On Mon, Mar 01, 2004 at 03:56:01PM +0100, Robert Lillack wrote:
> Dick Davies wrote:
> 
> > What's the overhead then? I'd guess 11Mbps / 8 =~ 1.5MBps - the rest is 
> > lost in collisions, is it?
> 
> There is no 'collision detection' on wireless LANs. You
> have to use 'collision avoidance' because you would need
> to be able to receive and transmit at the same time (full
> duplex) *and* you can not be sure that if another stations
> can hear you that you can hear it's answers.
> 
> So, if a station wants to send a packet it waits for
> silence then sends a RTS packet (request to send) waits
> for the CTS packet (clear to send) of the receiver and
> then has a specific time frame to send the real packet
> which contains the data. (During that time all the other
> stations will power down their radios to save energy)

RTS/CTS does slow things down a lot, but the 802.11 WLAN overhead does
not usually come from RTS/CTS.  RTS/CTS is optional. Ordinarily it
is turned off.  Most of the notorious 802.11 overhead comes from the
144-bit preamble and 48-bit PLCP header, which are sent at 1Mb/s (unless
"short preamble" is enabled). That's 192us overhead in each packet. At
11Mb/s, you can send 264 bytes in 192us! 802.11's large-ish header also
contributes to the overhead. So do the per-packet acknowledgements.

Dave

-- 
David Young             OJC Technologies
dyoung@ojctech.com      Urbana, IL * (217) 278-3933