Key considerations for 802.11n upgrades

Moving to 802.11n requires a major network upgrade, some elements of which we detail in this how-to guide.

According to In-Stat, draft 802.11n sales grew 340% last year. By 2009, standard 802.11n products will push legacy 802.11a/g devices onto the discount rack. The question now facing most companies is not whether to migrate to 802.11n, but where, how and when to do so. This checklist will point out critical factors that should be considered when planning your 802.11n upgrade in order to maximize benefit and minimize pain.

Assess your 802.11n needs

802.11n physical and link layer enhancements could significantly increase your WLAN's speed, capacity, reach and reliability. But realizing this potential requires proper deployment: Your 802.11n upgrade must be explicitly designed to satisfy your business needs. Before purchasing access points (APs) or allocating channels, address these questions:

checkmark Which applications must be supported? Many enterprises will use 802.11n to support more diverse and demanding applications, but a WLAN that's perfect for email can still be lousy for voice or streaming video. Start by identifying all applications that your upgraded WLAN must support, and quantify what each requires in terms of throughput, latency and coverage (minimum speed/delay when roaming).

checkmark How many users must be supported and where? Break your facility into coverage areas, and identify the total number of users in each area, as well as the percentage online at any time. When doing so, group users by device and required application mix (e.g., voice handsets on the second floor, meeting room laptops using Web and IM) so that capacity can be planned accordingly.

checkmark What client devices will be used? Upgrading a laptop fleet to standard 802.11n can take two to three years, and some legacy clients (e.g., handsets, scanners) will linger longer. Divvy existing devices into groups, based on protocol (a/b/g/draft n/final n) and bandwidth (2.4 GHz/5 GHz/both), and forecast how proportions will evolve over time.

checkmark Do you have 802.11 backhaul needs? Many companies use 802.11 as an edge access technology, but high-throughput 5 GHz 802.11n is also useful for backhaul. Identify your existing legacy wireless bridge links (if any) and where 802.11n backhaul could be applied to better meet old and new business needs.

Plan for legacy coexistence

Few of us have the luxury of deploying a new 802.11n WLAN from scratch. Most will need to coexist with legacy 802.11a/g devices -- from existing APs and clients to devices owned by neighbors and guests. To accomplish this, consider these questions:

checkmark Do you need protection? The 802.11n standard defines high-throughput (greenfield), non-high throughput (HT) (legacy), and HT mixed operating modes. If you plan to deploy 802.11n in the same location and band now occupied by legacy devices, you must use HT mixed-mode protection mechanisms for peaceful cohabitation.

checkmark Where can you avoid protection? HT mixed-mode avoids legacy interference but reduces 802.11n throughput. To get the most out of 802.11n, deploy new APs in greenfield mode, using 20 or 40 MHz channels in the 5 GHz band. This also requires rapid deployment of new 802.11n clients, however, since legacy clients cannot speak to greenfield APs.

checkmark Should you double up? Use business needs to determine the best coexistence approach. If 802.11n users are spread throughout your facility, consider dual-radio APs, using one radio for legacy a/g clients and the other for new 802.11n clients. If little overlap exists between your old WLAN and what your new WLAN must cover, consider an independent 802.11n-only deployment.

checkmark When will you "sunset" coexistence? Legacy clients get better range when associated to mixed-mode 802.11n APs. But speeds over 54 Mbps require new hardware, and the sooner legacy clients retire, the better your entire WLAN will perform. Establish target date(s) for turning a/g off.

Identify network infrastructure impacts

Those bigger, faster 802.11n APs can carry more traffic and require more juice. Before investing in this new wireless gear, decide where and how your existing distribution network equipment must be upgraded to support 802.11n:

checkmark Will your controller be a bottleneck? Many enterprises have migrated from autonomous APs to thin APs. But WLANs that route all traffic through one central WLAN controller could be overwhelmed by 802.11n traffic. To avoid this, consider direct AP-to-AP traffic forwarding and/or distributed controllers.

checkmark When will you need more backhaul? In theory, an 802.11n AP could send 600 Mbps onto your wired network. In practice, mixed mode will start under 200 Mbps and grow over time. Predict when your 10/100 Ethernet edge switches will require upgrade to gigabit backhaul.

checkmark How will you slake 802.11n's thirst for power? 802.3af PoE cannot deliver enough wattage to drive most 802.11n APs at full capacity. Possible solutions include using fewer MIMO antennas or radios, proprietary PoE, two PoE ports per AP, midspan PoE injectors, or AC power.

Engineer your new radio network

802.11n APs are not simply faster. They use MIMO antennas, wider channels to make better use of the air and listen more effectively, and signal processing techniques like spatial multiplexing, space-time block coding and transmit beam forming. These capabilities change what an optimal WLAN looks like, and results can vary across buildings due to multi-path. To avoid expensive mistakes and post-install revisions, take the time to formally plan your new WLAN:

checkmark Where will you place 802.11n APs? Some vendors recommend swapping legacy APs for 802.11n APs. That is simple, but may or may not be most effective. With 802.11n, cell shape and size is far less intuitive. For best results, use a predictive planner to model 802.11n and suggest AP locations that create the coverage areas and capacities needed by your users and applications.

checkmark Does your plan really work? Models are helpful but are no substitute for reality. Use a site survey to visualize actual RF coverage, based on measurements taken when people and other obstacles are present, using representative clients (a/b/g and n). Because a strong signal may not mean good user experience with MIMO, measure application performance in both directions.

checkmark What bands and channels will you use? 802.11n can use 20 or 40 MHz channels in the 2.4 or 5 GHz bands. The 2.4 GHz band used by most clients is overcrowded, though. Stick to 20 MHz channels there, saving 40 MHz for high-throughput applications in the 5 GHz band, and utilize automatic channel selection and power adjustment to simplify planning.

checkmark Should you prioritize traffic? Most business-grade 802.11n APs also support 802.11e Wi-Fi multimedia prioritization. If your WLAN must support voice, video and data using shared APs, map your applications onto WMM access categories and 802.1p LAN classes of service.

Anticipate management and security upgrades

After 802.11n installation, the maintenance, monitoring and security practices used with your old WLAN still apply, but tools used to support these tasks must be upgraded to deal with the complexities and threats associated with the new standard.

checkmark Is your WLAN manager up to the job? Large enterprises can leverage existing WLAN managers, but others may find that the time has come to invest in new tools. 802.11n is more dynamic, and capabilities like automated tuning and client locationing become more important.

checkmark Is your wireless intrusion prevention system (WIPS) ready for 802.11n? Protocol extensions and 40 MHz channels create something of a blind spot for intrusion prevention systems that use a/b/g sensors to monitor the air. Upgrade your WIPS sensors (or APs used as sensors) to listen and respond to 802.11n threats in both bands.

checkmark Can you troubleshoot 802.11n problems? WLAN analyzers, spectrum analyzers, and performance monitoring systems must also be upgraded -- from new 802.11n adapters and protocol decoders to analysis engines that understand 802.11n options and coexistence impacts. Training on 802.11n may also be a worthwhile investment.



Moving to 802.11n is a major network upgrade. Many will take this opportunity to expand how wireless is used, moving from casual Internet access in isolated locations to more comprehensive coverage and mission-critical business applications. Making this change cost-effectively and efficiently, with minimal disruption, requires careful planning and network engineering. We hope this checklist provides a starting point for making your own next-generation WLAN bigger, faster and better.

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