Fundamental of Computers
& Information Technology
Unit-IV
Use of communication and It
communication -Computer communications describes
a process in which two or more computers or devices transfer data,
instructions, and information. Figure 8-1 shows a sample communications system.
Some communications involve cables and wires; others are sent wirelessly
through the air. As illustrated in this figure, communications systems contain
all types of computers and computing devices. For successful communications, you
need the following:Computer communications are everywhere. Many require that
users subscribe to an Internet access provider. With other computer
communications, an organization such as a business or school provides
communications services to employees, students, or customers.
Basic Element of a
Communication System: - Communication is the process of transferring message
from one point to another. As shown in figure, the three basic elements of any
communication process are:
1.
A sender (source), which creates the message to be
transmitted.
2.
A medium, which carries the message
3.
A receiver (sink), which receives the message.
Receiver (Sink) Sender (Source)
Medium
For example, when
you speak to your friend on the telephone, you are the sender. The telephone
line through which your voice is transmitted is the medium and your friend is
receiver.
Data communication is the function of
transporting data from one point to another. In this case, the sender and the
receiver are normally machines, in particular, computer devices (computers,
terminals, peripheral devices like line printers, plotters etc) and the
transmission medium may be telephone lines, microwave links, satellite links
etc. Thus, the electronic systems that transfer data from one point to another
are called data communication system.
Analog and Digital Signals:
-
Data is propagated from one point to another by means of electrical signals,
which may be in digital or analog form. Almost everything in the world can be
described or represented in one of the two forms: Analog or Digital. The
principal features of analog representation are that they are continuous. In
contrast, digital representation consists of values measured at discrete
(isolated) intervals.
In
an analog signal, the transmitted power varies over a continuous range for
example sound, light and radio waves. The sine wave is the most fundamental
form of an analog signal. Sine waves can be described by three characteristics,
namely, amplitude, frequency and wavelength. Amplitude is the value of the
signal at any point on the wave. The maximum amplitude of a sine wave is the
highest value it reaches on the vertical axis. The amplitude (v) of analog
signal is measured in volts. Frequency refers to the number of cycles a signal
completes in one seconds. In other words, frequency means the number of times a
signal wave goes up and down in a second and it is measured in Hertz (Hz). For
example, if a signal wave completes one cycle in one second, its frequency in 1
Hz. Wavelength refers to the distance between successive similar points of a
given wave, that is, one cycle of the waveform.
Analog
signals are perfect for carrying data such as voice or sound. However, these
signals are prone to error or noise, which are caused from an outside source.
Attenuation is another problem with analog signals because the amplitude of the
wave naturally changes over distance.
Amplitude
Time
Wavelength
(One cycle)
Digital data is the data stored in the form of 0s
and 1s. When the signal is at a
Amplitude Time
To transmit data over analog
phone lines, a modem is required to convert the digital data signals to analog
signals. When transmitted over long distances, analog signals require to be
amplified, which can possibly distort the value of the data transmitted. When analog data is converted to digital
data, it can be transmitted over digital signals faster and without distortion.
Modulation: - Modulation is most often used for
superimposing digital data on analog waves. Modulation is necessary in
communication systems due to the following reasons: (1) in order to transmit a
wave effectively, the length of the transmitting antenna should be
approximately equal to the wavelength of the wave. So for transmitting audio
waves (whose wavelength is very high) we need to have very long antennas, which
is impractical. (2) The energy of a wave depends upon its frequency. The
greater the frequency of the wave, the greater the energy possessed by it. As
the audio signal frequencies are small they cannot be transmitted over long
distances.
There are three forms of modulation-
Amplitude, Frequency, and Phase modulation
Amplitude
Modulation (AM): - In amplitude modulation, only the amplitude of
the carrier wave is changed. However, the frequency of the modulated wave
remains the same. In other words, Two binary values (0 and 1) of digital data
are represented by two different amplitudes of the carrier signal keeping the frequency
and phase constant. On voice grade lines, it is used up to 1200 bits per
seconds. However, amplitude modulated signals are sensitive to impulse noise
that arise due to electrical sparks near the transmission line.
Frequency
Modulation (FM): - When the frequency of the
carrier wave is changed in accordance with the intensity of the signal it is
called frequency modulation. In frequency modulation, only the frequency of the
carrier wave is changed. In other words, Two binary values of digital data are
represented by two different frequencies while the amplitude and phase are kept
constant. Also known as frequency shift keying (FSK), this method is less
susceptible to error than amplitude modulation. Another advantage of this
method is that it is easier to design devices based on FSK because
discrimination between two frequencies is simpler than detecting phase changes.
Thus for medium speed communication (1200 to 2400 bits per second) the FSK
scheme is preferred. It gives noiseless reception. Noise is a form of amplitude
vibration and FM receivers reject such signals. Its operating range is quite
large.
Phase
Modulation (PM): - Phase modulation is a change in
the carrier phase angle. The phase angle cannot change without affecting a
change in frequency. Therefore phase modulation is in reality a second form of
frequency modulation. In other words, two binary values or digital data are
represented by the shift in phase of the carrier signal. That is, a sine wave
with phase=00
Represents a
digital 1 and a sine wave with phase=1800
represents a digital 0. This technique is more noise resistant and
efficient than both AM and FM. For higher speed transmission of 9600 bits per
second, phase modulation is more reliable and is preferred to other methods.
Communication Processors: In a data communication network, the task of network designers is
to select and coordinate the network components so that the necessary data is
moved to the right place, at the right time, with a minimum of errors, and at
the lowest possible cost. A number of communication processors are used by
network designers to achieve these goals. The functions of some of the commonly
used communication processors are described below:
Multiplexers: - there are
many applications in which several terminals are connected to a computer. If
each terminal is operating at 300 bits per second over a communication line
(channel) that can operate at 9600 bits per second, then we see a very
inefficient operation. It has been found that the capacity of a channel exceeds
that required for a single signal. A channel is an expensive resource. Hence
for its optimal utilization, the channel can be shared in such a way so as to
simultaneously transmit multiple signals over it. The method of dividing a
physical channel into multiple logical channels, so that a number of
independent signals may be simultaneously transmitted on it, is known as
multiplexing. The electronic device
that performs this task is known as Multiplexers.
Multiplexers take several data
communication lines or signals and convert them into one data communication
line or signal at the sending location. For example, as shown in figure,
Modem
Multiplexer Mux
There may be 4 terminals connected to
Multiplexers. The Multiplexers takes the signals from the 4 terminals and
converts them into one large signal, which can be transmitted over one
communication line. Then, at the receiving location, Multiplexers takes the one
large signal and breaks it into the original 4 signals. Thus, with multiplexing
it is possible for a single transmission medium to parallel transmit data
between several transmitters and receivers. There are two basic methods of
multiplexing channels. They are frequency division multiplexing and time
division multiplexing.
- Frequency-Division
Multiplexing (FDM): - FDM is used when the bandwidth of the
transmission medium is much greater than any given signal. In FDM, the
available bandwidth of a physical medium is split up into several smaller,
disjoint logical bandwidths. Each of the components bandwidth is used as a
separate communication line (channels). Process of FDM is shown in figure,
Modulator Demodulator
40 KHz 40 KHz
Signal-1 Signal-1
50 KHz 50 KHz
Channel
Signal-2 Signal-2
60 KHz 60 KHz
Signal-3 Signal-3
70 KHz 70 KHz
Signal-4 Signal-4
80 KHz 80 KHz
Signal-5 Signal-5
The best example of FDM is
the way we receive various station in a radio. Each radio station is assigned a
frequency range within a bandwidth of radio frequencies. Several radio stations
may be transmitting speech signals simultaneously over the physical channels. A
radio receiver antenna receives signals transmitted by all the stations.
Finally, the tuning dial in the radio is used to isolate the speech signal of
the station tuned. In FDM, the signals to be transmitted must be analog
signals. Thus, digital signals must be converted to analog form if they are to
use FDM.
2. Time-Division Signals: -TDM divides
the main signal into time slots, with each time slot carrying a separate
signal. The bit rate of transmission medium always exceeds the required rate of
the digital signals. This fact is utilized for time division multiplexing. In
TDM, the total time available in the channel is divided between several users
and each user of the channel is allotted a time slice (a small time interval)
during which he/she may transmit a message. Figure shows multiplexing of three
different signals using TDM. The same idea may be extended for multiplexing
several signals.
A3 A2 A1
Signal A
Channel B1
B3 B2
Signal B
C3 C2 C1
Signal C
Although TDM may be used to
multiplex digital or analog signals, its usage is more appropriate for data
transmission. It is also very much suitable for communication between computers
because such communication occur in short fast bursts.
Whether or not to use multiplexing usually depends upon
economics. The cost of high-speed modems and Multiplexers is very high compared
to the cost of low speed modems. However, if line costs are high due to long
distances, then multiplexing is cost effective. One disadvantage with multiplexing
relates to a transmission line failure. If the line goes out, everything is
dead. With individual lines only one terminal is likely to be lost.
Concentrators: - A
concentrators performs the same function as Multiplexers, but concentrators
have the ability to actually reduce the number of signals. For example, 100
signals from different devices coming into the concentrator could leave as only
70 or 80 signals. The signals are concentrated to form a fewer number of
signals. This requires intelligence. This intelligence normally takes the form
of microprocessors or even minicomputers. Thus, a concentrator is basically
intelligent Multiplexers. The advantage of using concentrators is that devices
of varying speeds and types are connected to the concentrator, which in turn is
connected to the host computer by high-speed lines. Concentrators are
especially useful where data communication cost is high, such as long distances
international communications. In general, concentrators are more expensive than
Multiplexers. However, for some long-distance and international data
communication applications, the use of concentrators is cost justified.
Communication Protocols: - Imagine yourself standing near a traffic
crossing. You can notice that for the smooth movement of the traffic, a
functioning traffic light is essential. The red signal sends a message to stop,
a yellow signal to wait and a green signal to cross. This set of rules, which
tell a driver when to move and when to stop, are traffic protocols. Similarly,
computers have certain protocols that define the manner in which communication
take place. A computer protocol is a set of rules that coordinates the exchange
of information. If one computer is sending information to another and they both
follow the same protocol, the message gets through; regardless of what types of
machines they are and on what operating systems they are running. As long as
the machines have software that can manage the protocol, communication is
possible. In fact, it is the data communication software that is responsible
for holding all data communication systems together. It instructs computer
system and devices as to exactly how the data is to be transferred from one
place to another. These procedures embedded in software are commonly called
protocols.
Roles of Protocol: - In any computer network, data communications
software normally performs the following functions for the efficient and error
free transmission of data:
- Data
Sequencing: - It refers to breaking a long transmission into smaller
blocks and maintaining control. That is a long message is split up into
smaller packets of fixed size. These packets are further fragmented into
data frames. This technique is widely used in conjunction with error
control techniques to reduce the amount of data that must be retransmitted
in case of a detect error.
- Data
routing: - Routing algorithms are designed to fine the most efficient
paths between sources and destinations.
- Flow
Control: - A communication protocols also prevents a fast sender from
overwhelming a slow receiver. It ensures resources sharing and protection
against congestion by regulating the flow of data on the communication
lines.
- Error
Control: - Error detecting and recovering routines are also an important
element of communications protocols. The most common method for correcting
errors is to retransmit a block. This method requires coordination of the
two stations that the block having error is discarding by the receiving
station and is repeated by the transmitting station.
- Precedence
and order of transmission: - There are well-defined rules to condition all
stations about when to transmit their data and when to receive data from
other stations. It is ensured that all stations get a change to use the
communication lines and other resources of the network depending upon the
priorities assigned to them.
- Connection
establishment: - When two stations of a network want to communicate with
each other, the communication protocol establishes and verifies a
connection between the two.
- Data
security: - Providing data security and privacy is also built into most
communications software package. It prevents access of data by
unauthorized users because it is relatively easy to tap a data
communication line.
Log information: - Data communication software can
also develop log information, which consists of all jobs, and data
communication tasks that have taken place. Such information is normally used
for financial purposes and the various users of the network are charged
accordingly
Direction of Transmission
Flow: - There are three ways, or modes of transmitting data from one
point to another. These are Simplex, Half Duplex and Full Duplex.
Simplex: - Simplex
transmission is Unidirectional. If transmission is simplex, communication can
take place in only one direction. Devices connected to such a circuit are
either a send only or a receive only. However, in almost all data processing
applications, communication in both directions is required. Even for a one-way
flow of information from a terminal to a computer, the system will be designed
to allow the computer to signal the terminal that data has been received.
Without this capability, the remote user might enter data and never know that
it was not received by the computer (due to some problem somewhere). Hence,
simplex circuits are seldom used because a return path is generally needed to
send acknowledgement, control, or error signals.
Receiver Sender
Television transmission can be considered, as an example of simplex
mode of transmission where the satellite only transmits the data to the
television, vice versa is not possible.
Half Duplex:-In half duplex
mode, each communications device can receive and transmits information’s, but
not at the same time. In other words, A half duplex system can transmit data in
both directions, but only in one direction at a time. Thus, a half duplex line
can alternately send and receive data. It requires two wires. This is the most
common type of transmission for voice communication because only one person is
supposed to speak at a time. It is also used to connect a terminal with a
computer. The terminal might transmit data and then the computer responds with
an acknowledgement.
The most common example of
half duplex transmission is the wireless handsets (generally used by military
person) where one user talks at a time and another listens.
Sender (Or
Receiver) Receiver (Or Sender)
OR
Full Duplex:-In a half duplex system,
the line must be turned-around each time the direction is reversed. This
involves a special switching circuit and requires a small amount of time
(almost 150 milliseconds). With high-speed capabilities of the computer, this
turn around time is unacceptable in many cases. Also, some applications require
simultaneous transmission in both directions. In such cases, a full duplex system
is used that allows information to flow simultaneously transmission in both
directions on the transmission path. It requires four wires. A full duplex mode can be compared to a
two-way road with traffic flowing in both directions.
A common example of full duplex transmitting
mode is the telephone network, where two people communicate over a telephone
line, both can talk as well as listen at the same time.
Sender (And
Receiver) Receiver (And Sender)
AND
Communication Software: -Software is a
virtual component of all communication networks. Communications control
software includes programs stored in the host computer as well as programs in
the front-end computers and other communication software. To communicate via a
modem, your microcomputer requires communication software. Communication software
manages the transmission of data between computers or video display terminals.
Popular microcomputer communication programs are Smartcom, Crosstalk, ProComm,
PC-Dial, Blast and PC Talk. Often the software comes on floppy disks bundled
with (sold along with) the modem. Besides establishing connections between
computers, communication software may perform other functions.
Access Control: - this function
establishes the connection between terminals and computers in a network. The
software works with a communication processor (such as a modem) to connect and
disconnect communication links and establish limitations (parameters) such as
transmission speeds, mode and direction. This function may also involve
automatic telephone dialing and redialing, logging on and off with appropriate
account numbers and security code and automatic answering of telephone calls
from another computer.
Error Correction: - Static on
telephone lines can introduce errors into data transmission or noise. Noise is
anything that causes distortion in the signal when it is received. When
acquiring a modem and its accompanying software. You should inquire whether it
incorporates error correction features.
Data Compression: - Data
compression reduces the volume of data in a message thereby reducing the amount
of time required to send data from one modem to another. The software does this
by replacing repeating pattern with symbols that indicate what the pattern is
and how often it is repeated and the full message is restored. With text and
graphics, a message may be compressed to as much as one tenth of its original
size.
Remote Control: - Remote control
software allows you to control a microcomputer from another microcomputer in a
different location, perhaps even thousands of miles away. One part of the
program is in the machine in front of you, the other in the remote machine.
Such software is useful for travelers who want to use their borne machines from
a far. It is also helpful for technicians trying to assist users with support
problems. Examples of remote control software for microcomputers are Carbon
Copy, Commute, Norton PCAnywhere and Tim.
Terminal Emulation: - Mainframes
and minicomputers are designed to access by terminals, not by microcomputers.
Thus, the operating systems are different for both. Terminal emulation software
allows you to use your microcomputer to simulate (copy) a mainframe or
minicomputer terminal.
Security Control: - Security
control protects a communication network from unauthorized access. Data
transmission can also be protected by coding techniques called encryption. Data
is scrambled (twisted) into a coded form before transmission and decoded upon
arrival.
- .
Communications Channels
(Transmission Media): - Transmission media refers to the physical media
through which communication signals are transmitted. The sender-medium-receiver
concept has actually been with us for a very long time. For example, shouting
to another person does in fact involve voice transmission over a distance via
the medium air, which carries the sound wave. Needless to say, the use of
telephone lines as a transmission medium considerably enhances the possible
distance. Like telephone lines, there are several types of physical channels
(communication media) through which data can be transmitted from one place to
another. Some of the most common data transmission medium is:
Twisted-Pair Wire: - This is the oldest and
still most common transmission line and consists of copper wires twisted into
pairs. These lines are used in established communications networks throughout
the world for both voice and data transmission. Each pair consists of a wire,
used for receiving data signal, and a wire used for transmitting data signal.
The wires are twisted in order to reduce noise (unwanted signal) and
interference from external sources. Twisted pairs are used in a short distance
communication (less than 100 meters) and they are available in two forms:
Unshielded and Shielded
- Unshielded Twisted Pair (UTP) Cable: - UTP
cable is the most common type of telecommunication medium in use today. It
is commonly used in telephone system. UTP cables are an inexpensive medium
of data transmission. They are easy to install and use. The twisted pair
consists of two metal conductors (usually copper) that are insulated
separately with plastic insulation. UTP cables have a maximum transmission
speed of up to 9600 bps. However their use is limited because they easily
pick up noise signals, which results in high error rates when the line
length extends beyond 100 meters.
- Shielded Twisted Pair (STP) Cables: - STP cable
has a metal foil or braided mesh covering that covers each pair of
insulated conductors. The metal foil is used to prevent infiltration of
electromagnetic noise. This shield also helps to eliminate crosstalk, a
phenomenon that can be experienced during telephone conversation when one
can hear another conversation in the background.
Coaxial Cable: -Unlike twisted
pairs that have two wires; coaxial cables have a single central conductor,
which is made up of solid wire (usually copper). Coaxial cables are groups of
specially wrapped and insulated wire lines that are able to transmit data at
high rates. As shown in figure, they consist of a central copper wire
surrounded by a PVC insulation over which a sleeve of copper mesh is placed.
The metal sleeve is again shielded by an outer shield of thick PVC material.
The signal is transmitted by the inner copper wire and is electrically shielded
by the outer metal sleeve. Coaxial cables offer much higher bandwidths than UTP
cables and are capable of transmitting digital signals at rates of 10 mega bits
per second. They are extensively used in long distance telephone lines and as
cables for cable TV. They are also used by telephone companies to transmit
data. In many cases, several coaxial cables are packaged into a very large
cable that can handle over 40,000 telephone calls simultaneously. Furthermore,
coaxial cables have much higher noise immunity and can offer cleaner and
crisper data transmission without distortion or loss of signal.
Optical Fibers: - Optical fibers are hair
thin threads of glass or plastic that can serve as a data transmission medium
as copper wires or coaxial cables. The basic difference in those optical fibers
transmits light signals instead of electrical signals. Because light travels
much faster than electricity, optical fibers can transmit data at much higher
speed than copper wires or coaxial cables. Optical fibers are made of glass,
plastic or silica. Plastic fibers are least efficient, but tend to be cheaper
and more rugged (strong). Glass or silica fibers are much smaller and their
lower attenuation makes them more suited for very high capacity channels. Physically, a fiber optic cable consists of
three concentric layers- the inner core, a cladding around it and the outer
protective coating. The inner core, which has a diameter of 8 to 200
micrometers, consists of a bunch of optical fibers. The cladding (shield)
around it is made of plastic or glass and has a refractive index less than that
of the core. The outer protective coating is made up of plastic. The main
components of an optical fiber communication system are shown in figure.
Light to Electrical wave converter Electrical to light wave converter
Optical Fiber
Electrical Electrical
Signal
Signal
Amplifier
Towards its source side is a
converter that converts electrical signals into light waves. The converter uses
either a light emitting diode (LED) or a laser diode to convert electric into
light signals. These light waves are then transmitted over the optical fiber to
the receiver’s end. At the receiver’s end, another converter is placed that
detects the light waves and converts them back to electrical signals. It uses
photoelectric diodes for this purpose. These electric signals are then
amplified using an amplifier and sent to the receiver.
Optical fibers have the
following advantages:
- Since transmission is light-based rather than
electricity, it is immune to noise interference.
- Transmission distance is greater than other
guided media because of less signal attenuation (degradation in quality
over distance).
- It is more secure because it is extremely
difficult and expensive to tap optical signals.
- They are smaller and lighter than copper wire
and are free from corrosion as well.
- Fiber optic offers, by far, the greatest
bandwidth of any transmission system.
- Both analog and digital signals can be
transmitted by the use of optical fibers. In analog transmission, the
light intensity is varied continuously. On the other hand, in digital
transmission the light source is turned on or off.
Disadvantage of Optical
Fiber:
- Fiber optic is expensive as it is costly to
produce, maintain and install.
- They are more fragile as fiber optic tends to
break easily as compared to copper wires.
- Aligning and joining two fiber optic cables is
not so simple and easy as for twisted copper wire pairs or coaxial cables.
It requires special equipment to do so.
Serial & Parallel
Transmission: - Data is transmitted
in two ways- serially and in parallel.
1. Serial Data Transmission: - One by one. Serial data transfer refers
to transmitting data one bit at a time. The opposite of serial is parallel, in
which several bits are transmitted concurrently.
2. Parallel Data Transmission: - Refers to processes that occur
simultaneously. Printers and other devices are said to be either parallel or
serial. Parallel means the device is capable of receiving more than one bit at
a time (that is, it receives several bits in parallel). Most modern printers
are parallel.
Modem: - The process of modulation and demodulation,
that is, the conversion of digital data to analog form and vice-versa, is
carried out by a special device called a modem (Modulator/Demodulator). Hence,
when an analog facility is used for data communication between two digital
device (A terminal and A computer), two modems are required, one near each
digital device. The digital signal generated at the terminal is converted to
analog form by the modulator of the modem placed near it. The analog signal is
transmitted through the telephone line, which is converted to digital form by
the demodulator of the modem placed near the computer. This digital data is processed by the
computer. The processed digital data is modulated to analog form and returned
via the telephone line to the terminal, where the analog signal are demodulated
to digital form for display on the terminal. Hence the modem is essential piece
of hardware for any application in which two digital devices (for example two
computer) want to communicate over an analog transmission channel (for example
Telephone line).
There
are two main reasons for using modems. They allow higher speeds of transmission
on a given analog line, and they reduce the effects of noise and distortion.
Modems can do a lot more than simply transport data across telephone line. A
good modem can also perform tests and checks on how it is operating. Some of
the more expensive modems actually contain microprocessors that allow them to
operate and function under a large number of different circumstances. These
modems are called smart modems.
When
all the telephone line becomes digital (in the future), the modem will no
longer be necessary. Most modems today are capable of sending and receiving
faxes. These modems are called Fax-Modems. Most manufactures supply the
software for their modems.
The
speed at which the modems can transfer data is measured in bits per second
(BPS). The modems that one usually uses for connecting to the Internet have
transfer rates in the range of 14,400 bps (14.4 kbps). But modems with transfer
rate of 28.8 kbps, 33.6 kbps and 56 kbps are now available in the market.
Modems are of two types:
- Internal: - It is a card that is fitted inside
the computer with a lead running directly from the computer to the phone
socket.
- External: - It is a small external box wired
between the computer and phone socket.
Types of Connections: -
- Dial-Up
line/Connection: - Dial up
line is a service that operates in a manner similar to a telephone call.
That is, a user of a computer willing to communicate with a remote
computer first makes a connection request by dialing up the remote
computer. A circuit is then established between the two computers via the
telephone company’s switching system. The modem attached to the user’s
computer then sends and receives data over the telephone line. Just as in
the case of a telephone call, the charge for data transmission service in
this case depends on the duration of communication and the distance
between the two computers.
- Leased
Line: - Leased
line (also known as dedicated line) is a special conditioned telephone
line that directly and permanently connects two computers. A leased line
can be used for both voice and data transmission. Hence, if an
organization has two offices in two different cities, it is often cheaper
for the organization to acquire its own leased line between the two
offices which can be used by the
organization for all its voice (telephone calls) and data transmission
between the two offices. The charges for a leased line are often based on
channel capacity (bps) and distance (air miles). This facility provides
reliable, high-speed Internet access ranging from 2.4 Kbps to 45Mbps. A
leased line connection is an affordable way to link two or more sites for
a fixed monthly charge.
- ISDN: - ISDN stands
for Integrated Services Digital Network and is more common in business and
commercial use. The ISDN is a telephone system that provides digital (not
analog) telephone and data services. As it supports digital services
(including digitized voice), the ISDN telephone users enjoy noise free,
CD-quality sound. Moreover, with the ISDN, no modem is necessary because
it supports digital transmission of all types of data (including voice).
This also results in very short call set-up time between two ISDN
subscribers. It was the first high-speed alternative to regular analog
phone modems. ISDN involves the digitization of telephone network so that
voice, graphics, text, and other data can be provided to users from a
single terminal over existing telephone wiring. Digital connections have
fewer errors in transmission, which means speed of downloading graphics,
web pages, sound etc is increased to four times faster than with dial-up
modems.
- DSL(Digital
Subscriber Line):- DSL
connections to ISPs use a standard telephone line but special equipment on
each end to create always-on Internet connections at blindingly fast
speeds, especially when compared with analog dial-up connections. Services
levels vary around the
DSL is a family of technologies that
provide digital data trasmission over the wires of a local telephone
network. DSL originally stood for
digital subscriber loop, although in recent years, many have adopted digital
subscriber line as a more marketing-friendly term for the most popular version
of consumer-ready DSL, ADSL. Typically, the download speed of consumer DSL
services ranges from 256 kilobytes per second (kbit/s) to 24,000 kbit/s,
depending on DSL technology, line conditions and service level implemented.
Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber
Line (ASDL) and equal to download speed for Symmetric Digital Subscriber
Line(SDSL), but telecom companies charge a lot more for the privilege. DSL
encompasses many such variations, so you will often see it refereed to as xDSL.
RF:-
An RF connector
is an electrical connector
designed to work at radio frequencies
in the multi-megahertz range. RF connectors are typically used with coaxial cables and are designed to maintain the
shielding that the coaxial design offers. Better models also minimize the
change in transmission line impedance
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transceivers.
Broad Band: Broadband in data communications
can refer to broadband networks
or broadband Internet
and may have the same meaning as above, so that data transmission over a fiber optic cable would be referred to as
broadband as compared to a telephone modem operating at 56,000 bits per second.
However, broadband in data communications
is frequently used in a more technical sense to refer to data transmission
where multiple pieces of data are sent simultaneously to increase the effective
rate of transmission, regardless of data signaling rate.
In network engineering
this term is used for methods where two or more signals share a medium.
Broadband Internet access, often shortened to just broadband, is high data rate Internet
access—typically contrasted with dial-up access
over a modem.
Dial-up modems are generally only
capable of a maximum bitrate of 56 kbit/s (kilobits
per second) and require the full use of a telephone line—whereas broadband
technologies supply at least double this bandwidth and generally without
disrupting telephone use.
Types of Network: -A computer network is a collection of two
or more computers, which are connected together to share information and
resources. Computers in a network are interconnected by telephone lines,
coaxial cables, satellite links, radio and/or some other communication technique.
There are three types of networks available today.
- Local
Area Network (LAN)
- Metropolitan
Area Network (MAN)
- Wide Area
Network (WAN)
These categories are defined depending upon various
factors like the size of the network, the distance it covers, and the type of
link used in interconnection.
Local Area Network: - A LAN is a digital communication system
capable of interconnecting a large number of computers, terminals and other
peripheral devices within a limited geographical area, typically under 1 KM
across. LANs normally operate within a compact area such as an office building
or a campus etc. In LANs, one computer is designated as the file server, which
stores all the software that controls the network along with the software that
can be shared by the computers attached to the network. Other computers
connected to the file server are called workstations. The configuration of a
LAN can be a Star, a Ring, or simply devices attached along a length of cable.
In LAN, transmission channels generally use coaxial or fiber optic cables and
special interface units rather than telephone lines and modems. Thus the
transmission speed is very high. The attached computers may be of different
types and be performing a variety of functions such as data processing, word
processing and electronic mail. The two main purpose of the LAN are to link
computers within an organization or campus so that they may share expensive
peripheral devices, for example high speed printers or magnetic disks holding
the data base and to allow these computers to communicate with other. Ethernet
& Omninet is the example of LAN.
Metropolitan Area Network: - A MAN is a network of computers spread over
a Metropolitan area such as a city. This network is usually reserved for
metropolitan areas where the city bridges its LAN with a series of backbones,
making one large network for the entire city. It may be a single network such
as a cable television network or it may be a means of connecting a number of
LANs.
Wide Area Network: - A WAN is a digital communication system,
which interconnects different sites, computer installations and user terminals
and may also enable LANs to communicate with other. This type of communication
network may be developed to operate nationwide or worldwide. In a WAN, the
transmission medium used are normally public system such as telephone lines,
microwave and satellite links. Packet switching is most effective as public
connection services are available for packet switched WANs. A WAN is also known
as a long haul network. For Example, a company with offices in
VPN: A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections
or virtual circuits
in some larger network (e.g., the Internet) instead of by physical wires. The
link-layer protocols of the virtual network are said to be tunneled
through the larger network when this is the case. One common application is
secure communications through the public Internet, but a VPN need not have
explicit security features, such as authentication or content encryption. VPNs,
for example, can be used to separate the traffic of different user communities
over an underlying network with strong security features.
A VPN may have best-effort
performance, or may have a defined service level
agreement (SLA) between the VPN customer and the VPN service
provider. Generally, a VPN has a topology more complex than point-to-point.
Network Topologies: - A network is logical extension of a data
communication system. In a computer network, two or more computers (often known
as nodes) are linked together for the purpose of communicating data and sharing
resources. The term network topology refers to the way in which the nodes of a
network are linked together. It is the geometric representation of the
relationship of all the links. There are five basic topologies: Bus, Ring,
Star, Tree and Mesh.
Ring Topology:-> In ring topology, computers are placed in a
circular arrangement without any terminating ends since there are no
unconnected ends. In this topology, each computer in the network has
communicating subordinates, but within the ring there is no master computer for
controlling other computers. A node receives data from one of its two adjacent
nodes. The only decision a node has to take is whether the data is for its own
use or not. If it is addressed to it, it utilizes it. Otherwise, it merely
passes it on to the next node.
Advantages: -
- Ring topology is easy to install and
reconfigure
- The ring network works well where there is no
central cite computer system.
- It is more reliable than a star network because
communication is not dependent on a single host computer. If a link
between any two computers breaks down, or if one of the computers breaks
down, alternate routing is possible.
Disadvantages: -
- In a ring network, communication delay is
directly proportional to the number of nodes in the network. Hence
addition of new nodes in the network increases the communication delay.
- The ring network requires more complicated
control software than star network.
Bus Topology: -Bus topology uses
a common bus or backbone (a single cable) to connect all devices. In this type
of network, a single transmission medium is shared by all nodes. When a
particular computer wants to send a message to another computer, it appends the
destination address to the message and checks whether the communication line is
free. As soon as the line becomes free, it broadcasts the message on the line.
As the message travels on the line, each computer checks whether it is
addressed to it. The message is picked up by the addressee computer, which
sends an acknowledgement to the source computer and frees the line. This type
of network is also known as ‘multipoint’ or broadcasting network. It is
appropriate for use in a local area network where a high-speed communication
channel is used and computers are confined to a small area.
Advantages: -
- The main advantage of a multi-access bus
network is the reduction in physical lines therefore less expensive than
others.
- The failure of a computer in the network does
not affect the network functioning for other computers.
- Addition of new computers to the network is
easy.
Disadvantages: -
- All computers in the network must have good
communication and decision-making capability.
- If the communication line fails, the entire system
breaks down.
- Heavy traffic can slow down a bus because
computers on such networks do not coordinate with each other to reserve
time to transmit.
Star Topology: - In this
configuration, multiple computers are connected to a host computer. That is,
the computers in the network are not linked directly to each other and can
communicate only via the host computer. The routing function is performed by
the host computer, which centrally controls communication between any two other
computers by establishing a logical path between them. This topology commonly
uses twisted pair cable, however, coaxial cable or fiber optic cable can also
be used.
Advantages: -
- Star topology is easy to install and wire.
- Star topology has minimal line cost because
only n-1 lines are required for connecting n nodes.
- Transmission delays between two nodes do not
increase by adding new nodes to the network because any two nodes may be
connected via two links only.
- The network is not disrupted even if a node fails
or is removed from the network.
Disadvantages: -
- It requires a longer length of cable
- The system crucially depends on the central
node. If the host computer fails, the entire network fails.
Mesh Topology: - In a mesh
topology, every node has a dedicated point-to-point link to every other node.
Message sent on a mesh network can take any if several possible paths from
source to destination. A fully connected mesh network has n (n-1)/2 physical
links to link n devices. For example, if an organization has 5 nodes and wants
to implement a mesh topology, 5(5-1)/2 that is, 10 links are required.
Advantages: -
- This type of network is very reliable, as any
link breakdown will affect only communication between the connected
computers.
- Communication is very fast between any two
nodes.
- Each node of the network need not have
individual routing capability.
Disadvantage: -
- The amount of required cabling is very large.
- As every node is connected to the other,
installation and reconfiguration is very difficult.
- The amount of hardware required in this type of
topology can make it expensive to implement.
Tree Topology: - A tree topology
combines characteristic of linear bus and star topologies. It consists of
groups of star configured workstations connected to a bus backbone cable. The
shape of the network is that of an inverted tree with the central root
branching and sub-branching to the intermediates of the network. The main
difference in this network and bus is the presence of a root to the tree. When
a node transmits, the root receives the signal and re-broadcasts it through the
entire network.
Advantage: -
- It is easy to extend because the tree is
divided into sub units, it is easier to add new nodes or branches to it.
- Easy fault isolation. It is possible to
disconnect whole branches of the network from the main structure. This
makes it easier to isolate a defective node.
Disadvantage: -
- If the backbone line breaks, the entire segment
goes down.
- It is more difficult to configure and wire than
other topologies.
Components of LAN: -
Media: - A variety of media
link devices in a communication network. The communication channels that these
provide can carry either analog or digital signals. All communications media
fall into one of the two broad classes: physical lines or wireless media. The
most commonly used media are:
- Twisted pair wires
- Coaxial Cables
- Telephone lines
- Microwave links
- Fiber optic cables
- Satellite links
NIC (Network Interface
Card): - NIC is the first contact between a machine and the network. It
connects clients, servers, and peripherals to the network via a port. Most
network interface card come as small circuit board that can be inserted onto
one of the computer motherboard’s slots. Alternatively, modern computers
sometimes include the network interface as part of their main circuit boards
(motherboards). Each network interface is associated with a unique address
called its media access control (MAC) address. The MAC address helps in sending
information to its intended destination.
NOS: - Just as we
need DOS or some other operating system to manage applications in a stand-alone
computer, we need a network operating system to control the flow of message
between workstation and servers. The network operating system may make a LAN
printer in another room appear to be locally attached to your workstation.
Examples of NOS are: Novell Netware, Microsoft Windows NT Server
Bridges: - A device that
connects two local area networks (LANs) or two segments of the same LAN. A
bridge filters data traffic at a network boundary. It reduces the amount of
traffic on a LAN by dividing it into two segments. Bridges operate at the data
link layer of the OSI Model. It inspects incoming traffic and decides whether
to forward or discard it.
Unlike routers, bridges are protocol independent. They
simply forward packets without analyzing and re-routing messages. Consequently,
they are faster than routers, but also less versatile.
HUB: - A hub is a
small box that connects individual devices on a network so that they can
communicate with one another. The hub operates by gathering the signals from
individual network devices and then sending them onto all other connected
devices. A hub can be thought of as the center of a bicycle wheel, where the
spokes (individual computers) meet. A hub works on the physical layer of the
OSI model.
Routers: - A router is
an essential network device for interconnecting two or more networks. Router’s
sole aim is to trace the best route for information to travel. As network
traffic changes during the day, routers can redirect information to take less
congested routes. A router creates and/or maintains a table, called a routing
table that stores the best routes to certain network destination. Routers are
generally expensive and difficult to configure and maintain. Most routers
operate by examining incoming or outgoing signals for information at the
network layer. In addition, they can permit or deny network communication with
a particular network.
Repeaters: - A repeater is
an electronic device that operates on the physical layer of the OSI model. A
repeater installed on the link receives signal, regenerates it, and sends the
refreshed copy back to the link. Nowadays, the terms repeater and hub are used
synonymously, but they are actually not the same. Although at its very basic
level, a hub can be thought of as a multiport repeater.
Gateways: - A gateway is
an internetworking device, which joins two different network protocols
together. It works on all the seven layers of the OSI Model. Gateways are also
known as protocol converters. A gateway accepts the packet formatted for one
protocol and converts the formatted packet into another protocol. A network
gateway can be implemented completely in software, hardware or as a combination
of both.
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