New mesh throughput record?
wmb at laptop.org
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
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