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

  • From: Michael.Dillon
  • Date: Wed Aug 31 09:09:14 2005

> With VoIP, packet loss and delay 
> eventually make the service useless. So VoIP fails harder than either 
> traditional IP apps and PSTN.

That is only in theory. In practice, during times of
impending congestion collapse, IP network operators 
reconfigure the network to cope. For instance when
DDoS is detected, people set up ACLs and trigger black 
hole routes. I think that it is possible for network
operators to define an analogous action plan to stave
off congestion collapse in an emergency situation.

I'm not sure exactly what that action plan would look
like, but I'm sure other list members will have plenty 
of good ideas. If you'll recall, just a few days ago
people were talking about how they informally identified
IP connectivity to emergency response sites so that those
sites could be given priority in restoring service.

We just need to sit down and talk these things over with
our local emergency response organizations and learn
where network operators can become part of the solution.

> 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.

MPLS may have the edge here because you can have backup paths
and fast reroute to keep traffic flowing if you have an
orderly plan for rebooting routers.

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

Experience with the Inter-NOC phone system
seems to suggest otherwise. Some kinds of packet
infrastructure only introduce insignificant delays.
It would be interesting to know if any of the academics
among us have studied the behavior of a SIP-based 
VoIP network during various types of failure and 
congestion scenarios. I suspect that problems will
be mostly found under certain specific sets of conditions
and if we know what those conditions are and how they
impact voice services, then we can plan actions to
mitigate the problems. One thing that IP network operators
can do is throw bandwidth at a problem by "shedding load", 
i.e. killing traffic that is deemed non-essential. This
would free bandwidth for traffic that is deemed "important".
This has nothing to do with QoS per se becaus it can be
implemented in many ways up to and including unplugging
sites that generate non-essential traffic.

All indications are that the next few decades will see
an increased number of emergency situation like the 
tsunami, terror attacks in major cities, hurricanes,
earthquakes. We have gotten very good at running the 
network through normal times, maybe we should now focus
on how to keep it running through times of extreme stress.

--Michael Dillon

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