Date: Sun, 14 Jul 2002 23:09:54 +0100 (BST) From: Duncan Barclay <dmlb@dmlb.org> To: Vinod <geekvinod@yahoo.com> Cc: freebsd-mobile@freebsd.org Subject: Re: wi TX rate mechanism Message-ID: <XFMail.20020714230954.dmlb@computer.my.domain> In-Reply-To: <20020714190625.79562.qmail@web21108.mail.yahoo.com>
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On 14-Jul-2002 Vinod wrote: > Hi there all.I had a question about the wi TX > mechanism. I have observed that sometimes the TX rate > of my wi card fluctuates between 11Mbps, 6Mbps and > 2Mbps.Why does this happen? > if its due to interference why should the TX rate be > affected?shouldnt just the receiving station be > affected? > Was wondering if the receiving station tells the > sender to slow down.does it? 802.11 has automatic rate throttling in the MAC (medium access controller). The firmware in the card will initally try to send a packet at the highest rate supported by the BSS. If the TX does not receive and acknowledgement in a given time, it will fall back to lower rates because lower rates have a better chance of being received when the received signal is corrupted by interference and noise. The mechanism is "independent" of the receiver, in that the TX makes all the decesions based on whether it gets acks. > Another question i had is about the throughput i get > and what the wicontrol command shows my TX rate is. > I have seen that i get about 5Mbps throughput when the > TX rate is shown to be 11Mbps.so can i assume i will > always get about 5Mbps when wicontrol shows the rate > to be 11Mbps? if i start getting lesser throughput > then will wicontrol also be showing a lesser TX rate > like 6Mbps or 2Mbps ? What i mean in short is will > wicontrol automatically show smaller TX rates at the > sender when my receiver throughput falls.this is > related to my first question actually. > Thanks in advance, > Vinod When using an access point (infrastructure) all traffic from station to station must go through the AP. I.e. sta1 -> ap, ap->sta2. This essentially halves the system through put compared with the on air bit rate. Using ad-hoc networks removes this and you get close to the on air rate. Duncan -- ________________________________________________________________________ Duncan Barclay | God smiles upon the little children, dmlb@dmlb.org | the alcoholics, and the permanently stoned. dmlb@freebsd.org| Steven King To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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