Networking allows one computer to send information to and receive information
from another. We may not always be aware of the numerous times we access
information on computer networks. Certainly the Internet is the most
conspicuous example of computer networking, linking millions of computers
around the world, but smaller networks play a roll in information access
on a daily basis. Many public libraries have replaced their card catalogs
with computer terminals that allow patrons to search for books far more
quickly and easily. Airports have numerous screens displaying information
regarding arriving and departing flights. Many retail stores feature
specialized computers that handle point-of-sale transactions. In each
of these cases, networking allows many different devices in multiple
locations to access a shared repository of data.
Before getting into the details of a networking standard like Ethernet,
we must first understand some basic terms and classifications that describe
and differentiate network technologies -- so let's get started!
Local Area vs. Wide Area
We can classify network technologies as belonging to one of two basic
groups. Local area network (LAN) technologies connect many devices that
are relatively close to each other, usually in the same building. The
library terminals that display book information would connect over a
local area network. Wide area network (WAN) technologies connect a smaller
number of devices that can be many kilometers apart. For example, if
two libraries at the opposite ends of a city wanted to share their book
catalog information, they would most likely make use of a wide area
network technology, which could be a dedicated line leased from the
local telephone company, intended solely to carry their data.
In comparison to WANs, LANs are faster and more reliable, but improvements
in technology continue to blur the line of demarcation. Fiber optic
cables have allowed LAN technologies to connect devices tens of kilometers
apart, while at the same time greatly improving the speed and reliability
In 1973, at Xerox Corporation’s Palo Alto Research Center (more commonly
known as PARC), researcher Bob Metcalfe designed and tested the first
Ethernet network. While working on a way to link Xerox’s "Alto" computer
to a printer, Metcalfe developed the physical method of cabling that
connected devices on the Ethernet as well as the standards that governed
communication on the cable. Ethernet has since become the most popular
and most widely deployed network technology in the world. Many of the
issues involved with Ethernet are common to many network technologies,
and understanding how Ethernet addressed these issues can provide a
foundation that will improve your understanding of networking in general.
The Ethernet standard has grown to encompass new technologies as computer
networking has matured, but the mechanics of operation for every Ethernet
network today stem from Metcalfe’s original design. The original Ethernet
described communication over a single cable shared by all devices on
the network. Once a device attached to this cable, it had the ability
to communicate with any other attached device. This allows the network
to expand to accommodate new devices without requiring any modification
to those devices already on the network.
Ethernet is a local area technology, with networks traditionally operating
within a single building, connecting devices in close proximity. At
most, Ethernet devices could have only a few hundred meters of cable
between them, making it impractical to connect geographically dispersed
locations. Modern advancements have increased these distances considerably,
allowing Ethernet networks to span tens of kilometers.
In networking, the term protocol refers to a set of rules that govern
communications. Protocols are to computers what language is to humans.
Since this article is in English, to understand it you must be able
to read English. Similarly, for two devices on a network to successfully
communicate, they must both understand the same protocols
Ethernet follows a simple set of rules that govern its basic operation.
To better understand these rules, it is important to understand the
basics of Ethernet terminology.
• Medium - Ethernet devices attach to a common medium that provides
a path along which the electronic signals will travel. Historically,
this medium has been coaxial copper cable, but today it is more commonly
a twisted pair or fiber optic cabling.
• Segment - We refer to a single shared medium as an Ethernet segment.
• Node - Devices that attach to that segment are stations or nodes.
• Frame - The nodes communicate in short messages called frames, which
are variably sized chunks of information.
Frames are analogous to sentences in human language. In English, we
have rules for constructing our sentences: We know that each sentence
must contain a subject and a predicate. The Ethernet protocol specifies
a set of rules for constructing frames. There are explicit minimum and
maximum lengths for frames, and a set of required pieces of information
that must appear in the frame. Each frame must include, for example,
both a destination address and a source address, which identify the
recipient and the sender of the message. The address uniquely identifies
the node, just as a name identifies a particular person. No two Ethernet
devices should ever h
Since a signal on the Ethernet medium reaches every attached node, the
destination address is critical to identify the intended recipient of
For example, in the figure above, when
computer B transmits to printer C, computers A and D will still receive
and examine the frame. However, when a station first receives a frame,
it checks the destination address to see if the frame is intended for
itself. If it is not, the station discards the frame without even examining
One interesting thing about Ethernet addressing is the implementation
of a broadcast address. A frame with a destination address equal to
the broadcast address (simply called a broadcast, for short) is intended
for every node on the network, and every node will both receive and
process this type of frame.
The acronym CSMA/CD signifies carrier-sense multiple access with collision
detection and describes how the Ethernet protocol regulates communication
among nodes. While the term may seem intimidating, if we break it apart
into its component concepts we will see that it describes rules very
similar to those that people use in polite conversation. To help illustrate
the operation of Ethernet, we will use an analogy of a dinner table
Let’s represent our Ethernet segment as a dinner table, and let several
people engaged in polite conversation at the table represent the nodes.
The term multiple access covers what we already discussed above: When
one Ethernet station transmits, all the stations on the medium hear
the transmission, just as when one person at the table talks, everyone
present is able to hear him or her.
Now let's imagine that you are at the table and you have something you
would like to say. At the moment, however, I am talking. Since this
is a polite conversation, rather than immediately speak up and interrupt,
you would wait until I finished talking before making your statement.
This is the same concept described in the Ethernet protocol as carrier
sense. Before a station transmits, it "listens" to the medium to determine
if another station is transmitting. If the medium is quiet, the station
recognizes that this is an appropriate time to transmit.
Carrier-sense multiple access gives us a good start in regulating our
conversation, but there is one scenario we still need to address. Let’s
go back to our dinner table analogy and imagine that there is a momentary
silence in the conversation. You and I both have something we would
like to add, and we both "sense the carrier" based on the silence, so
we begin speaking at approximately the same time. In Ethernet terminology,
a collision occurs when we both spoke at once.
In our conversation, we can handle this situation gracefully. We both
hear the other speak at the same time we are speaking, so we can stop
to give the other person a chance to go on. Ethernet nodes also listen
to the medium while they transmit to ensure that they are the only station
transmitting at that time. If the stations hear their own transmission
returning in a garbled form, as would happen if some other station had
begun to transmit its own message at the same time, then they know that
a collision occurred. A single Ethernet segment is sometimes called
a collision domain because no two stations on the segment can transmit
at the same time without causing a collision. When stations detect a
collision, they cease transmission, wait a random amount of time, and
attempt to transmit when they again detect silence on the medium.
The random pause and retry is an important part of the protocol. If
two stations collide when transmitting once, then both will need to
transmit again. At the next appropriate chance to transmit, both stations
involved with the previous collision will have data ready to transmit.
If they transmitted again at the first opportunity, they would most
likely collide again and again indefinitely. Instead, the random delay
makes it unlikely that any two stations will collide more than a few
times in a row.
Limitations of Ethernet
A single shared cable can serve as the basis for a complete Ethernet
network, which is what we discussed above. However, there are practical
limits to the size of our Ethernet network in this case. A primary concern
is the length of the shared cable.
Electrical signals propagate along a cable very quickly, but they weaken
as they travel, and electrical interference from neighboring devices
(fluorescent lights, for example) can scramble the signal. A network
cable must be short enough that devices at opposite ends can receive
each other's signals clearly and with minimal delay. This places a distance
limitation on the maximum separation between two devices (called the
network diameter) on an Ethernet network. Additionally, since in CSMA/CD
only a single device can transmit at a given time, there are practical
limits to the number of devices that can coexist in a single network.
Attach too many devices to one shared segment and contention for the
medium will increase. Every device may have to wait an extremely long
time before getting a chance to transmit.
Engineers have developed a number of network devices that alleviate
these difficulties. Many of these devices are not specific to Ethernet,
but play roles in other network technologies as well.
The first popular Ethernet medium was a copper coaxial cable known as
"thicknet." The maximum length of a thicknet cable was 500 meters. In
large building or campus environments, a 500-meter cable could not always
reach every network device. A repeater addresses this problem.
Repeaters connect multiple Ethernet segments, listening to each segment
and repeating the signal heard on one segment onto every other segment
connected to the repeater. By running multiple cables and joining them
with repeaters, you can significantly increase your network diameter.
In our dinner table analogy, we had only a few people at a table carrying
out the conversation, so restricting ourselves to a single speaker at
any given time was not a significant barrier to communication. But what
if there were many people at the table and only one were allowed to
speak at any given time?
In practice, we know that the analogy breaks down in circumstances such
as these. With larger groups of people, it is common for several different
conversations to occur simultaneously. If only one person in a crowded
room or at a banquet dinner were able to speak at any time, many people
would get frustrated waiting for a chance to talk. For humans, the problem
is self-correcting: Voices only carry so far, and the ear is adept at
picking out a particular conversation from the surrounding noise. This
makes it easy for us to have many small groups at a party converse in
the same room; but network cables carry signals quickly and efficiently
over long distances, so this natural segregation of conversations does
Ethernet networks faced congestion problems as they increased in size.
If a large number of stations connected to the same segment and each
generated a sizable amount of traffic, many stations may attempt to
transmit whenever there was an opportunity. Under these circumstances,
collisions would become more frequent and could begin to choke out successful
transmissions, which could take inordinately large amounts of time to
complete. One way to reduce congestion would be to split a single segment
into multiple segments, thus creating multiple collision domains. This
solution creates a different problem, as now these now separate segments
are not able to share information with each other.
To alleviate problems with segmentation, Ethernet networks implemented
bridges. Bridges connect two or more network segments, increasing the
network diameter as a repeater does, but bridges also help regulate
traffic. They can send and receive transmissions just like any other
node, but they do not function the same as a normal node. The bridge
does not originate any traffic of its own; like a repeater, it only
echoes what it hears from other stations. (That last statement is not
entirely accurate: Bridges do create a special Ethernet frame that allows
them to communicate with other bridges, but that is outside the scope
of this article.)
Remember how the multiple access and shared medium of Ethernet meant
that every station on the wire received every transmission, whether
it was the intended recipient or not? Bridges make use of this feature
to relay traffic between segments. In the figure above, the bridge connects
segments 1 and 2. If station A or B were to transmit, the bridge would
also receive the transmission on segment 1. How should the bridge respond
to this traffic? It could automatically transmit the frame onto segment
2, like a repeater, but that would not relieve congestion, as the network
would behave like one long segment.
One goal of the bridge is to reduce unnecessary traffic on both segments.
It does this by examining the destination address of the frame before
deciding how to handle it. If the destination address is that of station
A or B, then there is no need for the frame to appear on segment 2.
In this case, the bridge does nothing. We can say that the bridge filters
or drops the frame. If the destination address is that of station C
or D, or if it is the broadcast address, then the bridge will transmit,
or forward the frame on to segment 2. By forwarding packets, the bridge
allows any of the four devices in the figure to communicate. Additionally,
by filtering packets when appropriate, the bridge makes it possible
for station A to transmit to station B at the same time that station
C transmits to station D, allowing two conversations to occur simultaneously!
Switches are the modern counterparts of bridges, functionally equivalent
but offering a dedicated segment for every node on the network (more
on switches later in the article).
Alternative Network Technologies: Token Ring
The most common local area network alternative to Ethernet is a network
technology developed by IBM, called token ring. Where Ethernet relies
on the random gaps between transmissions to regulate access to the medium,
token ring implements a strict, orderly access method. A token-ring
network arranges nodes in a logical ring, as shown below. The nodes
forward frames in one direction around the ring, removing a frame when
it has circled the ring once.
1. The ring initializes by creating a token,
which is a special type of frame that gives a station permission to
2. The token circles the ring like any frame until it encounters a station
that wishes to transmit data.
3. This station then "captures" the token by replacing the token frame
with a data-carrying frame, which encircles the network.
4. Once that data frame returns to the transmitting station, that station
removes the data frame, creates a new token and forwards that token
on to the next node in the ring.
Token-ring nodes do not look for a carrier signal or listen for collisions;
the presence of the token frame provides assurance that the station
can transmit a data frame without fear of another station interrupting.
Because a station transmits only a single data frame before passing
the token along, each station on the ring will get a turn to communicate
in a deterministic and fair manner. Token-ring networks typically transmit
data at either 4 or 16 Mbps.
Fiber-distributed data interface (FDDI) is another
token-passing technology that operates over a pair of fiber optic rings,
with each ring passing a token in opposite directions. FDDI networks
offered transmission speeds of 100 Mbps, which initially made them quite
popular for high-speed networking. With the advent of 100-Mbps Ethernet,
which is cheaper and easier to administer, FDDI has waned in popularity.
courtesy : www.howstuffworks.com