Replacing PSTN with VoIP wise? Was Re: Phone networks struggle in Hurricane Katrina's wake

Wed Aug 31 10:27:20 UTC 2005

On 31-aug-2005, at 10:04, Mark Foster wrote:

> I do see a risk in the move toward IP systems at the edge. At the  
> core is a different story to at least some degree.
> Twas also pointed out that British Telecom are heading down the  
> same track as Telecom NZ, and their rollout should be completed  
> earlier.  I trust therefore that it has all been thought out in  
> terms of robustness and the like.

There are two types of VoIP: voice over a private, tightly controlled  
IP network, and voice over the public internet. Now obviously the  
latter is a risky proposition, as it imports all the limitations of  
the internet into the voice service. Apart from the fact that many  
parts of the internet aren't all that robust (but some are), this is  
a problem because voice and IP react differently to congestion  
collapse, which invariably happens to some degree in big emergencies.  
With IP, delays and packet loss build up, slowing everything down,  
but allowing many protocols to continue to work at a reduced rate.  
With PSTN, initiating calls starts failing more and more, but when  
you get through, you generally get to talk because you get a reserved  
piece of the scarce bandwidth. With VoIP, packet loss and delay  
eventually make the service useless. So VoIP fails harder than either  
traditional IP apps and PSTN.

However, voice over a private network isn't entirely trouble-free,  
even though the private network can be designed such that congestion  
is a less fatal problem. And it does have the advantage that it  
allows IP routing protocols to route ongoing calls around failed  
parts of the network. On the other hand, in a circuit switched  
network you can do all kinds of interesting stuff (such as restarting  
all your control software) without breaking your sessions. We're only  
now seeing this in IP, and I think it's not really possible to reach  
the same levels with IP routing even in the long run. And then there  
is all this SIP stuff, which I'm (thankfully) only superficially  
familiar with, but never seemed particular robust to me.

And voice over any kind of packet infrastructure introduces  
significant additional delays.

I think in 10 years or so we'll realize that TDM isn't so bad after all.