Example Rule for Sending the Bits
When Fred's computer was using Wilma's computer (the file
server) in Chapter 2, the network had
to take the file from the hard disk at the server, move it across the network,
and deliver it to the word processor on Fred's computer. Before the file server
could transmit the contents of the file across the network, it had to read the
file from the disk. Then it had to transmit the bits over the network.
Any time electricity goes over a wire, you can put a device on
the end of the wire that can sense different things about the electricity. One
thing that can be sensed is voltage. Voltage refers to the
amount of electrical power running over the wire. In networking, to send bits,
the sender can change the voltage to one level to mean a binary 0, and another
to mean a binary 1.
For example, to transmit the bits, the NIC inside Wilma's
computer sends some electrical signal over the cable. The device on the other
end of the cableFred's PC NIC in this casethen interprets the incoming
electrical signal. For this to work, both NICs must agree to some standard means
of transmission. For instance, imagine that Wilma's NIC sends a +5 volts
electrical signal to transmit a binary 0 and +10 volts to send a binary 1. If
Fred's NIC expects to receive a +2 volt signal for binary 0 and +4 volts for
binary 1, the network will not work because Fred will not understand what Wilma
is sending him.
The physical transmission of bits can be a lot more complicated
than this example, but the basic idea remains the same. The sending device puts
some electricity on the cable, which is interpreted as binary numbers by the
device at the other end of the cable. The term encoding refers to the general process of taking
a binary value and generating the correct electrical signal to be able to
transmit the bits across the network. The encoding rules used to transmit data
are defined by networking standards.
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