General Discussion
In reply to the discussion: Why Cell Phones Went Dead After Hurricane Sandy - horrifying. [View all]FarCenter
(19,429 posts)In the old days, land lines were a twisted copper pair of wires between the customer and the central office. It was powered from the central office by -48 volts DC on one conductor and ground on the other, and it could deliver up to 200 milliamps to the telephone set although the actual current depended on distance and the resistance of the wires.
Thus, an old fashioned telephone connected directly to the phone jack would operate so long as:
- the central office continued to provide DC power (they had batteries and diesel backup, but they could flood, be blown away, run out of fuel, etc.)
- the central office continued to operate (the switch continued to function, everyone didn't go off-hook at once, outgoing trunk circuits to other central offices were available, etc.)
- the cables between the central office and the customer were still intact (they could be ripped down by tree limbs, dug up by backhoes, flooded and insulation impaired, etc.)
Later, to reduce the expense of providing a pair per customer, the carriers introduced electronic systems into the connection between the central office and the customer. These would generally serve 25 or 48 customers over 2 pairs of wire. The electronics could be pole mounted or in controlled environment vaults (manholes). The electronics would be powered locally and backed up by batteries and sometimes generators. These introduced new vulnerabilities that affected service availability.
The latest "wired" systems use fiber to the neighborhood, fiber to the curb, or fiber to the home architectures. These also require local power, with or without backup.
Wireless is even more complicated.
Locally, the best information was available by walking over to the police station or to city hall and reading the bulletins.