hosted PBX/VOIP thru VPN?

Nathan Ward nanog at daork.net
Wed Nov 12 06:01:19 CST 2008


On 13/11/2008, at 12:39 AM, Aaron Wolfe wrote:

> Because the broadband connection was so fast, TCP was able to
> repair the impairments without reducing voice quality. "


That works fine if latency+window size is low, so that segments are  
retransmitted quickly.

You really should also do the math and factor in the latency that  
comes from doing something like this, assuming you lose a packet. G. 
114 recommends an end to end latency of no more than 150ms for voice  
applications, where over 400ms is unacceptable (between 150 and 400  
you should indicate that performance is not ideal).

Finally, some audio codecs work well with fairly high amounts of loss  
- I'd recommend doing something like that first. iLBC does this really  
well. G.729 etc. do not - they rely on context, so a single packet  
lost results in several packets of lost audio (and so, silence). iLBC  
doesn't rely on context, and quality degrades during packet loss  
before you get silence.

The i stands for Internet - so no surprise it works great in typical  
Internet conditions.

--
Nathan Ward





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