Unanswered Question
Sep 25th, 2009
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Welcome to the Cisco Networking Professionals Ask the Expert conversation. This is an opportunity to learn how to scale wireless performance with 802.11n with Cisco experts Frederick Niehaus and Jeevan Patil. Fred is a technical marketing engineer for the Wireless Networking Business Unit at Cisco Systems, Inc. Mr. Niehaus has extensive customer contact and is responsible for developing and marketing enterprise class wireless solutions using Cisco Aironet and Airespace Series Wireless LAN products. He has actively participated in some of Cisco's largest Wireless LAN deployments in education and retail with such customers as the New York Board of Education and the Home Depot stores. In addition to live deployments, Mr. Niehaus has also served as technical editor for several Cisco Press books including the Cisco 802.11 Wireless Networking Reference Guide by Toby J. Velte and The Business Case for Enterprise-Class Wireless LANs by David Castaneda, Oisin Mac Alasdair and Christopher Vinckier. Prior to joining Cisco's Wireless Networking Business Unit, via the acquisition of Aironet, Mr. Niehaus worked as a support engineer for Telxon Corporation, supporting some of the very first wireless implementations for customers such as Wal*Mart, Ford, Hertz Rent-A-Car and others. Mr. Niehaus has been in the data communications and networking industry for the past 20 years and holds a Radio Amateur (Ham) License "N8CPI". Between 1983 and 1995, prior to joining Cisco, he was a senior technical support engineer for Tecmar Inc. one of the first companies to offer aftermarket products for the Apple and IBM personal computers. Jeevan Patil has been with Cisco for the past 10 years. For the first 5 years Cisco was a software engineer working on security, network management and wireless. In the past 5 years Jeevan has been the Product Manager on various initiatives such as Client hardware, CCX, standalone(Autonomous) software, WLSE hardware and software, Access Points 521 series, 1250, 802.11n etc. Currently Jeevan is focused on 802.11n standards and Access Points.

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Overall Rating: 5 (4 ratings)
hrios Sun, 09/27/2009 - 10:22
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In a real-world setup where you have to support 802.11a/b/g/n clients, how important is it to have 802.11n radios connected to gig switch uplinks? Is throughput significantly reduced by only providing 100mbps?

fredn Mon, 09/28/2009 - 12:24
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Well it depends on how much traffic you expect from the WLAN network and which AP you were going to use. If you were using an AP such as the AP-1250 but you decided to order it with only one radio and only run the one radio in 20 MHz mode then data rates up to MCS 15 would allow for 145 Mbps of throughput (and we all know real data rate is slower then radio data rate) so that would work provided you used only one radio (5 GHz or 2.4 GHz) as soon as you enabled the second radio, then your data would certainly exceed 100 Mbps. Now if you were using 40 MHz wide channels and say MCS 13, then your data rate is around 240 with only ONE RADIO so you can can see rather quickly why we recommend gig switch uplinks. If your data requirements are light or you are doing a staged roll-out, it would indeed work with 100 Mbps provided you didn't have a lot of users but I would only do this as a temporary solution.

Robert.N.Barrett_2 Sun, 09/27/2009 - 12:30
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Is Cisco going to support Greenfield mode (no legacy client support) for the 5GHz or 2.4Ghz radios in a WLC environment? We would like leave 2.4GHz for legacy clients and use 5GHz radios for bonded-channel 802.11n only (1140 and/or 1250 AP's).

fredn Mon, 09/28/2009 - 12:41
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Cisco currently does not support Greenfield mode for the following reasons. 1. Many of the companies that make WiFi cards (chipset vendors) either do not support Greenfield mode or do so only on their later designs so to support this could be problematic for those clients resulting in support problems as many of these clients require at least one legacy data rate to be supported for connectivity. 2. When you are in full Greenfield mode (and you choose not to support any legacy data rates) some Access Points that are in non-Greenfield mode (say a company next door for instance) would not be able to successfully detect these devices as WiFi and this would appear to be noise that those earlier Access Points cannot decode. When such a thing happens, the AP can be "fooled" into believing it is hearing a "radar" signal or event and may force those APs off channel. Given it can be problematic for them and given Cisco believes it is important to be a good "RF" neighbor and promotes co-location of such devices, we try to avoid Greenfield to minimize potential customer problems. That said, we do give customers a lot of control over their devices and if we see this as a problem or as more chipsets start to support this, we do reserve the right to perhaps enable it in the future and if we do so, it is likely to be a "hidden" command at first as there are no plans (today) to promote or default devices into Greenfield modes due to the current issues described.

Robert.N.Barrett_2 Tue, 09/29/2009 - 07:18
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Thanks, Fred!

Point #1 is most interesting. Can you give an example of a card that exhibits the behavior you describe? I would like to be able to show customers this behavior.

For point #2 - I think there are a large number of installations where peaceful co-existence is not an issue with the 5GHz frequencies (in the US, at least). Many of my customers do not currently use 802.11a, so the 5GHz frequency space seems to be a perfect opportunity for Greenfield mode (issues around point #1 not withstanding).

At least one of your competitors supports Greenfield mode.

fredn Tue, 09/29/2009 - 08:44
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On point #1 you would need to setup an AP in Greenfield mode with clients passing traffic (as I mentioned you can not do that today with our products) we have not brought that interface out to customers today (we tested this with our AP in such mode)and with other third party APs in this mode.

Then using a variety of different APs and client cards you can see this behaviour. I'd rather not list or cite our specific testing (as that is done internal here) and this is not the venue to identify specific vendors makes/model numbers etc.

I'd suggest you reach out to the WiFi Alliance and see what testing they have done with regard to this (as they are the authority for third party interoperability)or perhaps you can set it up with the "competitor" you mention that does support Greenfield mode?

Again we do not support this at this time because customers can disable most legacy rates essentially achieving high throughput similar to Greenfield mode.

Regarding point #2 I do not dispute there may be some corner cases whereas customers may wish to use Greenfield mode - but as I mentioned, our testing has found it to be problematic (at this time) and it is Cisco's desire to be a good RF neighbor for the reasons I've previously described. If your position is to support it regardless then please reach out to your Cisco account team and feature request via that channel.

zhenningx Mon, 09/28/2009 - 15:00
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As we know that the throughput for mixed b/g is about 8M-12Mbps, what are the throuput for mixed b/g/n and a/n with 1142 and 1252 APs?

fredn Thu, 10/01/2009 - 06:02
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Well when talking about actual throughput (not radio data rate) it is usually a bit higher then that around 12-16 Mbps (assuming .11b clients) and in a .11g only mode around 24-26 Mbps.

Now when we come to the question of 802.11n it changes with regard to what modes are being used with 802.11n for example, are you using 20 MHz wide or 40 MHz wide (channel bonding)? is the guard interval short or long? Has one of the clients that is being used (decided to enable protection mechanisms) as some clients do while others don't and this can diminish upstream traffic.

I really don't have a good answer as this also changes with regard to the customer's environment. Rule of thumb might be to say that even under bad conditions you still will realize a much higher performance with .11n (around 60 Mbps) compared to the 24/26 Mbps of 802.11g.

Of course the more legacy clients .11b you have on the air the longer they will take up air time transmitting data at a much slower rate.

You can certainly performance tune your system by limiting legacy clients to faster data rates or eliminating 802.11b clients altogether. These are decisions that are driven by the overall network performance requirements desired and of course how much data and number of users you have to service.

rmoneal Wed, 09/30/2009 - 11:35
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I have WCS and WLC are at and I have several .11N 1140's and 1252's how do I turn on ClientLink?

fredn Thu, 10/01/2009 - 09:52
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In the latest version of software that is posted on CCO, ClientLink is a configurable radio button under 11n Parameters. You will see a checkbox for “Beam Forming” select it to enable.

For a good overview on ClientLink see the following papers

ClientLink Whitepaper:

ClientLink Miercom Report:


rmoneal Thu, 10/01/2009 - 11:29
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I see under 11N parameters Client Link enable. Do I need to change the channel width to 40 MHZ?

fredn Fri, 10/02/2009 - 07:29
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No you do not.

40 MHz is a mode by which two channels are bonded together to allow for higher bandwidth.

It is different from ClientLink which is beamforming or the ability to direct a more powerful signal to the client.

king06aaa Fri, 10/02/2009 - 11:23
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I am making my first foray into configuring an 802.11n access point - 1252.

I have it configured and am connecting with the latest Intel client and card. The client will always connect to the 2.4 radio (and thereby 144 mbps) if both radios are enabled. I want it to connect to the 5.0 radio if it is present.

If I disable the 2.4 radio on the AP, I will connect to the 5.0 radio.

There are three options in the client for connecting - Mixed, 2.4 and 5.0 GHz.

If I have both radios enabled and select "Mixed" it will always connect to the 2.4 radio. However, if I select 5.0, it will connect to 5.0 and give me 300 mbps.

It doesn't seem to be the Intel client's problem because it shifts back and forth seamlessly each time a change is made on the AP or client.

Any ideas?

fredn Mon, 10/05/2009 - 06:10
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Generally speaking the client makes the decision on what band 2.4 GHz or 5 GHz it will use (and many clients start out looking at the 2.4 GHz band first).

Starting with the 6.0 release and above using Cisco AP-1140 and AP-1250 Access Points you can enable a feature called BandSelect. BandSelect is off by default.

BandSelect is used primarily for data clients and it motivates the client to go to 5 GHz. Note: there is a warning when enabling it that states it may impact time sensitive applications such as voice as it is ideally suited for data clients.

To enable (Cisco Controller)

config band-select probe-response enable

To see it the command is show band-select.

You can find more on BandSelect at the following URL

zhenningx Fri, 10/02/2009 - 11:37
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Can 802.11n APs (1142 and 1252) handle more users(in mixed environment with legacy clients) than 1131 APs? What is your commendation for the number of users on 1142 and 1252 AP?

fredn Mon, 10/05/2009 - 06:22
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Number of clients really depends on the applications (and data requirements of the users) as well as how you have configured your WLAN network.

For example, disabling older .11b client data rates or limiting slower data rates on your WLAN network can increase the overall performance which would improve the user's experience perhaps allowing more clients without any perceived degradation in performance.

So the short answer is yes (however; we like to stick to the same number of clients that we recommend in the Cisco deployment guides).

Given the AP is now using GigE ports and and it's fair to say that older non 802.11n Access Points generally used 100 Mbps ports) this helps and with the additional improved thoughput and distance of 802.11n it is easy to see that performance has increased.

bjohnson5@partn... Fri, 10/02/2009 - 20:15
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Hi Fred,

I'm looking at the power levels on the 1140 radios and amazed at the variations in power by data rate. These are in addition to the UNII-band EIRP rules, with some additional antenna gain assumptions on Cisco's part.

Are these really FCC-regulated levels? Does MIMO/MRC/ClientLink overcome these limitations to deliver higher sustained legacy rates at range?

Active power levels by rate

6.0 to 18.0 , 14 dBm, changed due to regulatory maximum

24.0 to 36.0 , 13 dBm, changed due to regulatory maximum

48.0 to 48.0 , 12 dBm, changed due to regulatory maximum

54.0 to 54.0 , 11 dBm, changed due to regulatory maximum

6.0-bf to 18.0-b, 14 dBm, changed due to regulatory maximum

24.0-b to 36.0-b, 13 dBm, changed due to regulatory maximum

48.0-b to 48.0-b, 12 dBm, changed due to regulatory maximum

54.0-b to m6. , 11 dBm, changed due to regulatory maximum

m7. to m7. , 10 dBm, changed due to regulatory maximum

m8. to m14. , 11 dBm, changed due to regulatory maximum

m15. to m15. , 10 dBm, changed due to regulatory maximum

m0.-4 to m3.-4 , 14 dBm, changed due to regulatory maximum

m4.-4 to m4.-4 , 13 dBm, changed due to regulatory maximum

m5.-4 to m5.-4 , 12 dBm, changed due to regulatory maximum

m6.-4 to m6.-4 , 11 dBm, changed due to regulatory maximum

m7.-4 to m7.-4 , 10 dBm, changed due to regulatory maximum

m8.-4 to m11.-4, 14 dBm, changed due to regulatory maximum

m12.-4 to m12.-4, 13 dBm, changed due to regulatory maximum

m13.-4 to m13.-4, 12 dBm, changed due to regulatory maximum

m14.-4 to m14.-4, 11 dBm, changed due to regulatory maximum

m15.-4 to m15.-4, 10 dBm, changed due to regulatory maximum

fredn Thu, 10/08/2009 - 12:32
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Yes this power levels are real (don't be amazed) it's pretty much the same across the board with our competitors as well. What you are seeing here is not an FCC regulated limitation but rather one of PoE. When we design products, such as the 1140 we design to a power of approx 12.5 Watts (yes 802.3af is 15.4 Watts) but the device is designed less as there is loss in Ethernet cable etc. As the data rates go lower the transmitter power goes up since the transmitter EVM limit is relaxed.

EVM is the linear or distortion factor, the higher the data rate the less distortion is tolerated. Similar to receiver sensitivity gets better as the data rates go down (since it can decode better through the distortion).

If you have a need for higher transmitter power, take a look at the AP-1250 product which can accept a higher PoE rating (beyond that of 802.3af) using our power injector.

bjohnson5@partn... Fri, 10/09/2009 - 04:52
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Thank you Fred,

I am enlightened (if not empowered). Great information. Its a necessary evil to accomplish PoE. With this limitation, do we still think the 1140 extends the coverage for legacy clients? What's the gain on the 1140 radios? Is the EIRP vs. a 1250 with standard dipoles a wash?

Thanks again,

zhenningx Mon, 10/05/2009 - 17:15
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Do Cisco APs have the build in mechanism to provide fairness access to the radio bandwidth? Say we have 10 clients connecting to the save AP, we want each of them has equal shared percentage of the radio bandwith, rather than one host dominates the radio bandwidth. Has Cisco done such testings? I haven't seen any Cisco documents talking about this. thanks.

Robert.N.Barrett_2 Mon, 10/05/2009 - 19:44
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"Fair" access to the channel is something that is a part of the Wi-Fi standard, and is controlled just as much (if not more) by the clients as it is by the AP.

fredn Thu, 10/08/2009 - 12:41
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Yes - Wi-Fi is a shared medium and as such clients have an equal chance at bandwidth, that said there are challenges when the client decides for example that it wants to use 2.4 GHz when it is capable of using say a channel on 5 GHz with less clients. This is where features like BandSelect can be used. For more information on deployments and BandSelect see the following URLs.


fredn Thu, 10/08/2009 - 12:40
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Yes - Wi-Fi is a shared medium and as such clients have an equal chance at bandwidth, that said there are challenges when the client decides for example that it wants to use 2.4 GHz when it is capable of using say a channel on 5 GHz with less clients. This is where features like BandSelect can be used. For more information on deployments and BandSelect see the following URLs.


peggyjackson Wed, 10/07/2009 - 15:04
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What is the best way to setup for high density WLAN area? Is it possible to use several 802.11n AP's in close range.


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