The History and Technology of Landline Telephones
I have three telephones on my desk: a slim beige push-button model with redial and speed-dial buttons purchased in the late 1990s, a squat black 1950s “Modern Telephone with letters along the dial and a real bell inside, and a smartphone. One phone is for my work line, one for my home line, and one for travel. Yet despite the diversity of devices and the half century that separates their manufacture, all work over the same network, commonly called the Public Switched Telephone Network or PSTN. While everyone knows that Alexander Graham Bell invented the telephone, his more important work was the development of the network.
A telephone by itself is not worth much; its value lies in its ability to connect its user with others. The Internet’s broad functionality stretched the meaning of communications network. I would like to outline here the history and technology of the Landline telephone with two purposes in mind: explaining how it is that the network supports such a broad set of applications, and developing an understanding of why securing the network is so hard.
The Telephone Network
The first device to rely on a networked system was the telegraph. The telegraph functioned very differently from the telephone. For one thing, the telegraph was not for use by unskilled people; only experts (who knew Morse code among other things) could use the system. As a result, although telegraphs were quickly taken up by businesses and other institutions, they were not for home use. Nonetheless the networks for the two communications systems are similar. Both are also similar to a network with a completely different purpose: the railroad. Such a similarity should not be surprising; the telegraph was not only modeled on the railroad, in many parts of the world early telegraph networks and railway systems were inseparable.
Telegraph wires traveled along railroad rights of way, railroad stations served as telegraph offices, and the telegraph was used to let stations up the line know when the train would be in. Telegraph networks were “decentralized”: networks with hubs or clusters and with some, but limited, connectivity between the hubs. Decentralized networks look like railroad connections between major cities and the suburbs. There are railway connections between a city and its suburbs, and between the cities, but typically there are no direct connections between one city’s suburbs and another’s.
Initially networks were quite local. The subscriber would ring the local switch and tell the operator the name of the party with whom they wanted to speak. The first switches were manual, consisting of panels with jacks and cables between them. (Remember scenes from old black and white films featuring women inside telephone exchanges switching cables!) The operator would ring that party and then connect the two lines on the switchboard via patch cords. While the original operators were teenage boys, their antics soon made clear that more responsible people were needed, and young women became the telephone operators of choice across the world.
A undertaker from the American State of Missouri designed the first automated telephone switch.”We tend to think of a phone number as the name of the phone at a particular location, but it is actually something else entirely. As Van Jacobson, one of the early designers of Internet protocols, once put it, “A phone number is not the name of your mom’s phone; it’s a program for the end-office switch fabric to build a path to the destination line card.
Centralized versus decentralized networks.
For example, take our office board line number: 022-401-09656. The first three digits-the area code-establish the general area of the phone number; in this case it is Mumbai. The next three digits, normally called the telephone exchange, represent a smaller geographic area. In our example the last four digits are, indeed, the local exchange’s name for the phone. Taken as a whole, the set of ten digits constitute a route description; the switching equipment within the network interprets that information much like a program and uses it to form a connection
The first thing a modern telephone-and I will start by describing just landline phones-must do is signal that it is “off hook” and thus ready to make a call. This happens when the receiver is lifted, which closes a circuit, creating a dial tone and signaling the central office (the local phone exchange). Then the subscriber can dial the phone number she wishes to reach (“dial,” of course, being an anachronism from the era of rotary telephones). When the central office receives this number, its job is to determine where to route the call. If the call is local-that is, within the same area code-then the switches at the central office need to determine which trunk line, or communication channel, should be used to route the call to an appropriate intermediate telephone exchange. This new exchange repeats the process, but this time connects to the recipient’s local exchange. Since the first three digits denote the local exchange and are thus unnecessary, only the last four digits of the number are transmitted. The local exchange determines if the recipient’s line is free; if so, it “rings” the line. If the recipient answers, her receiver closes a circuit to the local exchange, which establishes the call.
The speakers have a fixed circuit for the call, the one that was created during the call setup.This is, of course, a simplified example: the call did not use an area code, let alone an international code. The other simplification is that the call described above had only two “hops”- that is, it only went through two telephone exchanges.The key goal of the network design was to provide quality of voice service. Engineers needed to factor in that each time a call goes through an exchange, it needs to use a repeater to amplify the voice signal. Passing through a repeater causes the signal to change slightly. Thus the network needed to minimize the number of times a call would go through an exchange.
The telephone company limits calls to five hops, after which it deems the degradation in voice quality unacceptable. Digital signals do not face this problem and thus can travel through an arbitrary number of repeaters. This small engineering difference leads to a remarkable freedom in system design. Messages can traverse an arbitrarily long path to reach a destination, enabling a more robust network. The telephone system is built from highly reliable components. The telephone company believed in service that allowed a user’s calls to go through ninety-nine times out of a hundred. Since central office switches served ten thousand lines, this meant “five 9s” reliability (otherwise the 1 percent blocking could not be satisfied). Of course, more than central office switches are needed to service a call that travels between two destinations with different central offices.
At the height of the Cold War, some engineers began thinking about reliability differently. After all, you might care less about talking to a particular person at a particular moment than about getting the message through eventually. That is, presuming the other party is in and willing to answer the phone, you might not be concerned about always being able to connect each time you dialed, but you might want to ensure that the message you are attempting to send eventually gets through. This was the problem that the designers of the Internet tried to solve.
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