A rare large-scale, wireless LAN stress test using three vendors' equipment found many WLANs could run into a performance ceiling as they grow in size and traffic. The study's authors say the problem lies not with the vendors' gear but with the design of the 802.11 protocol.

The tests confirm two troubling issues for high-density nets, according to Novarum, a consulting firm that developed and ran the tests (PDF version). First, co-channel radio interference among the access points clobbers the aggregate throughput of the WLAN. Second, the conventional thin access point/controller architecture doesn't scale well as the number of access points increase in a given area.

According to Novarum, performance degrades because of radio interference in dense deployments, and because the conventional model for enterprise wireless LANs - thin access points linked with a controller - scales poorly.

These problems are related to the way the 802.11 media access control (MAC) layer is designed, how it handles acknowledgements and retries, and how these become problematic under high and sustained traffic loads. In many WLAN products, and the WLANs built with them, this protocol inefficiency hasn't been an issue, because there have been relatively few wireless clients and their traffic has consisted of relatively light, and bursty, data traffic.

High-throughput WLANs based on 802.11n, especially running in the 5GHz band, will partly mitigate these effects, according to the study's authors, but not entirely. Because 11n offers a larger 'data pipe,' it takes more traffic to reach network overload in a dense WLAN. But because of 11n's higher throughput, enterprises are looking to do more with it, such as voice and streaming video, which can push the net toward overload.

"If you don't deal with co-channel interference and [the need for] access point coordination, it will put more pressure on the protocols supporting [wireless] voice and video," says Phil Belanger, co-founder of Novarum.

Designing a real-world test

Novarum's tests, conducted in the fall of 2007 in a vacant second-floor office in Sunnyvale, California, are unusual because of the number of access points and clients actually deployed. As the report points out, WLAN tests typically involve a single access point and about a dozen clients deployed in a lab-like environment. (Though Network World in 2006 conducted a large-scale WLAN test of 25 access points.)

In this case, Novarum used 72 wireless laptops and up to 54 wireless VoIP handsets, linking to a typical office wireless LAN first with 15 and then with 10 access points. The net was recreated each time with access points from Aruba Networks, Cisco, and Meru Networks.