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Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Two channel plan vs Three channel plan?

I have a dense AP deployment, a total of 6 APs on two floors with 3 APs in each floor which is an L shaped office space with each AP no more than 30 feet apart. With this configuration I was still experiencing issues with some clients when roaming away (to another floor or to a spot in the same floor where they would get a better signal on another AP but were still associated to an AP with a weaker signal).

What I ended up doing to help this was disable lower transmit & basic rates on the SSID to get the AP to drop the client if the signal drops too much, however I am not sure that's the best way to do it. I also wanted to try out "Handoff Assist" but can't make sense of the threshold.

The RSSI to my APs (even ones that are close) is usually >-50, not sure if this is because of interference (there is a lot of traffic on channels 1,6,11) so I tried going to a two channel plan on ch 3 & 9. This has improved throughput but I am not sure if this is the best configuration either. I have read the 3 vs 4 channel plan article and it kind of sells me on the 3 channel plan but my throughput was much lower before I switched to a 2 channel plan - though I should add that I just recently disabled broadcast traffic and minimized arp traffic in the air as per some posts I saw here.

So my questions are:

1. Is a 2 ch plan better than 3 in my scenario (a lot of traffic most of it on 1,6,11) or should I stick to a 3 ch plan?
2. Should I be using station hand off or limiting the power of my APs? My currently solution to this is to use Mode Aware ARM and have it switch two of the APs to be AMs.

Thank you
Guru Elite
Posts: 21,271
Registered: ‎03-29-2007

Re: Two channel plan vs Three channel plan?

A two channel plan is not better than a three channel plan, because you lose the bandwidth of an entire non-overlapping channel when you do that.

If you have APs mounted in halls where they can see themselves very well (at 30 feet), and right over each other on the floors below and above, it is possible and likely that they will turn down their power to compensate, or clients will see the access points below just as good as the access points on the same floor. I would turn on Mode Aware for the 2.4ghz band, use 3 channels, turn on band steering and take it from there. Thirty feet can be close if you are just designing for coverage. Typically you want access points about 50 to 60 feet apart in a coverage model. A capacity model dictates that they be closer than that.


Colin Joseph
Aruba Customer Engineering

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Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Re: Two channel plan vs Three channel plan?

The problem is that there is so much congestion that sitting 10 feet away from the access point (when it's on channel 6) I can't get more than 9Mbps at times and a "Good" signal quality.

9Mbps is the speed I am currently getting to transfer a file, terrible speeds!

I've the min EIRP to 3 and the max to 127 however in RF Plan I still see the coverage to be overwhelming and a lot of co-interference. Should I just convert two of the APs (the ones in the middle of the other two) to AMs? Is there anything else I can do to help?
Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Re: Two channel plan vs Three channel plan?

By the way, what I did is to make two of the APs air monitors and I see the signal quality is much better now, but there are still some APs that are causing interference with each other I think. However, again, I think the coverage is better.

What is a good value for the station handoff assist threshold? That's the only thing I think still needs some work to help us when moving away from APs... this being a dense environment it is hard to set a good threshold because I can walk to the other end of the office on one floor and still be connected to the AP on the other floor.... I also don't really understand the low threshold for the station handoff assist, the language is not very clear: low threshold ... deauth should never be sent above this value.

Does that mean that if the rssi is -40dBM and I set the threshold to 50 that the station will be sent a deauth if it goes to -60dBM?
Occasional Contributor II
Posts: 44
Registered: ‎04-02-2007

Re: Two channel plan vs Three channel plan?

Miguel,

I'm trying to understand your situation here. You say that all of your AP's ("including the close ones") have RSSI's greater than -50. Do you mean greater than -50dBm or less than -50dBm?

It sounds like you've got two issues at hand - interference and roaming.

Re: interference - 9 Mbps when you're 10 feet from the AP? I'm wondering if something else is being missed? Do you see this with multiple clients? Have you confirmed the client is on the AP that is nearby, and not another, subpar AP across the building?

In the failed state, what is the AP's txpower? What does "show ap arm rf-summary" show for the AP? Have you performed a spectrum analysis? Sorry to ask so many questions, but we have a building where we coexist with 50-60 other AP's, and never see this much performance impact. I'm wondering if you have some other interferer causing trouble.

In the long term, I think your 2-channel plan is going to be a problem. Reducing the number of available non-overlapping channels means your APs have to reuse channels more often, which dramatically increases interference, and will only get worse if you need to increase capacity. Additionally, if you've got ambient interference on 1,6,11, it's still going to impact your AP's on 3 and 9 in the form of ACI.

Re: roaming - Station handoff assist is a hack that often breaks a lot of things. It works by sending deauths, which a lot of clients handle very poorly (think dropped sessions, DHCP renewals, etc). My advice: Update your wireless card drivers. If you can, turn up roaming aggressiveness for your card (I know you can do this for Intel chipsets). Turn the Max TX EIRP down (try 18 or even 15 dBm), so your cell size is smaller. Do everything Colin recommends.
Guru Elite
Posts: 21,271
Registered: ‎03-29-2007

Re: Two channel plan vs Three channel plan?


By the way, what I did is to make two of the APs air monitors and I see the signal quality is much better now, but there are still some APs that are causing interference with each other I think. However, again, I think the coverage is better.

What is a good value for the station handoff assist threshold? That's the only thing I think still needs some work to help us when moving away from APs... this being a dense environment it is hard to set a good threshold because I can walk to the other end of the office on one floor and still be connected to the AP on the other floor.... I also don't really understand the low threshold for the station handoff assist, the language is not very clear: low threshold ... deauth should never be sent above this value.

Does that mean that if the rssi is -40dBM and I set the threshold to 50 that the station will be sent a deauth if it goes to -60dBM?




Miguel,

You need to take these issues step by step.

Are you dropping broadcasts in your Virtual AP profile for that wireless LAN? If you are not, you should consider it. It helps greatly in reducing needless broadcast traffic.


Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base

Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Re: Two channel plan vs Three channel plan?


Miguel,

You need to take these issues step by step.

Are you dropping broadcasts in your Virtual AP profile for that wireless LAN? If you are not, you should consider it. It helps greatly in reducing needless broadcast traffic.




I am dropping broadcasts.
Guru Elite
Posts: 21,271
Registered: ‎03-29-2007

Re: Two channel plan vs Three channel plan?

Is the problem manifesting itself on 2.4 ghz (b/g) or 5ghz (a) clients or both? Did you try to enable Mode Aware in the ARM profile to see if any access points turn into APMs?


Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base

Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Re: Two channel plan vs Three channel plan?

Adam/Joseph,

Sorry, let me take a step back and try to explain everything with as much detail as possible.

I currently have 6 AP61s in my office space (which consists of two L shaped floors about 96' x 74' at each end of the L); originally all of the AP61s were configured to be APs with ARM (at first mode aware ARM wasnt enabled, but now it is). I was originally on a three channel plan, but in troubleshooting I switched to a 2 channel plan, then after cjoseph advised against it and after I read a little more, I switched back to a 3 channel plan. Now, I have enabled mode aware ARM, I have changed the Min EIRP to 3 and left Max at 127 (which I think goes up to 23 only) and made two of the 6 APs air monitors. Please see attachments for the location of the APs and the current coverage and the proximity of each AP, these APs are all in a drop ceiling, there is about 15' of space between floors.

Prior to making the Min EIRP 3, it was at 9 & Max was at 127; at this point on the three channel plan and having all six AP61s configured as APs I think all the co-channel interference was causing the issue where I was unable to get past 9Mbps on a connection no matter how close I was to the AP, I saw the 9Mbps issue with multiple clients and the signal quality was bad overall. Once I changed two of the AP61s to air monitors (I also made the Min EIRP 3 for all of the APs) I noticed the signal quality getting better, I can only attribute that to the fact that there was less co-interference. However, after making these changes I dont think this is an issue anymore as I see great signal quality everywhere now with less active APs in the environment.

Now I am facing the final of the issues, which is roaming, or getting my clients to roam to another AP. Most of my clients are wireless devices (iPads, iPhones, Android phones) and a few of them are laptops, but most of the laptops are Macbook Airs, with one Lenovo that I use for testing which has Intel drivers.... If station handoff assist is a hack I'd rather not do it, however the problem is that due to the proximity of everything in the office (this is me assuming btw), the client sees the signal strong enough to not change to an AP with a stronger signal. I will give you an example that I saw last night actually, I started under hhap3 (refer to the attachments) and I walked up to the 15/F and sat in the room next to hhap2. The signal strength of hhap2 was -44dBM and hhap3 became -80dBM and yet I was still on hhap3 in that room! I was downloading a file and while I was close enough to hhap3 I was getting speeds upwards of 1MB/s (which is good because before I'd be stuck in the 200KB/s range) but when I moved next to hhap2 I saw the speed drop to like 300KB/s but I know that had I switched to the AP that was closer the speed would have increased again (I was downloading from the LAN).

By the way, in the attachments, the APs on 14 have the following stats:
hhap3 - Access Point / 6 / 18
hhap5 - Access Point / 11 / 12
hhap6 - Air Monitor

and on 15:
hhap1 - Air Monitor
hhap2 - Access Point / 11 / 23
hhap4 - Access Point / 1 / 18

Thoughts?
Occasional Contributor II
Posts: 13
Registered: ‎01-11-2011

Re: Two channel plan vs Three channel plan?




2.4ghz (b/g) but our clients are all g. I did enable Mode Aware ARM and I did see two of the APs turn in the APMs at one point but that stopped after I switched to a three channel plan so I left it on but I manually set two of the APs to AMs.

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