Concentrators
The main function of a concentrator is to serve as a
termination point for cable running from individual
nodes in a network. The cable connects to the network
or to another wire center.
A concentrator may have multiple boards or boxes
mounted on a rack. Each board is essentially a hub, a
wiring center for a single network’s nodes. Such boards
generally include light-emitting diodes (LEDs) to
indicate the status of each port on the board.
Hubs
A hub is a box with a number of connectors to which
multiple nodes (PCs) are attached. It serves as a
common termination point that can relay signals along
the appropriate paths. All hubs provide connectivity,
and some even provide management capabilities. A hub
usually connects nodes that have a common
architecture. Although the boundary between
concentrators and hubs is not always clear, hubs are
generally simpler and cheaper than concentrators.
Modems
In module 2, we introduced you to modems and
how they are used in a data communications
environment. They translate data from digital to analog
form at the sending end of the communications path and
from analog to digital at the receiving end. From a
conceptual standpoint, this explanation is sufficient.
However, if you are going to install a modem, you need
MODEMS AT WORK.— Put simply, the object of
a modem is to change the characteristics of a simple sine
wave, referred to as a carrier signal. We know this
carrier signal has several properties that can be altered
to represent data. It has amplitude (height); it has
frequency (a unit of time); and it has phase (a relative
starting point). Modems are capable of altering one or
more of these characteristics to represent data.
The job a modem performs can be divided into two
discrete parts or phases at each end of the
communications link. At the sending end, it converts
digital bit streams (strings of 0’s and 1’s) into analog sine
waves. This is the encoding process. Another
component within the modem then changes
(modulates) the analog signal so the data may be
transmitted simultaneously with other data and voice
traffic that has also been modulated. This process is
basically reversed at the receiving end. There, the
analog signal is brought back to its basic level
(demodulated), and the analog sine waves are
reconverted (decoded) back into their corresponding bit
streams (see figure 2-3).
CODECs.— In today’s digital communications
lines, voice traffic is considered the outsider that digital
data used to be to analog lines. Voice can enter the data
communications lines only after being encoded into
digital form. It then must be decoded to be audible again
at the receiving end. The device used to perform the
encoding and decoding functions is known as a codec.
This is simply another black box conversion device that
has always been in existence in a slightly different form
to know some of the technical aspects of modems.
as part of a modem.
Figure 2-3.—Digital data as it is encoded, modulated, transmitted, demodulated, and decoded.
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