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Performance issues - RTS threshold, Fragmentation Threshold

Posted: 09:08 02 Feb 04 #1
  • alnug
  • 7 Posts
  • Join Date: 29 January 2004
Anyone know of these settings? I know there is no wy of altering this in the 604+ but can anyone point in me in a direction as to how I can find out about these settings?

I am getting data flow problems using my wireless card. Using DUmeter i see the speed vary between 480kps and 100kps and it fluctuates constantly until the data has finished.

When i look in the properties for the PC-card wireless device i have (650+) then i see that the RTS threshold, Fragmentation Threshold is both set to 4096 (i think this is something to do with 4x mode but i do not have this switched on).

I then try setting both these settings to 2436 and find that data flow is more consistent.

It is strange that many wireless settings still seem to be not talked about?

Hope someone can help.
Posted: 22:45 02 Feb 04 #2
  • alnug
  • 7 Posts
  • Join Date: 29 January 2004
Anyone?

Someone must know something of these settings? Thanks
Posted: 23:58 02 Feb 04 #3
  • rperkin
  • 2535 Posts
  • Join Date: 12 August 2003
  • Location: UK
"many wireless settings still seem to be not talked about?"
I believe there are a number of reasons why CTS/RTS and the Fragmentation Threshold are not talked about. In no particular order:
- no-one's heard of them :)
- they are settings whose value can be changed in the driver, but are not visible elsewhere
- with D-Link kit we are talking about (mostly) entry-level consumer oriented products, so everything is designed or expected to work almost straight out of the box
- there is no documentation
- there are few readily available tools which allow tweaking and the measurement of results

"how I can find out about these settings?"
If you want the detail, then the IEEE 802.11 standard is where to start. Find it via this link.

Or you can look at some Cisco documentation, which will give a few pointers from a manufacturer of industrial-strength equipment. Find some sample info here.

Or you can look at this article and related pages.

This is what IEEE 802.11 says:
"The process of partitioning a MAC service data unit (MSDU) or a MAC management protocol data unit (MMPDU) into smaller MAC level frames, MAC protocol data units (MPDUs), is called fragmentation. Fragmentation creates MPDUs smaller than the original MSDU or MMPDU length to increase reliability, by increasing the probability of successful transmission of the MSDU or MMPDU in cases where channel characteristics limit reception reliability for longer frames. Fragmentation is accomplished at each immediate transmitter. The process of recombining MPDUs into a single MSDU or MMPDU is defined as defragmentation. Defragmentation is accomplished at each immediate recipient."

My understanding is as follows:

- as it says above, fragmentation is used "in cases where channel characteristics limit reception reliability"

- what this means is that when transmitting a packet over a network medium, sometimes the packet is broken into several segments if the size of packet exceeds that allowed by the network medium.

- sending smaller frames reduces the probability of collisions, so potentially improving performance

- The Fragmentation Threshold defines the number of bytes used for the fragmentation boundary

- the threshold value should be reduced in small decrements to try to ease high rates of collisions

- however, the lower the threshold is set the more overhead will be incurred, so resulting in decreased performance. For bad collision problems this could improve performance; otherwise you will just see a trade off in speed

- RTS/CTS is intended to deal with traffic congestion problems. For example, the throughput of your machine might be suffering when others are doing large downloads or file transfers or streaming media

- transmitters contending for the medium may not be aware of each other; the RTS/CTS mechanism can solve this by effictively notifying their presence

- if the packet size is smaller than the preset RTS threshold size, the RTS/CTS mechanism will not be enabled.

- to enable RTS/CTS you should lower the RTS threshold to a level below the MTU (Maximum Transmission Unit) value

- again, the lower you set the threshold, the higher the overheads, so there is a trade off

I'm not aware of any tools which allow such tweaking of driver settings and give immediate feedback. There are a number available for tweaking the MTU and other settings to gain better throughput.
Posted: 23:40 03 Feb 04 #4
  • alnug
  • 7 Posts
  • Join Date: 29 January 2004
Cheers for that rperkin....i was pretty much aware of what you mentioned..thanks all the same...I was just wondering if anyone has been playing with these settings that's all.

Again thanks
Posted: 18:17 04 Feb 04 #5
  • rperkin
  • 2535 Posts
  • Join Date: 12 August 2003
  • Location: UK
"wondering if anyone has been playing with these settings"

You could be the first!

The default settings in the router itself are:
rts_threshold 2432
frag_threshold 2346

Hope this helps - have fun :)
Posted: 10:46 05 Feb 04 #6
  • rperkin
  • 2535 Posts
  • Join Date: 12 August 2003
  • Location: UK
I've just spotted that at the top of this thread you said "there is no wy of altering this in the 604+"
But there is - just not in the web-based interface.

Use the following CLI commands:
wireless set rts_threshold <n>
wireless set frag_threshold <n>


where <n> is the value you wish to set.

To ensure that any changes you make to these settings are saved, use the following commands:
config save
flashfs update
restart


If you need info on how to gain access to the CLI, read this thread.

Hope this helps - let us know if you find some rules of thumb for improving performance.
Posted: 14:08 05 Feb 04 #7
  • NormanB
  • 178 Posts
  • Join Date: 21 April 2003
Wow!
Posted: 10:38 06 Feb 04 #11
  • rperkin
  • 2535 Posts
  • Join Date: 12 August 2003
  • Location: UK
So, are you going to test it out Norman ?
Posted: 11:59 06 Feb 04 #12
  • NormanB
  • 178 Posts
  • Join Date: 21 April 2003
Richard

Sometimes I just feel so inadequate ;-)

I can cope with the basics but this stuff is doing 600knots at an altitude of 1000 feet and is now well astern of me!

NormanB
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