802.11n Coverage and Range in the Enterprise
June 21, 2007
Think simply swapping old APs for new 11n APs is all you have to worry about? Think again.
Ten years ago, designing a wireless LAN was more art than science. Most of the early wireless LAN deployments were hand-crafted in the sense that RF engineers designed the layout and placement of access points (APs) with simple tools and their good instincts.
Back then, when 802.11 networks operated at 1 and 2 Mbps and APs were fat, the rule of thumb for coverage planning in WLANs was 20% overlap of adjacent cells. That would result in a continuous coverage area, but the access points were far enough apart that they would not directly interfere with each other. Client roaming behavior was smooth and predictable. As wireless clients moved away from their access point and into these overlapping areas, they were able to detect that they were in the fringe coverage and would scan for better access points. In a well-designed network, a roaming 802.11 client would find a neighboring AP with better RF signal and automatically associate to the new AP.
Over the last few years, WLAN systems for the enterprise have become much more sophisticated. APs are now thin, and a central WLAN controller consolidates management, security and deployment functions. Modern APs are capable of automatically setting their channel, and have detailed control of their transmit power levels, even at deployment. Tools are available for 802.11a/b/g to plan networks, predict wireless coverage and recommend AP placement. The central controller can take a system-wide view and make adjustments as necessary to assure adequate coverage throughout the network. There is still some debate about whether this automated approach results in a better wireless network design than the good old handcrafted approach. However, the WLAN controller architecture has clearly driven the success of WLANs in the enterprise by reducing complexity and lowering deployment and operating costs.
802.11n for the enterprise changes everything. There are many different facets of this change. New 802.11n APs capable of delivering ten times the capacity of the current generation will create tremendous load on enterprise networks and force redesigns for both wired and wireless networks. The debate about WLAN architectures - controllers and thin APs versus standalone fat APs - has resurfaced as WLAN infrastructure vendors migrate to 802.11n and optimize their systems to take advantage of the new capabilities.
Lets focus on the range and coverage capabilities of 802.11n to illustrate how a new 802.11n deployment will be different from current 802.11a/b/g network designs. We previously covered the outstanding range of draft 802.11n products coming to market today for consumers and small businesses. They really do deliver at least twice the range of 802.11g products, and enterprise APs and clients based on 802.11n could do even better. But combining tens or hundreds of new 802.11n APs in an office may have unpredictable results in terms of the coverage achieved and the behavior of wireless applications.
There will be a wide variance of capabilities in the client devices using 802.11n. Certainly, legacy 802.11a/b/g clients will have very different range capabilities than 802.11n clients when communicating with new 11n APs. Most enterprise WLAN deployments will plan to support both legacy and new 11n clients for the next few years. However, even a greenfield 11n deployment could have a variety of client devices with differing range. There will be a significant range difference between an 11n dual-mode Wi-Fi/cellular handset and an enterprise 11n Wi-Fi client in a notebook PC. The notebook client will have multiple antennas built into the case and multiple radios operating at higher power. The dual-mode handset will likely only have one radio dedicated to Wi-Fi, and is not large enough to support the many antennas that are physically separated. The range of these products will be very different.
When designing an enterprise WLAN, how do you do coverage planning? Do you design your network for the least capable client? When 802.11n systems move into the enterprise, the disparity between the best-performing client and the worst-performing client will be greater than ever before, and new network designs must address this.