CME - Software Conferencing vs. DSP

Unanswered Question
Feb 10th, 2009

We are having some issues with conferencing and had a few questions...

It is my understanding that CME has a built-in software conferencing capability to conference 3 users without requiring DSP resources. This appears to work fine as long as all clients are using g711 for their codec. But as soon as we introduce a phone that crosses a dial-peer that has a default codec not g711, the conferences fail ("can't complete conference").

The 3 user conferencing (with a limit on the total number of conferences based on CME Hardware platform) works for us. But not being able to conference the remote users is an issue.

Does this mean that we must either convert the dial-peer to g711 or setup full HW based ad-hoc conferencing using DSP resources?

Is there any way to force the voip dial-peer to "transcode" to g711 only before participating in the software conference (we have limited DSP transcode resources available).

If we setup HW conferencing, it appears based on the DSP calculator that it requires 1 DSP per 8 g729a conference participants. Does this mean 8 non-g711 phones or 8 phones total?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (4 ratings)
Chris Deren Tue, 02/10/2009 - 10:26

I believe you should be able to configure xcoders that will do the job, the xcoders will provide you alot more for your buck from DSPs than dedicated hardware conferences.


Nicholas Matthews Tue, 02/10/2009 - 10:40

When you configure conferencing, it is going to require a full DSP no matter what. This is different from transcoding, which will use DSPs as needed per codec required and as calls request it.

If you add a transcoder, it should allow for you to match the 1 sided capability of the conferencing with G711 and the remote phone with G729.

It will be 8 phones total.



cjbaker Tue, 02/10/2009 - 12:42

Okay, now I am really confused...

On two CME's I have conferencing setup as software (or so I believe):


max-conferences 8 gain 6

I have DSPs setup for transcoding only (not conferencing ... 28xx so PVDM2's on motherboard):

voice-card 0

dsp services dspfarm

sccp local GigabitEthernet0/0.120

sccp ccm identifier 11 version 7.0



sccp ccm group 1

description Transcoding DSP Resources

bind interface GigabitEthernet0/0.120

associate ccm 11 priority 1

associate profile 1 register mtp00241432fc80


dspfarm profile 1 transcode

description Transcoding DSP Profile

codec g711ulaw

codec g711alaw

codec g729ar8

codec g729abr8

codec g729r8

maximum sessions 5

associate application SCCP



sdspfarm units 5

sdspfarm tag 1 mtp00241432fc80

If I place a call from the remote site to the local site (going over a dial-peer with the default g729r8 codec) and try to "confrn" I get a "cannot complete conference" message.

But if I change the dial-peer to "codec g711ulaw" and try the same thing, it works fine.

So from my point of view, I am not using any DSP resources for conferencing. I believe based on the CME System Admin Guide (01-20-2009) on pg 694 "Conferencing Overview" ...

"Conferencing allows you to join three or more parties in a telephone conversation. Two types of

conferencing are available in Cisco Unified CME: ad hoc and meet-me.

Ad hoc conferences are created when one party calls another party, then either party adds one or more

parties to the conference call. Ad hoc conferences can be hardware-based or software-based, depending

on the number of parties. Hardware-based ad hoc conferencing uses digital signal processors (DSPs) to

allow more parties than software-based ad hoc conferencing, which allows three parties only."

Your email implies I have to use hardware based no matter what. Can't I use software ad-hoc at 3 parties max without using DSPs?

Nicholas Matthews Tue, 02/10/2009 - 12:49

Yes, you can do software for 3 G711 parties.

If you want to do any G729 conferencing it will require hardware.

The only question is if the transcoder will convert G729 to G711 for a software conference. I would have to play with this in the lab, but it looks like you already have this configured.

Looking at the configuration guide, it looks like this is relevant:

The codec g729r8 dspfarm-assist seems in order here.


cjbaker Wed, 02/11/2009 - 11:57

This is a production system and I don't want to "play" ... where do you think that goes .... on the dial-peer between buildings or the transcoding resource or ???

Nicholas Matthews Wed, 02/11/2009 - 12:20

Hi CJ,

No playing required on your part - I just haven't tried this particular feature out before.

Add this to your transcoding profile:

dsp profile x transcoding

g729r8 dspfarm-assist

What this command does is assist the software conferencing with transcoding for this specific situation. If you had hardware conferencing configured also, it would not make any sense to use a transcoder to send it to a hardware conference.

So, what this command says that if a conferencing session needs assistance - lend it. The transcoder by default just doesn't have the context to know whether it should or not, so you need the codec "g729r8 dspfarm-assist" configured under the dspfarm profile.



ogor Fri, 04/10/2009 - 19:15

Hi Manuel,

Please where did you find this CME tech tips ? do you have others regarding CME ?

Best Regards



This Discussion