Chapter 6 - OSI Model
Have you ever heard of the OSI model?
Up to this point, we've mostly talked about layer one, the Physical layer. The phone lines which carry bits that go high and low are Physical layer. Serial lines that carry bits that go high and low are Physical layer. It's how you arrange those bits that you get to the second layer, Data Link. Technically, the Baudot code and UART codes are primitive second layer Data Link codes, but it gets much more involved than that.
OSI Model Layer 2: Data Link
So far, we've been using serial to send raw characters back and forth over a wire. That's data, but with just two computers you only have a loop not a link. Linking multiple computers together is not as straightforward as you might hope, and early computer networks had wild link designs. One of the most popular before Ethernet came along was called token ring.
A serial line makes it easy for two computers to talk to each other, but if you add a third computer it gets much more complicated. The naive solution is to wire a transmit and receive serial line between every pairing of computers so that everyone can talk to each other directly, but that scales very poorly. Token ring was one solution for how to link together multiple computers without the madness of direct wiring. The general premise was that each computer in the network had two lines that would carry serial data. One of those lines would go to the previous computer in the ring, and the other line would go into the next computer in the ring.
Token ring network architecture used a virtual talking stick, called the token, which was a virtual signal that was passed around the ring. Whoever had the token was allowed to send a message, which would get passed along from computer to computer in the forward direction until it reached the destination computer. Once the token holder was done talking, it would signal to give the token to the next computer in the loop.
This architecture was popular since it was easy to conceptualize, but it was hated because if any one segment the loop was broken the entire network stopped working. The joke has been made from time to time about people "dropping the token". It's funny because it's something that really happened and made everyone upset.