The Public Switched Telephone Network (PSTN) was designed for high reliability and availability and has achieved an impressive record of service over its history. People today expect continuous telephone service, even in emergencies such as the loss of commercial power or construction cable cuts. The PSTN, however, was not designed for survivability in the face of malicious or catastrophic faults that cause total failure of one of its subscriber-line switching offices. Such a failure could potentially disrupt phone service to thousands of customers for days or weeks, depending on the extent of the damage. Fortunately, modern switching equipment is designed for distribution over a wide geographic area and several subscriber-line architectures are available to address survivability issues. This paper defines the issue of subscriber-line survivability and identifies architectures that improve survivability.
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