society. From letting you call John in Calgary to letting you make a withdraw
complicated and expensive networks did not start out big and complicated but
rather as a wire and two terminals back in 1844. From these simple networks to
and the basis on which it functions.
2. The Beginnings
2.1. Dot Dot Dot Dash Dash Dash Dot Dot Dot
can exchange information. The first such exchange of information was on May 24,
1844 when Samuel Morse sent the famous message “What hath God wrought” from the
telegraph. The telegraph is basically an electromagnet connected to a battery
via a switch. When the switch is down the current flows from the battery
line. By itself the telegraph could express only two states, on or off. This
limitation was eliminated by the fact that it was the duration of the connection
respectively. From these combinations of dots and dashes the Morse code was
numbers and several punctuation marks. A variation to the telegraph was a
receiving module that Morse had invented. The module consisted of a
mechanically operated pencil and a roll of paper. When a message was received
the pencil would draw the corresponding dashes and dots on the paper to be
This machine when receiving Morse code would print letters corresponding to the
Morse code on a roll of paper hence eliminating the need for decoding the code.
2.2. Mr. Watson, Come Here!
arrived a few hours ahead of gray thus getting the patent on the telephone. The
patent issued to Bell was number 174,465, and is considered the most valuable
patent ever issued. Bell quickly tried to sell his invention to Western Union
but they declined and hired Elisha Gray and Thomas Edison to invent a better
telephone. A telephone battle began between Western Union and Bell. Soon after
Bell filed suit against Western Union and won since he had possessed the basic
rights and patents to the telephone. As a settlement Western Union handed over
market. During his experiments to create a functional telephone Bell pursued
two separate designs for the telephone transmitter. The first used a membrane
attached to a metal rod. The metal rod was submerged in a cup of mild acid. As
the user spoke into the transmitter the membrane vibrated which in turn moved
One of the greatest drawbacks to this model was that the cup of acid would have
to be constantly refilled. The second of Bell’s prototypes was the induction
telephone transmitter. It used the principle of magnetic induction to change
surrounded by a coil of wire. The movement of the rod in the coil produced a
weak electric current. An advantage was that theoretically it could also be
used both as a transmitter and a receiver. But since the current produced was
so weak, it was unsuccessful as a transmitter. Most modern day telephones still
use a variation of Bell’s design. The first practical transmitter was invented
by Thomas Edison while he was working for the Western Union. During his
resistance when subjected to varying pressure. So he sandwiched a carbon button
between a metal membrane and a metal support. The motion of the membrane
changed the pressure on the carbon button, varying the flow of electricity
through the microphone. When the Bell Vs. Western Union lawsuit was settled the
rights to this transmitter were also taken over by Bell.
2.3. Please Wait, I’ll Connect You.
The first network of telephones consisted of switchboards. When a
customer wanted to place a call he would turn a crank on his telephone terminal
at home. This would produce a current through the line. A light at the
to call and she would connect him by means of inserting a plug into a jack
corresponding to the desired phone. In earlier years he found that he could use
the ground as the return part of the circuit, but this left the telephone very
susceptible to interference from anything electrical. So in the mid 1880s Bell
realized that he would have to change the telephone networks from one wire to
two wire. In 1889 Almon Brown Strowger invented the telephone dial which
eliminated the use for telephone operators.
French inventor Emile Baudot created the first efficient printing
telegraph. The printing telegraph was the first to use a typewriter like
keyboard and allowed eight users to use the same line. More importantly, his
machines did not use Morse code. Baudot’s five level code sent five pulses for
eliminating the need for operators. After some improvements by Donald Murray
the rights to the machine were sold to Western Union and Western Electric. The
machine was named the teletypewriter and was also known by it’s nickname TTY. A
service called telex was offered by Western Union. It allowed subscribers to
exchange typed messages with one another.
3. From The Carterfone to the 14,400
3.1. I’ll Patch Her Up On The Carterfone, Captain.
The first practical computers used the means of punched cards as a
method of storing data. These punched cards held 80 characters each. They
dated back to the mechanical vote-counting machine invented by Hermen Hollerith
They were very slow in computing speed and the punch cards could be very easily
lost or destroyed. One of the first VDTs (Video Display Terminal) was the Lear-
Siegler ADM-3A. It could display 24 lines of 80 characters each (a remarkable
feat of technology). One of the regulations that AT&T passed was that no other
company’s equipment could be physically connected to any of it’s lines or
equipment. This meant that unless AT&T invented a peripheral it would not be
Carterfone invented a simple device that could go around these regulations. The
Carterfone allowed for a company’s radio to be connected to the telephone system.
The top portion of the Carterfone consisted of molded plastic. When a radio
user needed to use the telephone, the radio operator at the base station placed
the receiver in the Carterfone and dialed the number. This allowed the user to
They were specifications that, if met would allow third party companies to sell
and hook up their equipment to the telephone network. This turned the telephone
industry upside down and challenged AT&T’s monopoly in the telephone business.
3.2. So Gentelmen A’ Will Be 65
With more and more electronic communication and the invention of VDTs
the shortcomings of the Baudot code were realized. So in 1966, several
telecommunications companies devised a replacement for the Baudot code. The
result was the American Standard Code for Information Interchange, or ASCII.
ASCII uses 7 bits of code, allowing it to represent 128 characters without a
shift code. The code defined 96 printable characters (A through Z in upper- and
lowercase, numbers from 0 to 9, and various punctuation marks) and several
control characters such as carriage return, line feed, backspace etc. ASCII
also included an error checking mechanism. An extra bit, called the parity bit,
is added to each character. When in even parity mode, the bit would have a
number of ones. IBM invented it’s own code which used 8 bits of code giving 256
character possibilities. The code was called EBCDIC, for Extended Binary Coded
Decimal Interchange Code and was not sequential. The Extended ASCII was
designed so that PCs could again attain compatibility with the IBM machines.
The other upper 128 characters of the EASCII code include pictures such as lines,
hearts and scientific notation. In 1969 guidelines were set for the
construction of serial ports. The RS-232C standard was established to define a
transmit ASCII code but can also transmit Baudot and EBCDIC data. The connector
normally uses a 25 pin D shell connector with a male plug on the DTE (Data
Terminal Equipment) and a female plug on the DCE (Data Communications Equipment).
3.3. Hello Joshua, Would You Like To Play A Game…
In the 1950s a need arose to connect computer terminals across ordinary
telephone lines. This need was fulfilled by AT&T’s Bell 103 modem. A modem
(modulator/demodulator) is used to convert the on-off digital pulses of computer
data into on-off analog tones that can be transmitted over a normal telephone
circuit. The Bell 103 operated at a speed of 300 bits per second, which at that
time was more than ample for the slow printing terminals of the day. The Bell
103 used two pairs of tones to represent the on-off states of the RS-232C data
line. One pair for the modem that is calling and the other pair for the modem
answering the call. The calling modem sends data by switching between 1070 and
1270 hertz, and the answering modem by switching between 2025 and 2225 hertz.
The principle on which the Bell 103 operated is still in use today. During the
sixties and seventies the concept of mainframe networks arose. A mainframe
consisted of a very powerful computer to which thousands of terminals were
connected. The mainframe worked on a timesharing process. Timesharing was when
many users on terminals could use limited amounts of the host computer’s
resources, thus letting many parties access the host at the same time. This
type of network, however, was very expensive, and since on time sharing you
could only use small amounts of the host’s total computing power (CPU), the use
of the terminal was slow and sluggish. In the late seventies the personal
monitor, a keyboard, a CPU (Central Processing Unit), and various other
have to share their CPU and that the operating costs of these systems were much
less that that of their predecessors. The computers could, with a software
package, emulate terminals, and be connected to the mainframe network. Bell
laboratories came up with the 212a unit which operated at the speed of 1200 bits
per second. This unit, however, was very susceptible to noise interference.
3.4. Hey Bell! I Can Hang Myself Up!
many other companies started to create new designs of modems. Hayes
Microcomputer Products, took the lead in the PC modem business. Hayes pioneered
the use of microprocessor chips inside the modem itself. The Hayes Smartmodem,
introduced in 1981, used a Zilog Z-8 CPU chip to control the modem circuitry and
to provide automatic dialing and answering. The Hayes unit could take the phone
off the hook, wait for the dialtone, and dial a telephone number all by itself.
The Hayes Smartmodems sometimes had more powerful CPUs than the computers that
they were connected to. The next advancement was the invention of the 2400 bits
per second modem. The specifications came from the CCITT, an industry standard
setting organization composed of hundreds of companies world wide. The new
standard was designated as V.22bis and is still in use today. Other CCITT
standards that followed were the V.32 (9600 bps), the V.32bis (14400 bps), the
V42 (error control), and the V42bis (data compression). Virtually all modems
today conform to these standards. The next big computer invention was the fax
modem. It uses the on-off data transmission just as a modem but for the purpose
white area on the image. The image is sent as a set of zeros and ones and is
then reconstructed on the receiving end.
4.1. I Donnwanna File-Share!
give computers and peripherals the ability to accept requests for service across
a network and give other computers the ability to correctly use those services.
Servers share their hard disks, attached peripherals such as printers and
optical drives, and communication devices. They inspect requests for proper
service. There is a multitude of different types of servers. File servers are
equipped with large hard drives that are used to share files and information, as
specific segments of the data files under controlled conditions. Print servers
accept print jobs sent by anyone on the network. These servers are equipped
with spooling software (saving data to disk until the printer is ready to accept
it) that is vital in the situations where many requests can pour in at the same
time. Network Operating Systems package requests from the keyboard and from
applications in a succession of data envelopes for transmission across the
IPX (Internetwork Packet Exchange) packet, and the LAN adapter will then package
the IPX request into an Ethernet frame. In each step information about data and
error control data is added to the packet.
4.2. Eight Go In One Comes Out
The Network Interface Card or LAN adapter, is an interface between the
computer and the network cabling. Within the computer it is responsible for the
movement of data between the RAM (Random Access Memory) and the card itself.
Externally it is responsible for the control of the flow of data in and out of
the network cabling system. Since typically computers are faster than the
network, the LAN adapter must also function as a buffer between the two. It is
also responsible for the change of the form of data from a wide parallel stream
and out of the network port. To handle these tasks the LAN adapters are
equipped with a microprocessor and 8-64K of RAM. Some of the cards include
sockets for ROM chips called Boot ROM. These chips allow computers without hard
drives to boot operating systems from the file server.
4.3. Take Your Turn!
Ethernet and Token Ring network adapters use similar systems of
electrical signaling over the network cable. These signals are very similar to
the Baudot and Morse codes. A technique called Manchester encoding uses voltage
pulses ranging from -15v to +15v in order to transmit the zeros and ones. The
network cable has only one drawback, it can only carry signals from one network
card at a time. So each LAN architecture needs a media-access control (MAC)
scheme in order to make the network cards take turns transmitting into the cable.
Ethernet cards listen to the traffic on the cable and transmit only if there is
a break in the traffic when the channel is quiet. This technique is called
Carrier-Sense Multiple Access With Collision detection (CSMA/CD). With
collision detection, if two cards start transmitting at the same time, they see
the collision, stop, and resume some time later. Token Ring networks use a much
more complex process called token passing. Token Ring cards wait for permission
in order to transmit into the cable that forms an electrical loop. The cards
use their serial numbers in order to find the master interface card. This card
starts a message called a token. When a card with information to send receives
the token, it sends the data across the network. After the addressed interface
card receives the information and returns it to the originating card, the token
is given back to the master to be passed onto the next card. The ARCnet network
uses a very similar system to that of the Token Ring. Instead of using a token,
the master card keeps a table of all active cards and polls each one in turn,
giving permission to transmit.
4.4. Tied In A Knot
Various types of cabling are used to connect the LAN adapters to the
servers. Unshielded twisted pair wires offer rather slow speed, are very
inexpensive, are small, and can only span very short distances. These cables
use the RJ-45 connector. Coaxial cable offers fast speed, is rather expensive,
has a medium sized diameter, and can span medium distances. Coaxial cable uses
BNC connectors. The shielded twisted pair cable offers fast speed, is more
expensive than the coaxial cable, has a large diameter, and can only span short
distances. These cables use the IBM data connector. The fiber optic cable is
has a tiny diameter, and can span very long distances. This cable uses the ST
fiber optic connector. Wiring hubs are used as central points for the cables
from the network interface cards.
5.5. Loves Me, Loves Me Not, Server Based, Peer To Peer…
There are two general types of LANs. The Server-based networks rely on
one major server to store data, offer access to perhiperals, handle the printing
based networks have a high start up cost, but offer high security as well as
ease of operation. These networks become more economical as more computers are
added to the network. In Peer to peer networks the network responsibilities are
divided among many computers. Some act as file servers, others as print servers,
some as CD-ROM servers, tape drive servers, etc. The Startup cost of these
networks is much cheaper, but when more computers are added to the network, some
of the servers may not be able to handle the extra activity.
5. Links Between LANs
5.1. She Just Won’t Send Sysop!
Most networks have very short information transfer ranges. But, in an
ever shrinking world the need for links between LANs has never been higher.
This section will explain the components and information needed to link LANs.
When an electric current travels over a long length, it’s charge decreases, and
it is susceptible to electromagnetic interference. To combat the length problem
a component has been devised. A repeater is a little box that is inserted
between a cable. It’s primary function is to amplify the weakening pulse and
send it on it’s way. Bridges are used to analyze the station address of each
Ethernet packet and determine the destination of the message. The Routers strip
the outer Ethernet packets of a data packet in order to get the data. This data
is sent to other routers in other places of the world and then repackaged by
those routers. The removal of the excess data packets by the routers decreases
the time required to transfer that data. If networks use the same addressing
protocol, bridges can be used to link them, however, if they use different
addressing protocols, only routers may be used. During these times MANs
(Metropolitan Area Networks) are in use and development today. These use
routers that are connected preferably via a fiber optic cable, to create one
5.2. Pluto Calling Earth!
Any networks larger than 1000m typically rely on telephone digital lines
for data transfer. These networks are called Circuit Switched Digital Networks .
Circuit Switched Digital Networks utilize a switching matrix at the central
office of a telephone company that connects local calls to long distance
services. The Telephone companies now offer dial up circuits with signaling
rates of 56, 64, and 384 kilobits per second as well as 1.544 megabits per
second. Another type of LAN to LAN connections are packet switching networks.
These are services that a network router calls up on a digital line. They
consist of a group of packet switches that are connected via intraswitch trunks
(usually fiber optic) that relay addressed packets of information between them.
Once the packet reaches the destination packet switch, it sends it via another
digital connection to the receiving router.