Rule #1: It never goes as fast as they say it does
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 (at least) two.
This means that for the most popular wireless networking standard right now (802.11g), you take the 54 Mbps quoted number, divide by two, and get 27 Mbps. 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 we talk about Rule #2.)
And truth be told, in actual use, you'll be lucky to get about 40% of whatever number you see prominently displayed on the front of the product box.
"Enhanced" modes that have been added to 802.11a, b and g products promise even higher speeds, although they usually throw in a caution that all of your equipment has to come from the same manufacturer for the higher speeds to work. These "turbo" modes actually, do work, but are worth a rule of their own:
Rule #1B: You can't depend on quoted "Turbo" or "enhanced" mode speeds in mixed networks.
The main reason for this rule is that speed enhancement techniques rely on working with their own kind in order to maximize network speed. Even though the most popular 802.11g techniques - Broadcom's AfterBurner, and Atheros' Super-G - use some of the same speed-enhancement techniques, they are implemented differently and therefore aren't interoperable. And when the techniques don't work, the products default back to using the standard (and slower) 802.11g protocols that they must be able to implement.
Tip: Broadcom and Atheros are manufacturers of the wireless chipsets found in many wireless products.
And, thanks to the marketing folks at your favorite manufacturer of networking products, even when the same speed-boosting techniques are used, the names are changed so that you, the consumer, can't tell that they are the same.
For example, Broadcom's AfterBurner technology is called "SpeedBooster" by Linksys, and "125* High-Speed Mode" by Buffalo Technology. Products using Atheros' Super G chips are a little easier to keep track of, since most manufacturers either use "108 Mbps" somewhere in the product name or even use the actual "Super G " term somewhere in their product literature.