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New Member

CCI in Hi-density classroom environment

Looking for input on how others are tackling this common scenario of Co-channel Interference in High-density.

The Environment:

40 seats per classroom. Higher-ed, so extremely mixed client base.

AP-on-a-stick set to 24Mb lowest Mandatory data rate & lowest Tx power setting (2 dBm)

The Problem:

Needing to disable 2.4GHz radio's to achieve the required density does not seem optimal. Am I going about this wrong?

The plan:

I've used the following formula for ap density and number of radios:

  • target actual TCP/UDP throughput per client / throughput capabilities of the client / max realistic channel utilization * number of clients = # of Radio's

At 40 seats per classroom, assuming each student has a laptop and a smartphone or tablet, with a target throughput of 3Mbps for laptops & 1Mbps for smartphone/tablets:

Laptops:

  • 3Mbps / 150 / .8 * 40 = 1

Smartphone/Tablet:

  • 1Mbps / 30 / .8 * 40 = 1.6

Total radios needed per classroom = 2.6. Each AP having 2 radios, this brings me to 1 AP per classroom; if rounding down. All classrooms are in use, and fully populated every day. Current client base shows about 30% 5GHz capable, campus-wide. I expect this number to increase rapidly as most clients are becoming 5GHz capable.

Antenna in use: AIR-ANT2566P4W-R (Dual-band patch): http://www.cisco.com/en/US/docs/wireless/antenna/installation/guide/ant2566p4w.pdf

PDF picture is result of placing the AP/Antenna at the front of classroom (facing East, or away from the hallway).

There is very little attenuation between the rooms (sheetrock walls).

This is a 3-story building, with same layout on each floor.

Current plan:

One AP per classroom, with patch antenna mounted facing east, for classrooms which have an exterior wall (see diagram), except using Mechanical downtilt on first floor to reduce the signal in the rooms 'behind' the Anetanne. 2nd & 3rd floors have antenna mounted as drawn, w/ no downtilt. Using 20MHz 5GHz channels, and disabling some of the 2.4GHz radio's, to reduce CCI.

Interior classrooms will utilize same patch antenna, again using downtilt on first floor and not on upper floors. Disabling some of the 2.4GHz radio's. If downtilt is utilized on 2nd/3rd floors, the signal peneetrates the floors too much.

Options I've considered:

  • Purposefully creating attenuation 'behind' the antenna (mesh on wall, RF blocking paint etc.) to reduce the signal 'behind').
  • Using non-cisco antennae, with a better front-to-back ratio, which would reduce CCI as a result of the radiation of signal 'behind'. Not sure if an equivelant one exists. Essentially, I'd benefit from a similiar radiation patter, minus the back-lobes.
  • Using internnal antennae, and just disabling more 2.4GHz radio's (ea. patch antenna is ~500$)

The attached pdf represents the results of having an antenna mounted in room 222, pointing out/east/towards the exterior wall; and client RSSI readings from varios locations.

11 REPLIES
Hall of Fame Super Silver

Re: CCI in Hi-density classroom environment

I have done many installs in schools that are one for one. I also haven't had to disable any 2.4ghz not did I have to use patch antennas. Most one for one installs have been done with AP's placed in the middle of the classroom and or multiple APs in larger areas. Tweaking the data rated and tx power is all I had to do. Yes you get some overlap, but it doesn't kill the RF. You have to play around with having 24mbps as the lowest but maybe just supported and have 36mbps as mandatory and then tweak the TX power that allows some overlap to adjacent rooms. This gives your clients this ability to associate to adjacent AP's that are not loaded with users. I typically test with one AP being enabled to get the coverage I want and the add a few adjacent APs to see how the rf environment is. After tweaking, I will enable all the AP's. You should also use the max and min tx power threshold since you know what your max and min should be when testing earlier.

Sent from Cisco Technical Support iPhone App

-Scott
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New Member

Re: CCI in Hi-density classroom environment

Scott Fella wrote:

Yes you get some overlap, but it doesn't kill the RF.

Thanks for the input, exactly what I was looking for.

That is surprising. All omni's on multiple stories yielding acceptable CCI. I'll give this a shot on a survey, and analyze RF. Even at low power/high datarate, I anticipate AP's on the same channel being -72 to -75 from each other. Curious, what would you consider acceptable channel utilization, without clients?

I do plan on using RF profiles to set min/max Tx powers and higher mandatory rates (as you said, 24, or 36 with the two lower 'supported').

Hall of Fame Super Silver

Re: CCI in Hi-density classroom environment

In worse scenario, like stadiums, the channel utilization will hit 100%. In downtown areas, you might see 50% or higher. I haven't had complaints in schools where they have APs in every classroom. The key is to shrink the cells down, but not too much where clients can't connect. That is the issue with dropping power down too much. The AP is basically installed but not doing anything.

Sent from Cisco Technical Support iPhone App

-Scott
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New Member

CCI in Hi-density classroom environment

Scott Fella wrote:

In worse scenario, like stadiums, the channel utilization will hit 100%. In downtown areas, you might see 50% or higher.

So with 0 clients, even if Channel Util. is as high as 50% (or higher), due to mgmnt frames, that's more disirable than disabling some 2.4 radios?

The obvious caveat being that even w/ some radios disabled, I need to ensure clients in neighboring rooms can still connect, which they should be able to do, since -72 is what I see even at lowest power/24Mb mandatory.

Hall of Fame Super Silver

CCI in Hi-density classroom environment

Its your preference.  I have never disabled the 2.4 radios.  You can set the mandatory higher at 36 mbps or 48 mbps and allow 24 mbps as supported if you want, depending on the room size.  In other environments, you can't control channel utilization, that doesn't mean because your neighbors in a downtown area also has wifi, that you need to disable your 2.4ghz, you need to tweak the environment to work for you.  Testing is the best way.

Thanks,

Scott

Help out other by using the rating system and marking answered questions as "Answered"

-Scott
*** Please rate helpful posts ***
New Member

Re: CCI in Hi-density classroom environment

What is missing on your drawing is the reference scale (ruller) thus making a guess at this point of how large are the rooms.

Having the RF Utilization at 50% only due to management traffic is high. I am not sure if the answer above was ment to be with 0 clients or during the events. Scott: could you please clearifty your answer?

I like your option of placing patch antennas facing outdoors on the outside rooms. Rooms in the middle probably could use omni directional internal antennas instead of patch to minimize the inter-floor penetration. Every third room or so can have their 2.4GHz radios disabled to minimize the impact of CCI. You may try passive attenuators between the patch antennas and the AP to further reduce the side lobs and back side lobs of the antennas. As Scott has mentioned the end result may still have some overlaps that are not as small as recommended (Co-Channel APs -86dBm apart), but if you can minimize it - do that. At the end if you cannot achieve X amount of mbps/user in 2.4GHz while using minimum power settings at the AP and enforcing high data rates - that is just that :-(

You probably have seen this guide but just in case here is the link:

http://www.cisco.com/web/strategy/docs/education/cisco_wlan_design_guide.pdf

Hall of Fame Super Silver

Re: CCI in Hi-density classroom environment

I didn't mean 0 clients and high utilization. I see high utilization in areas with many users. I also see high utilization when you have neighboring wifi.

Sent from Cisco Technical Support iPhone App

-Scott
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New Member

Re: CCI in Hi-density classroom environment

Scott,

I thought so - just wanted to clear it to the topic author as I think he mis-read your answer.

Hall of Fame Super Silver

Re: CCI in Hi-density classroom environment

Haha... Maybe:) in a school environment, the only wireless typically you see is your own. I don't know how you can see channel utilization that high unless they have existing wireless and that is throwing off the tool.

Sent from Cisco Technical Support iPhone App

-Scott
*** Please rate helpful posts ***
New Member

Re: CCI in Hi-density classroom environment

Vlad,

Thanks for pointing that out, I was in fact interpreting the response of high utilization, even with 0 clients.

Scott, I agree 50%+ CU is quite normal under client load.

I've read the hi-density guides from Cisco/Aerohive/Ruckus...they are all good with relevant info. In the end, I'll do some more surveying & testing, but will likely go with the option of patches on the exterior rooms & omni's in the interior.

I saw the reference to attenuators in the cisco guide, if you have a good source for where to get them (or who has good quality ones) pls send along a link.

I didn't include drawing scale, as RSSI & seat-count seemed like enough data, but they are approx.  30'x30'.

Thanks for the input, sounds like I'm headed down the correct path...or at least am approaching the problem correctly.

New Member

Re: CCI in Hi-density classroom environment

TerraWave makes them and  also I think L-Com:

http://www.terra-wave.com/shop/attenuators-c-98.html

I didn't use them yet so please do further checks..

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