NETWORKING
Networking,
was first conceived in late 1960s, when U.S. Department of Defense began
developing the Advance Research Projects Agency (ARPA). From this agency
was created ARPANET, which was the first packet-switched network. The
first version of ARPANET began operations in 1969. This project was a
success and developed into what is today called the capital
"I", or Internet.
In
1983, Novell Inc. released a network operating system that
revolutionized the computer network. The system was created by a group
of Brigham Young University graduates who called themselves
"Superset Software." This operation system was known by many
names, but was the forerunner of the network OS know as NetWare.
Current
Networking technology offers a cost-effective method for distributing
services and sharing equipment. From the early ARPANET project,
networking was found its way into small family businesses and large
corporate environments.
A
network is a number of computers connected by cable that can share
information and peripheral devices, such as printer. Networking is a
communication system that enables users to transfer or share similar
types of data and programs. Networking also applies to any multi-user
system. Local area networking (LAN) connects PCs together for sharing
real-time information, storage, applications and peripherals. Novell
NetWare puts these networking functions together and offer a wide range
of products for networks of any size. Some of the main features and
benefits of networks are given below:
-
The
growth of networks from main frame and mini computers.
-
Getting
updated information immediately.
-
Using
new technology while keeping your exiting equipment.
-
Using
networks to share programs, data and backups among different users.
-
protecting
vital information by using networks.
-
Sharing
expensive peripherals on networks.
Networking
can be as simple or as complex as you need for your company. Small
offices can use the network for sharing data and printers among users.
With medium-sized networks, you can have gateway links to minicomputers
from multiple vendors and links to offices across town or across
the country. Because each type of multi-user environment has its own
rules and utilities, gateways enable dissimilar systems to communicate
with each other through hardware and software communications.
Even
large networks, commonly referred to as Enterprise Networks, enable
users to access data located throughout the world from any number of
minicomputers, mainframes or remote LANs. Local Area Network is the
linkage of personal and other computers within a limited area by high
performance cable.
LAN
permits users on the network to share limited resources, such as an
expensive color graphic printer. If one computer in a network shares its
printer with a network, every computer connected to the network can use
it. You have access to the printer as if it was connected directly to
your machine.
COMPUTER
PROCESSING SYSTEMS
Current
technology has produced many forms of data processing, and a company can
select a form uniquely suited to its needs. The processing systems are:
a) Interactive Processing System
b) Real Time Processing Systems
-
Time
Sharing Systems.
-
Multi-User
Systems.
-
Multi
Tasking Systems.
BATCH
PROCESSING SYSTEMS
Batch
processing, a widely used technique, involves the regular processing of
large amounts of data. In batch processing, data is collected for a
predetermined period of time, after which it is processed.
ON-LINE
PROCESSING SYSTEMS
We
can connect our small personal computer with the large computer with the
help of an operating system to do some work is referred to as On-line
Processing. Online processing is used by airlines to accept and confirm
customer reservations in seconds. As online banking system is another
example of this type of processing.
CHARACTERISTICS
An
on-line system consists of terminal(s) connected to and controlled by
the computer. Communication lines connect the various departments of a
business directly to a computer for specific department on the nature of
the functions performed. It is important to appreciate that an on-line
system need not be a real-time system but a real-time system must be an
on-line system.
APPLICATIONS
On-line
systems may be used for:
(a)
Interactive Processing (Transaction Processing)
-
On-line
order processing
-
On-line
building society transactions
-
On-line
payroll processing
-
On-line
point of scale (supermarket) check-out systems
(b)
Real-time Processing
-
Airlines
seat reservation system
-
On-line
warehouse stock control
-
On-line
hotel accommodation system
-
On-line
banking
(c)
Random Enquiries
-
On-line
credit enquiries
-
On-line
product availability enquiries
-
On-line
account enquiries
-
On-line
package holiday availability enquiries
INTERACTIVE
PROCESSING SYSTEM
It
handles transactions individually in what is referred to as
Conversational Mode Processing, unlike batch processing which deals with
them in batches. The software prompts the user, indicating the needs of
the application, which may specify the data required for processing, ask
if it is a debit or a credit or if there any more transactions for the
item currently being processed, to which the user responds
interactively. The system may then ask that if there are any more
transactions and the user respond accordingly. At the end of the posting
run back-up copies of master files are produced for security purposes.
Interactive processing can be done on a stand-alone basis by a single
user or on a multi user basis, which allow each user to enter and
process data according to his or her specific needs. This type of
processing technique receive and process data at random time intervals.
If transactions are dealt with as event occur and the files are updated
with details immediately, it is a real-time system. Transactions need to
be processed as they occur; for example dispatches to customers can be
input the following day for the production of invoices. Transactions are
still dealt with on an individual basis, however, and this is the key
factor distinguishing this type of processing with batch processing.
REAL
TIME PROCESSING SYSTEM
It
is a parallel time relationship with an ongoing activity and is
production information quickly enough to be useful in controlling this
current live and dynamic activity. Thus, the word real time describes an
interactive processing system will severe time limitations. A real time
system uses interactive processing, but an interactive system need be
operating in real time. The difference is that real time processing
require immediate transaction input from all input-originating
terminals. Many terminals are tied directly by high-speed
telecommunications lines into one or more processors. Several stations
can operate at same time. Files are updated each minute, and inquires
are answered by split second access to up-to-the-minute records. But its
possible to have an interactive system that combine immediate access to
records from a central collecting source. Such an interactive system
would meet many needs would be similar and less expensive than real time
system.
TIME
SHARING SYSTEMS
It
is a general term used to describe a processing system with a number of
independent, relatively low-speed, online and simultaneously usable
stations. Each station provides direct access to the processor. The use
of special programs allows the processor to switch from one station to
another and to do a part of each job in an allocated "time
slice" until the work is completed. The process is frequently so
fast that a user has, the illusion that nobody else is using the
computer.
MULTI
USER SYSTEMS
The
Capabilities PCs offer to small businesses can be greatly enhanced when
several independent workstations are linked into a larger system. The
ability to send and receive messages and shares programs and data makes
connecting PCs a wish choice for small companies unable to afford a mini
computer.
A
variety of systems are available for linking several workstations. One
economic approach is a multi-user system. This system uses a single
processing unit to link several dump terminals.
Since
multi-user systems link inexpensive terminals instead of the more
expensive PCs used with other systems, multi-user systems provide cost
savings for small businesses purchasing a complete system or adding
users to a system currently in operation. Yet for a small business that
already has compatible PC hardware and software, multi-user systems may
not be the best choice because these systems require special equipment
and the exiting equipment cannot be used.
MULTI
TASKING SYSTEMS
Techniques
that facilitates the running of two or more tasks concurrently on
microcomputers is called Multi-Tasking. In mainframe environments
multi-tasking is referred to as "Multi-Programming", but in
both instances the technique allows high speed switching between
different tasks while affording access to multiple sources of
information as with integration with text files, functional files,
graphics and database etc. Each task has its own window on the screen,
which act as a viewing area of the computer's memory. Multi-tasking
provides facilities for the transfer of information from one application
to another with high speed switching between them by means of special
operating systems.
Data communications is the transfer of
encoded information from one location to another by means of a
communications channel. Like other types of communications, it involves
three basic elements: 1. A Sending Unit 2. A Transfer Unit 3. A
Receiving Unit The Sending and the receiving units are usually computers
or terminals. The transmission channel is commonly a telephone line
,though data can also be transmitted in the form of radio waves,
microwaves or laser beams, at the present time most data communications
networks handle data exclusively in electronic form.
ADVANTAGES
"
Data capture at the information source. " Centralized control of
business data. " Rapid transmission of information. " Support
of rapid expansion (into dispersed locations). " Improved
management control of business data (by linking terminals to a central
database).
The
market manager using a data communications system that gives him the
following major advantages:
DATA
CAPTURE AT THE INFORMATION CENTRE
Using
data communications makes it possible to capture data at the point of
origin .That is, data can be entered directly into the system at their
source instead of being sent to some distant location for data entry and
processing.
RAPID
DISSEMINATION OF INFORMATION
A
data communications network provides immediate access to information
when it is needed. If the company's distribution centers are connected
by a data communications network, the clerk can use a terminal to find
out the current inventory of the requested item in the other centers -
and therefore how best to fill the order.
DISPERSED
OPERATION
A
firm using a data communications network can operate more efficiently
over a wide geographical area than it could not work without such a
network.
DATA
COMMUNICATIONS APPLICATION
The
use of data communications systems in the business environment has
increase rapidly. Following is a list appeared in order of increasing
complexity, starting with source-data collection and ending with
networks involving data communications between two or more computers.
SOURCE
DATA COLLECTION
As
its name indicates, source data collection involves the acquisition if
data at its point of origin. Information is entered at a remote terminal
and stored on floppy disks for later transmission to a central computer
where it can be processed. This application permits the collection of
large amounts of data during those hours most convenient for capturing
it and transmission costs are lowest. Source data collection is often
used for inventory control. Data on shipping and receiving transactions
can b entered at a terminal is the local warehouse of distribution
center, stored, and transmitted at a latter time.
REMOTE
JOB ENTRY
Remote
job entry is a batch-processing application. The job is entered into the
system at a remote terminal and the output is returned to a remote
printer at some later time. Routing instructions that specify the output
destinations are submitted along with the job. Remote job entry usually
sized volumes of data and a response time of one to two hours. This
application tends to be vulnerable to error because the input is
difficult to check and errors show up only in the output.
MESSAGE
SWITCHING
Message
switching permits the sending of a message from one terminal to anoyher
on a data communications network.All the sender needs to know is the
recipients mailbox ID, or identification code.Mesaage switching also
makes it possible to send a single message to mailing list consisting of
two or more mailbox IDs.The volume of data involved in anyone
transmission tends to be low, while the response time may vary from
minutes to hours .Electronic mail is an application of message switching
in computerized office environments using company-wide networks.
ON-LINE
QUERY
On-line
inquiry makes information available to a user who has keyed in a request
or inquiry at a terminal connected by a communications like a host
computer. The volume of a data transmitted per inquiry is usually quite
low, and response time is measured in seconds. Retailers to check the
credit status of customers making credit-card purchases extensively use
this application. But the usefulness and value of online is not limited
to making business transactions more convenient. In on-line inquiry with
overnight update users can enter update transactions, and these
transactions are used to update the appropriate master files once a day
or less often. In on-line inquiry with immediate update the user can
transact to update master files as soon as new information becomes
available .The best known application of this type is the airline
reservations system.
TIME
SHARING
Time
sharing refers to the sharing of the computer's time by multiple users.
In its most popular form, conversational time sharing, this application
allows the user to communicate interactively with the computer, keying
in data and instructions at a terminal, and receiving a response in few
seconds, and often faster. Because of this quick response time, the user
has the impression that no one else is using the computer. In fact, the
computer is dividing its time among many users, giving each a tiny slice
of its time before going on to the text. The process, appropriate called
Time Slicing, is so fast that computer often executes an instruction
from one user, makes the rounds of the other users, and returns in time
to execute the first user's next instruction.
COMPUTER
TO COMPUTER NETWORK
The
most complex data communications network consist of distributed
computers linked together. Such a network provides users at many
different locations with the advantages of having access of all the
files, programs, and processing capabilities of the network.
DISTRIBUTED
DATA PROCESSING
Another
environment in which data communications is used is in distributed data
processing networks which link centralized computer systems to
decentralized mini- and microcomputers with local processing ability.
DATA
COMMUNICATION NETWORK'S COMPONENT
The
structure of a data communications network is more easily understood if
it is first compared with a traditional data processing system. In a
data communications system, a communication channel links a
communication control unit to one or more devices on the network. The
communication control unit handles the input/output processing required
by dozens of hundreds of terminals trying to send data or to receive
data from the computer system.
To
understand how a data communications system functions, we need to
concentrate on three main types of components:
-
The
Controller,
-
The
Devices, and
-
The
Communication lines.
COMMUNICATION
CONTROL UNIT
A
large central computer is seldom the ideal instrument for handling data
communications, which involve short bursts of activity in sending and
receiving single messages. To make central computer effective in data
communications, additional hardware has been developed in the form of
front end modules. These modules control data transmission between the
central computers and remote devices. They are designed to perform as
many communication functions as possible, freeing the computer to
perform the data processing operations for which it is better equipped.
The
important function of the communication control unit includes the
following:
-
Connecting
up to several hundred communications lines to the central computer.
-
Adapting
the main computer to a data network by converting the transmissions
from remote sites into a form that the computer can accept.
-
Polling
(monitoring) remote devices to determine their status-ready to send
a message or receive one.
-
Storing
and holding data intended for a device that is busy or temporarily
out of service.
-
Providing
data protection and accountability by maintaining a message log of
all transmission.
-
Detecting
errors in messages and either connecting them or ordering a
retransmission.
-
Adding
communications control codes to outgoing transmission and deleting
from incoming ones.
-
Determining
which devices to receive a transmission (one, small, or all).
-
Controlling
the message priority system so that the more important transmissions
are processed ahead of less important ones.
TERMINALS
The
terminal used may be any one of several types of input/output devices.
Teletypewriter terminal have a keyboard for input and can print hard
copy output. Video terminals have a CRT
screen for displaying input from the keyboard and output from the
computer. Remote job entry terminals are stations consisting of a card
reader, line printer, often some time of storage capacity for the local
user, and a operator console that has CRT screen, all of which are
connected to a control unit. Transaction terminal, such as
point-of-scale terminals in retail stores, are linked to a controller or
mini computer in the transaction environment itself. Intelligent
terminals have a built-in microcomputer that enables them to perform
such functions as transaction editing, verification, and even database
enquiry or data processing.
MODEM
The
term MoDem is an abbreviation for Modulator/demodulator. It is easier to
understand what a modem does if you think of it as a two-way adapter
between a transmission channel and either a communication control unit
or a terminal. The modem converts a digital signal received from a
communication control or terminal into an analog signal that can be
transmitted over a telephone line. The modem converts an analog signal
received over a communications channel back into a digital signal, which
is the kind , that computer and terminals understand.
COMMUNICATIONS
LINES
A
communication line or a channel that links terminal to the communication
control unit and the central computer can be one of several kinds. For
the time being, however, it is necessary to only that there are two
types of arrangements of lines in a network: point-to-point and
multi-point.
In
a point- to-point configuration a single terminal is linked to the
central computer. In multi-point configurations the line to the computer
is shared among several terminals.
MULTIPLEXER
A
communication device is designed to carry only one message at a time;
that is, it cannot carry two or more messages simultaneously. However,
this limitation can in effect by passed with the use of a multiplexer. A
multiplexer enables a single communication device to carry several
combined signals. It does so by converting several low speed signals and
transmitting them over a high speed line. Given the costs of
transmission, it is clearly cheaper to send several signals lines at a
high speed over a single line than to send them at low speed over
separate lines. Thus multiplexing uses channels more efficiently and
reduces the per-message cost.
CONCENTRATORS
Concentrators
serve much the same basic function as multiplexers, but they have
additional capabilities. In effect, they are intelligent multiplexers.
Besides combining messages they can check for errors, change message
codes and formats deletes extraneous characters, and temporarily store
messages or parts of messages. The ability of concentrators to combine
messages makes it possible to use the full capacity of a transmission
line.
WHAT
IS CHANNEL?
In
the simplest terms a communication channel can be defined as a path for
the transmission of a signal between two point. A channel is also
referred to as a line, circuit, or link. Of course, not all channels are
the same or transit signals at the same aped. There are six type of
communications channels listed below.
TELEPHONE
WIRE PAIRS
Telephone
wire pairs are the oldest types of communication channel. Telephone
lines contain many such pairs twisted around one another and wrapped in
a protective covering. A single wire pair constitutes a channel for the
transmission of voice communication. For that reason they are often
referred to as voice grade channels.
Telephone
channels used for data communications have a bandwidth, or frequency
range, of 3000 cycles per second. Transmission is measured in terms of
wave frequency of the transmitted signal. One Hertz(1 Hz) represents a
frequency of one cycle, or one complete wave, per second. The larger the
bandwidth, the higher the frequency, the more waves are transmitted per
unit of time and, therefore, the more information can be carried by a
transmission. Consequently, even though transmission can be said to be
faster because it carries more information in the same unit of time than
does a low-frequency transmission. By the same token, a line can carry a
greater number of frequencies can be said to be faster than one that can
carry fewer frequencies.
WIRE
CABLES
Wire
cables also contain wire pairs, usually numbering in the hundreds.
However, insulation and special twisting methods minimize the
interference, or cross talk, between individual pairs. The resulting
improvement in the quality of transmission has led to the replacement of
earlier, open-wire pairs by cable wires. Today the latter are the type
used in most data transmission channels.
COAXIAL
CABLES
A
coaxial cable provides a much higher quality of transmission than do
wires pairs and transmits at much higher frequencies. A coaxial cable
consists of a cylinder with a copper wire conductor running down its
center. Insulation separates the conductor from the cylinder. A large
cable can hold twenty operating coaxial cables. Often used for
underground transmission lines, a single large cable can carry more than
100,000 telephone calls at one time.
MICROWAVE
LINKS
Microwave
links are the chief competition of coaxial cables for transmitting large
volumes of data. A microwave link typically consists of several
microwave towers spaced twenty-five to thirty miles apart. Signals are
transmitted on a line-of-sight path from one tower to the next. The path
between towers must be clear of any obstruction, because solid objects
easily reflect microwaves. Towers cannot be spaced apart more than
thirty miles, because microwaves travel
in a straight line and will not bend with the curvature of the
earth.
When
a tower receives a signal, it amplifies the signal and retransmits it to
the next tower. One hundred towers can carry a signal from coast to
coast. Coaxial links are at a disadvantage in this respect, since a
coast to coast coaxial link requires about 1000 amplifications and
transmissions. Microwave links can carry thousands of voice channels and
are widely used for television transmission.
COMMUNICATIONS
SATTELITE
These
can be thought of as microwave towers placed many miles above the earth.
Because of their height satellite links can transmit microwave signals
over great distances, unhampered by the earth’s curvature or by
mountains and other objects that limit microwave links.
In-practice
a communications satellites hangs in stationary orbit over a given
geographic area. Signals sent from one station in this are bounced off
the satellite to distant receivers in the same area. A combination
receiver does the bouncing /transmitter called a Transponder. This
device receives a signal form earth and retransmits it at a different
frequency. The change of frequency is necessary to prevent incoming and
outgoing signals from interfering with each other.
If
the intended receiver lies outside the geographic area covered by a
satellite, a second satellite can be used. The original signal is simply
bounced back to an intermediate station, which transmits it to the
second satellite, which in turn sends, it to the intended destination.
Three such satellites are sufficient to enable a signal to hop to almost
any part of the world.
LASERS
AND OPTICAL FIBERS
Lasers
are the optical fibers are the products of new technologies that are now
being studied possible applications to data communications.
A
laser generates a beam of light that is almost perfectly monochromatic,
which means that almost all the light rays
in the beam have exactly the same frequency. Laser light is also
coherent. Generally speaking, this means that all the light waves in the
beam are “in-step” and therefore interference between waves in
minimal. In practice terms, coherence means a minimum of distortion in
the information transmitted by the laser beam. These features make a
laser beam capable of carrying 100,000
times as much information as a microwave link.
Optical
fibers are extremely fine filaments that act as a kind of pipe for the
transmission of light. Made of glass or glass like materials, optical
fibers are particularly effective in transmitting laser light with
minimum distortion or loss of light. Consequently, the possibility of
combining the two is of great interest to researchers in data
transmission.
TRANSMISSION
FUNDAMENTALS
In
order to understand the basic functioning of data communications
networks, we need to know more about how a message is sent over a
communication link:
-
Modes
of Transmission
-
Types
of Transmission
-
Protocols
MODES
OF TRANSMISSION
The
mode of tansmission refers to the manner in which the characters of a
message are transmitted. The problem for the receiving unit is to
separate one character from the text. There are three ways, or modes,
for this kind of decoding:
·
Synchronous
·
Asynchronous
·
Isochronous
Synchronous
Transmission
Synchronous transmission
sends character in groups, or blocks. A block of data may contain
thousands of bit. Each block begins with a set of bits that enables the
receiver to synchronize itself with the transmission. In effect, the
receiver times or measures the transmission as it received and separates
the individual characters. In other words, the receiver sets it to
divide the transmitted message into time intervals, or characters.
Timing is clearly important.
Asynchronous
Transmission
Asynchronous transmission
sends characters one at a time. A start bit identifies each character
and a stop bit. This type of transmission is said to be asynchronous
(unsynchronized) because the receiver can identify a character by its
start and stop bits regardless of when it arrives; that is, characters
can be sent at irregular intervals. Asynchronous transmission is much
slower than the synchronous type, being limited to about 1800 bits a
second.
Isochronous
Transmission
Isochronous
transmission combines features of the other two. Characters have
identifying start and stop bits, and the sender and receiver are
synchronized. Rates of isochronous transmission go as high as 9600 bits
a second- slower than synchronous but faster than asynchronous.
protocol is a predetermined procedure or routine used by sending and
receiving units to ensure that they communicate properly. Protocols are
the feature of the hardware equipment and are usually decided on at the
design stage by the manufacture. The protocol chosen may be that of the
manufacture or one previously established by a standard s committee.
Protocols
define the correct procedure and rules for executing transmissions and
related operations. Specifically, they are used to define:
-
The
proper steps for beginning and terminating a transmission;
-
The
control characters to be used and how to use them in “laying
out” a message;
-
The
procedure to be followed for one line dialogue;
and
-
The
procedure for detecting errors and the corrective action to be
taken.
Value
Added Networks (VANs)
This
type of communication network provides additional services to the
communication channels by third-party vendors under a government
license. The additional services include automatic error detection and
correction as well as ‘store and forward’ message services,
electronic mail and protocol conversations to access different computers
to network.
Local
Area Network (LAN)
In
general terms, a local are network (LAN) is an electronic communications
network that uses private branch exchange (PBXs) or other local
communications channel for transmission instead of public communication
facilities such as long distance links or satellites communications.
Perhaps its greatest advantage is that it permits the linking up of
variety of devices- word processors, desktop computers, printers and
storage devices- in a small area such as a business office. The
importance of this advantage is made clear by single statistic: 60% of
office communication involves individuals in he local organization:
Specifically,
a small personal or office computer in a LAN is able to;
·
Gain access to data stored in a central storage disk devices and
store data in those devices;
·
Transit documents prepared on the computer to an electronic
printer;
·
Transit electronic messages to other computers in the network;
and
·
Send information to and receive information from a CPU in the
network.
To
understand the functioning of LANs better, let’s look at the three
types of configuration, or layout, of these networks.
WIDE
AREA NETWORK (WAN)
There
are many variations in data communications network arrangement, but the
basic types are the wide area network (WAN), and the local area network
(LAN). The first WANs was established to provide timesharing services.
With the introduction of microcomputers, the networks facilitated the
distribution of computing sources throughout the film. Today, both WANs
and LANs are being used for client/server computing.
In
a LAN, the firm owns all of the hardware and circuitry, but in a WAN the
channel is provided by a common carrier. Common carriers provide a
variety of products and services. A product that is revolutionizing how
data and information are communicated is the integrated services digital
network (ISDN).
The
STAR Network
The
star configuration links each device in the network to a central control
unit. Star networks represent the traditional approach to network
linkage. Most star networks use PBX links (telephone lines). This type
of link has advantages and disadvantages. The main advantage is that
telephone lines already exist. The main disadvantage is the limited
capacity of such lines.
The
RING Network
Ring
networks link devices by cable in a loop or circle. The central
controller or computer is part of the loop. Data is transmitted around
the ring, with the attached devices picking out from the information
stream those messages addressed to them.
The
particular type of line control used by ring networks is known as Token
passing. Each device is allowed to transmit only when it holds a logical
token. The token is passed from device to device. The function of token
passing is to prevent multiple devices from transmitting at the same
time, a condition that can result in data collisions.
The
BUS Network
BUS
networks consist of a cable to which devices are connected by a cable
interface unit. The network has no central controller, and data can be
passed from one device to another through the bus. Because there is no
central controller, the prevention of collisions between messages is a
major concern. Bus networks use a form of line control known as CSMA/CD,
or Carrier Sense Multiple/CD require a device to first check the
transmission like for traffic before it sends a message. This procedure
is called
Listening
Before Transmitting. If the line is free, the device transmits.
CSMD/CD
also requires the device to check for collisions. If one does occur, the
device retransmits its message at the next opportunity.
Networking