Data communications over telegraph circuits 1916
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about the samme time we were looking at the previous example
we were doing ha-cmp
and we spent some time with a telco organization that had a five-nines operation. they were currently had a system with triple redundant hardware connected to a SS7 with replicated T1 links.
Anne & Lynn Wheeler Sometimes the introduction of modern technology leads to a loss of backup capability and failures become more serious...
the problem was that any (scheduled) software system maint. for the box consumed close to ten years worth of allowed down time.
replicated high availability system addressed the system down time issue related to doing software maint. ... and while the individual hardware boxes might not quite have five-nines ... the SS7 logic already would redrive request on the 2nd T1 if it didn't get an answer on the first T1. The SS7 redrive logic would mask any individual outage ... and while an individual backend box might not be quite five-nines ... the overall infrastructure (becuase of the SS7 redrive logic) would be much better than five-nines.
Morten Reistad These are excellent points and many technical people don't understand them. Every computer program should have appropriate (depending on criticality) failure exits, even for things that are never supposed to happen...
the solution by the replicated hardware group ... was to install a pair of replicated hardware boxes ... in order to meet the overall system five-nines objective.
however, once resorting to a pair of backend boxes to handle the overall system outage issue ... then it was no longer necessary to have the individual boxes be done with replicated hardware ... aka a high availability backend configuration (relying on the SS7 fault masking redrive) made it redundant and superfluous for having redundant hardware for the individual computers.
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