Repeaters and Bridges

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

Data Link Addresses

In a certain community in Colorado, two individuals are named Jeff Doyle. One Jeff Doyle frequently
receives telephone calls for the person with whom he shares a name—so much so that his clever wife has
posted the correct number next to the phone to redirect errant callers to their desired destination. In other
words, because two individuals cannot be uniquely identified, data is occasionally delivered incorrectly
and a process must be implemented to correct the error.
Among family, friends, and associates, a given name is usually sufficient for accurately distinguishing
individuals. However, as this example shows, most names become inaccurate over a larger population. A
more unique identifier, such as a United States Social Security number, is needed to distinguish one
person from every other.
NOTE
Frame
Devices on a LAN must also be uniquely and individually identified or they, like humans sharing the
same name, will receive data not intended for them. When data is to be delivered on aLAN , it is
encapsulated within an entity called a frame, a kind of binary envelope. Think of data encapsulation as
being the digital equivalent of placing a letter inside an envelope, as in Figure 1.1[1] . A destination address
and a return (source) address are written on the outside of the envelope. Without a destination address, the
postal service would have no idea where to deliver the letter. Likewise, when a frame is placed on a data
link, all devices attached to the link "see" the frame; therefore, some mechanism must indicate which
device should pick up the frame and read the enclosed data.

Bicycles with Motors

One of the difficulties of decentralized computing is that it isolates users from one another and from the
data and applications they may need to use in common. When a file is created, how is it shared with Tom,
Dick, and Harriet down the hall? The early solution to this was the storied SneakerNet: Put the file on
floppy disks and hand carry them to the necessary destinations. But what happens when Tom, Dick, and
Harriet modify their copies of the file? How does one ensure that all information in all versions are
synchronized? What if those three coworkers are on different floors or in different buildings or cities?
What if the file needs to be updated several times a day? What if there are not three coworkers, but 300
people? What if all 300 people occasionally need to print a hard copy of some modification they have
made to the file?
The local-area network, or LAN, is a small step back to centralization. LANs are a means of pooling and
sharing resources. Servers enable everyone to access a common copy of a file or a common database; no
more "walkabouts" with floppies, no more worries about inconsistent information. E-mail furnishes a
compromise between phone calls, which require the presence of the recipient, and physical mail service,
which is called snail mail for a good reason. The sharing of printers and modem pools eliminates the need
for expensive, periodically used services on every desk.
Of course, in their infancy, LANs met with more than a little derision from the mainframe manufacturers.
A commonly heard jibe during the early years was, "A LAN is like a bike with a motor, and we don't
make Mopeds!" What a difference a few years and a few billion dollars would make.
Physically, a LAN accomplishes resource pooling among a group of devices by connecting them to a
common, shared medium, or datalink. This medium may be twisted-pair wires (shielded or unshielded),
coaxial cable, optical fiber, infrared light, or whatever. What matters is that all devices attach commonly
to the data link through some sort of network interface.
A shared physical medium is not enough. Rules must govern how the data link is shared. As in any
community, a set of rules is necessary to keep life orderly, to ensure that all parties behave themselves,
and to guarantee that everyone gets a fair share of the available resources. For a local-area network, this
set of rules, or protocol, is generally called a Media Access Control (MAC). The MAC, as the name
implies, dictates how each machine will access and share a given medium.
So far, a LAN has been defined as being a community of devices such as PCs, printers, and servers
coexisting on a common communications medium and following a common protocol that regulates how
they access the medium. But there is one last requirement: As in any community, each individual must be
uniquely identifiable.

Internetworks, Routers, and Addresses

Basic Concepts: Internetworks, Routers, and
Addresses
Bicycles with Motors
Data Link Addresses
Repeaters and Bridges
Routers
Network Addresses
Once upon a time, computing power and data storage were centralized. Mainframes were locked away in
climate-controlled, highly secure rooms, watched over by a priesthood of IS administrators. Contact with
a computer was typically accomplished by bringing a stack of Hollerith cards to the priests, who
interceded on our behalf with the Big Kahuna.
The advent of the minicomputer took the computers out of the IS temple of corporations and universities
and brought them to the departmental level. For a mere $100K or two, engineering and accounting and
any other department with a need for data processing could have their own machines.
Following on the heels of the minicomputers were microcomputers, bringing data processing right to the
desktop. Affordability and accessibility dropped from the departmental level to the individual level,
making the phrase personal computer part of everyone's vocabulary.
Desktop computing has evolved at a mind-boggling pace, but it was certainly not an immediate
alternative to centralized, mainframe-based computing. There was a ramping-up period in which both
software and hardware had to be developed to a level where personal computers could be taken seriously.