New mesh throughput record?

Mitch Bradley wmb at
Wed Oct 22 06:01:33 EDT 2008

I just achieved a mesh throughput rate - XO to XO - of 2.8 MBytes/sec 
using multicast with TTL = 1.  That's twice as fast as the best I was 
able to do through the Linux driver, using the same parameters.

I did it using Open Firmware, paying careful attention to queuing USB 
bulk-out requests so the USB host controller doesn't have to wait for a 
USB frame boundary before sending the next one.

The packet loss rate was quite low - on the order of 0.1% .  The 
machines were less than a meter apart.

The broadcast bit rate can be 36, 48, or 54 Mbits/s to achieve that 
throughput.  At lower bit rates the throughput starts to drop off.

I also measured the rate at which it's possible to push data to the wlan 
device over the USB bus without transmitting - that rate is 4.2 
MBytes/s.  I did that by tricking the chip into accepting the USB 
packets but not transmitting them. 

Using the Linux driver, the fastest transmit rate I was able to achieve 
was 1.4 Mbytes/s (multicast, TTL=1, high broadcast bit rates, i.e. the 
same parameters).  That makes me suspect that the Linux driver is 
inadvertently throttling the rate.  The "throttled" rate is almost 
exactly one ethernet frame per millisecond.  I'm not sure what's causing 
it.  Perhaps some characteristic of the EHCI driver is synchronizing the 
bulk-out transactions to the USB macro frames.  It might also be a Linux 
scheduler artifact, perhaps related to the multithreaded nature of the 
Libertas driver.


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