You're still not answering his question. :-)
The TJ320 has no buffering capability but the Silabs parts have nothing to do
with that, and neither do the framers on the higher-density TDM cards. I
think his question is more "why don't these cards have bigger PCM buffers and
interrupt less often, or at least have deeper buffers so if an interrupt is
delayed I don't get overruns?"
I believe the answer lies in latency. You do *not* want deeper buffers.
Better interrupt handlers, perhaps, but not any deeper buffering on the
hardware, as that just increases latency.
This is a tradeoff. You would ideally want a 0.125 ms latency which
actually is possible (not on these boards thought), but since you need
to transport through a PCI and use a host you need to adjust to a
reasonable tradeoff. I would prefer to use 6ms (48 bytes), which is a
more common handrule for PABX design and a far more reasonable interrupt
speed for a Pentium.
You will hit 2 perfomance issues with a 1ms packet.
- No pentium are particulary efficient moving packets below 50 bytes.
Make a small C program that move from 1 to 100 bytes and you will see
that the xfer capacity increase dramatically as you increase packet
size. This don't really matter if you use DMA, but it matters a lot if
the Pentium is doing the job.
- Some of the older type pentiums which you typically find in embedded
boxes are very inefficient handling fast interrupts and might use 10-20%
of their capacity just serving the interrupt in this case. This drops
dramatically if you are able to increase to 6 or 10 ms packets, which
also is a reasonable tradeoff as far as latency goes.
Also remember that echo cancel, dtmf, G.729.A etc all need more that 1ms
packets to work on, meaning you have to buffer anyway. dtmf can happen
outside the voicepath, but echo cancel and g.729.a has to be in your
voicepath.
But, thanks for your answers - I realize there is little that can be
done to address this issue as none of these boards seems to have dynamic
buffering.
jan
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