Voice termination - DSP

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Feb 13th, 2008
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Voice termination applies to a call that has two call legs, one leg on a time-division multiplexing (TDM)

interface and the second leg on a Voice over IP (VoIP) connection. This termination function is

performed by digital signal processor (DSP) resources.

My question is:

If I have 2 PRI, and my voice call comes from first and terminates on second, do I consume DSP resources for that? The call will not terminate on LAN so are DSPs needed?

Best regards

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paolo bevilacqua Wed, 02/13/2008 - 03:20
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Yes DSP are always needed, for the simple reasons that IOS will not let you create voice port unless you have enough DSP for each channel enabled.

Why is that, on the calls, although hairpinned, you might need to have tone detection / generation, an other thing osnly a DSP can do. See following document for an example:


Hope this helps, please rate post if it does!

goranr Wed, 02/13/2008 - 04:23
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Yes, I understand it, but if, as you say so, call is hairpinned, but with Direct inward dial, and with same signaling on both PRI links, why call takes DSP-s?

Yes, I agree with you from start, but, if call does not need resources, why they are used.

See: ///TDM switching allows the drop of the Digital Signal Processor (DSP) resources from the media path for the duration of the call. However, the provision of DSPs on the router for the initial call setup is a requirement. ///Once the call connects, the DSPs are removed from the media stream. ///

Best regards,

paolo bevilacqua Wed, 02/13/2008 - 04:29
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Hi, the router cannot know if you have DID or not, if you the call will be hairpinned, etc.

So DSP is a firm requirement.

goranr Wed, 02/13/2008 - 04:37
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I only wanted to know if they are used through the call. Now I see they are not, so I can use DSP-s for other purpose (e.g. conferencing).

Best regards and thanks.


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