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

[David Josephson]

>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.
>  
>
Not off-base, but you haven't made it all the way home yet. This is 
another layer of the puzzle, and again we are not talking about apples 
and apples here. "Circuit switched" means that there is a (real or 
virtual) circuit that takes data on an input port and delivers it to an 
output port somewhere. "Packet switched" means that each packet of data 
is examined by each port it passes, to see where it should be sent. 
Normally this layer of VoIP traffic is handled not in Asterisk, but in a 
router. You could run the router on the same Linux box that's running 
Asterisk (and send packets to different Ethernet ports depending on 
their destination address) but normally this task is handled by a 
separate router. There is a small computational overhead associated with 
adding and decoding Ethernet packets but the main routing work is done 
outside Asterisk, and isn't too intensive. You could read up on TCP/IP 
routing and understand how this works in more detail.

>>It's too bad. A lot of people without any telephone background try to 
>>make up stuff using pieces of the old terminology and wonder why they 
>>stay confused. They could look it up, but they don't. For instance 
>>DID's. DID has a specific meaning and inward service from the PSTN 
>>handed off on VOIP isn't it.
>>    
>>
>
>Do you have a good, reliable source that I could take a look at?
>  
>
It's not something you can "take a look at" in my experience. Some of 
the Bell System training material that comes up on eBay is good. You 
need to follow the progress from circuit-switched voice telephony circa 
1930 through modern TDM, and then look at the development of TCP/IP 
switching separately.

>Is there a way to specify the format?  What if there is no sound card on 
>the Asterisk server?
>  
>
No sound card, no monitor. Recording to the various file formats is 
possible, as Herman mentioned.

Your reference picture is fine ... but note that Asterisk can be the 
TDM/VoIP gateway, particularly when Digium releases their DS3 card (644 
voice channels!) working, a lot more cheaply than a standalone box from 
some hardware vendor.

>  
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