Om Malik reported on the outages of Vonage and Lingo last weekend, and also a similar instance last year with AT&T's CallVantage service.  He wonders what the impact will be in terms of customer satisfaction, once users realize that current VoIP services will be hard pressed to deliver a service that matches the 99.999% reliability of a carrier. 

While the above mentioned service outages make one ponder on the availability of VoIP, I wonder whether the bulk of the current VoIP users really have such high expectations about the reliability of the service.  Or, even more importantly if they are able to live with less than 99%+ reliability.  The question is: is that carrier-grade reliability necessary for the current early adopters of VoIP?  Particularly when considering that with the advent of mobile communications, the end-user tolerance for noisy lines, dropped calls, etc. has gone way up.

Columbia University Computer Science Professor Henning Schulzrinne has been determining the availability of an average VoIP call as early as April 2003.  One of his findings is that when the Internet is used as the transport network, net VoIP service availability is approximately 98 percent.

Recall that VoIP service availability is defined as the likelihood of a VoIP call being successfully established on the caller’s first attempt. To put the 98 percent number in context, the PSTN runs at an availability rate of 99.99% to 99.999%, whereas mobile networks in the UK run at a rate of 97 to 98 percent service availability (according to a 2002 survey ran by Ofcom, the Office of Communications, which is the is the regulator for the UK telecom industry).

Professor Schulzrinne's findings show that the early adopters of VoIP services relying on the Internet for transport are already used to living with less than even two nines worth of availability.  So how important really is the 99.999% figure?  Perhaps all that is required for the VoIP SPs using their own networks is sufficient CAPEX to fine tune these networks to offer just enough availability.  And how much is "just enough"?  I would say somewhere in the middle between the availability of wireless and POTS services.  It would be interesting exercise to estimate how much CAPEX each 9 is worth...