Data communications over telegraph circuits 1890

Things evolved over time. Until about 1950 they had a lot of duplexed ground-return single wire circuits that were time-division multiplexed and ran Baudot. Then they moved into frequency division multiplex, which required voice-grade circuits but also was compatible with the microwave system they were putting in. And local loops might be their own or leased from the phone company but were basically voice-grade lines. There were the undersea cables which being so costly merited elaborate multiplexing and regeneration facilities. They also had varioplex, in some ways a forerunner to packet switching. In varioplex got to use a channel for a while and then it was switched to another user after an arbitrary number of characters. Real packet switching of course requires computers with substantial memory to deal with fragmented messages and packets arriving out of sequence and retransmission of errored packets.

The public message network, used for ordinary telegrams, evolved from Morse to teleprinter store and forward, with messages being retyped at each relay point. Then they got into reperforators so messages didn't have to be retyped for relaying; but the layout of the system was rather ad hoc. To send a message to Peoria you had to know to send it to Chicago (or maybe Indianapolis) and an operator there knew how to send it on to Peoria. Then in 1949 they went to Plan 21-A which provided for fifteen switching centers and automatic switching from the center of origin to the center of destination. Operators at the destination centers would semi-automatically route messages to the nearest destination office. This couldn't be done automatically from the originating office because destination routing was too complicated. And then there were Desk-Fax and teleprinter tielines between W.U. offices and customer offices. Some of it they built themselves, and some they had contract manufacturers build. See the W.U.T.R. articles on the Chattanooga Works. There was a time when they built their own teleprinters, the 100-series. I don't know if they did this trying to save money, or if they objected to letting the Bell System profit through Teltype from their purchases. Other times they built to their own designs because Teletype didn't make just what they wanted. When Telex came in they bought some teleprinters from Siemens, and all the switching equipment from Siemens, because they were basically using the German Telex network as their model. I don't know if other people are interested in this discussion, so maybe we should take it to private email.

jhhaynes at earthlink dot net --

jhhaynes at earthlink dot net