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What Every Information Manager Should Know About Wireless

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Issue: 
Fall 2002

- John Porter, Virginia Coast Reserve (VCR)

First of all, wireless spread-spectrum networking is not magic. Sure, it works in mysterious ways with radios leaping from frequency to frequency. Sure, it can do amazing things – like connecting a remote field site at speeds usually seen only in a LAN-wired building. Sure, wireless wizards pull off miraculous tricks. However, it is not magic…. (I’m almost sure)!

Wireless networking is accomplished using several different frequencies and technologies with different capabilities. First let’s talk capabilities. There are two basic modes of communication using spread spectrum radio – serial and IP. The first replicates the use of a serial cable to connect devices. In fact, you can think of it as one really, really long cable! It is used by hooking up a device with serial RS232 output (e.g., a data logger) to a radio in the field, then connecting a paired radio to the serial port on a PC in your lab. As far as the PC or logger is concerned, they are just hooked together by a cable – they don’t notice the radios in the system. Speeds can be high (in serial terms) with speeds of 115,000 bits per second. More than fast enough for most data loggers. There are elaborations that can be used. Freewave radios (the Cadillac of serial spread spectrum radios) have options that allow a master radio to query various “slave” radios – just as you might have the serial port on you computer hooked to a switch box connected to various serial devices. The one difference is that it can be set up to be automatic – you don’t need to be there to turn the switch.

The second mode of communication, IP, uses spread spectrum radios as if they were hubs or network cards on a wired net. The most common standard is called 802.11b or “Wi-Fi”. Radios using this standard can interoperate (although not all features from different vendors may be supported) and exchange data at high rates of speed, typically 11 megabits per second (slower if the radio connection is weak). There are also specialized network hubs and bridges that use proprietary, non-standard protocols that may provide higher speed or greater ranges. The wireless equivalent of a network hub is called an “access point.” It contains a radio capable of communicating with up to 255 wireless clients. It is typically then hooked to a wired LAN with a standard network cable. The equivalent of a wired network card is usually a PCMCIA card suitable for use in a laptop computer, or a small box connected via a USB port. Again, as far as the PC is concerned, it is simply connected to the network as if by a wire. For serial devices rather than PCs, there are “bridges” that allow you to go from serial to IP. Then, the trick is to get software on a PC to talk via IP to that device. Fortunately, many hardware manufacturers (e.g., Campbell Scientific) are realizing this and providing capabilities in their software for IP/ Serial connections.

Regardless of the mode (serial or IP) you use, radio frequencies are relevant. First, you don’t need a license to run wireless networking. The positive side of this is that the networking is therefore free – you don’t need to pay a cell-phone company. The downside of this is that the frequencies where unlicensed spread spectrum networking is allowed are “garbage frequencies” where no one wanted to purchase the rights to the spectrum (usually because of undesirable characteristics). In the US, these are in frequencies around 900 MHz, 2.4 GHz and 5 GHz (outside the US, often only 2.4 and 5 GHz are available). These frequencies are used for a variety of other applications (e.g., portable phones, microwave ovens).

A special challenge for ecological researchers is that all these frequencies require line-of-sight between radios and are easily blocked by vegetation. For example, a microwave oven works by heating water molecules using a strong 2.4 GHz radio signal. When your wireless LAN card uses a much weaker signal in that same frequency to transmit data through trees, it ends up heating up the water in the leaves and stems which eats up the minimal power in the signal. 900 MHz is a little better in this respect, but neither frequency will punch its way through either a forest or a hillside! Adding to the challenge, spread spectrum radios are limited by law to low power (less than 1 watt), but most equipment makers, driven by a desire to increase battery life in laptop computers (the main wireless market), are using much less than that – almost all are less than 0.1 watts and some are at 0.03 watts.

Deploying spread spectrum radios in real-life ecological field situations requires some cleverness and trial and error. First, site selection is critical. If you can get high enough on a tower or hilltop, you can provide that critical line-of-sight between radio antennas. Ranges can also be improved by using directional antennas that channel all the available power into a beam aimed at your receiver. Additionally, the power of transmissions from very low power wireless network cards can be boosted by external amplifiers up to the full allowable 1-watt. Depending on that critical line-of-sight, this allows data to be sent at high speeds for several miles. Combining these techniques can provide realistic ranges in the tens of kilometers.

With respect to strategies for deploying wireless, here are a couple pointers based on our experience at the VCR/LTER. First, try to define your current and future needs as clearly as possible before starting. You need to know your destination before you start a journey. Second, find out as much as you can by talking to folks that are already involved in wireless projects. We benefited immensely from the expertise that Dave Hughes and Tom Williams brought to the project. Their WWW site (http://wireless.oldcolo.com) has lots of valuable resources and even step-by-step diaries from different projects in which they were involved. Finally, experiment! During our stepwise deployment of wireless technologies at the VCR/LTER, we tried out a lot of different radios and antennas, including a set of Freewave radios borrowed from John Vande Castle at the LTER Network Office. We figure that if it turns out a radio or antenna won’t work in one situation, another situation will be found that can still use the equipment – so nothing will be wasted.