|Our First Analogy
For our first analogy, visualize a rope stretched tightly between two
springs. Electrical engineers have known for a long time that pulses
traveling on wires behave a lot like waves traveling on springs or
ropes. So if you can visualize a rope stretched between two springs, as
illustrated in this picture, there’s really no need for you to worry
about the electrons or the magnetic waves or the molecular structure and
physics of a cable. We’ll leave those secrets and those details to the
engineers, and we’ll just talk about the concepts here.
If you grasped the rope as shown in this illustration, you could feel
any vibrations that happen to be passing through it. Or you could
generate some vibrations or pulses of your own- by tugging on it,
pulling to the left, or to the right. If somebody else placed their hand
on the other end of the rope, they could feel the motion that you impart
to it when you tug it to the left or to the right. The two of you could
agree on the way you communicate the most basic bits of information. For
example, you might agree that if you tug the rope to the right you are
sending a message that says, “One,” whereas if you tug the rope to the
left, you’re sending a message that says, “Zero.” In both cases you
could just relax the rope, and let it return to its center point between
Observe that in this diagram, there are only two of you sharing the
rope. We call this a “Point to Point” connection, or a “Point to Point”
network. In a Point to Point network, the recipient of every message is
obvious. When one party on a Point to Point network transmits a message,
it’s obvious that that message must be destined for the other party, so
there’s no need for the message to contain any address information.
But now, suppose that a third person joined the network by placing his
hand on the rope, as illustrated here...
Now when you send a message, it is no longer obvious which of the other
two parties is the intended recipient.