Surveillance or Security?: The Risks Posed by New Wiretapping Technologies Read Online Free PDF

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.13
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.14 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 path15 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.
    2.2 Creating the Internet
    In the 1960s, physicists had realized that the electromagnetic pulse from
a high-altitude nuclear explosion would disrupt, and quite possibly destroy,
electrical systems in a large area. Any centralized communication network
such as the phone system that the United States (and the rest of the world)
used would be in trouble." RAND researcher Paul Baran went to work on
this problem.
    Since the frequency of AM radio stations would not be disrupted by the
blast, Baran realized the stations could be used to relay messages. He implemented this using a dozen radio stations." Meanwhile, using digital networks, Donald Davies of the United Kingdom's National Physical Laboratory
found another solution to the problem. Davies solved Baran's problem
while trying to address a completely different question.
    Davies was interested in transmitting large data files across networked
computers.18 The problem is different from voice communications. Data
traffic is bursty: lots of data for a short time, then nothing, then lots again.
Dedicating a telephone circuit to a data transfer did not make a lot of sense;
the line would just not be used to its full extent19 and, unlike with voice,
a small delay is not a major issue in transmitting data files.
    Both Baran and Davies hit upon the same solution. Redundancy in the
network-multiple distinct ways of going between the sender and the
recipient-was key.