It's more or less random, and depends on the particular carrier's prefernce for numebring schemes. For example, MCI/Worldcom has a different numbering schme for its major switches than AT&T or Sprint.
It doesn't work, really, and is actually becoming less and less relevant as the network topology flattens. The New Orleans 4ESS is probably among the last of its breed, and I remember hearing that AT&T removed a lot fo them from service long ago as they shifted towards a decentralized network.
No, those are MCI offices, and their equivalent of class 4 switches. (MCIWorldcom uses/used the NAA labeling system for their class 4 switches, i.e. "2BM" "2CU" "2CX" etc.) MCI is probably the last major carrier to have a hierarchical structure in place, as it's widely known that during the big telecom bubble that they helped create and then burst, they were too busy cooking their books to actually invest in restructuring and upgrading their network, and instead merely spent only the money they needed to keep it running at a semi-decent level. That practice continues to this day, pending the buyout of MCI by Verizon, at which time it becomes Verizon's headache.
Only in vestigial references. The Number 5 ESS is still around and its reference is still used when operated as a class 5 end office, but then you'll find lots of 5ESS (the model number, but not acting as the class) performing tasks that resemble what a class 4 switch would do.
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