Over the VPN the bandwith is twice the bandwith used over an ISDN

Unanswered Question
Feb 25th, 2007


After analyze, when i perform a call over a VPN the bandwith used is twice the bandwith used the same configuration using an ISDN link. On my configuration i use on one side an ASA 5520 ans on the other side an 3640. The phoneset use H323 protocol. Using the codec G729 the bandwith is 50 K over the VPN ans 25 over the ISDN. Using 711 the bandwidth is 200 K over the VPN and 100 K over the ISDN. I know than encryption add overhead but twice it seem very higth.

Do you have an idea.


I have this problem too.
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rtappend_2 Sun, 02/25/2007 - 05:11

Hi Claude,

Check the Voip Bandwidth calculation tool here:


You can play around with the impact of different voice payload sizes vs overhead to determine the resulting voice call bandwidth.

Note that IPSec overheads can be up to 58 bytes, which when added to the IP/UDP/RTP and Layer 2 overheads can have a significant impact for each 20 byte voice payload packet.

Increasing the voice payload size can make quite a difference.


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