Selecting Wireless Networking

by Phil Hughes

What you are about to read here is the result of research I have been doing to solve a local problem. By local, I mean that I am looking for the right answer for a system in the suburbs of the second largest city in Costa Rica. I realize my conclusion is unlikely to fit your needs, so I will confine this article to some facts that may help you head in the right direction.

The biggest buzzword today is 802.11, a topic is which the value of something is either a, b or g. So, that is where I started.

The most mature version of this technology is 802.11b. Yes, it seems strange that 802.11b would be more mature than 802.11a, but it is. Within the 802.11b standard there is what you could call a sub-group, known as WiFi, which is actually an interoperability agreement. That is, products from one vendor that are branded WiFi should talk to WiFi products from other vendors. Thus, if interoperability is your issue, WiFi is a big win.

But what is 802.11b anyway? It is a standard, with the spectrum regulation on the part of the Federal Communications Commission (FCC) well established. So it's not about to go away or change. It operates at approximately 2.4GHz, and the band is organized so there are only three non-overlapping channels. This means you can have a maximum of three networks operating in the same space before there is interference.

Note that I said interference, which is not to say that you can't have 10 or 20 different networks in the same space. But they will all suffer because, while they won't be able to communicate with each other, they will interfere with each other. The result of the interference is, at least, reduced throughput, and it could mean one or more networks simply do not work.

As for speed, the maximum speed of 802.11b is 11Mbps. Keep in mind, too, that speed is a function of range as well as interference. Cards will automatically reduce their data rate if signals are weak. In addition, treat the 11Mbps number a lot like 10Mb Ethernet numbers. More precisely, when you add retransmissions because of collisions and protocol overhead, you will not get anywhere close to the 11Mb rate.

Moving on to 802.11a, there is a host of advantages and disadvantages. The two big advantages are frequency and data rate. 802.11a cards operate in the 5GHz band; currently this chunk of spectrum is much less crowded. In addition, there are 12 non-overlapping channels, as opposed to the three available with 802.11b. The data rate is 54Mbps, and some vendors have offered extensions to 72Mb and faster.

So, is there a downside to 802.11a? In a word, yes. First, the higher frequency tends to mean shorter range. Although it is easier to build high-gain antennas (that is, you get more gain for the same size antenna), signals at 5GHz tend to be easily blocked by almost anything. So, in addition to a higher cost for the cards themselves, you are going to need more of them in order to cover the same area.

Also, if you have an existing 802.11b network and you introduce 802.11a radios, you need to bridge between the networks because the radios can't talk to each other. Again, this translates into more money.

Enter 802.11g. This new standard offers some advantages but has its own downsides. On the positive side, 802.11g offers the 54Mbps speed of 802.11a but is frequency-compatible with 802.11b. This means you can add 802.11g cards to an existing 802.11b network, and they will play nicely with each other. Although they won't make existing 802.11b cards faster, the new 802.11g cards can talk to each other at the faster data rate.

The downsides to 802.11g are how crowded the 2.4GHz spectrum is and the availability of only three non-overlapping channels. If you are in rural Nebraska and only want a fast network for your house, 802.11g likely is a good answer, particularly if you have existing 802.11b cards in operation. On the other hand, don't expect to have a clear channel with your 802.11g cards in San Jose (California or Costa Rica) or any other heavily populated area.

While the information I have covered is fairly generic, it applies most closely to wanting to construct a network containing one or more access points and some systems that talk to the access point(s). (To clarify, by access point I mean something that connects your network to another network or the outside world, rather than a specific product you buy. Generally, you can use your Linux box with a regular client card in it as an access point.) My particular need is for a point-to-point link, which changes the considerations a little bit.

First, I can use directional antennas to help reduce interference and cover a longer distance. (Before you get carried away, check with your local regulatory agency--the FCC in the US--to see what you can and can't do.) That the antenna offer significant directivity to eliminate interference may be more important than the signal gain realized. This may be good, however, as a 2.4 or 5 GHz signal loss in coaxial cable is significant.

If you plan to use an external antenna, here's a tip: shop carefully for the card you intend to use. Many cards do not offer an external antenna connector. In addition, those antennas that do offer it but were built for the US market will have a non-standard antenna connector, as required by the FCC. You can find these connectors, but it will raise your stress level.

Next, because only two radios, rather than a whole host of them, are talking on the network, the number of collisions is reduced. So, the effective throughput of the network is increased.

Finally, compatibility is a non-issue. That is, if only two radios are hooked up in a point-to-point system, then it makes no difference if they interoperate with other radios. In fact, from an interference point of view, it may be better if they don't interoperate.

Are there alternatives to 802.11? Sure. The most well-known alternative is the RangeLan2 series of radios from Proxim. Although they are speced at only 1.6Mbps, they are a mature and stable product. In addition to the typical PCMCIA cards, the RangeLan2 is a self-contained box with an Ethernet interface. At SSC, we used a set of these radios to communicate between the Editorial offices and the rest of the company for about two years, until we managed to get everyone moved into a single building.

A lot of issues and considerations are involved when trying to select the right wireless solution to best fit your needs. I hope this article helps you become aware of some of the options available and saves you some initial research time.

Phil Hughes is the publisher of Linux Journal.


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