[Asterisk-biz] Re: T1/DS1/ISDN PRI

[Herman Webley]

On Thu, 2005-04-28 at 16:41 -0400, Matt Roth wrote:
> It is my understanding that TDM is circuit switched and VoIP is packet 
> switched.  It would seem to me that at some point in a TDM-VoIP gateway, 
> a change from circuit switching to packet switching is happening, and 
> vice versa depending on the direction of the signals.  I was just 
> wondering if anyone could detail that process and tell me if it is 
> resource-intensive.  If I'm completely off-base, please point me in the 
> right direction. 

TDM is circuit switched. And of course VoIP is packet switched (IP
packets no less :) ). All that needs to be done is to convert from
circuit to packet switched is grouping the frames of the voice stream
(now packets) and adding IP/UDP headers to each packet. In the other
direction it is a little more complex because the receiver may need to
reorder the packets (the packets can get disordered in transport).

The process is analgous to a water pipe versus bottled water. The pipe
is dedicated and can always carry that amount of water. Also the water
doesn't get disordered. Its basically first in first out. On the other
hand with the bottles, disorder can happen. But on the plus side with
bottles, you can think of many other ways to carry the water. It allows
more versatility. I guess putting water into bottles, and filling big
containers with water may be harder than VOIP. Maybe this isn't really a
good analogy at all :).

In any case this process isn't resource consuming, unless the voice
stream is compressed when converting to VoIP. (This ability to be
compressed is one of the versatility benefits of VoIP, it could be
implemented with TDM as well though)

> Is there a way to specify the format?  What if there is no sound card on 
> the Asterisk server?

I think 'show file formats' from the * CLI will give you an idea about
formats (it can't record to all of those, but it knows of those).
Asterisk can record to many formats. Check the Record and Monitor
application pages on VoIP-info.org

Best regards,
Herman Webley