title: Muttering about VoIP


by: Scott Bradner  


What is it about voice over IP (VoIP) that creates such passionate dogmatism?  It seems to me that far too much of what passes for debate about the future of VpIP is dominated by assertions of perfect foreknowledge.  Considering the track records of most prognosticators I will not add to the conclusions as much as sketching some of the factors that will enter into determining the future -- which I do predict will be done in real-time.


Debate about VoIP is dominated by the subject of quality of service (QoS), at least if anyone who has ever worked for a phone company is part of the crowd.  Some have an almost anal fixation on the topic, almost as if they had never heard of cell phones.  The lesson of cell phones, that QoS is not the only factor, should have been internalized by phone people by now but it does not seem to have been.  Too many of them seem to think that no one will use a less-than-perfect phone no matter what other factors might be there.  This did not turn out to be the case with cell phones.  In the case of cell phones the main competing factor was convenience, in the case of VoIP it is cost and will be features.  At this point VoIP is no extra cost -- free service might just cure some selectivity.


What factors do effect the quality of VoIP?  First there is latency.  A round trip from me speaking to your hearing then from your talking to my hearing, needs to be under about 300 ms or we tend to talk over each other.  That is not all that hard to meet in today's networks.  The voice coder/decoders (codecs) can take about 25 ms combined each way so that leaves about 250 ms left for network latency.  As I write this I'm seeing a network round trip time between east & west coasts of 101 ms ( 21 hops) so that would leave 150 ms to spare. 


Another factor is reliability.  Modern codecs can easily deal with 4 to 5% packet loss with no discernable loss of quality.  Since I normally get less than 2% packet loss on the 'Net that should also be fine.  I should get quality at least as good as a normal phone and better than a cell phone.


So what's wrong with this picture?  I'm working late Saturday night - the results might be rather different at 2pm on a weekday.  How about prioritizing the voice data on the Net?  That might just help quite a bit but why should someone other than my own Internet service provider honor the prioritization  - since there is no way for me to pay them to do so?


Then there are the regulators.  VoIP is against the law in more than 40 countries because it takes revenue away from the telephone companies (and taxes away from the governments). 


Not a clear picture but "free" is a factor to remember.


disclaimer:  Harvard does not generally comment of free things and has not in this case.