codec question

[msulieman]

i have an ip phone on a LAN segment in one site uses G.711 codec, wants to call another ip phone on another LAN segment in a remote site through an IP WAN the first LAN has 28xx gateway, and also the second LAN has 28xx gateway, I have the following question:

if I want the codec of the calls between the two sites to be G.729, what exactly will happen:

will the two phones negotiate before starting the call on what codec to use, they accept G.729 and the call takes place, or

the call will be converted from G.711 to G.729 by dsp on the first site 28xx gateway, passes through WAN as G.729, then when arrived to the remote site gateway 28xx it will be then converted back to G.711 ?

Thanks

[webberr]

Hi,

Assuming both phone support the G.729 codec (and they most likely do) then G.729 will be negotiated before the call starts and both phones will natively use G.729 without any external DSPs or transcoders.

HTH, Rob.

[msulieman]

thanks for ur clear answer,

now, user on LAN segment on site 1 wants to call pstn and assume now this LAN has a gateway (dsp within) to PSTN with, as a concept how exactly codec will function in this case, how the initiating or negotiation for the required codec will occur?

thanks

[thisisshanky]

Most phones (like 7940, 60, 70, 71 etc) are capable of g729 and g711 codesc. So when you setup each office into its own region and select g729 as the codec between the two regions, the phones will use g729 for calls between the two offices. No need for transcoding.

[msulieman]

thanks for ur clear answers,

now, user on LAN segment on site 1 wants to call pstn and assume now this LAN has a gateway (dsp within) to PSTN with, as a concept how exactly codec will function in this case, how the initiating or negotiation for the required codec will occur?

thanks

[thisisshanky]

Gateways have a device pool and hence a region and hence codec setting. Typically with in a site you have plenty of bandwidth in the lan, so no issues using g711 codec. So in this case for the pstn call, the region setting of the phone and the gateway is looked at to determine the type of codec.

HTH

Sankar.

[msulieman]

thanks sankar but can u explain more by what do u mean by

(the region setting of the phone and the gateway is looked at to determine the type of codec.

)

[thisisshanky]

Think of gateway as an endpoint. Every endpoint has a region setting. When a call is placed between two endpoints, one in region A and one in region B, the codec setting specified in the region configuration determines the type of codec used between the two devices. Same applies here. Since your phone and gateway is at site1, the codec used should be g711 (unless you used g729 as the default codec within the region)

[msulieman]

my question now is why does there is no transcoding between 2 regions, although the calls within the same region is G711 for example and between 2 regions is G.729, why should not it be reconverted back to g711?

what is the reason

[thisisshanky]

Because Phones are capable of using both codecs. Gateways can be configured to support both codecs. Hope that make sense.

Sankar

PS: please remember to rate posts!

[msulieman]

of course thanks

sometimes one needs to ask even if he may expect the answer to ansure the knowledge from experts

thanks again