[cisco-voip] When is Transcoding Required?

[Daniel]

Hi Aaron,

If you have a site with an ISDN 10 with a LAN that has phones configured in
the same region as the gateway set to use G729, and the gateway voice dial
peers (H323) set to use G729 then you won't need transcoding as such. You
will still need DSP's for the packetisation from ISDN to IP packets. If you
use G729 then you'll require a PVDM2-32 as you'll use twice the amount
because of the codec complexity. If you used G711 from the gateway to phones
then you'll only need a PVDM2-16.

To answer your question if a call comes in from the gateway requesting only
G711 to a device that is only G729 then yes a transcoder is required
preferably at the same LAN as the gateway and phone. For instance if a site
gateway recevies a call for a G711 UCCX call and the UCCX server is in
another region that requires the gateway to use G729 to UCCX then a
transcoder is required to transcode the G729 call to G711 so that the UCCX
prompts can be heard.

This would be the same for software conferencing from a branch site to the
core server, the branch phone and gateway would talk G729 to a transcoder
which would talk G711 to the software conference bridge. In this situation a
hardware conference bridge would be better utilised.

Use this link below about number of calls per DSP etc... High Complexity
codec would mean G729 to name one.
http://www.cisco.com/en/US/prod/collateral/modules/ps3115/ps6024/prod_qas0900aecd8016c6ad_ps3115_Products_Q_and_A_Item.html

hope that helps.

cheers,

Daniel



On Fri, Dec 11, 2009 at 12:36 PM, Aaron Riemer <ariemer at wesenergy.com.au>wrote:

>  Hi Guys,
>
>
>
> Can someone clarify exactly when transcoding between codec’s will actually
> occur on voice gateways? For example if I have a branch site with Cisco IP
> Telephony and the phones are using G.729 and the site has a voice gateway
> and PSTN services in what situation will transcoding occur? Will all inbound
> and outbound calls through the PSTN require transcoding? i.e. G.729 to G.711
> and vice versa? If the site has 10 lines will I require at least 10
> transcoding sessions to cater for 10 simultaneous PSTN calls?
>
>
>
> Thanks,
>
>
>
> Aaron.
>
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