Maximizing 802.11b throuput

Dave Edwards Dave.Edwards at abicom-international.com
Thu Mar 13 10:30:16 EST 2003


You also need to factor in the card's io performance. For the pccard 
based prism2 devices this can be as long as 200uS to write a frame into 
the device. I'm not sure if it's an issue with the PCI based devices but 
be aware, the pccard (16bit) interface is painfully slow!

This can be a problem if you are implementing a polling protocol with 
the radios.

Also, some versions of the Prism 2 firmware will only send multicast 
frames after a beacon. The AP firmware will allow multicast frames to be 
sent at other times, but it's difficult getting a legal copy of it!


Bichot Guillaume (Princeton) wrote:

> In theory the overal bit rate would be not so far from 11 Mbits. If 
> I'm not wrong, based on payload size of 1500 bytes, a total overhead 
> of 1148 bytes max (including PHY (guard interval + 1Mbit/s overhead) , 
> MAC (DFIS + data frame header), LLC header, SNAP header , IP header, 
> UDP header)  would give you a total throughput of 7.8 Mbit/s.
> This is for one way streaming in multicast/broadcast without any 
> IEEE802.11 association attempt (no uplink traffic at all) and good 
> channel quality.
>  
> In multicast however I've constated that the bit rate is sometime 
> limited by the firmware of my card (PCI prism2 Linksys). I cannot 
> stream more than 1Mbit/s. 
>  
> If you stream in unicast this is another story. MAC acknowledgment may 
> reduce considerably the bandwidth depending on the quality of your 
> channel. Practically the 3Mbit/s number seems often raised.
>  
> In theory transmitting in multicast with the maximum packet size is 
> the way to maximize the bandwidth usage. However you have to cope in 
> some way with error correction.
>  
> Guillaume Bichot
> -------------------------------------------------------------------------------- 
> As of February 12th, 2003 Thomson unified its email addresses on a 
> worldwide basis. Please note my new email address: 
> guillaume.bichot at thomson.net http://www.thomson.net/
>
>     -----Original Message-----
>     From: André Luiz Ribeiro Moutinho
>     [mailto:andre.moutinho at compsisnet.com.br]
>     Sent: Thursday, March 13, 2003 8:25 AM
>     To: hostap at shmoo.com
>     Cc: Mauricio Micoski; Marcelo Ferreira Vinhas; Marco Antônio Fernandes
>     Subject: Maximizing 802.11b throuput
>
>     Hi,
>
>     I am implementing a realtime video streamming application using
>     802.11b in order to transmit video data.
>     Transmitting MJPEG 320x240 at 30fps and 40% quality results a
>     3Mbit/s total bandwith. When I try to increase the video data
>     bandwith (for instance, rising JPEG quality), the radio channel
>     starts
>     failing to transmit all video information. I would like to know what
>     is the REAL 802.11 data transmittion throuput (4 or 5Mbit/s ??) and
>     what could be done in order to maximize the data throuput. For
>     instance,
>     trying to optimize the transmitted data block size.
>     I need to use the maximum data bandwith available in order to
>     transmit
>     the maximum mount of video sessions possible.
>
>     Thanks a lot,
>
>     Andre Moutinho
>






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