How was Tymnet different from the Internet?
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When we think about “the Internet” today, we are referring to three “levels” of connectivity: a physical link (your wifi or cell data connection), a virtual link (routers, particularly the “backbone” routers interconnecting all of the endpoints throughout the whole Internet in an arbitrary mesh) and a software (or *application*) connection, which is usually HTTP/HTML, aka the World Wide Web, between the server host and (all of) the client browser/app(s).
Tymnet (and many many similar systems) was a system of *terminal servers* accessible through standard phone lines. It acted as a physical transmission system, interconnecting computers (“real” computers, mainframes and “mini-computers” like the PDP-10s which were the OG Unix hosts) to “dumb terminals” (or terminal emulation software running on microcomputers). A dumb terminal was just an IO (input/output) device, originally a teletype and later a CRT and keyboard, which had no real computing power of its own. The *terminal server* was a piece of equipment that connected to the host (computer) as if it were a terminal, or a bunch of terminals, and could be “dialed up” from a remote terminal, allowing the remote terminal to connect to the host, with neither the host or terminal being ‘aware’ the terminal server/Tymnet system was involved once the link was set up. These legacy systems used *switching* rather than *routing*; the distinction is esoteric for our purposes, you just need to know they are different.
In comparison to the Internet, all of Tymnet (or any other system of terminal servers) was the equivalent of a single physical link, “one hop” between routers, just like a single Ethernet local area network (LAN, what in today’s system would equate to a WIFI system). So the Internet could be used through Tymnet, with all of the packets (and the datagrams carried by those packets) being just text characters being transmitted between a computer (the server) and a terminal (the client) as far as Tymnet was concerned. Or, likewise, the Internet could be used to access Tymnet (through an Internet server or client emulating a host or terminal) to connect to legacy computer systems that didn’t do Internet routing themselves.
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