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

[Jiang Qian]

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