Optimizing throughput on Aironet 1252 standalone for 802.11n only

[Ivanleonel]

Hi all!

I’m trying to set 2,4ghz radio to use 802.11n only, but AP tells me that I must choose one of data rates 6, 9, 12, 18, 24, 36, 48, or 54 Mbit/s. I confused. Aren’t those data rates for 11g?

I want to enable only MCS Rates 1-15 which is 11n.

Now my radio in mixed mode. How to change it to native 802.11n mode to get benefits of N?

[Rasika Nayanajith · Accepted solution]

You cannot do that. 802.11n mandatory to be backward compatible & you have to enable at least one 802.11a/g data rate.

HTH

Rasika

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[Ivanleonel]

rasikanayanajith, thanks for clarification!

I will think how to do something with AP to improve performance. 

[Rasika Nayanajith]

What's the exact issue ?  How much throughput you are getting ?

Typically I would disable all data rates upto 12Mbps & make 12Mbps mandatory. All other high data rates supported.

If your 802.11n client get good RSSI & SNR, then it will be dynamically set to a MCS rate. Refer below for MCS rates

MCS Index - 802.11n and 802.11ac

For 802.11n, you have to configure following two things & make sure it is the case.

1. Open Auth or WPA2/AES

2. WMM to be enable.

HTH

Rasika

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[Ivanleonel]

Thanks for guidelines. Yes I have enabled 11n properly.

My AIR-AP1252AG-E-K9 has 6 antennas.

In total I have about 55 associations.

36 clients use 2,4ghz and 17 clients use 5ghz.

15 clients are laptops. Another 40 are phones and they don’t use Wi-Fi actively.

Now I disabled rates up to 12Mbps & make 12Mbps mandatory as you mentioned above. Definitely it feels better.

Actual average speed is 2-3 mbytes.

You have told that “802.11n mandatory to be backward compatible” Is it requirement only for cisco APs and there is another vendors which are able to enable strictly 802.11N without backward compatibility?

[Rasika Nayanajith · Accepted solution]

Actual average speed is 2-3 mbytes.

So you are getting 16-24Mbps in average. is that correct ? If you have that many clients on a single AP, I think this average value is reasonable.

You have told that “802.11n mandatory to be backward compatible” Is it requirement only for cisco APs and there is another vendors which are able to enable strictly 802.11N without backward compatibility?

This is not Cisco restriction, It is defined by IEEE standard & all vendor 802.11n implementation follows that.

To get better throughput in 5GHz, you can enable channel bonding as well. In AP CLI, it is like this.

interface Dot11Radio1
 channel width 40-above

HTH

Rasika

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[Ivanleonel]

Thanks for spend you time!

[George Stefanick]

Is like to add a few comment for clarification. 

1 the standard for 802.11n states that legacy, mix and green field preambles are supported. However cisco, like many other vendors support legacy amd mix. This is why you can have just 802.11n MCS rates. I'm other words, with mix you will support n ame atleast one legacy PHY. Ras this is indeed a cisco thing not a standard. If it was a standard then explain the greenfield preamble :) 

2 you have 6 antennas but 3 are for 2.4 and 3 are for 5 ghz. Your biggest bottles neck will always be medium contention when you have a properly designed network. The more clients on channel the slower she will be. Everyone has to share the medium.

[Rasika Nayanajith]

Hi George,

Ras this is indeed a cisco thing not a standard. If it was a standard then explain the greenfield preamble :) 

This is why I like you spending more time on this CSC, every time I learn something.from your response.

Yes, Greenfield HT, you do not want to have backward compatibility, but question is does any vendor implement that ? 

Rasika

[George Stefanick]

Enterprise gear I looked at does legacy and mix preamble .. No support for greenfield ht only .. 

It's all about the details :) 

[George Stefanick]

Vendors implementation of 802.11n and 802.11ac pick and choose what they want to implement .. Think of a list to ouck and choose from and yiu as Cisco can pick and choose different components ..

[George Stefanick]

  Let me add ... Don't confuse using higher phy rates... RTS/CTS and nav timers are used to quite down the legacy clients once quite the ht clients will punch frames but they still use the mix preamble .. So legacy client can back off .. 

Does does that make sense ?

[Rasika Nayanajith]

Yes all that make sense :)

I would like to clarify  this from you.

1. This is from 9.23.2 from IEEE 2012 standard, I understood this (may be wrong) as if no protection mode mean they can use green field PPDU, if protection enabled then has to use mixed-mode PPDU.

The HT Protection field may be set to no protection mode only if the following are true:
— All STAs detected (by any means) in the primary or the secondary channel are HT STAs, and
— All STAs that are known by the transmitting STA to be a member of this BSS are either
— 20/40 MHz HT STAs in a 20/40 MHz BSS, or
— 20 MHz HT STAs in a 20 MHz BSS.

2. What are the differences in HT protection mode enabled PPDU & protection mode disabled PPDU ?

Rasika

[George Stefanick · Accepted solution]

1. States ALL stations must be HT in both comments. Meaning they are configured for HT GREENFIELD and no protection is required. Remember STA's also mean access points (radios).

Remember Cisco access points aren't allowing HT only. They are doing mix or legacy. In most cases users configure as mixed. So not ALL STAs are HT so No greenfield HT only and protection must be set because the access point is set that way .. Make sense ? 

Now if the ap could set to greenfield only then yes HT would be set .. Why because all STA would be greenfield 

2. PPDU is where the PHY and preambles live. In other words if protection is set then 1 preamble with a Legacy section (L-STF, L-LTF, L-SIG) and Ht section will accompany a frame transmission so both legacy devices and HT devices can sync to the pending phy header and next the pending frame .. No protection then only a preamble with HT will be sent .. 

Make sense ? 

[George Stefanick]

Remember STA also means access points ..