 The magazine of the Melbourne PC User Group Why Go Underground? Ian Watts |
Wireless LANs are proving useful in applications where the cost of cabling is high or the need to frequently change the physical location of computers is required. Such situations can be found in historical buildings, construction sites, shopping centres and so on.
The performance is forging ahead at a rapid rate. It had seemed that the "standard" speed of 2 Mbit/s was well entrenched. However almost overnight it all but disappeared and has been replaced by 11 Mbit/s products. Not only are they faster, but also cheaper. The 11 Mbit/s wireless LAN is backwards compatible with 2 Mbit/s, in fact the 11 Mbit/s will automatically scale back to 5.5 Mbit/s, 2 Mbit/s and 1 Mbit/s, dependent upon the signal strength.
Wireless LANs can be configured in a number of different ways. In its simplest form, a number of PCs are connected to form a small self- contained network with every node able to communicate with every other node. A wireless LAN network card is installed in each computer. This is defined by the IEEE 802.11b standard as an "ad-hoc" configuration.
The Infrastructure configuration uses the same wireless LAN network cards, but they communicate with an Access Point. The access point provides a connection between the wireless nodes and an existing wired LAN, it is the gateway through which the wireless stations access a main file server.
Access points can also be configured as a Bridge. In this situation, two wired LANs may be connected together, eg two buildings on either side of a road or even between suburbs, provided line of sight is maintained.
As wireless LANs operate in the 2.4 GHz ISM (Industrial, Scientific and Medical) band, there are no charges or on going rentals associated with Bridges or point to point links. This makes them a far more attractive proposition than renting cable or fibre from a Teleco company.
DSSS (Direct Sequence Spread Spectrum) technology is used for the wireless transmission and this spreads the radio signal across the band to the extent that it appears to be part of the general noise, it can in fact be below the level of the ambient noise and so is very difficult to detect or intercept. Privacy is further enhanced by selecting either WEP-64 or WEP-128 (64 bit or 128 bit Wired Equivalent Privacy).
MicroGram Computers recently carried out tests on a pair of
Bridging Access Points and a station adapter, This was done partly to satisfy ourselves that they
worked as expected and to demonstrate the operation to a couple of customers. We had two 18 dBi
high gain antennas in stock, and commandeered these for the job. One was mounted one on top of our
office building, see Figure 1. The access point was connected to the antenna and, this in turn, to
our office LAN. The other antenna was mounted on a short mast, Figure 2.
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We then took to the hills of the NSW Central Coast and about 1km from the office connected a notebook to a station adapter and a dipole antenna. Signal strength software, included with the station adapter indicated 20% signal strength, not bad for a dipole at one end at this distance. It should be noted however that we sat the antenna on the roof of the car, and so picked up a reasonably good ground plane.
At this point we reconfigured the office access point as a bridge. This was done via the wireless LAN using the remote station. A quick phone call to the office and the power to the access point was turned off and on again to boot with the new configuration. The station was configured with the remote bridge. With some difficulty we managed to find an "almost" line of sight location between 2.5 km and 3.0 km from the office. Trees and scrub blocked a good line of sight as can be seen in the Figure 3 (from the office roof) and Figure 4 (from remote site looking towards the office).
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Initial tests were terrible and we wondered where the problem lay. One of our customers had a friend who was in the radio game. He said "Make sure you have a good earth on the antenna!" The only spare wire in the area was a bit of rusty barbed wire, this was connected to the antenna and the other end buried about 150 mm in damp soil. What a difference, the link worked perfectly even though we could not see the office and we were not really sure that the antennas were pointing in the right direction (in either direction).
Looking at a map later we concluded that they weren't properly lined up, Figure 3. A flashing torch at either end after dark would have allowed an accurate alignment, but one of the customers wanted to take the equipment away, satisfied it would do what he wanted - and so were we.
About the Author
Ian Watts Dip. Elec. Eng., M.I.E. Aust,
ian@mgram.com.au gained early computer experience with a Queensland Power Authority from the
late 1960s until 1980. He is now Managing Director of MicroGram Computers, a company he and his
wife started in 1986.
Reprinted from the June 2001 issue of PC Update,
the magazine of Melbourne PC User Group, Australia
