I believe many of the issues have been addressed. The key is network
connectivity and availability. When determining network connectivity you
need to be able to subtract one from the number of paths and still have a
positive number. One circuit does not make a network. Maybe two. Next is
calculating reliability. First start with what you want it to be. Now ask
your network providers or equipment suppliers to provide these numbers. If
the question cannot be answered with a number. You need to be careful about
using this vendor or service provider. Now, as far a protocol of choice,
are we talking a replacement for an old "Fire Bar" or bridge, or are we
talking a PSAP. If we are talking a PSAP, the network provider may make
this a real easy exercise in that you must be able to talk to the E911
tandem. Keep in mind that due to liability issues, in the past, audible
ringing was returned from the PSAP. I expect that this is still the case.
When people started suing when no one responded to the emergency, or
answered the call. The phone company attorneys asked if the person heard
audible ringing, if the answer was yes the telephone network had done its
job. Sue the PSAP provider.
One final thing to keep in mind is that when you buy the circuits from the
telephone company or the 911 network provider, there is usually a special
way of tracking all the circuits so that where possible no two circuits wind
up in the same cable. Further, as the carrier or provider evolve their
network, they maintain the physical separation of redundant circuits. Of
course, if you want something done right, periodically auditing how the
facility providers provide the circuit should be done. This may be like
pushing a rope, but this information should be available.
