|
North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: VoIP over IPsec
In message <[email protected]>, "Stephen Sprunk" writes: > >Thus spake "Charles Youse" <[email protected]> >> In order to cut costs in our telecom budget I'm toying with the idea >> of replacing a lot of our inter-office leased lines with VPN >> connections over the public Internet. [...] >> Assume for the moment that latency and bandwidth are not an issue; >> e.g., any two points that will be exchanging voice data will both have >> transit from the same provider with an aggressive SLA. > >Latency, bandwidth, and packet loss are moot. Jitter is VoIP's enemy. > While jitter is more important, you can't ignore the others. The traditional phone network has long had a delay budget -- each component is supposed to bound how long it takes to process voice. Bandwidth matters, too, because the serialization delay for a packet increases the latency at that hop. This causes problems for compression and IPsec. You can get better compression -- and hence reduce bandwidth demands -- by compressing longer samples. Of course, this means that you can't finish the compression until the end of that time interval, which increases the latency. The obvious solution is to compress shorter segments, but that means that the IPsec overhead is a significant portion of the bandwidth consumed, which again has latency implications for bandwidth-limited channels. I'm not saying you can't do this; I am saying that under certain circumstances, there may be issues. Latency, for example, is a psychoacoustic phenomena (did you enjoy the last call you made over a satellite circuit? I didn't.) And yes, I had to crunch the numbers on this for my day job a few years ago. --Steve Bellovin, http://www.research.att.com/~smb (me) http://www.wilyhacker.com (2nd edition of "Firewalls" book)
|