Help understanding DSP requirments

Unanswered Question
Apr 30th, 2007


I have an E1 connected to a 2800 series voice enabled router (from a PBX using FXS/FXO loop start signalling).

Now, I need to terminate calls on the 2800, and play an announcment (using TCL or VXML).

I also need to transfer calls out back to the PBX, after the announcment.

Are DSP resources required only when playing announcments ?

Or are they also required when doing a hair-pin transfer (no codec change involved in the transfer)?

And if required on a transfer, is 1 DSP's required per leg of the transfer ?


I have this problem too.
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Paolo Bevilacqua Mon, 04/30/2007 - 05:20


If you are supporting voice in the router, the DSP will be needed either for playing annoucements, and hairpinning to other port.

In G.711, on the 28xx, one DSP supports 8 calls.

Hope this helps, if so please rate post!

shahedvoicerite Mon, 04/30/2007 - 05:27

Thanks for your reply.

I however cannot understand why a DSP should be required for hair-pinning, since its a TDM to TDM transfer (and no conversion to packet voice seems to be necessary)

Is there any link that explains how DSP's are used ? (I have tried searching this site, but have not found a good doc yet)


Paolo Bevilacqua Mon, 04/30/2007 - 05:41

Well there is more than one reasons. First of all when there is an analog port involved, of course a dsp must be involved. Another reason is tone detection and generation. However, if all ports are digital, the router will try to cross-connect using TDM resources if available on the platform, see:

Understanding how and when DSP are used required knowledge of IOS internals, this is not something that cisco discloses to the public, although with experience you can get to know may things...

A good tool is the DSP calculator on CCO, however it requires customer-level access.

Thank you for the nice rating and good luck!


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