[Asterisk-Users] PCI front mount chassis?

[Steven Critchfield]

On Fri, 2004-03-12 at 08:55, Bruno Haas wrote:
> Wow, 1000 IRQs a second. I'm quite amazed. Does anybody know which 
> applications would require such a low latency ? It does seem to me that 
> this way of doing things is rather dangerous and prone to problems. 
> Anybody can comment ?

It isn't so much that the software needs it but the hardware. Part of
what makes other telephony hardware so expensive is the extra cpu power
on card and the memory on card to use for buffering. The 1000 interrupts
a second is so you can pick up a byte of data per channel each time. 

BTW, any modem technology is going to require at least a consistent
latency if not a near nill added latency. 

> Nicolas Bougues wrote:
> 
> >On Fri, Mar 12, 2004 at 08:41:36AM -0500, Walt Reed wrote:
> >  
> >
> >>The voice cards generate an order of magnitude more interrupts than
> >>anything else. This "may" be why it's not recommended to share
> >>interrupts on voice cards. Don't know if the T1 cards have a similar
> >>issue. I would hope not. The x100p's are a pretty simplistic device.
> >>They probably generate an interrupt for every byte. The x100p's are also
> >>used for timing in things like MOH and MM conferences AFAIK. It seems
> >>like it would be nice to only put one card in "timer mode" if that is
> >>indeed what is generating all those interrupts. Could someone "in the
> >>know" enlighten us?
> >>
> >>    
> >>
> >
> >Digium boards usually generate 1000 interrupts/sec. This is mostly a
> >"timer" interrupt, so that the driver can poll the board for 8 bytes
> >per channel 1000 times per second. The polling is either slave (PIO),
> >or busmaster (DMA).
> >
> >Each board generates 1000 interrupts/second, no matter the kind/number
> >of ports on the boards. The driver knows if that's a single channel
> >board, with 8 bytes to fetch, or quad E1, with 128*8 bytes to get on
> >each interrupt.
> >
> >There are quite strict timing requirements : if one interrupt is lost,
> >1/1000th of sound on the line(s) is lost, which can be quite bad for
> >things like HDLC (on T1/E1) or modem sound (no matter the kind of
> >channel).
> >
> >Sharing an IRQ may introduce some latency, because the IRQ has to be
> >handled by several drivers, and if each of them does not behave
> >properly, frames can be late/lost.
> >
> >  
> >
> 
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-- 
Steven Critchfield  <critch at basesys.com>