[ltp] Slightly OT: wifi througput, overhead and goodput?

[Stefan Blum]

Hi, though I am not at all a network guru, I recently read an article in a 
German IT-newsmag. Here's a short courtesy translation of the paragraph 
relating to your question:

"The transmission of a payload of 1500 bytes with 54 MBit/s takes 325 
microseconds, equaling a net-rate of approx. 37 MBit/s. If you take into 
account TCP/IP-Overhead (additional 40 bytes per packet, TCP-ACKs) and 
repeated transmissions due to radio-interferences, you get net-rates of 
approx. 25 MBit/s, which 802.11a usually acchieves on a good 
radio-connection. ... For 802.11g, that in principle uses the same radio 
technique as 11a, it might get worse."
(German original: http://www.heise.de/netze/artikel/80521/0)

It seems like you get pretty much the max out of your 54MBit/s connection if 
you have a throughput of about 24MBit/s.

Hope that helps! Regards,

Stefan


On Friday 16 March 2007 04:09, Jiang Qian wrote:
> Hi all:
> 	To all the network gurus out there, these questions has been
> bugging me for a while and haven't been resolved through all the google
> search and wiki reading. I've been trying to make a sense of how good
> the ipw2200 card of my T43 talk with my WRT54GL router and what is the
> possible bottleneck of the network.
>
> 1. I read on wikipedia[1] that the maximum throughput of 802.11g is
> 24.7Mb/s, in contrast with the maximum data rate of 54Mb/s. How does the
> discrepancy come about? I thought this has something to do with only
> half of the channel is available, but that should give 27Mb/s.
>
> 2. As I read here[2], even the throughput is not the unidirectional data
> transfer rate. Goodput is a better measure. For example, due to tcp
> overhead, only 94.92Mb/s is possible for the 100Mb network. So does the
> similar overhead apply to wireless network? Or does the wireless network
> have an even bigger overhead? Is this overhead already taken into
> account in the reduction from 54 to 24.7Mb/s in question 1.?
>
> 3. To put it all together, when I download one 131MB file via ftp from
> my desktop(connected to wired port of router) to my laptop(connected to
> the wireless port), the log of vsftp report a download rate of
> 2893Kbyte/sec.  That is 2893*1024*8/1,000,000=23.7Mb/sec. This is the
> one direction effective goodput[2], I suppose. What percentage am I from
> the theoretical maximum of the unidirectional tcp goodput, combining
> consideration of the previous two points?
>
> 	Any comment or enlightenment would be most appreciated. If you
> find wikipedia entries cited here to be wrong, please correct them or I
> can do the typing for you:)
> 								  Jiang
>
> Reference:
> [1]http://en.wikipedia.org/wiki/802.11g#802.11g
> [2]http://en.wikipedia.org/wiki/Goodput