Re: phone numbers [telecom]

The proportion of telephone lines served by manual, SxS, panel, crossbar, and ESS varied over time.

The Bell System Engineering Textbook gives a breakdown, but I don't have it handy. However, I believe in the 1970s SxS served roughly 45% of the lines, the rest shared between panel and crossbar, with ESS growing and a few manual exchanges.

The Bell System served many small towns and rural areas in addition to big cities. Before the war SxS was the way to go and continued to be into the 1970s, when the number of SxS lines peaked in 1974. Also, in cities No. 1 crossbar, developed in the late 1930s, was generally used instead of panel for new installations.

Bell System literature focuses heavily on panel and crossbar as they required sophisticated designs, but SxS was a major player. Indeed, in the 1970s Bell Labs worked on add-ons to extend the service life and functionality of SxS. Ironically, SxS could and was used for Centrex service in addition to No. 5xbar.

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>>> Toll alerting is a remnant of dialing plans associated with crossbar >>> or panel. >> >> No, it's a PUC mandate. > > Actually, it's an artifact of the way that toll dialing on SxS > exchanges worked, subsequently elevated - cargo cult style - into > Holy Regulatory Writ. > > Bell was mostly panel which didn't need 1+, independents were mostly > SxS which did. That's why states where most people lived in Bell > areas, e.g., New York, Illinois, and California, never had toll > alerting.

By land area, most of the country was/is independent. But, by population, most of the country was/is Bell, regardless of state. Yet some states mandated toll alerting while other states did not.

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Stephen Sprunk

Panel peaked in 1958 at 3,830,000 lines. Step peaked in 1973 at 24,440,000 lines, more than any other switching system. No. 5 surpassed step in 1974 in number of lines.

Also, note that for the small community dial office, there were 3,500 of them serving 4,000,000 lines. The book notes that a common-control swtich was uneconomical for small exchanges for years.

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