Battery power support today [telecom]

The Bell Labs history 1925-75 talks about concentrators. In brief, the 1A provided service to 50 or 100 lines over 10 or 20 trunks with crossbar links. Starting in 1961 4,300 units were built, but no more than half were in use at any one time. They were primarily used to defer installing new cable. The LSS was developed in 1977 providing for 96 lines on 32 trunks using graded multiple concepts. I think these units allowed subscribers to connect at the concentrator level instead of tying up two trunks all the way to the central office; that is, if two neighbors wanted to talk to each other, the concentrator (along with CO logic control), handled the call.

One anonymous critic of the Bell System wrote that being served by a concentrated line was a service disaster. Whether that was true I don't know. But the Bell System did get hit with a service crisis in various parts of the country in the 1970s, partly because service demands outpaced capacity. (There were other factors, too). That is, new communities were built faster than adequate trunks and switching could be placed to serve them, and people used their telephones more. Some more affluent families obtained second lines for their kids. A physical plant engineered and built for 1950s usage won't do well in the 1970s.

Today of course swicthgear is far, far cheaper in price so there is more capacity.