[Asterisk-Users] four wildcards in a single pc

[Jim Van Meggelen]

asterisk-users-bounces at lists.digium.com wrote:
> Hi.
> Please excuse me asking this again. But it really puzzles me.

It is puzzling, no denying it. The development team is still struggling
with these issues, and so far there has not been found a foolproof
solution (at least I can't recall having seen one).

> We're installing asterisk at a branch office at NJ (HQ is at
> Petach-Tikva) It'll need to support 5 POTS lines, 11 analog
> extensions and four VOIP phones. 
> 
> I wanted to go with a T1 card from digium and a channel bank,
> but we have a dead line. It has to be up and running by
> January 1st. I don't have the time to start shopping at ebay,
> where you don't know what you'll get, and you need to
> install, under time pressure something you not familiar with.

For sure, but you should also consider the experience of those who have
been there before you.

> So I thought of installing a combination of four pci cards in
> the machine, and everybody on the list just keeps telling me it won't
> work. 

It _might_ work, but it is almost guaranteed not to work _well_. The
Digium PCI cards are rather different from any PCI card you may have
used in the past.

> I have installed successfully more then four cards in a
> machine before. I had a firewall with eight network
> interfaces (one quad card, one duo and two singles) I have
> machines with two dialogic boards, a pci display card, and a
> network interface. And I know I've had machines at home that
> had a display adaptor, modem, network, scsi, and soundblaster all
> together. 

Yep, so have we all.

The thing is, just ONE of these Digium cards will request more
interrupts all by its lonesome than every single PCI card you've ever
installed in all the machines you've ever owned, all put together! Well,
perhaps not, but seriously, these things work very differently from any
PCI card you've ever seen before.

> Yet, people claim it won't work because of lack of IRQs, and
> that it's not related to Digium.

That isn't strictly correct, but the problem does pertain to IRQs. 

OK, look, you _might_ be able to free up enough IRQs on a PIC-based
motherboard -- if you disable the serial ports, mouse, parallel port and
USB. It's not recommended, but it's theoretically possible. 

And if you have a MoBo that is APIC-compliant, you should be able to
have all the IRQs you can handle, so lack of IRQs doesn't need to be an
issue (make sure you have a BIOS and chipset that's up to the task).

BUT . . . 

Getting dedicated IRQs for the cards is a minor problem compared to what
happens when you have four cards hammering away mercilessly at the
chipset and CPU of your motherboard; 1000 IRQs per second, per card.
Nobody's really sure what's wrong, but it causes problems for pretty
nearly everyone.

What everyone here is saying is that we're all pretty sure you're gonna
run into problems; problems that could easily be avoided by avoiding the
whole TDM400 mess in the first place.

> What am I missing?

The Digium cards are unique in the world of telephony, because instead
of having an expensive DSP chip on board, they use the CPU to provide
this functionality. The challenge comes from the fact that voice is
intolerant of delay. In order to ensure that the voice processing that
goes on in the CPU is handled with no perceivable delay, the zaptel
cards have to establish a kind of pseudo-synchronous clocking with the
CPU. Unfortunately, the signalling bus on a PC isn't synchronous, at
least not in that way. The "clock" that the zaptel cards use is the IRQ
of the card, literally requesting the CPU interrupt what it's doing and
pay attention to it 1000 times per second, regardless of what it's
doing. You are proposing the use of FOUR of these cards. 

Since this has caused trouble for nearly everyone who has tried it,
everyone is suggesting that you might want to give the matter some
careful thought. There _are_ less painful ways.

Cheers,

Jim.