▶ 11.1 Building Data Networks
Some people believe the global telegraph network was the first internet, but the telephone network was the first to reach into most homes and workplaces. Unlike the telegraph, the telephone became part of the social and cultural fabric of modern life in the mid-20th century, much like the internet has become today.
Because of this, classic elements of telephone technology became familiar social and cultural concepts, much like the word “multitasking” was inherited from computing. These concepts include switchboards, party lines, area codes, and toy “tin can” telephones. Each of these reflect a particular way of organizing a network. Each arose during the 19th and 20th centuries as telephone engineers faced the problem of connecting phones together into a network.
The telephone exchange was the network’s fundamental building block (FIGURE 11.1). Every phone was wired to an exchange, which connected it to other phones. Although phone calls, especially on wired phones, may seem familiar and even trivial, the underlying network has always demanded the highest performance from available technology.
FIGURE 11.1 Switchboards were the first exchanges.
Library of Congress, Prints & Photographs Division, LC-DIG-ggbain-01423.
To make a call, the telephone network establishes a connection between the two telephones. A telephone transmitter converts the speaker’s voice into electrical signals that follow the connection across the network. The receiver at the other end converts the signals into sounds. The signals travel across the network with very little delay. If the network delays the signal too much, people have trouble carrying on a conversation.
Evolution of the Phone Network
It’s easy to connect things together. It’s hard to maintain order while connecting them together. A network’s topology refers to the structure of its connections. The simplest phone system might contain only phones and wires, but larger networks need intermediate devices to make order out of chaos.
By the end of the 20th century, the phone network had tried every major technique for connecting calls. The challenge was to interconnect the telephone exchanges so that calls could be handled quickly and efficiently.
Traditionally, the phone network uses different terms than we use in computer networking. While we talk about hosts on a computer network, we talk about phones in the phone network. What we call nodes on a computer network are called exchanges or “central offices” on the phone network. Links on a computer network are called “lines” or “trunks” on the phone network.