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ejansson
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Small MTU better with interference?

Thu Dec 07, 2006 4:59 am

Just wondering if any one has tried this and if so what the results were. If you turn down the MTU to 64 or even 32 bytes does it improve (diminish) retries in a noisy enviroment? My thinking would be yes as many of you know a link can work fine with a regular ping, but a sure sign of rf problems is if you us a large ping size like 1400. Also canopy uses a very small packet size and is a major reason it does well in a noisy enviroment. Basicly a small packet is less likely to get stepped on then a large one....
 
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sten
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Thu Dec 07, 2006 1:23 pm

In a sense that is true. But i doubt you could or should turn it lower than 576 bytes. Normally one would let the radio fragment large packets, or apply Forward Error Correction in noisy environments but no such feature is available in routeros. Perhaps using nstreme together with a very small frame size limit and fixed size policy?
Move along. Nothing to see here.
 
ste
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Re: Small MTU better with interference?

Thu Dec 07, 2006 2:27 pm

Just wondering if any one has tried this and if so what the results were. If you turn down the MTU to 64 or even 32 bytes does it improve (diminish) retries in a noisy enviroment? My thinking would be yes as many of you know a link can work fine with a regular ping, but a sure sign of rf problems is if you us a large ping size like 1400. Also canopy uses a very small packet size and is a major reason it does well in a noisy enviroment. Basicly a small packet is less likely to get stepped on then a large one....
This will help for pings, but realworld traffic gets fragmented and loosing
one fragment is fatal for the whole packet. Lowering MTU is no real
goog Idea because you might see problems with path MTU discovery
which are ugly to fix. You have to fight firewalls which block the
ICMP messages which says that the sender has to lower packet size.

Stefan
 
UniKyrn
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Thu Dec 07, 2006 5:25 pm

Dropping the MTU will help to a certain extent, the packet has less air time so the chances of it being interfered with are smaller. While it might seem odd, you can also dial down the speed of the link and while the packets spend more time "in the air", the slower speeds are more resistant to interference. The radios drop the speed themselves because of interference and signal strength, you can force it slower manually. Note, not all CPE's obey the AP's request to drop the link speed.

However, either of these changes is going to radically reduce your throughput to your customers. If the interference is at the customer end, you're limiting all your customers because of one or two problem links. If the interference is at the AP side, you can probably improve the situation slightly with changes like these, but you'd be better off in the long run if you dealt with the interference problem directly.
 
maxfava
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Mon Dec 11, 2006 12:46 pm

Hi
As I have interference problem (just I can I image, not sure) could you tell if you have set MTU on the AP or on client or on both?

thanks
Max
 
JR
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Re: Small MTU better with interference?

Fri Jul 04, 2008 10:16 pm

Hi Geroge,

I have set the packet size under the MTU, on General tab. As it is the AP that is most effected, doing it on the client side (ap rx) seems to help the most, but I have played with both directions at various times. I have also had some success with running nstreme with no framing and 1500 packet size all the way down to 128k, Kills the trough put and is not a perfect solution but it can provide some level of service, rather than loosing the connection totally. Depending on the area and interference, going to a wide channel can help some too. Spread Spectrum will often get data through "in between" the interference.......But using the entire 20mhz with small packets and nstreme will generate a lot of calls from pissed offed competitors.
With reduced power density, 40Mhz (turbo) would be a great spread too!
Less chances for those 25mhz gear to follow you, but you have to carefull, don't step on your toes!
(just a bump to bring the mtu over here)

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