[Asterisk-Users] Backup Proxy & Automatic Failover

Nicolas Bougues nbougues-listes at axialys.net
Tue Dec 30 12:17:55 MST 2003

On Tue, Dec 30, 2003 at 06:49:51PM +0000, Adthrawn wrote:
> Hi,
> The term TDM is banded around too, but from my knowledge, TDM is 
> trunking (probably some clever acronym relating to trunking), and in 
> Asterisk's case, using the IAX protocol. This leads me to the big 
> question;

TDM is time division multiplex. It's how phone calls are sliced on
digital lines.

FDM is frequency domain multiplex : to have two (or more) phone calls
on the same wire, you shift the frequencies used for each call. That's
similar to the way ADSL can work at the same time as telephone.

TDM is far more efficient : you digitalize the phone data (sound), it
gets you 8000 bytes per second (64kbps/s). Then, you take a T1 link
for instance, you put on it a carrier that allows you to transfer 1536
kbits/s. And you realize that every 8000th of a second, your T1 can
transport 24 bytes. This way, every 8000th of a second, you send 24
bytes from 24 simultaneous phone calls. That's TDM.

Asterisk (zaptel) does TDM over Ethernet as well : instead of using a
sync link such as a T1 or E1, you send multiplexed frames across the

> Is there anyway of shifting the load of one Asterisk server to another 
> without breaking or loosing a call?

No. If you're talking about ISDN, that would mean asking the telco to
transfer the call from one span to another, transparently. If you're
talking about VoIP, that would mean doing tricky re-routing, which
could be nasty if NAT is involved.

In any case, that would mean transfering a whole call context as well,
like a running AGI script for instance, which is simply not possible.

> I know that with Survivable Routing (Cisco's big on this), the ISDN 
> interface is actually a router; so the Proxy is just used to decide the 
> destination and LCR functions, and then hands off to a router. This of 
> course, if a Proxy went down, would just prevent new calls from being 
> made, whilst existing calls can continue merrily - until someone 
> switches the Router off, or corrupts the IOS settings :-)

What if the "ISDN router" goes down ?? Asterisk is much more of an
ISDN router kind than a proxy kind. Maybe you could try to use SER as
a front end ?

Nicolas Bougues
Axialys Interactive

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