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How to plan for 802.11n wireless LAN upgrades

Deploying 802.11n access points (APs) may be a big step, but it's just one element of a much broader network upgrade. From greater speed and capacity to real-time application support and mission-critical corporate network access, 802.11n raises the bar for wireless LANs (WLANs). To reap the business benefits of 802.11n, enterprises must plan to invest not only in new AP hardware but also in client device refresh, network infrastructure expansion, updated management tools, and training.

Engineer WLAN upgrades for increased workload

Upgrading an existing WLAN to faster 802.11n APs will consume more wireless controller capacity and backhaul network bandwidth. Furthermore, many businesses will use 802.11n to expand wireless services to support additional users, new applications and broader coverage areas. To avoid unpleasant surprises, predict the traffic load that wireless will deliver to your wired network and how it will grow over time.

A typical 802.11a/b/g AP operates at a maximum data rate of 54 Mbps, delivering roughly half that application throughput due to overhead. Depending on product and configuration, draft 2.0 802.11n APs can boost maximum data rate to 300 Mbps, yielding 150+ Mbps of application throughput. Therefore, replacing a dual-radio legacy AP with an 802.11n-capable AP could potentially increase maximum offered load from about 54 Mbps to roughly 300 Mbps.

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More on 802.11n wireless network upgrades
Deploying 802.11n access points: Best practices  

Understanding 802.11n antennas

However, few WLANs will immediately cut over to new clients and applications that fully utilize available wireless capacity. In the near term, a more realistic calculation might involve using one radio to support 802.11g clients and the other to support 802.11n clients, yielding a combined output of no more than 204 Mbps.

Thus, you may want to plan network upgrades to accommodate a fourfold workload increase on WLAN controllers, links used to carry traffic onto your distribution network, and distribution layer switches. This growth will probably occur gradually as client devices are upgraded to 11n, so examine notebook and handset refresh cycles to anticipate migration over time for your own workforce.

Break through bottlenecks in WLAN network upgrades

When planning network upgrades, it is important to consider where new 802.11n APs will be located and connected to your corporate network.

Some new APs may be cabled to existing WLAN controllers that have spare capacity, but others will require WLAN controller upgrades -- especially when using 802.11n to expand coverage. In certain locations (e.g., small/branch offices), it can be desirable to install autonomous 802.11n APs that do not aggregate traffic through a local controller. In all of these cases, try to identify and eliminate potential bottlenecks through WLAN design and configuration. For example, route intra-WLAN traffic directly from AP to AP to reduce latency and controller workload.

Next, consider backhaul links that connect WLAN controllers (and autonomous APs) to your wired distribution network. Some 10/100 Ethernet links will eventually need to be upgraded to Gigabit Ethernet -- if not between the controller and switch, then possibly between switches. Alternatively, consider using 802.11n to implement backhaul over 5 GHz wireless -- especially in hard-to-wire venues where new cable drops are infeasible or sub-optimal. With higher capacities and faster data rates, 802.11n is more than capable of carrying backhaul traffic traditionally relayed over 10/100 Ethernet. But don't forget AP redundancy to ensure backhaul availability.

Plan for increased 802.11n power demand

When used at maximum capacity, 802.11n APs tend to require more power to drive their additional multiple-input multiple-output (MIMO) transmit/receive chains and sophisticated CPUs. Enterprises that drive legacy APs using standard 802.3af Power over Ethernet (PoE) may find that a new 3 x 3 MIMO 802.11n AP draws more than the maximum 12.95 W per port.

Although today's APs implement stable capabilities defined by draft 2.0 of the 802.11n standard, many are the first in an entirely new generation of enterprise WLAN products. Next year's APs may well incorporate more efficient chipsets and find other ways to optimize power consumption. Thus, PoE may or may not be an issue for your 802.11n deployment, depending on the products that you choose, the PoE-capable switches that you use, and how you configure your WLAN.

Ask your WLAN vendor to estimate the PoE draw for the APs that you have selected, in the specific configurations that you plan to deploy. For example, if a dual-radio AP with both radios in 3 x 3 MIMO mode would exceed your power budget, consider temporarily using one of those radios in 2 x 2 mode instead. You may not need to tap the full potential of all new 802.11n APs immediately anyway. Alternatively, plan to deploy more PoE ports as distribution switches are upgraded, and/or insert mid-span power injectors where really needed.

Expand your 802.11n management toolbox

Because 802.11n APs essentially "speak a new language," they require updated network planning, monitoring and troubleshooting tools. Existing WLAN tools can be applied to some 802.11n APs -- for example, WLAN analyzers and wireless intrusion prevention systems (WIPS) will detect the presence of 802.11n APs operating in legacy or mixed modes. However, the legacy 802.11a/b/g radios used by those systems cannot receive or fully decode transmissions sent with 802.11n improvements like 40 MHz-wide channels or frame aggregation. To do their jobs well, those tools must be upgraded to full participation in 802.11n.

As noted in "802.11n AP deployment best practices," optimum performance and return on investment can be achieved by planning WLAN upgrades with a predictive modeling tool. Invest in contemporary RF planning tools that understand 802.11n protocol capabilities and MIMO signal propagation and can recommend configurations that optimize placement and options. Particularly when moving from a best-effort data WLAN to a business-critical mixed-media WLAN, use planning tools to map users/devices onto application coverage areas, satisfying specified quality-of-service objectives.

During WLAN deployment, handheld tools are required to support site surveys and connection debugging. Not only must these tools be equipped with new 802.11n adapters, they must understand the consequences of 802.11n changes. For example, use site survey tools that let you measure not just signal strength but application throughput -- and not just in one direction but in both. Use diagnostic tools that can pinpoint 802.11n feature support mismatches instead of making you compare PDU fields and bit maps. Leverage monitoring capabilities offered by new 802.11n APs -- they have a more persistent view of the RF environment and may record useful observations at the time when a transient problem occurs.

Improve WLAN monitoring for 802.11n

After deployment, use full-time, centralized security and performance monitoring systems to keep an eye on your bigger, faster 802.11n network. 802.11n WLANs are more dynamic and will use techniques like spatial diversity and dynamic frequency selection to bypass problems on their own. When problems do occur, however, they will happen at higher speeds and affect more users and more sensitive applications. The only effective way to deal with these challenges is to keep a 24/7 watch over your WLAN in a way that enables rapid analysis and response.

For example, because 802.11n expands WLAN reach, rogue and accidental connections become more likely. Upgrade WIPS sensors (and APs configured to operate as sensors) to parse 802.11n traffic and cover all affected areas. Configure automated response policies to block rogues (including 802.11n Greenfield APs). Use locationing techniques that understand multi-path to quickly assess rogue impacts and find/remove them.

Finally, most companies will experience immediate throughput and reach improvement when deploying 802.11n. However, 802.11n protocols and options are so complex that WLANs are unlikely to operate at maximum utilization and effectiveness without additional investment in training and tools. Upgrade RF performance monitoring tools to analyze and assess 802.11n options -- not just in general, but with an understanding of the specific APs and clients used within your WLAN. Look for tools that can not only recommend potential optimizations but help administrators visualize the impact of possible changes -- and then implement them by defining and pushing configuration updates in an automated fashion.

While these network and management infrastructure updates are not strictly mandatory when moving to 802.11n, they are sensible investments. Upgrading to 802.11n without network capacity, visibility and control would be akin to driving a high-performance race car on a narrow back road with an obscured windshield and no brake pedal. Plan to surround your WLAN with the right tools to avoid nasty surprises and maximize your chances of success.

About the author:
Lisa Phifer is president and co-owner of Core Competence, a consulting firm focused on business use of emerging network and security technologies. At Core Competence, Lisa draws upon her 27 years of network design, implementation and testing experience to provide a range of services, from vulnerability assessment and product evaluation to user education and white paper development. She has advised companies large and small regarding the use of network technologies and security best practices to manage risk and meet business needs. Lisa teaches and writes extensively about a wide range of technologies, from wireless/mobile security and intrusion prevention to virtual private networking and network access control. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.

This was first published in September 2009

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