Cisco IP phones over wan routers - dsps ?

Unanswered Question

I have a query re routers and cisco phones. I am going to run @200 Cisco IP phones over a point-point wan link to a Cisco IPT system. My question is does the router need to have dsp chips to aid processing ? Thought these were only if you had e1 cards in the router ? Do they help with processing of voice packets just with IP phones directly connecting over the router or ar ethey simply not required and QoS and normal router processor will take care of it ?

Hopefully an easy one to answer !

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Jaime Valencia Mon, 07/06/2009 - 03:53
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DSPs are necessary for:

Voice termination (E1, T1, FXS, FXO, etc)



They have nothing to do with QoS or and are not involved whatsoever if ip phones are talking directly between them in a codec both understand.



if this helps, please rate

Wilson Samuel Mon, 07/06/2009 - 07:00
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also, in simple terms, if at any point if the Codec differs (e.g G.729 v/s G.711) and Tele Conf. is required, one will need DSPs.


Wilson Samuel

Sushil Kumar Katre Mon, 07/06/2009 - 19:54
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Hi Roger,

One suggestion, I would suggest you to have DSPs on the remote router for conferencing purpose. In case you are planning to have conference facility. That way conference calls within the office (remote site users) will not have to traverse the WAN and go to the central conference resource for conferencing.

-> Sushil

The main concept to understand here is "Codec Complexity".

g711/g722 = Low Codec Complexity = Low DSP Resource requirement.

g729 = High Codec Complexity = High DSP Resource requirement.

With 200 phones over the WAN, and a E1 I would say put a PVDM 2-64 into the router.

(to allow for Voice Conferencing and Transcodeing Profiles)

At minimum use a PVDM 2-48

(will not allow for Voice Conferencing and Transcodeing Profiles)

If you debug the "dsp-resource-manager" you will find the "out of resource" where the PSTN call will fail. This indicate you do not have enough DSP resource to facilitate multiple 15-30 high complexity codec g729 calls.


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