[net2-wg] 600 pps, high throughput

[Jonathan Hui]

On 3/13/06, Philip Levis <pal at cs.stanford.edu> wrote:
> As Joe's pointed out in the past, the best solution is to use DMA
> (assuming you have a free DMA channel). If you have CSMA enabled, none
> of this makes a big difference, though, as the CSMA latency is greater
> than the loading time. I think Jonathan was planning on checking in a
> DMA version at some point, based on the re-architecture of the MSP430
> USART system Vlado, he, and I came up with after the TTX. He's been
> doing some pretty serious performance tradeoff comparisons, so probably
> has some good thoughts on the matter.
>
> Jonathan?

The tradeoffs are pretty obvious. As Phil pointed out, the expected
backoff time can far exceed any lost time by not using the DMA
controller. Utilizing DMA would actually be more useful to pull
packets out of the radio, but contention under CSMA often keeps
packets from coming in back to back. If backoffs are turned off,
utilizing DMA would definitely have a greater impact on overall packet
throughput (both transmitting and receiving).

One issue is that the msp430 only provides three DMA channels, two of
which would be required for full DMA support on the SPI. While, the
DMA can be helpful for packet training optimizations it also limits
their use for other potentially crucial tasks as sampling the ADC.

--
Jonathan W. Hui
jwhui at cs.berkeley.edu
http://www.cs.berkeley.edu/~jwhui/