When humans talk on the phone, they make a call, talk for a while, and then hang up. Statistically, most of the time, humans are not talking on the phone. At least they weren’t before everyone had smartphones. But computers, including the applications on your smartphone, communicate differently than humans do. Sometimes computers send short messages to check if another computer is available. Computers sometimes send medium sized information like a single picture or a long email message. And sometimes computers send a lot of information like a whole movie or a piece of software to install that might take minutes or even hours to download. So messages between computers can be short, medium, or long. In the earliest days of connecting computers to one another, pairs of computers were connected with wires. The simplest way to send data from one computer to another was to line up the outgoing messages in a queue and send the messages one after another as fast as the computers and the wires could carry the data. Each message would wait for its turn until the messages ahead of it were sent, and then it would get its chance to be sent across the connection.
When the computers were in the same building, the building owner could run wires to connect them. If the computers were in the same town, the owners of the computers generally had to lease wires from the telephone companies to connect their computers. They often would have the phone company connect the wires together in their central office so that it was not necessary for one computer to “dial” the other computer to send data. These leased lines were convenient for computer communications because they were “always on”, but they were also quite expensive because they were used 24 hours a day.
When the computers were even farther away, in different cities, the leased lines were extended using the longer wires connecting the central offices. Since there were so few wires between central offices, these long-distance leased lines were quite expensive and their cost increased dramatically as the length of the leased line increased. But if you had enough money, you could lease direct connections between your computers so they could exchange data. This worked pretty well as long as you were only using one brand of computers, because each computer company had their own way of using telephone wires to connect their computers together and send data.