The graph shows that the voltage level on the wire changes
continuously over time. Because the voltage changes continuously, the signal is
considered to be an analog electrical signal. You
might recall that Chapter 4, "How to
Build a Local (Network) Roadway," covered how Ethernet uses a digital electrical
signal to transmit data, with the digital signal having a discrete value for a
time period, and then changing to another discrete value so that the graph shows
a bunch of right angles. Digital transmission is convenient for transmitting
data because when you transmit data, it's already in digits0s and 1swhich are
also discrete values.
The telcos created the local loop to support voice; that's why
the loop uses analog electrical signals. The sounds that your voice makes happen
to be a continuously changing sound wave, so analog electrical signals that
continuously change work well for voice. In short, the word analog refers to a continuously changing signal,
and the term digital refers to a signal
that has discrete or exact states that imply a particular value, or digit.
The analog signal has a couple of characteristics that I'll
define briefly, and then we'll talk about how to transmit data using such a
signal:
-
Frequency Defined as how the
number of times the signal repeats itself, from peak to peak, in one second
(assuming that the sound the human makes doesn't change for a whole second). Figure 16-2 shows a frequency of 3 Hertz
(Hz), or 3 per second. The greater the frequency of the electrical signal, the
higher the pitch of the sound being represented.
-
Amplitude Represents how
strong the signal is; a higher amplitude peak represents a louder sound.
The goal of the original telco was to create a circuit between
any two phones. Each circuit consisted of an electrical path between two phones,
which in turn supported the sending of an analog electrical signal in each
direction, allowing the people on the circuit to have a conversation. Remember:
The original telco predated the first vacuum tube computers, so the concept of
support data communication between computers wasn't a consideration for the
original telco. The original telco just wanted to get these analog electrical
signals, which represented sounds, from one place to the other.
Sections
Comments (0 posted)
Syndication
