High Channel utilization but no RX or TX utilisation

Endorsed Question
Feb 19th, 2009
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Hello WLAN experts,

looking at our Top-Ten Access Points statistics, I often see APs with the following Load statistics:

RX Utilisation : 0

TX Utilisation : 0

Channel Utilisation : 18


Do You have any explanation for the high Channel utilisation ?

What is causing this ?

Detailed traffic statistics contain the following figures:

TX Frame count: 380606

RX Frame Count: 77222980

FCS Error Count: 3281837

TX Failed count: 157979

Retry Count: 145735


Noise vs. Channel: -98

Interference: -90



Thank You in advance


Cisco Endorsed by Shankar Ramanathan
George Stefanick about 8 years 5 months ago

High utilization can be caused by the following:


High density of access points on channel or co- channel.

Example- excessive access points on the same channel that contribute to interference, beacons, probe responses and probe request. All of which takes up medium.


Excessive use of SSIDs

if you broadcast for example 10 SSIDS you increase your management over head as above - broadcast, probe responses, and request. I did a study and found that by adding a single SSID you could increase over head by 3-7% (w/ 1Mb frames)



Low data rates enabled (1/2 mb)

By standard your managment frames (broadcast) are transmitted on the lowest basic PHY rate. If you enable 1/2Mb frames you are allowing these to go out in a very slow manner. Disabling 1/2 rates you can improve utilization significantly. In most you can see utilzation at 40-50% with no traffic be lowered to 5%.



High TX power is always a big problem. If your APs are at 100mW. They are far extending and could be adding to other same channel APs utilization. So those beacons are going further and clients are responding to them .. smaller cells are better ...


And of course data transmissions...



I would use a protocol analyzer like airmagnet. The controllers dont always do a good job in showing utilization.


I hope you find this helpful ...



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dennischolmes Thu, 02/19/2009 - 05:33
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Channel utilization can occur from broadcast beacons as well as data transmitted and received. I be you have 1 and 2mb datarates enabled on your controllers.

Victor900502 Wed, 05/25/2016 - 16:43
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Hi :

Do you know how can I disable 1/2 rate via PI or WLC ?

Thanks a lot.

Tulga Bat Mon, 05/30/2016 - 00:46
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Hi,  I see 50 clients connected to 1 AP, sounds ridiculous i know, but when i check Channel Utilization = 40%, AP uplink is around 10Mb/5Mb and our computers couldn't download 10MB file from our file server, is that related with this? how to measure and check it? we have PI and WLC but still couldn't find related stuff 

Scott Fella Mon, 05/30/2016 - 01:27
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Check the channel utilization while downloading the file, but look at it from the controller CLI (show ap auto-rf 802.11a <ap name>)  you can also look at the 802.11b. This command will show you the utilization real time and you will see how downloading a file can also increase the utilization.  This is why you need to determine how many clients can connect and still have good user experience.  The long time recommend number is 25, but again, it will depend on the clients and their activity on the wireless.  I have found the 30 is a safe number before the experience starts to fall fast.

-Scott 

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Tulga Bat Thu, 06/02/2016 - 00:30
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These are what i am getting


Number Of Slots.................................. 2
AP Name.......................................... 
MAC Address...................................... cc:ef:48:20:72:0d
Slot ID........................................ 1
Radio Type..................................... RADIO_TYPE_80211a
Sub-band Type.................................. All
Noise Information
Noise Profile................................ PASSED
Channel 36................................... -97 dBm
Channel 40................................... -96 dBm
Channel 44................................... -92 dBm
Channel 48................................... -93 dBm
Channel 52................................... -98 dBm
Channel 56................................... -97 dBm
Channel 60................................... -96 dBm
Channel 64................................... -95 dBm
Channel 149.................................. -94 dBm
Channel 153.................................. -94 dBm
Channel 157.................................. -92 dBm
Channel 161.................................. -93 dBm
Channel 165.................................. -94 dBm

--More-- or (q)uit
Interference Information
Interference Profile......................... PASSED
Channel 36................................... -87 dBm @ 1 % busy
Channel 40................................... -128 dBm @ 0 % busy
Channel 44................................... -128 dBm @ 0 % busy
Channel 48................................... -128 dBm @ 0 % busy
Channel 52................................... -128 dBm @ 0 % busy
Channel 56................................... -128 dBm @ 0 % busy
Channel 60................................... -128 dBm @ 0 % busy
Channel 64................................... -128 dBm @ 0 % busy
Channel 149.................................. -128 dBm @ 0 % busy
Channel 153.................................. -128 dBm @ 0 % busy
Channel 157.................................. -128 dBm @ 0 % busy
Channel 161.................................. -128 dBm @ 0 % busy
Channel 165.................................. -128 dBm @ 0 % busy
Rogue Histogram (20/40/80)
.............................................
Channel 36................................... 3/ 0/ 0
Channel 40................................... 94/ 0/ 0
Channel 44................................... 0/ 0/ 0
Channel 48................................... 0/ 0/ 0
Channel 52................................... 0/ 0/ 0
Channel 56................................... 0/ 0/ 0

--More-- or (q)uit
Channel 60................................... 0/ 0/ 0
Channel 64................................... 0/ 0/ 0
Channel 149.................................. 0/ 0/ 0
Channel 153.................................. 0/ 0/ 0
Channel 157.................................. 0/ 0/ 0
Channel 161.................................. 0/ 0/ 0
Channel 165.................................. 0/ 0/ 0
Load Information
Load Profile................................. FAILED
Receive Utilization.......................... 0 %
Transmit Utilization......................... 0 %
Channel Utilization.......................... 3 %
Attached Clients............................. 24 clients
Coverage Information
Coverage Profile............................. PASSED
Failed Clients............................... 0 clients
Client Signal Strengths
RSSI -100 dbm................................ 0 clients
RSSI -92 dbm................................ 0 clients
RSSI -84 dbm................................ 0 clients
RSSI -76 dbm................................ 0 clients
RSSI -68 dbm................................ 7 clients
RSSI -60 dbm................................ 9 clients

--More-- or (q)uit
RSSI -52 dbm................................ 6 clients
Client Signal To Noise Ratios
SNR 0 dB.................................. 0 clients
SNR 5 dB.................................. 0 clients
SNR 10 dB.................................. 0 clients
SNR 15 dB.................................. 0 clients
SNR 20 dB.................................. 0 clients
SNR 25 dB.................................. 1 clients
SNR 30 dB.................................. 5 clients
SNR 35 dB.................................. 4 clients
SNR 40 dB.................................. 6 clients
SNR 45 dB.................................. 6 clients
Nearby APs
AP 64:a0:e7:5d:7a:20 slot 1.................. -51 dBm on 161 (10.98.4.43)
AP f0:29:29:ea:fb:50 slot 1.................. -73 dBm on 44 (10.98.4.43)
Radar Information
Channel Assignment Information
Current Channel Average Energy............... -80 dBm
Previous Channel Average Energy.............. -57 dBm
Channel Change Count......................... 22
Last Channel Change Time..................... Sat May 21 06:04:58 2016
Recommended Best Channel..................... 40
RF Parameter Recommendations

--More-- or (q)uit
Power Level.................................. 1
RTS/CTS Threshold............................ 2347
Fragmentation Threshold...................... 2346
Antenna Pattern.............................. 0

Persistent Interference Devices
Class Type Channel DC (%%) RSSI (dBm) Last Update Time
------------------------- ------- ------ ---------- ------------------------
All third party trademarks are the property of their respective owners.

() >show ap auto-rf 802.11b 

Number Of Slots.................................. 2
AP Name.......................................... 
MAC Address...................................... cc:ef:48:20:72:0d
Slot ID........................................ 0
Radio Type..................................... RADIO_TYPE_80211b/g
Sub-band Type.................................. All
Noise Information
Noise Profile................................ PASSED
Channel 1.................................... -80 dBm
Channel 2.................................... -84 dBm
Channel 3.................................... -71 dBm
Channel 4.................................... -77 dBm
Channel 5.................................... -63 dBm
Channel 6.................................... -87 dBm
Channel 7.................................... -70 dBm
Channel 8.................................... -63 dBm
Channel 9.................................... -83 dBm
Channel 10................................... -90 dBm
Channel 11................................... -88 dBm
Interference Information
Interference Profile......................... PASSED

--More-- or (q)uit
Channel 1.................................... -68 dBm @ 8 % busy
Channel 2.................................... -68 dBm @ 1 % busy
Channel 3.................................... -66 dBm @ 1 % busy
Channel 4.................................... -128 dBm @ 0 % busy
Channel 5.................................... -63 dBm @ 34 % busy
Channel 6.................................... -67 dBm @ 19 % busy
Channel 7.................................... -72 dBm @ 6 % busy
Channel 8.................................... -70 dBm @ 2 % busy
Channel 9.................................... -75 dBm @ 6 % busy
Channel 10................................... -74 dBm @ 15 % busy
Channel 11................................... -75 dBm @ 9 % busy
Load Information
Load Profile................................. FAILED
Receive Utilization.......................... 0 %
Transmit Utilization......................... 4 %
Channel Utilization.......................... 78 %
Attached Clients............................. 24 clients
Coverage Information
Coverage Profile............................. PASSED
Failed Clients............................... 2 clients
Client Signal Strengths
RSSI -100 dbm................................ 0 clients
RSSI -92 dbm................................ 0 clients

--More-- or (q)uit
RSSI -84 dbm................................ 2 clients
RSSI -76 dbm................................ 1 clients
RSSI -68 dbm................................ 1 clients
RSSI -60 dbm................................ 10 clients
RSSI -52 dbm................................ 10 clients
Client Signal To Noise Ratios
SNR 0 dB.................................. 0 clients
SNR 5 dB.................................. 2 clients
SNR 10 dB.................................. 0 clients
SNR 15 dB.................................. 3 clients
SNR 20 dB.................................. 3 clients
SNR 25 dB.................................. 3 clients
SNR 30 dB.................................. 1 clients
SNR 35 dB.................................. 3 clients
SNR 40 dB.................................. 3 clients
SNR 45 dB.................................. 6 clients
Nearby APs
AP 00:1a:30:33:7c:20 slot 0.................. -84 dBm on 11 ()
AP 04:c5:a4:f3:da:00 slot 0.................. -74 dBm on 1 ()
AP 64:a0:e7:5d:7a:20 slot 0.................. -39 dBm on 6 ()
AP 68:bc:0c:f6:6a:20 slot 0.................. -85 dBm on 6 ()
AP 68:bc:0c:f8:5c:70 slot 0.................. -85 dBm on 1 ()
AP 88:f0:77:fc:b8:c0 slot 0.................. -72 dBm on 11 ()

--More-- or (q)uit
AP f0:29:29:ea:fb:50 slot 0.................. -62 dBm on 1 ()
Radar Information
Channel Assignment Information
Current Channel Average Energy............... unknown
Previous Channel Average Energy.............. unknown
Channel Change Count......................... 0
Last Channel Change Time..................... Sat Nov 7 19:42:24 2015
Recommended Best Channel..................... 1
RF Parameter Recommendations
Power Level.................................. 2
RTS/CTS Threshold............................ 2347
Fragmentation Threshold...................... 2346
Antenna Pattern.............................. 0

Persistent Interference Devices
Class Type Channel DC (%%) RSSI (dBm) Last Update Time
------------------------- ------- ------ ---------- ------------------------
All third party trademarks are the property of their respective owners.

George Stefanick Sat, 02/21/2009 - 08:59
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High utilization can be caused by the following:


High density of access points on channel or co- channel.

Example- excessive access points on the same channel that contribute to interference, beacons, probe responses and probe request. All of which takes up medium.


Excessive use of SSIDs

if you broadcast for example 10 SSIDS you increase your management over head as above - broadcast, probe responses, and request. I did a study and found that by adding a single SSID you could increase over head by 3-7% (w/ 1Mb frames)



Low data rates enabled (1/2 mb)

By standard your managment frames (broadcast) are transmitted on the lowest basic PHY rate. If you enable 1/2Mb frames you are allowing these to go out in a very slow manner. Disabling 1/2 rates you can improve utilization significantly. In most you can see utilzation at 40-50% with no traffic be lowered to 5%.



High TX power is always a big problem. If your APs are at 100mW. They are far extending and could be adding to other same channel APs utilization. So those beacons are going further and clients are responding to them .. smaller cells are better ...


And of course data transmissions...



I would use a protocol analyzer like airmagnet. The controllers dont always do a good job in showing utilization.


I hope you find this helpful ...



Victor900502 Wed, 05/25/2016 - 16:42
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Hi :

Do you know how can I disable 1/2 rate via PI or WLC ?

Thanks a lot.

George Stefanick Wed, 05/25/2016 - 17:20
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Hello 


under the wirlees tab on the WLC. Left hand side you will see your 2 networks. 802.11bgn and 802.11anac 


click on 802.21gbn and hit network. You will see the phy rates where you can disable. 

Victor900502 Thu, 05/26/2016 - 11:43
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Thanks a lot.

For improving the channel utilization, is your suggestion turning of 1Mbps and 2Mbps of 11b or the 1/2 FEC coding like following : ?


In 11ac MCS 0, MCS 1 and MCS 3 are 1/2 rate codes

 In 11n this is

MCS 0, 8, 16

MCS 1, 9, 17

MCS 3, 11, 19

George Stefanick Thu, 05/26/2016 - 11:47
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It's my advice if your deisgn can support higher rates you should disabled everything below 12. Don't mess with other rates. Again, if your deisgn supports it. 

Victor900502 Thu, 05/26/2016 - 11:51
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Your 12  means "12Mbps", am I right ?

You are not talking about the FEC rate (1/2, 3/4 etc...), right?

George Stefanick Thu, 05/26/2016 - 11:54
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Yup ! 12 PHY. Don't mess with anything higher ..


I've had a lot of success with 12 supported and 24 mandatory. Rest supported .. Keep in mind you need to insure your RF will support it .. If it doesn't you will create issues for yourself .. 

George Stefanick Thu, 05/26/2016 - 12:06
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If you found this helpful please consider rating helpful post ! Thanks ! 

George Stefanick Thu, 05/26/2016 - 12:13
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Wow thanks ! 

Victor900502 Wed, 06/01/2016 - 09:44
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Hi,George:


Another thing is very strange,  from the report I found the Channel Utilization are much higher than the sum of Rx and Tx. In both band for all of the AP. 

From my understanding, Rx+Tx reflecting the traffic on the AP. So , all of the others are interference ? You can refer to the attached picture.

Thanks.

Attachment: 
Scott Fella Wed, 06/01/2016 - 10:02
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That is typical as channel utilization has various number of things in the calculation.  Traffic, co-channel, broadcast, beacons all affect the channel utilization.

-Scott 

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Victor900502 Wed, 06/01/2016 - 10:06
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So, the beacons and probe(from own AP) are not parts of Rx /Tx ?

In my reports, Rx and Tx always very low (less than 5%) but channel utilization sometimes are higher like 70%.

dennischolmes Wed, 06/01/2016 - 10:18
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Hey Victor,


Remember that high channel utilization can come from a lot of different places. Client devices, APs, and even a high level of the noise floor (causing excessive retries) can be the source of the issue. If you have a copy of Airmagnet WiFi Analyzer Pro you can dig deep into what is happening. Depending solely on the WLC tools is difficult at best to determine the issue. Airmagnet looks at the basic stuff and then allows you to dig down all the way to the packet level if necessary.

George Stefanick Wed, 06/01/2016 - 11:31
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Hey look who is back! The old thread kicked started Dennis! LOL Hi Dennis...


Vic, Think of it this way. I just validated this with my Cisco guy. Utilization it the sum of all WiFi and non WiFI traffic, say 70%. The TX/RX is specific to the ap radio you are looking at. That is why you see the difference. 


Does that help ?

Victor900502 Wed, 06/01/2016 - 11:59
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thanks George.

I understood that, I have checked non-wifi interference too via clean air. The non-wifi percentage is very low,around 2%.

So , most of contributor should be the co-channel interference . But what I cannot understand is the similar things happening on 5GHz band. I believe we should have few co-channel interference in 5GHz.

Again, are the beacon and probe considered as Rx or Tx ?

George Stefanick Wed, 06/01/2016 - 12:04
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Are you bonding on 5 Ghz and what channels are you using UNII1,2,2E and 3 ?

Victor900502 Wed, 06/01/2016 - 13:34
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No bonding.


We are using all the channels, UNII 1, 2, 2E and 3.   DCA enabled

Scott Fella Wed, 06/01/2016 - 10:21
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That is also what we see all around the world. The channel utilization is what we look at to determine end user experience and the ability to make changes to lower overall channel utilization throughout the day.  Every site is different and we treat them differently because of what the users are doing on the wireless.

The best way is to be onsite and test for yourself.  Everyone's experience is different and the number of clients on a given AP or radio and utilization will vary.  This is how you can determine what is the threshold for your environment.

-Scott

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Victor900502 Wed, 05/25/2016 - 16:47
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Hi :

Do you know how can I disable 1/2 rate via PI or WLC ?

Thanks a lot.

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