802.11n Coverage and Range in the Enterprise - Page 2

By Phil Belanger

June 21, 2007

Environmental factors are more influential in coverage with 802.11n networks than with legacy 802.11 systems. The new physical layer for 11n employs MIMO (multiple in, multiple out) techniques to improve the link margin, and actually takes advantage of multi-path to improve the signal. Multi-path is essentially multiple reflections of the RF signal arriving at the receiver at slightly different times. Indoors, multi-path is caused by the radio signal bouncing off walls and other obstructions. 11n benefits from multi-path.

However, multi-path is a signal impairment for earlier versions of 802.11 – it’s essentially interference. A difficult location that was a “dead spot” or “coverage hole” in an 11g network may be a great location with excellent signal strength in an 11n network. Our intuition about designing network coverage for 11a/b/g networks is all wrong when it comes to designing 11n networks. The current set of automated network planning and site survey tools for 11a/b/g will likely produce strange results. The size and shape of the coverage areas with this new infrastructure will be very different. The coverage achieved by 802.11n will be more sensitive to building design, and will vary more from one installation to another. We will need to develop new tools and best practices for installing large enterprise WLANs based on 11n.

The method for achieving extreme range in 802.11n changes the behavior at the system level. With all of the optional capabilities enabled, an 11n client may be communicating to its AP with multiple spatial streams using multiple antennas and beam forming. Beam forming concentrates the RF energy in a particular direction, toward the intended recipient. The beam is formed dynamically and can be pointed automatically, creating the effect of a temporary high gain antenna between the two communicating stations. Range extension in this manner will change the shape of the effective coverage area even more. Coverage will literally follow a mobile client with smart antennas beyond the normal coverage area.

AP-to-AP roaming behavior will be different for new 11n clients with smart antenna systems. In current WLANs, the mobile client is usually associated to the closest access point. That may not be the norm for 11n networks. As a result, applications may perform differently. Overall, interference from adjacent APs operating in this manner may occur less frequently than with current systems, but when an enhanced mobile client is nearby and active, it will create interference for adjacent APs at a higher power level. Even interference in the enterprise will be different with 11n.

These are not huge problems. They just make the system behavior of WLANs in the enterprise very different than it has been for the last ten years. It is possible to deploy an 802.11n system in the enterprise without understanding or addressing any of these issues -- existing 802.11 networks could upgrade their APs from 11g to 11n, placing all of the new APs in exactly the same locations. Legacy Wi-Fi devices would continue to work, though there may be funny behavior with some applications. Old trouble areas for coverage may be improved, and new coverage holes may emerge. New 11n Wi-Fi clients would be able to use the same infrastructure and operate at higher data rates.

Since new 11n APs will be more expensive than current thin APs, the simple AP swap-out could be an expensive system upgrade for an incremental performance improvement. It may be possible to achieve the same performance with fewer APs, or to have much better performance with the same investment in a system tuned to address the 802.11n system behavior.

To fully exploit the promise of 802.11n in the enterprise, we need to develop new tools, new intuition about range and coverage, and new best practices for WLAN design and deployment.

The technical part of the IEEE 802.11n standard is complete, though the official standard won’t be published for more than a year. Wireless chip vendors are already shipping their second generation 11n chipsets. Now, further innovation will come at the system level and there will be many new and different 11n enterprise WLAN systems introduced in the next few months.

It will be interesting to see how WLAN infrastructure vendors deal with the complexity, challenges and new capabilities of 11n. Fat APs may come back in style. WLAN architectures and deployment strategies that were once considered strange may become the new best practices for building WLANs.


N is for Novarum. Stay tuned for future installments in our wireless series as we examine the promise and pitfalls of 11n in more detail.

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