Random 802.11n Observances

Random 802.11n Observances

By CWNP On 08/09/2007 - 10 Comments

The 802.11n amendment allows for up to 4x4 MIMO.  That's 4 transmitters and 4 receivers working at the same time.  Due to cost, nobody is building a 4x4 system.  Instead, we're seeing 2x2 on the cheap stuff, 2x3 on the higher-end client radios and the lower-end APs, and 3x3 on the higher-end APs.  More transmitters and receivers means better quality: better reception, higher throughput, the works.  Always look for 2x3 stations and 3x3 APs whenever possible.

Some systems do not have 40 MHz operation, and while that isn't the only feature that adds to the advantages of 802.11n over 802.11a/g, it's a biggy.  To add to the confusion, some systems, like Apple's new extreme basestation only give you 40 MHz operation in 5 GHz, but not in 2.4 GHz...and did I mention that Apple doesn't let you choose your channel in 5 GHz?  Grrr.  The Wi-Fi Alliance certification test plan says that 40 MHz operation in 2.4 and 5 GHz is optional.

The 802.11n draft specifies up to 4 spatial streams - streams of bits transmitted over separate physical paths.  Manufacturers are only building chipsets to support 2 spatial streams currently, and the Wi-Fi Alliance is only requiring certified APs to support 2 spatial streams in Tx and Rx modes and Stations to support 2 spatial streams in Rx only.  

If you're planning on installing 802.11n anytime soon, consider the fact that you need a dual-band 802.11n-capable protocol analyzer and/or WIPS.  Without these tools, you're flying blind.  Connectivity troubleshooting - toast.  Performance optimization - no go.  Security monitoring - ha!  Deploying a WLAN system without the tools to perform troubleshooting, optimization, and security monitoring doesn't make any sense.  When will these tools be available?  

Wildpackets, AirMagnet, Tamosoft, and AirPcap have given us laptop-based 802.11n protocol analysis, but the most significant hang-up thus far is sparse client radio support.  Keep in mind that dual-band is what REALLY matters because half of the picture is more or less no picture at all.  Broadcom's Intensi-Fi chipset comes in CardBus format thankfully.  You can buy this card from Buffalo Technologies or one of their resellers.  So far Broadcom's development department has only just begun to work with most of the protocol analyzer vendors on the market.  Thus far they work with Wildpackets, but that's all I can find so far.  AirMagnet and Tamosoft are working toward this (as I understand), but Network Chemistry (now part of Aruba Networks), AirDefense, Network Instruments, and Network General, haven't announced anything yet on the dual-band front.  

What about the Atheros AR5008-3NX chipset (undoubtedly a market favorite)?  The only place I've seen one of these so far is inside an Apple Extreme Basestation.  http://www.vonwentzel.net/ABS/Dissection-Extreme-n/index.html  I'd love to have the CardBus version of this chipset, but there's nothing available to the public yet.  The only vendor saying that it's coming shortly is AirPcap - in September 2007.   What about Marvell's 88W8060 chipset or Qualcomm/Airgo's AGN400 chipsets?  Nada.  The Wi-Fi Alliance seems to be the only organization on the planet to have any of these chipsets in a useable format.  So, at present, we're pretty much stuck in limbo.  Grrr.

10 Responses to Random 802.11n Observances

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Manufacturers are only building chipsets to support 2 spatial streams currently, and the Wi-Fi Alliance is Visit This Web Page only requiring certified APs to support 2 spatial streams in Tx and Rx modes and Stations to support 2 spatial streams in Rx only.

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08/24/2007 at 13:47pm
While cramming new radios into an AP may seem like a good thing, you also want to consider processor performance. More radios and antennas means more functions to perform on the RF as it enters and exits the AP. You need a solution that can handle the packets both at the AP and wherever they transit the wired network. This means you not only have to consider what the AP is doing but also how those increased packets are going to affect your controller, if you are using controller-based APs. There's a massive difference between a dual-band AP running .11g and .11a radios pushing maybe 50 mbps (20-26mbps throughput per radio) and a dual-band, dual-radio (.11a/n and .11b/g/n) AP that might push well over 200mbps (100 mbps throughput per radio).

As far as the 2x3 vs 3x3 discussion goes, 3x3 really only begins to matter if there are three spatial streams being used. Current chip sets are limited to two spatial streams; adding another TX radio for 3x3 only provides about 1dB more coverage. Until three spatial stream solutions are available, there are miniscule advantages for using three transmitters.

You also need to weigh power requirements and product costs. APs require power in the form of Power over Ethernet, a.k.a. IEEE 802.3af (more accurately, IEEE 802.3-2005, clause 33 PoE), inline power, or via a local electrical outlet (a.k.a, the "wall wart"). IEEE 802.3-2005, clause 33 POE can only handle about 13 watts. This isn't enough power for an AP with multiple .11n-capable radios like you'd see in a 2x3 or 3x3 design. Newer POE capabilities are coming (IEEE draft 802.3at) that will be able to handle about 28 watts. Until that standard emerges though, you'll either need to go with 802.3at draft products, fewer radios, or use wall warts or inline power injectors to achieve the required power for your APs.

A 3x3 MIMO AP will only get slightly more throughput than a 2x3, but at what cost? The increase in throughput acceleration for 3x3 when compared to 2x3, for the foreseeable future, is minimal, especially when you consider adding advanced wireless technologies like VoWLAN to the mix. It is more important to have an AP with a powerful CPU with lots of available memory than an AP with another TX radio. Adding another TX radio will likely add significantly to the product manufacturing cost which ultimately will be passed on to customers. Most importantly, consider the future. We're still at Draft 2.0 of 802.11n. There could be future changes that would impact 802.11n standards-based hardware designs. You should consider a modular radio design when discussing 802.11n products with your WLAN vendor. Make sure they can "future-proof" your WLAN by simply upgrading the radios rather than the entire access point.

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