Little-Known Wireless Facts

What you need to know to set up your network.

• By Don Jones
• January 01, 2004

802.11b provides up to 11Mbps of bandwidth; 802.11g and 802.11a each provide up to 54Mbps. However, that’s all shared bandwidth, meaning all clients within range of an access point (AP) share its capacity. Think of each access point as a single-lane highway: Everybody shares the same pipe when getting from one end of town to the other. If there’s an area—such as a conference room—where more users need better bandwidth, APs can be stacked, effectively adding “lanes” to the highway. Each AP provides a full bandwidth connection—either 11 or 54Mbps—to a wired network. However, there’s a limit to stacking: The b and g specifications provide only three channels, so there can’t be more than three APs in the same transmission area. While the a spec provides less overall range, it allows for eight channels, increasing the number of 54Mbps lanes that can be provided in any given area.

By the way, the 11 and 54Mbps rates are the data rate for the connection, not the actual throughput. Throughput in a b network ranges from .9Mbps to 5.8Mbps, depending on range, while throughput in a g network goes from about .9Mbps to 24.7Mbps; throughput in an a network is about the same.

All three wireless protocols provide fallback speeds whenever transmission quality is poor. 802.11b networks can fall back to 1 or 2Mbps; networks using a and g have a wide range of fallback speeds. You may also know that the Wi-Fi CERTIFIED™ designation requires all 802.11g equipment to be backward compatible with 802.11b signals. However, did you know that your 802.11b/g AP provides a special fallback speed known as mixed g? This mode activates automatically whenever an 802.11b client is within range of a b/g AP, and the mode forces all g clients to operate at a slower maximum throughput of about 15Mbps. That’s better than the b-only speeds, but only about half of what g can really do. What’s worse, this mixed mode will prevail throughout the entire interconnected network. For example, suppose you’ve placed your APs so their transmission areas overlap—a perfectly normal practice. A single b client within range of a b/g AP will result in all APs running on the same channel to go into mixed g mode!

The solution? Either get rid of every b client on your network—unlikely, as it’s built into so many devices like laptops and PDAs—or deploy a secondary, high-speed g network on a separate channel. That’s more complex and more expensive, but if you’re bound and determined to have the fastest wireless transmission rates possible, it’s the only choice.

We’ve all seen 802.11g offerings claiming speeds in excess of 100Mbps. Sound too good to be true? Well, it is and it isn’t. Those speeds are certainly possible, but not by strictly following the IEEE 802.11g specifications. Manufacturers offering faster data rates are going a bit outside the spec. That means the speed advantage will only be had if all the equipment involved—wireless network cards and APs alike—are using the same technique (generally meaning they must come from the same manufacturer).

While going outside the official 802.11g specification can provide faster speeds, it can also jeopardize the compatibility that the specification was designed for. Most outside-the-spec products still carry the Wi-Fi CERTIFIED designation, so they should work fine with other manufacturers’ equipment, but you won’t necessarily be able to run at the maximum advertised speed.