[asterisk-dev] TDM cross-connect.

Matthew Fredrickson creslin at digium.com
Sat Mar 31 08:29:09 MST 2007

On Mar 31, 2007, at 12:04 AM, Alex Balashov wrote:

> I guess it would help if I clarified my question a little bit:
> As I understand it, the difference between a switch and a DACS is that 
> a switch performs dynamic connection functions, while a DACS is 
> basically
> for nailed-up, static channel cross-connections amongst trunks -- 
> essentially a digital patch panel.
> My question intended to ask whether Asterisk/Zaptel can perform 
> switching functions between time slots on the Digium multispan T1 
> cards, and if so,
> across multiple cards?  Clearly, this cannot be purely a Zaptel 
> exercise;
> the switching portion would require higher-order dial plan logic, and
> probably continuous involvement in the media path.  But can it be 
> done, in
> principle?

The answer is the same as last time you asked it: Yes.  This is all 
done with a combination of zaptel and asterisk.  If the call is between 
two zap channels, asterisk requests zaptel to cross connect the two 
timeslot's data.  Zaptel in turn, either A) Cross connects the TDM data 
within the card, if both timeslots are within the same card and the 
card supports it or B) cross connects the TDM data within software, or 
rather, within the zaptel, kernel land infrastructure if the two zap 
channels are not on the same card or the card does not support TDM 
cross connect.  This is done dynamically when Asterisk finds out that 
the legs of the call are zap channels.  Or it can be done statically 
using the "dacs" channel type in zaptel.conf.

Also, at least for Digium cards that support it, you can not only cross 
connect the TDM data within the card or zaptel dynamically, but you can 
also monitor the TDM data in parallel to the cross connect operation.

Matthew Fredrickson

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