The information presented so far may be distilled into a few brief statements:
A data communication network is a group of two or more devices connected by a common, shared
medium.
These devices have an agreed-upon set of rules, usually called the Media Access Control, or
MAC, that govern how the media is shared.
Each and every device has an identifier, and each identifier is unique to only one device.
Using these identifiers, the devices communicate by encapsulating the data they need to send
within a virtual envelope called a frame.
So here's this wonderful resource-sharing tool called a LAN. It's so wonderful, in fact, that everyone
wants to be connected to it. And herein is the rub. As a LAN grows, new problems present themselves.
The first problem is one of physical distance. Figure 1.4 shows that three factors can influence an
electrical signal. These factors may decrease or eliminate any intelligence the signal represents:
Figure 1.4. Attenuation, interference, and distortion prevent a signal from arriving in the same shape it was
in when it left. Attenuation (a) is a function of the resistance of the wire. A certain amount of signal energy
must be spent "pushing past" the resistance. Interference (b) is a function of outside influences—noise—
which adds characteristics to the signal that should not be there. Distortion (c) is a function of the wire
impeding different frequency components of the signal in different ways.
Attenuation
Interference
Distortion
As the distance the signal must travel down the wire increases, so do the degrading effects of these three
factors. Photonic pulses traveling along an optical fiber are much less susceptible to interference but will
still succumb to attenuation and distortion.
Repeaters are added to the wire at certain intervals to alleviate the difficulties associated with excessive
distance. A repeater is placed on the media some distance from the signal source but still near enough to
be able to correctly interpret the signal (see Figure 1.5). It then repeats the signal by producing a new,
clean copy of the old degraded signal. Hence, the name repeater.
Figure 1.5. By placing a repeater in the link at a distance where the original signal can still be recognized,
despite the effects of attenuation, interference, and distortion, a fresh signal can be generated and the
length of the wire extended.
A repeater may be thought of as part of the physical medium. It has no real intelligence, but merely
regenerates a signal; a digital repeater is sometimes facetiously called a "bit spitter" for this reason.
The second problem associated with growing LANs is congestion. Repeaters are added to extend the
distance of the wire and to add devices; however, the fundamental reason for having a LAN is to share
resources. When a too-large population tries to share limited resources, the rules of polite behavior begin
to be violated and conflicts erupt. Among humans, poverty, crime, and warfare may result. On Ethernet
networks, collisions deplete the available bandwidth. On Token Ring and FDDI networks, the token
rotation time and timing jitter may become prohibitively high.
Drawing boundaries between populations of LAN devices is a solution to overcrowding. This task is
accomplished by the use of bridges.[6]
[6] If you cut through the marketing hype surrounding modern Ethernet and Token Ring switches, you'll find that these very useful tools are
merely high-performance bridges.
Figure 1.6 shows the most common type of bridge: a transparent bridge. It performs three simple
functions: learning, forwarding, and filtering. It is transparent in that end stations have no knowledge of
the bridge.
Figure 1.6. The transparent bridge segments network devices into manageable populations. A bridging
table tracks the members of each population and manages communication between the populations.
The bridge learns by listening promiscuously on all its ports. That is, every time a station transmits a
frame, the bridge examines the source identifier of the frame. It then records the identifier in a bridging
table, along with the port on which it was heard. The bridge therefore learns which stations are out port 1,
which are out port 2, and so on.
In Figure 1.6, the bridge uses the information in its bridging table to forward frames when a member of
one population—say, a station out port 1—wants to send a frame to a member of another population: a
station out port 2.
A bridge that only learns and forwards would have no use. The real utility of a bridge is in the third
function, filtering. Figure 1.6 shows that if a station out port 2 sends a frame to another station out port 2,
the bridge will examine the frame. The bridge consults its bridging table and sees that the destination
device is out the same port on which the frame was received and will not forward the frame. The frame is
filtered.
Bridges enable the addition of far more devices to a network than would be possible if all the devices
were in a single population, contending for the same bandwidth. Filtering means that only frames that 20