Designing for 802.11n

Unanswered Question

So far, we have been told to continue to use the guidelines for 802.11b/g when designing for 802.11n.

However, there are, no doubt, additional considerations that we will need to account for.

For example:

* If the coverage quality is increased with 802.11n, does this mean that we can place access points further apart and still get the same RF coverage?

* When more than one 2.4Ghz band is used on an access point, how will this impact legacy clients?

* How will the use of more than on 2.4Ghz band on a single access point affect co-channel interference among adjacent 802.11n access points?

* In an environment with a mix of both 802.11a/b/g and 802.11n (either an internal mix or foreign APs), are there any considerations to ensure that the different technologies play nicely together?

* When directional antennae are needed (for example when servicing long aisles in warehouse/manufacturing facilities), what options exist? Would three YAGI's per radio be required for indoor VoWiFi? etc.

* With the increased bandwidth, does this affect the maximum recommended number of users on the access point?

* When mixing legacy (b or g) clients on the N network, what considerations should we use?

* What sort of survey tools are recommended by Cisco? (Presumably AirMagnet).

This is just a limited list that comes to mind.

I have registered for the AirMagnet webinar in hopes of getting some additional insight, but am interested in Cisco's own recommendations.

If anyone is aware of new information available on the Cisco website, any links to this information would be appreciated.

- John

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Overall Rating: 4.9 (9 ratings)
dennischolmes Tue, 07/01/2008 - 13:55
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You ask a ton of good and important questions. First, let me ask one very big one. Do you intend to run 802.11n on 2.4 or 5ghz, or both? If you do 40mhz channel bonding you will only be able to use one channel on 2.4 and eleven on 5ghz. So 5ghz is the real way to go here. When designing space between APs a site survey is strongly suggested for .11n in 5ghz space. This will allow your 2.4 legacy clients to operate on the non channel bonded 2.4 space and provide adequate coverage for your .11n 5ghz network. You CAN NOT do channel bonding on 2.4! I can't emphasize that enough. Co-channel interference would kill you. Cisco does not at this time offer any real selection of antennae. I suggest you contact Tessco as they are developing several options for the 1252. The maximum number of users on an AP increases nearly 10 fold for data only but no published best practices numbers are out as of yet. I asked that myself of a Business Unit TME just last week. Airmagnet now supports 802.11n and I strongly suggest Survey PRO. If you have further questions feel free to contact me at the email address in my profile on this site.

Scott Fella Tue, 07/01/2008 - 18:16
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Let me give this a shot also... If you are doing greenfield (only 'N' client) then yeah, you should see some increase. But since you will be using 40mhz width on 5ghz, you still will get the same attenuation you get normally on the 5ghz. So looking at how most have done surveys on the 2.4ghz for data and voice and not really caring about coverage on the 5ghz (depends), now you have to use the 5ghz, so if you look at past surveys, you will probably need more ap's to cover and area now than before. The increase bandwidth as Dennis mentioned will increase the number of users you can have on the ap. This depends on what applications the wireless will support. I had a clinet that had users that need to be able to upload large video files. We were able to have three users per radio and they were able to upload 2gigs of video within 30 minutes. One user was able to upload 2 gigs in 12-15 minutes.

Again.... mixed environemt usually means you still have to survey for 802.11b/g and 802.11a. Like Dennis mentioned, AirMagnet just came out with Survey PRo that supports 'N'. They also have a card for around 125 bucks I think.

pmccubbin Wed, 07/02/2008 - 06:02
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Dennis and Scott,

Thank you both for your very well considered responses. I rate them each a "5" for the insights they provided.

Too bad I can't give John a "5" for asking these important questions.



Scott Fella Wed, 07/02/2008 - 08:47
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Will it enhance coverage... yeah in open areas. 5ghz still has the issues regarding attenuation. Also look at how legacy 802.11a standard is. You still have one receive and transmit radio.

Dennis / Fella5:

Thanks for your insights.

These comments are very much appreciated.

I am hoping to see more concrete information from Cisco at some point here. After all, the 1250 APs have been in the field now for eight months or so - so I would expect to see some real-world-inspired guidelines.

(Cisco? Are you listening out there?)

Again, thanks for your comments. The forum has been a terrific source of expert advice and I appreciate the time taken to post these entries.

- John

Scott Fella Wed, 07/02/2008 - 09:52
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Well hopefully when 802.11N gets standardize, it won't be a hardware replacement and just software. I have heard in the begining, that it would be a software change, now I hear it can be a hardware swap.... of couse you have to eat the cost.

Darren Ramsey Wed, 07/02/2008 - 21:53
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I attended a pretty good presentation last week at Cisco Networkers, BRKAGG-3012 (11n Protocol and Technology Overview) for those that have access to the presentations. I'll summarize some of the key points.

(1) Don't bother with 2.4G, deploy MIMO on 5G

(2) 11n introduces a new MAC and PHY.

(3) 11n speed increase is accomplished by channel bonding (40Mhz), packet aggregation and block acknowledge (A-MPDU), and multi-spatial streams (MSS).

(4) One to one replacement from 1240 to 1250. Even with legacy adapters, 11b client range was extended by 40 feet, and 11a clients extended by 10 feet. This is due to the AP adding the multipath client streams together for better SNR. The presenter actually showed a video using a legacy client dropping off behind some elevators. He swapped the 1240 with a 1250 and was able to maintain a good signal.

(5) 11n speeds require WPA2 and AES, WPA or TKIP will fall back to legacy speeds.

(6) You can run the 1250 today in AF mode (15.4w) and then activate the other antennas with EPOE or injectors (18.5w) later.

(7) Gig required for speeds greater then 90Mb.

(8) For 11n, run either 4.2.130 or the upcoming 5.1. Avoid 5.0, too buggy.


FYI - Jeff indicates that Transmit Beam Forming (TxBF) is at work here - which only makes sense as you need a client-side enhancement to compliment MRC at the AP. I've been told that TxBF is not going to be available until a later code release, and that this only works for stationary clients.


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