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Wireless Features


It's been almost two months since 802.11n was finalized and a bit less since the Wi-Fi Alliance began certifying products under its Wi-Fi CERTIFIED n program (Wi-Fi Alliance Kicks Off New 802.11n Test Program).

So I thought this would be a good opportunity to poke into what the finalized spec hath wrought, see if there are any tangible benefits to end users from the changes and, if so, figure out what you actually need to do to get those benefits.

First, if you're waiting for the maker of the Draft 11n products that you already purchased to release firmware and driver updates to bring them to final N status, stop waiting. There won't be any such releases because all existing "draft" products were automatically grandfathered to "final" N status. Congratulations!

You can also stop waiting for any updates that will magically enable three-stream (450 Mbps maximum link rate) capability in your three-antenna wireless router (such as the ever-popular D-Link DIR-655). That won't happen either.

It turns out that three-stream MIMO requires significantly more computational heavy lifting than two-stream. So even though your router may have three antennas and its radios have three transmit and receive chains, their MAC / Baseband devices just don't have enough computing poop. So you'll have to wait for routers with three-stream N chipsets to hit the shelves.

So far, D-Link's Xtreme N 450 Dual Band Router and Trendnet's TEW-773GR 450Mbps Wireless N Gigabit Router that were paraded around at this past January's Consumer Electronics Show have failed to materialize on store shelves. I checked with both vendors and Trendnet says their router was announced prematurely and won't ship this year. My Trendnet contact also speculated that there will probably be more three-stream announcements at January's upcoming CES, but that products won't actually ship until Q2 2010. D-Link didn't respond to my query.

One last important note for three spatial streams is that you'll need a three-stream capable client adapter to get the 450 Mbps link rate (and that's in 40 MHz bandwidth mode). So far, there are no such beasties available and a look at the WFA's Certification database turned up only one Ralink Reference design.

But even when three-stream client adapters are available, they are likely to be in the form of internal notebook / netbook adapters. Notebook upgrades will be problematic, however, since most (if not all) products currently have only two antennas and three will be required for full 450 Mbps linking. External USB format adapters will be hard to find because three-stream antenna separation will be hard to do in such a physically small format.

My guess is that Ethernet-connected bridges will be the easiest way to get a three-stream client connected. The physical size is right and they'll be able to use the PCIe bus interface that the chipsets will have.

Optional Features

But what if you don't care about buying a three-stream router and just want a two-stream model that implements some (or all) of the new 11n features that are supposed to provide improved performance?

Figure 1 is taken from this Wi-Fi Alliance whitepaper and summarizes all of 802.11n's optional features. The finalized N spec brought with it four new "optional" features (identified by ** in the table) that are starting to appear in products certified under the new Alliance program.

I've already discussed the gotchas in the new Three Stream optional feature. So let's see what the benefits are of the other options and what you need to do to get the benefits.

802.11n optional features

Figure 1: 802.11n optional features

In the same whitepaper, the Alliance provides a nice diagram of a simultation of performance improvement contributions from some of 11n's optional features. You can see that Transmit-A-MPDU theoretically provides the largest improvement.

802.11n optional feature throughput improvement - simulation

Figure 2: 802.11n optional feature throughput improvement - simulation

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