Re: [Madwifi-devel] Are wireless collisions inevitable?

[Derek S. <de...@in...>]

Hi,
  Wireless collisions are inevitable.

If you do a scan, you will see multiple APs listed. The APs may not 
coordinate between them, so there is a good chance of any two nodes
"deciding to transmit" after a CCA, at the same time..

Wireless collisions are one of the things that makes rate control 
algorithms so difficult.

packets are lost because of background interference - the symbol rate is 
too high.

packets are also lost because of collisions in the wireless medium. In 
this case, the symbol rate is fine.

The rate control algorithm cannot distinguish between the two loss 
mechanisms. - the rate control algorithm simply "knows" that the packet 
was lost.

Now, the rate control algorithm has to decide what to do -
  if it is the first case - use a slower symbol rate
  if it is the second case - try again at the same rate.

Derek.
==========================
In any environment, there is
On Thu, 7 May 2009, David Murray wrote:

> Hi,
>
> I have a question about the 802.11 MAC layer and I am unsure where I
> can ask. If this is too off topic and there is a more appropriate forum
> then let me know.
>
> Basically, I am wondering if wireless 802.11 collisions are inevitable.
> It seems to me that even in a simple network, with one STA, one AP and
> one TCP flow, packets will be generated in both directions (TCP data
> and TCP acks). So if both the AP and wireless STA are txing packets,
> they will each follow DIFS and pick random slots between 1 and 31.
>
> My question is, does this mean that on average, 1 in 31 packets will
> result in a collision because the two stations chose the same slot?
> This seems like a lot of packet loss (even though they will be
> recovered by 802.11). The only way for this not to occur is if the
> internal clock of the AP and client are slightly different such that
> even if they do select the same timeslot, one will begin transmitting
> slightly earlier. This might enable the CCA (Clear Channel Assessment)
> of the node with the slower clock to detect a transmission and backoff.
>
> Thanks for your time
> Dave
>
> ------------------------------------------------------------------------------
> The NEW KODAK i700 Series Scanners deliver under ANY circumstances! Your
> production scanning environment may not be a perfect world - but thanks to
> Kodak, there's a perfect scanner to get the job done! With the NEW KODAK i700
> Series Scanner you'll get full speed at 300 dpi even with all image
> processing features enabled. http://p.sf.net/sfu/kodak-com
> _______________________________________________
> Madwifi-devel mailing list
> Mad...@li...
> https://lists.sourceforge.net/lists/listinfo/madwifi-devel
>
>

-- 
Derek Smithies Ph.D.
IndraNet Technologies Ltd.
ph +64 3 365 6485
Web: http://www.indranet-technologies.com/

"The only thing IE should be used for is to download Fire Fox"