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VOIP using a VPN 1
- Thread starter Fixin
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Fixin,
what kind of vpn(gre, esp, pptp)?
I don't know that much about vpns, but, I'm pretty sure there's different overhead per-protocol.
Cisco has a pretty sweet calculator to fool around with the numbers on a voip call but you have to be a registered user to use it:
You could, for example, try using a larger sample, thus reducing the number of packets thus reducing ip overhead, plus maybe some rtp header compression(i guess that might be possible across a vpn tunnel)?
I tried a couple of calculations for a single g711 call across ethernet -
basic no-frills 160 byte/packet call:
Voice Packets Per Second 50(Codec Bit Rate / Voice Payload Size)
Bandwidth Per Call (RTP Only) 87.2 kbps ( Total Packet Size(bits) )* ( Packets Per Second )
5% Additional Overhead 4.36 kbps
(5% additional overhead per call to accomodate bandwidth for signaling (for example: RTCP/H225/H245 messages on H.323 networks).)
Bandwith Per Call + 5.0% Additional Overhead:
91.56 kbps Overhead + Bandwidth Per call
Then here's the same call with 240 byte packets:
Voice Packets Per Second 33.333 (Codec Bit Rate / Voice Payload Size)
Bandwidth Per Call (RTP Only) 79.467 kbps ( Total Packet Size(bits) )* ( Packets Per Second )
5% Additional Overhead 3.973 kbps 5% additional overhead per call to accomodate bandwidth for signaling (for example: RTCP/H225/H245 messages on H.323 networks).
Bandwith Per Call + 5.0% Additional Overhead 83.44 kbps Overhead + Bandwidth Per call
See if you can find out the overhead on the particular flavor of vpn you're using, and see if you can play with any of the voip-specific stuff to tune the bandwidth requirements. Use g729, if possible - it gives excellent fidelity for the low bandwidth usage, something like 26K per call.
what kind of vpn(gre, esp, pptp)?
I don't know that much about vpns, but, I'm pretty sure there's different overhead per-protocol.
Cisco has a pretty sweet calculator to fool around with the numbers on a voip call but you have to be a registered user to use it:
You could, for example, try using a larger sample, thus reducing the number of packets thus reducing ip overhead, plus maybe some rtp header compression(i guess that might be possible across a vpn tunnel)?
I tried a couple of calculations for a single g711 call across ethernet -
basic no-frills 160 byte/packet call:
Voice Packets Per Second 50(Codec Bit Rate / Voice Payload Size)
Bandwidth Per Call (RTP Only) 87.2 kbps ( Total Packet Size(bits) )* ( Packets Per Second )
5% Additional Overhead 4.36 kbps
(5% additional overhead per call to accomodate bandwidth for signaling (for example: RTCP/H225/H245 messages on H.323 networks).)
Bandwith Per Call + 5.0% Additional Overhead:
91.56 kbps Overhead + Bandwidth Per call
Then here's the same call with 240 byte packets:
Voice Packets Per Second 33.333 (Codec Bit Rate / Voice Payload Size)
Bandwidth Per Call (RTP Only) 79.467 kbps ( Total Packet Size(bits) )* ( Packets Per Second )
5% Additional Overhead 3.973 kbps 5% additional overhead per call to accomodate bandwidth for signaling (for example: RTCP/H225/H245 messages on H.323 networks).
Bandwith Per Call + 5.0% Additional Overhead 83.44 kbps Overhead + Bandwidth Per call
See if you can find out the overhead on the particular flavor of vpn you're using, and see if you can play with any of the voip-specific stuff to tune the bandwidth requirements. Use g729, if possible - it gives excellent fidelity for the low bandwidth usage, something like 26K per call.
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