[VoiceOps] G729 across ATM

[Kris Alberts]

Hi

 

I'm trying to ensure optimal bandwidth usage for our VoIP solution. The
last mile link to the customer is ATM.

I have done some calculations, and unless I'm missing something here it
seems 1 voice packet will always be split across two ATM cells. I'm not
finding a good doc that can break this down into detail.

 

Voice usage per packet:

Codec: G729a, using 20ms sample size, 20 byte payload = 20 bytes.

Layer3: IP/RTP/UDP header = 40 bytes
Bandwidth required =  at least 60 byte packet.

 

ATM Cell: 53 bytes - 5 overhead - 8 AAL trailer

Bandwidth available = at most 40 bytes.

 

My first question is: How does the Cisco voice bandwidth calculator fit
G729 on 10ms or 20ms or 30ms payload into 1 Cell? Do they assume cRTP is
enabled?

Then, I'm not finding a good doc on how to effectively calculate
bandwidth usage for cRTP across ATM. Configuring it via a
virtual-template interface seems straight forward though.
Lastly, if I manage to get a G729 packet squeezed into 1 cell, apart
from the obvious save in bandwidth consumption, should there be any
other improvement in the solution as a whole?

 

Regards,

kris 

"Morality is moral only when it is voluntary." Lincoln Steffens

 


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