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If I have a Linux setup with VPN in Paris and a Linux setup with VPN in London, both with high-speed internet, will packet-switching be slower for VOIP than if I didn't use VPN?

A few people have told me that it will be slower, but I can't find anything to back that claim up.

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high-speed is marketing term for more bandwidth, and doesn't really mean speed, though it can affect speed. Latency equals speed, and latency is usually affected most by the number of hops between points. The VPN will most likely cause a change in route that could either reduce the number of hops or increase the number of hops, which may or may not affect overall latency on the entire path. It is latency that adversely affects VoIP the most. If your VPN tunnel is extremely latent for any reason, that single hop alone could cause problems as well. –  MaQleod Sep 23 '11 at 15:09

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Everything will be a little slower over the VPN, since the connection naturally introduces several more hops to and from a remote location. For example, if you live in London, the VPN server is in USA, and you are connecting to another London-located server, then the packets will have to go overseas and back. Depending on the connection quality, the delay might be quite noticeable.

Other factors are the VPN server load (if it's a public VPN, it might be slow at processing packets), data encryption (very minor, though, as many recent HTTPS studies show), and VPN protocol overhead (resulting in more packets to be sent).

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If the VPN uses encryption, such as SSL or IPsec, then yes the network may be slower as the packets need to be encrypted and decrypted at each end. Whether this will interfere with your ability to carry out VOIP operations depends on the speed of the internet connection, how much bandwidth you give the application and the amount of CPU available on each end.

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With today's CPUs, encryption has very little actual impact on connection speed. The RSA key exchange is what slows down SSL the most -- and VPN protocols perform it much less often than HTTPS, usually only every few hours. The actual data encryption is fast. –  grawity Sep 23 '11 at 14:59
    
Not to mention that changing the route by using a VPN may have other adverse affects - both good and bad. For instance, sending the packets on a VPN may either shorten or lengthen your route, it could also filter you through a firewall that will cause one-way audio. –  MaQleod Sep 23 '11 at 15:00
    
British Telecom, AT&T. Provides millions of Audio traffic over VPN. They are very quality minded companies. Also they provide wholesale/premium quality over VoIP worldwide. Interconnection with such suppliers/vendor is must almost with VPN –  YumYumYum Sep 23 '11 at 18:23

Yes, there will be an increase in the time taken for data packets (of any type) to be processed across the VPN link. This does not mean that the voice will sound slower, just that the audio delay on the link will be milliseconds longer so there will be a bit more 'long distance' delay in the conversation flow. In practical terms, provided the VPN link is solid and reliable, you may not even notice the effect but the only way to find out is to try it.

In my last job, I ran a VoIP (Asterisk) phone system at Head Office, linked to a similar setup in another office about 150 miles away. The inter-site link was via standard 8Mbit ADSL broadband with an IPSec VPN tunnel created between a pair of Draytek 2800-series ADSL routers and there was no significant sound delay on calls made across the link.

I once did a test where I made a call from my desk, though the VPN and back to my colleague opposite me and the speech delay was comparable to calling your desk phone from your mobile and speaking to yourself.

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