Don’t Mess With WMM!

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Tim Higgins

A recent Forum post reported what appeared to be a bug involving the NETGEAR WNDR3700 and Intel 5300 WiFi Link mini PCIe client. The gist of the problem was that the reader could only get a 54 Mbps maximum connection rate between the two products in both the 2.4 and 5 GHz bands. But connecting the same notebook (with Intel 5300) to a Trendnet TEW-672GR 802.11n router yielded the expected 300 Mbps link rate (with 40 MHz mode enabled).

The reader later reported that enabling WMM on each radio, which he had previously disabled while setting up the router, immediately allowed the Intel 5300 client to link to the WNDR3700 at the same 300 Mbps rate provided by the Trendnet router.

Since I was unaware of any relationship between WMM and supported link rate, I reported the problem to NETGEAR as an possible bug. But after consulting with Atheros (maker of the WNDR3700’s wireless chipset), Intel and even the Wi-Fi Alliance, NETGEAR reported that the behavior found by the reader was normal!

I’ve seen recommendations from users to disable WMM in 11n routers to either improve performance or fix problems. So I asked NETGEAR for an explanation of why WMM is required to enable link rates higher than 54 Mbps. They sent along an explanation from Atheros, which I’m paraphrasing below.

Basically, the 802.11n spec requires devices to support 802.11e (Quality of Service [QoS] enhancements for wireless LAN) in order to use HT (High Throughput) link rates, i.e. higher than 54 Mbps. (WMM is a subset of 802.11e that was created by the Wi-Fi Alliance as a stop-gap measure while 802.11e made its way slowly through the IEEE review process.) WMM’s Traffic Identifier (TID) field is key to aggregation mechanisms, including block acknowledgement (block ACK), that enable 802.11n’s high throughput rates.

Since WMM support is required for products to be certified for 802.11n, WMM comes enabled by default in all Wi-Fi Certified n APs and wireless routers. So even if you don’t have any WMM-aware devices on your network, leave WMM enabled or you may find your clients connecting only at 54 Mbps rates.

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