[cisco-voip] When is Transcoding Required?

[Daniel]

Yes you can selectively configure what codec is used between phones,
gateways and sites etc.... You can do this by using regions, also on H323
dial peers you can set the codec.

Regions
Codecs can be configured between regions so that branch talking to core over
the WAN is a a G729 call between regions, you would set the service
parameter so that G729 is default between regions (inter region) and then
default that its G711 within a region (Intra region). You can then manually
configure specific regions, for example we use G711 to a Music on Hold
region so that we can do G711 MoH from a router flash and also in our
environment we have G711 configured to a tandberg trunk so that no matter
what site your at desktop video calls to our tandberg environment are G711
audio with a specific amount of video quality.

If the gateway and phones are on the same LAN then use G711 have them in the
same region. If the call is from a gateway to another site phone on
a different LAN over a WAN then use G729 between the regions.

My example for this would be a site gateway receives an ISDN call for a
phone, If the call is answered at this point it would be G711, but the user
is not available so the call diverts to voicemail, the voicemail server is
centrallised at the head office so the call traverses the WAN to conenct to
the voicemail box, this is now between regions so the call is now G729. The
gateway dial peer would need to support G729, the voicemail server would
need to support G729. If they don't both support G729 then a transcoder is
needed your preference here to reduce bandwidth needed would be to have the
gateway support G729 via the dial peer and then a trancoder at head office
next to the voicemail server so that the traffic over the WAN from the
gateway is G729 to the transcoder and G711 to the G711 only voicemail
server.







On Fri, Dec 11, 2009 at 1:49 PM, Aaron Riemer <ariemer at wesenergy.com.au>wrote:

>  Thanks Daniel. So to make sure I understand this correctly. Can Cisco IP
> phones dynamically use different codecs based on where calls are going or
> coming from? If calls are coming from the PSTN then would it make sense that
> the phone uses G.711 to reduce the level of transcoding required? If calls
> are coming from the IP network then G.729 is used? Can the call agent make
> this distinction of codec choice?
>
>
>
> As you can no doubt probably guess I am very new to the area of voice and
> it is difficult to know where to start!
>
>
>
> Thanks,
>
>
>
> Aaron.
>
> * *
>
>
>
> *From:* admin at danofive.id.au [mailto:admin at danofive.id.au] *On Behalf Of *
> Daniel
> *Sent:* Friday, 11 December 2009 11:32 AM
> *To:* Aaron Riemer
> *Cc:* cisco-voip at puck.nether.net
>
> *Subject:* Re: [cisco-voip] When is Transcoding Required?
>
>
>
> 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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