-240 cable, a nice middle ground between the inflexible and cumbersome LMR-400 which is usually used for amplifier-to-antenna runs; and the wimpy, high loss, but very flexible LMR-195 which is the cable of choice for pigtails and other very short connections.

The antenna, also from YDI, is an omnidirectional with 8 dBi gain,
chosen because its 18" length is still manageable, and also because a
directional antenna (Yagi or flat panel) will not do the trick on a
wobbling boat (or any other mobile application). We used 3M
marine-grade caulk to seal the hole in the roof.

Caulking the Connection

Part 3: War Sailing

The next day we performed a range survey from the boat. When one does this from a car, driving around town looking for unsecured 802.11b access points and logging their locations and signal strengths, it is called "War Driving." Hence the title for this section.

Indeed, we used classic war driving tool for this survey: a program
called Net Stumbler which, in addition to recording signal strength, noise level, access point name, and just about every other datum that is broadcast by an access point, also has the very nice feature of working with a GPS unit. As a result, once we had the GPS unit connected and the laptop appropriately aware of its location (serial port 1, 9600 bits per second, and so on) all we had to do was drive around the island.

As a result, we gathered some 50,000 data points. Of these, a good
two thirds were redundant due to our sampling three times as fast as
the GPS updated its position. After some filtering, the number was
reduced to about 17,000.

War sailing to, and around, Hog Island, which is the
southwest-to-northeast bone-shaped object on the right.

In the above illustration, darker colors indicate weaker signals.
Note that Machipongo Station's signal (in yellow) is visible from
nearly everywhere, while the signal from Broadwater Tower (in red) is
sporadic and often quite weak. The conclusion we drew is that for
this application the stronger Orinoco Access Point is a significantly
better bet.

-Tom Williams
Special Assistant
NSF Wireless Field Tests

PREVIOUS

NEXT