Since manufacturers know that people buying computer gear like to compare numbers, and that bigger numbers are usually more attractive than smaller numbers, they're more than happy to oblige! So they make sure you see the biggest throughput number that they can quote on all their marketing literature.

Tip: "Throughput" or "transfer rate" is the number of bits that move from one place to another in a given period of time. For wireless networking equipment, throughput is usually quoted in Mbps (Megabits per second).

This number, however, is usually the raw data rate, and is something that you'll never approach in your actual network. What number can you use? The answer is Rule 1A:

Rule #1A: Take the manufacturer's Mbps number and divide by two.

This means that for the most popular wireless networking standard right now (802.11b or "WiFi"), you take the 11Mbps quoted number, divide by two, and get 5.5Mbps. This would be the fastest speed that you'd most likely experience on your network, under best-case conditions. (We'll explain what we mean by "best case conditions" later, when weI talk about Rule #2.)

Newer standards, such as 802.11a and "enhanced" 802.11b have "turbo" or "2X" modes that promise even higher speeds, although they usually throw in a caution that all of your equipment has to come from them for the higher speed to work. These "turbo" modes actually, do work, but are worth a rule of their own:

Rule #1B: Don't select equipment based on "Turbo" or "enhanced" mode specs.

There are two reasons for this rule:

Manufacturers don't guarantee interoperability with equipment from other manufacturers for their "turbo" modes

Testing has shown that you need a strong signal to really get the benefit from these modes, and they can actually give you slower performance than the normal mode under weaker signal conditions

The "turbo" numbers are very optimistic. Top 802.11a "Turbo" modes are typically 30Mbps, and top "enhanced" 802.11b clocks in at around 7Mbps.

Pretty much all of that missing throughput is used to make sure all of your data gets from one place to another without any missing pieces. This overhead, which all communication protocols have, is heavier in wireless networking due to the tougher environment that your bits have to travel through.

As disappointing as losing half your throughput may be, you may never notice it in a small wireless network. It all depends on how many wireless clients you have, and what their data transfer requirements are. Here are a few scenarios for comparison.

Case 1: Probably won't notice

One to two wireless clients Occasional file and print sharing Network used mostly for Internet web browsing, email, instant messaging Internet connection via dialup

Explanation: These applications generally don't require high transfer rates for extended periods of time. Even if they did, the slow Internet connection is going to be the main factor in determining your effective client throughput.

Case 2: Might notice

Three to four wireless clients Occasional large Internet downloads Light local network file and print sharing One or two Internet audio streams One video stream

Explanation: With more wireless clients sharing the same 5Mbps (usually less) bandwidth, the chances of more than one client wanting a large chunk of bandwidth at the same time are increased. Add the higher bandwidth requirements of audio and video streams and file and printer sharing,you're your wireless users might occasionally notice some sluggishness on the network.

Note that I don't mention the Internet connection method as a wireless performance factor. Unless you are very fortunate (or paying a lot of money), your broadband connection probably tops out at 1 to 1.5Mbps. Most 802.11b wireless networks lose connection before they fall back to a reliable 1.1Mbps rate.

Case 3: Definitely will notice

More than four wireless clients Frequent large Internet file downloads Heavy local network file and print sharing activity More than one video stream More than two simultaneous audio streams

Explanation: More clients + larger data transfers = Frustrated users!

The other key factor that can affect the speed of your wireless network is enabling WEP encryption. WEP (Wired Equivalent Privacy) is a much maligned, but still useful, feature of 802.11a and b equipment. WEP's purpose is to keep your wireless data streams private, and involves the use of an encryption algorithm. The encryption algorithm requires some fairly hefty number crunching, which is usually done in hardware. Some hardware doesn't work as efficiently as others, so in some cases enabling WEP encryption can cut your throughput by 50 to 60%. Even with this problem, I wouldn't run a wireless network with WEP disabled, so make sure you look for gear that doesn't have this problem. Note that 802.11a products, all of which use chipsets from Atheros, don't show any throughput reduction when using WEP with any bit-length key.