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conchalnet
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High latency

Mon Dec 07, 2009 10:01 pm

Hi all,

I've a lot of problems with high latency in some APs. I'll show a example of on AP with problems:

This AP is running in a RB433Ah with 2 R52H cards with sectorial panels antennas. I've problem in just one card, the second card is OK and don't show the problem. (I replaced the cards yet, but the problem persists).

On the AP that I'm with high latency I've the follow informations:

- 40 clients associated (on the card that is running OK I've 41 clients and the latency is 8ms in average)
- All clients have good signals (between -50 and -65).
- The noise floor is -102dBm
- The Overrall TX CCQ is about 90%-98%
- The ACK Timeout is 30ms

On this card I've a latency of 300ms in average and this causes problems for me because the Internet of the customers is slow when the latency is high.

What is this problem? How can I detect the cause of the problem? And what is the best solution to prevent this kind of problems?

Thanks in advance

Fabrício
 
ejansson
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Re: High latency

Mon Dec 07, 2009 10:52 pm

Are you using Nstreme? What version of software on the AP and clients.
 
conchalnet
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Re: High latency

Tue Dec 08, 2009 2:29 pm

Hi ejason,

Don't. I'm not using nstream.

My customers don't use Mikrotik Router OS, they has Edimax 7209Abg or Ubiquiti Bullet 2 as CPEs.

On the AP I've a Routerboard runing RouterOS v4.3 (I updated it to try to solve the latency problem. The APs was runing 3.9 version before).

The AP is configurated as AP Bridge and the customers as station. Sometimes the problems with latency limits the throughput of my AP cell. The traffic on the AP is very small (something like 300kbps) and I've a very high latency on the customers radios (more than 400ms).

Thanks again!
 
ejansson
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Re: High latency

Thu Dec 10, 2009 3:44 am

Since you are not using Nstream and have 40 clients or so you issue is likely "hidden node". When using regular 802.11a/b/g you will run in to latency and through put problems at around 15 customers give or take depending on your exact configuration. Regular a/b/g is a contention based and is an indoor protocol. For it to function well each client needs to "see" most or all of the other clients. When used out doors normally no client can seen another and a result always think it is safe to send traffic. You end up with collisions and retries which lead to high latency and low through put. You can enable RTS / CTS on the clients (if they support it) to help reduce this problem.
 
popcorrin
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Re: High latency

Thu Dec 10, 2009 4:27 am

40 clients is quite a few.
I know you said you had a sector with 40 clients and it was working fine but is it possible that most of the users on that particular sector are light users. What about the layout of your clients. How far from the ap are they.

Are you shaping bandwidth.
 
conchalnet
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Re: High latency

Thu Dec 10, 2009 3:25 pm

Hi ejansson and popcorrin!

Thanks for your reply!

I make the bandwidth control using a simple queue for each customer (all customers have fixed IP address) and my customers don't "see" each other and they are distribuited in a 90 degrees sector in different distances of my AP (I've some customers a few meters of the AP and another ones in some kilometers of the AP).

ejansson, about the RTS/CTs, in some clients CPE I've using the Ubiquiti Bullet 2 equipment and I've the option to enable the RTS Threshold (it's disabled on all customers today). When I enabled this option I've to set a value wich is defined as 2346 by default. What's the ideal value in your opinion? I haven't this equipment on all customers, can I enable this option on the customers that I've the bullet 2? And on the RouterOS, what have I to enable to do use of this resource?

Thanks again!

Fabrício
 
ejansson
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Re: High latency

Tue Dec 15, 2009 5:27 am

As we have been using Nstreme I don't have any direct info on the thresh hold settings. My guess is you should be fairly low 512 to 128 range as smaller packets are less likely to be involved in a collision as they take less time to transmit. I would dig through the forum as a do recall coming across some suggested settings by I have not been able to find the post again...... if you find it let me know! It is best if all clients support RTS/CTS but you should have no problems mixing and matching you just won't get optimized performance, but every bit should help.
 
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jwcn
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Re: High latency

Tue Dec 15, 2009 7:03 am

Set your RTS to 256 and fragmentation threshholds to 768

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