Exactly, remember, these are protocols desinged for the LAN environment. Wired Ethernet uses collision detection. Wireless cannot do that because it can't discern whether the collision is from its own source or not. So instead, wireless takes a more novel collision avoidance approach. With CSMA/CA, devices listen to the media befor talking. If they discover the medium is clear, it talks. If not, it backs off for a certain contention window. As users aggregate, contention occurs more frequently. Actual throughput, when achieved, is no different than when the media is less full, but we generally measure performance over time, so when more users are on the air, each have less time to do their work.
In terms of net versus gross, the 11mbps is not myth, it is just eaten up in the overhead of the protocol. So while the modem rate is actually 11mbps, the total net payload for transmitted content frames is only somewhere between 5.5mbps and 7mbps in a perfect link. If you want to maximize throughput, engineer good links. Links that can only sustain a connection at the 2mbps fallback rate require several x more of the AP resources to send the same sized file as a client within the 5.5mbps rate, etc. Few understand this, but it is the key to understanding PMP capacity. If you are doing PTP, turn off all features required for PMP and you will reduce the link overhead, this increasing performance of the link. Patrick Alvarion -----Original Message----- From: Sameer Verma [mailto:[EMAIL PROTECTED] Sent: Tuesday, July 01, 2003 9:08 PM To: S Woodside Cc: Bulk John; 'BAWUG' Subject: Re: [BAWUG] Throughput question - 802.11b S Woodside wrote: > What happens is very similar to what happens on a shared 10 Mbps > ethernet bus (in fact the sharing protocol is the same CSMA/CD). Its actually CSMA/CA (carrier sense multiple access/collision avoidance), which is more along the lines of 100Mbps Ethernet. The traffic starts backing off as it starts to get crowded, and slows down until things get back to normal. Here's a nice explanation. http://alpha.fdu.edu/~kanoksri/IEEE80211b.html. > The maximum available rate would typically be divided by the number of > users. Reality is probably not quite that good. But most of the time > the backhaul is so much slower than the Wi-Fi bandwidth that it > doesn't matter. > If you want to maximize bandwidth, use multiple radios on different > frequencies. > > simon Sameer -- Dr. Sameer Verma, Ph.D. Asst. Professor of Information Systems San Francisco State University San Francisco CA 94132 USA http://verma.sfsu.edu/ -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless This mail passed through mail.alvarion.com **************************************************************************** ******** This footnote confirms that this email message has been scanned by PineApp Mail-SeCure for the presence of malicious code, vandals & computer viruses. **************************************************************************** ******** -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
