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Transmitting Data Using Twisted Pairs

While it is true that Ethernet sends data over UTP cables, the physical means to send the
data uses electricity that flows over the wires inside the UTP cable. To better understand
how Ethernet sends data using electricity, break the idea down into two parts: how to create
an electrical circuit and then how to make that electrical signal communicate 1s and 0s.

First, to create one electrical circuit, Ethernet defines how to use the two wires inside a
single twisted pair of wires, as shown in Figure 2-5. The figure does not show a UTP cable
between two nodes, but instead shows two individual wires that are inside the UTP cable.
An electrical circuit requires a complete loop, so the two nodes, using circuitry on their
Ethernet ports, connect the wires in one pair to complete a loop, allowing electricity to flow.
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Figure 2-5 Creating One Electrical Circuit over One Pair to Send in One Direction
To send data, the two devices follow some rules called an encoding scheme. The idea works
a lot like when two people talk using the same language: The speaker says some words in a
particular language, and the listener, because she speaks the same language, can understand
the spoken words. With an encoding scheme, the transmitting node changes the electrical
signal over time, while the other node, the receiver, using the same rules, interprets those
changes as either 0s or 1s. (For example, 10BASE-T uses an encoding scheme that encodes
a binary 0 as a transition from higher voltage to lower voltage during the middle of a
1/10,000,000th-of-a-second interval.)

Note that in an actual UTP cable, the wires will be twisted together, instead of being parallel
as shown in Figure 2-5. The twisting helps solve some important physical transmission issues.
When electrical current passes over any wire, it creates electromagnetic interference (EMI) that interferes with the electrical signals in nearby wires, including the wires in the same
cable. (EMI between wire pairs in the same cable is called crosstalk.) Twisting the wire pairs
together helps cancel out most of the EMI, so most networking physical links that use copper
wires use twisted pairs.

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