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802.11n WLAN design best practices

In the second part of our series you’ll discover best practices for designing and deploying 802.11n a wireless LAN (WLAN), from RF design and band/channel selection to access point placement for coverage and capacity. After reading this section, you will understand where to start when adding 802.11n to your wireless LAN and the key factors that must be considered during the design process.

Don't miss any of the articles in this tutorial on deploying 802.11n for improved wireless LAN performance:


Anyone deploying and designing a wireless LAN (WLAN) today should use 802.11n access points (APs). Not only do prices rival 11a/g, but indoor spaces may need fewer APs thanks to 11n's reach. Although some battery-powered devices have not yet been re-factored, most new clients ship with 11n as well. In short, buy 11n unless business needs absolutely dictate otherwise.

When it comes to designing your 802.11n WLAN, however, best practices are far more elusive. For example:

  • Channel architecture: Nearly all products use multiple channels to create micro-cells, but at least two vendors use a single channel to create one big RF layer. The latter clearly cuts roam time when clients move, but trade-offs are hotly debated.

  • Distribution of control: 802.11n exposes bottlenecks and inefficiencies in centrally controlled WLANs. Small controllers can be swamped by high throughput, tunneling everything to one controller causes latency, and extending QoS end-to-end can be hard. For best results, centralize administration and visibility while delegating real-time QoS and security control tasks to APs.

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  • Backhaul capacity: A fully utilized 11n AP may deliver an order of magnitude more traffic than 11a/g. Examine 10/100 Ethernet edge switches and backhaul links to plan Gigabit Ethernet upgrades where additional capacity is required.

No matter how IP packets flow through your network, when they reach an AP, they will be wrapped into 11n frames for transmission over a radio channel. Here, RF networks should be explicitly designed to meet coverage, capacity and application needs.

 

  • Band selection: When possible, use the 2.4 GHz band for 11g clients only; use portions of the 5 GHz band for 11a and new 11n clients. This practice reduces interference and delivers more capacity (including 40 MHz bonded channels) but requires purchasing only dual-band 11n clients.

  • Channel planning: Divvy 5 GHz channels to meet diverse application needs. For example, assign static 20 MHz UNII-1 channels to voice, giving handsets a small reliable scan list. Allocate UNII-2/2e channel blocks to data or file transfer apps that can tolerate dynamic frequency selection (DFS). Bond 5 GHz channels only for apps that need sustained high throughput, such as HD video. Never bond 2.4 GHz channels.

  • AP placement: Legacy APs were often placed by rules of thumb, but multipath causes 11n cells to be jagged and change over time. Use a WLAN planner to approximate number of 11n APs and where to place them to meet design targets. For best results, use a predictive planner that considers building impact on RF. To future-proof your plan, position dual-band APs based on 5 GHz cells instead of larger 2.4 GHz cells. Consider directional antennas to efficiently fill in coverage gaps like long hallways or stairwells.

  • Design targets: Always base AP plans on application-aware targets. For example, min signal strength -67 dBm with cell overlap 10% to 20% is often advised for voice. When choosing cell size, consider that large cells minimize roaming – good for latency-sensitive apps – but small cells are better for high-density areas like classrooms. Even when using auto-configured AP transmit power, choose a max that matches your weakest (often legacy) client's transmit power.

 

Experts debate 11n site survey benefits, but survey proponents recommend taking post-deployment measurements to verify your design as an 802.11n design best practice. Active survey techniques should be used with 11n, associating to measure downlink and uplink. Finally, always measure application performance with real client devices because 802.11n clients are very diverse, and signal strength is not always a reliable proxy for user experience.

 

Next: 802.11n migration: Supporting legacy devices

This was first published in December 2010

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