[asterisk-users] Where is the Digium DS3 card?

[Alex Balashov]

Steve Totaro wrote:

> A T3 MUXed into 28 T1 PRIs in one, or a few trunk groups inherently
> has redundancy.  If a box dies, the calls are dropped (unless you are
> doing reinvite) and any call backs go right to the
> Ts that are not in alarm.

True - and if you're simply using CT3 as an economical method of getting 
say, a dozen T1s into a gateway, that is probably an advantage.  But if 
that's the case, it would not be cost-effective to shell out extra money 
for additional PCs with quad T1 cards just to provide failover in the 
event that the primaries fail.  80/20 rule and all that.

The point is that most people that want a DS3 interface really do want 
to pump in a DS3's worth of calls, more or less, in which case they 
really can't afford to have those DS1s going spare just for redundancy's 
sake.  And if you are doing substantially less than a DS3's worth of 
calls, you probably shouldn't be looking at a DS3 interface to begin 
with unless that's just an incredibly lucrative way to get channelised 
PRIs in from a vendor - and with typical the cost of UNE DS3 loops vs 
T1s, that's not necessarily so.

Secondly, an industrial-grade ISDN media gateway designed for telco 
environments (like a Cisco AS, say) isn't going to go down frequently 
enough to merit this kind of concern.  Don't get me wrong, I am the last 
to go on record saying that Cisco voice equipment (or any other) doesn't 
fail from time to time -- ha.  But, again, 80/20 rule.  A PC is much 
more likely to fail within the same MTBF.

So yes, a single gateway handling a DS3 can go down.  But so can an M13 
mux.  You've got single points of failure either way.

If one is in the sort of environment where such high availability really 
is a concern (typically a telco setting), one probably needs to invest 
in a big DACS and redundant, protection-switched DS3 paths (and 
protection line cards for them on the DACS side) as well as redundant 
gateways, or at least redundant DS3 line cards in the chassis.  At that 
point of stringent availability, this discussion becomes a wee bit moot 
because most likely you would not be using Asterisk and PCs in such a 
setting anyway.

> Running stripped down Linux OS boxen with quad port T1 cards and four
> or five lines in extensions.conf, no unneeded modules or software
> loaded, asterisk 1.2, entries for zaptel and zapata, and a couple
> entries in sip.conf builds a worry free solution.  Asterisk uptime 2
> years, system uptime two years and twenty minutes.

Yes, but total cost of ownership goes up because you need someone to do 
all that, and even so, despite the impressive uptime you mention, PCs do 
need a lot more maintenance, upkeep and worry.

With dedicated media gateways, you just plug in, set up and it works.


-- 
Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
Tel    : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
Mobile : (+1) (706) 338-8599