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 Sizing Up Your WLAN By Jim Geier
March 14, 2002
When deploying wireless LANs, most installers ensure that there are enough access points to provide adequate RF coverage. This enables users to roam throughout the facility; however, it doesn't completely address capacity requirements. You should also plan to include a sufficient number of access points to support the wireless applications and the quantity of users expected to be operating within common areas. This leads us to the common question: How many users can each access point support? In order to answer this, you first need to understand the difference between data rate and throughput. Data rate is what we're generally referring to when identifying the performance of a wireless LAN, and it corresponds to the speed that data bits are sent. If we say that an 802.11b wireless LAN is operating at 11Mbps, we're talking about data rate. Each 802.11 frame in this case is sent at 11Mbps. Keep in mind that data rate only applies while a frame is being sent. Because of protocol overhead and shared medium access delays, each user can't continuously send information (using multiple frames) at 11Mbps. The true measure of performance is throughput, which is the speed of sending information over time. You can calculate throughput by dividing the number of information bits sent by the time it takes to send them. In regards to 802.11, information includes the bits sent in the frame body of information frames. Management and control frames don't count. As a result, throughput will always be less than the data rate. Information sent by users requires a certain amount of bandwidth that we call signal throughput. For example, the signal throughput of someone actively browsing the Web might be 100Kbps high quality and a streaming video signal could be as high as 2 Mbps. The aggregate throughput of all users (total of all signal throughputs) is what you can apply to determine the number of users that an access point can support. For 802.11b networks operating at 11Mbps, the total throughput capacity of an access point is about 6Mbps, due to protocol overhead and access delays as mentioned earlier. As a result, the access point in this example would support approximately 60 users (6Mbps/ 100Kbps) actively surfing the web. If all users were viewing high quality streaming video, then the access point would only effectively handle about three users (6Mbps/2Mbps). This gives you a rough idea of the maximum number of users. In actual practice, it's very difficult to accurately determine the throughput requirements of individual users. Overhead from non-802.11 protocols (e.g., TCP/IP) adds complexity, and it's hard to predict utilization levels and traffic patterns. To obtain a clearer picture of throughput before deploying the network or making changes, you can use an 802.11 simulation tool (e.g., Opnet) that lets you model traffic in a network and view throughput levels under various conditions. Of course the true test is whether or not users are happy with the performance. If they're complaining about delays, then go back to the drawing boards and consider adding more access points or migrating to a faster network. Author Biography: Jim Geier provides independent consulting services to companies developing and deploying wireless networks. He is the author of the book, Wireless LANs (2nd Edition), and regularly instructs workshops on wireless LANs.
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