Wireless local area networks allow employees to disconnect their Ethernet cables and work anywhere in the office. They can even be used for making phone calls with the latest technology, but one thing they don't do is perform at their advertised throughputs.
The Wi-Fi throughput rates are so well known that anyone even vaguely familiar with the technology can recite the speeds. 802.11b, the most commonly used standard, runs at 11 Mbps. 802.11a and 802.11g run at 54 Mbps, right?
While it is normal to expect some drop off in performance between the theoretical and the practical, Wi-Fi speeds are significantly slower than anyone might expect.
With the higher speed 802.11a and 802.11g systems, the lab environment maximum is 33 Mbps, he said, and real-world performance is closer to 22 Mbps, which is a far cry from the blazing 54 Mbps speeds touted by manufacturers.
In general, WLAN managers should expect top performance to reach only one-third to one-half of the advertised rate, said Craig Mathias, a principal with the Framingham, Mass.-based research firm, Farpoint Group.
The largest loss of throughput is due to management frames, said Graham Melville, director of product marketing in the wireless infrastructure division at Symbol Technologies Inc. in Holtsville, N.Y. Management frames contain the information the access points need to manage themselves and determine which user to listen to. The nodes use a portion of the airwaves to send and receive this management information, reducing the bandwidth available to users.
Interference is also a big problem, particularly for 802.11b and 802.11g networks, said Iain Gillott, Principal with Austin, Texas-based iGollott Research. Those systems use the 2.4 GHz spectrum band, which is also used by cordless phones and microwave ovens. Transmission from those devices can slow or even disrupt a Wi-Fi network.
Networks that use the 802.11a standard experience less interference because few other devices use the spectrum in the 5 GHz band, Gillott said.
Network devices that employ the 802.11g standard are also susceptible to losses of throughput. Though 802.11g runs at a much faster rate than 802.11b, it uses the same spectrum as 802.11b and can accommodate those devices. But when an 802.11b device logs on to an 802.11g network, it slows all traffic down considerably, said Mathias. He said 802.11 traffic will slow to the lowest common denominator.
The vendor defense
Despite the dramatic difference between theoretical rates and actual performance speeds, vendors make little effort to publicize the contrast in their promotional materials.
Cisco Systems Inc., the leading provider of enterprise Wi-Fi equipment, cites the top speeds in many of the product listings on its Web site. On Symbol's site, one has to dig deeper to find any mention of 802.11 speeds, but when throughput is mentioned the company uses the same inflated numbers.
"The industry is not doing enough to communicate the real-world speeds," said Mathias.
Though Cisco's Web-based brochures quote the maximum speeds, discussions with customers are different, said Ann Sun, Cisco's senior manager of wireless and mobility marketing. "We are always very frank with our customers about the real-world performance and throughput of wireless LANs," she said.
Melville said that Symbol works with its channel partners to ensure that they know how to help customers get the best performance from their wireless networks.
In the vendors' defense, they are in an awkward position, said Mathias. "How do you explain that the universe is a variable place?" he asks. "That is not what a network manager or anyone wants to hear."
Getting the extra speed
While the maximum WLAN speeds are usually unattainable, users can take steps to optimize throughput. Most experts agree that the first step to coaxing optimum performance from the network is conducting a site survey. Those designing the network need to consider not only the physical characteristics of the environment, but also the number of users during peak usage periods, said Sun. The more user that connect to an access point, the slower each of their connections will be.
Another way to boost 802.11 throughput is to use channels that are far apart from each other. For example, Surineni said that if a network is configured to use channels 1, 6 and 11, then it is less likely that users on those channels will interfere with each other and slow the network down.
Network managers should also make sure that they have not inadvertently limited the speeds of their access points. Each can be configured to transmit at varying speeds and strengths, Mathias said.