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Chapter-Three
3. Data Communications, Networks and Internet
Objectives:
To know basic component used in data transmission
To know, what a computer network is and different types of networks
To know what is Internet and what service Internet has.
Information is transmitted in the form of analog or digital. Most communication lines are
designed to carry analog signals. Digital transmission a rarely used for telecommunications.
Therefore, a technical must be used to represent a digital signal or an analog carrier
Modulation: Converting digital signals into analog signals.
Demodulation: Converting analog signals back into digital signals.
Modem: Short for modulator/demodulator: A communications device that converts one
form of a signal to another that is suitable for transmission over communication circuits,
typically from digital to analog and then from analog to digital
Types of Signals
a. Digital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values (a
quantified discrete-time signal), for example an arbitrary bit stream, or of a digitized (sampled).
A signal that is generated by means of a digital modulation method (digital passband
transmission), produced by a modem, is in the first case considered as a digital signal, and in the
second case as converted to an analog signal.
b. Analog signal
An analog signal is any continuous signal for which the time varying feature (variable) of the
signal is a representation of some other time varying quantity, i.e., analogous/similar to another
time varying signal. It differs from a digital signal in terms of small fluctuations in the signal
which are meaningful.
Any information may be conveyed by an analog signal; often such a signal is a measured
response to changes in physical phenomena, such as sound, light, temperature, or pressure. An
analog signal is one where at each point in time the value of the signal is significant, whereas a
digital signal is one where at each point in time, the value of the signal must be above or below
some discrete threshold
.
The primary disadvantage of analog signaling is that any system has noise – i.e., random
unwanted variation. As the signal is copied and re-copied, or transmitted over long distances,
these apparently random variations become dominant. Electrically, these losses can be
diminished by shielding, good connections, and several cable types such as coaxial or twisted
pair.
The effects of noise create signal loss and distortion. This is impossible to recover, since
amplifying the signal to recover attenuated parts of the signal amplifies the noise
(distortion/interference) as well. Even if the resolution of an analog signal is higher than a
comparable digital signal, the difference can be overshadowed by the noise in the signal.
3. 1. Data Transmission
The need of information has increased from time to time. This leads to the need of sharing of
information among different agents (individual), which may be at different places or locations.
Data communication is the exchange of information between two agents. For exchange of
information the information should be transmitted from one point to another through a
transmission media called Channel. The following figure shows the different components of
data communication.
Agent agent
Source System Destination system
Input
device
Transmi
tter
Transmiss
ion
medium
Receiver
Output
device
- Unguided Transmission media– Data transmission through air /space (i.e wireless
system)
- Guided Transmission media
Data transmission is through solid medium (wired system). Different types of cables are
used to connect devices
Commonly used cable types are:
Twisted pair
Coaxial cable
Optical fiber
a. Twisted Pair:
Insulated copper wires arranged in regular spiral pattern
The oldest, least expensive, and most commonly used media
Reduce susceptibility to interference than straight pair wires (two straight parallel wires
tend to act as an antenna and pick up extraneous signals when compared to twisted pairs)
Highly susceptible to electrical noise, interference, and ‘tapping’ of the signal as
compared to the other guided media
Arrangement of twisted pairs into group used for high-speed LAN
Types of Twisted pairs
Unshielded twisted pairs (UTP)
The most commonly used cable types in LANs
Made up of up to four twisted pairs enclosed in a plastic jacket
UTP is highly subjected to external electromagnetic interference.
Different types of UTP cables are used for different purposes. Currently, the most
commonly used UTP cables are Category 5 Cables:
Categories of UTP Cables
UTP cables according to the quality:
Category 1 ― the lowest quality, only good for voice, mainly found in very old
buildings, not recommended now
Category 2 ― good for voice and low data rates (up to 4Mbps for low-speed token ring
networks)
Category 3 ― at least 3 twists per foot, for up to 10 Mbps (common in phone networks in
residential buildings)
Category 4 ― up to 16 Mbps (mainly for token rings)
Category 5 (or 5e ) ― up to 100 Mbps (common for networks targeted for high-speed
data communications)
Category 6 ― more twists than Cat 5, up to 1 Gbps
Shielded Twisted Pairs (STP)
Are similar to UTP except the extra shield applied to the cables. The shield is used to
reduce external electrical interference and is suitable to be used in large industries
where there is a high level of electrical interference
b. Coaxial Cable
Most versatile medium used in LANs, Cable TV, VCR-to-TV connections
Noise immunity is better than twisted pair
Less susceptible to interference and cross talk but there still is attenuation and thermal
noise problem
Can go up to 185m or 500m without the need for an amplifier/repeater
c. Optical Fiber cables
Flexible, thin, made of very pure glass / plastic fiber capable of conducting optical (light)
rays
Extremely high bandwidth. They are used for high data transmission rate
Very high noise immunity, resistant to electromagnetic interference
Does not radiate energy/cause interference
Very difficult to tap
Better security but multipoint not easy
Need optical-electrical interface (more expensive than electrical interface) Fiber
Optimum transmission in 1 - 10 GHz range
Bandwidth of 100’s MHz
270msSignificant propagation delay about
VSAT ( Very small Aperture Terminal) :- High speed data transmission using satellite
c. Infrared Transmission
For short-range communication
o Remote controls for TVs, VCRs, and stereos
o Indoor wireless LANs
Do not pass through solid walls
o Better security and no interference (with a similar system in adjacent rooms)
No government license is needed
Cannot be used outdoors (due to the sunshine)
C
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When a computer system is processing data all by itself, without any interaction or
interconnection with any other computer system, it is called a stand-alone computer system. A
Network System is a system of two or more computers that are connected to each other for the
purpose of data communication and sharing of resources.
A network also consists of communication channels that are responsible for connecting devices
with each other and transmitting information...
Some of the most important advantages of a computer network system are:
Data sharing between different users of computers with certain amount of data
security and access control.
Sharing of software application systems.
Distribution of computing load on computers at distributed locations to reduce
peak load.
Sharing or high power computing resources situated at a central location on a
need basis.
Sharing of costly special devices by several computer users.
Components of Computer Networks
The components required to operate a network can be divided into two major categories -
Hardware and Software
Hardware
The hardware components of a network consist of the following five items
Server
Workstation
Cabling
Network Interface Cards (NIC)
Other Networking Devices
The Server
High capacity computers that function as the central repository of resources/services
the network provides.
Usually the server is also responsible for controlling which users on which computers
in the network are granted access to a service.
Workstations
A place where the services of the server are used (This is where the user is working)
Any computer where the user will be working at. It does not have to be a high capacity
computer
Cabling
Cables that interconnect the different devices on the network.
There are various types of cables, including coaxial cable, twisted-pair cable and fiber-
optic cables.
Wireless networks do not require physical cables
Network interface card (NIC)
The NIC is located in every workstation on the network as well as in the servers.
It is a card that plugs into one of the expansion slots on the motherboard of a network
device, thereby providing a connector on the back to connect the device to the network
It acts as a “transmitter” and “receiver” and therefore can be referred to as transceiver
Less disk storage space is required because the program is stored only once on the server,
instead of being stored on the hard disks of multiple standalone computers
When a new version of the software is released, it is easier to update on copy of the
program on the server than to update many copies stored on standalone computer.
Purchasing a software license for a network can be less expensive than purchasing a
single-user license for every workstation on the network
In short sharing programs on a network
Saves disk space
Reduces maintenance
Reduce licensing cost
b. Sharing Hardware
Computer networks enable us to share expensive hardware resource among several computers.
A typical example of shared resource is printer. For example, a company may prefer to acquire
one expensive printer and connect it to the network to provide high quality printing to users. This
avoids the need to have separate printer for each computer in the office.
3. 4. 2. Centralizing Administration and Support
Networking computers can simplify support tasks as well. It is far more efficient for technical
personnel to support one version of one operating system or application and to set up all
computers in the same manner than to support many individual and unique systems and setups.
Even if your computer is physically connected to a network, you cannot typically use network
resources until you log into the network. When you log in, you formally identify yourself to the
network by providing your ID and password.
3. 4. 3. Sharing Information/data
Users in a certain network environment have the liberty of sharing data and information across
the network. Data sharing enables different users to work on a certain file concurrently. Few
examples of data sharing are:
o Database : - databases are often managed centrally and several users can have
access to the database at same time. For example, in a networked banking
system, different bank branches can have access to the central account database.
This enables bank clients to carry their transactions on any branch bank office.
o E-mail : email communication can be achieved over the network enabling
networked users in the company to communicate messages across the network
using email.
o Intranet : Intranets are similar to World Wide Web (WWW) where centrally
stored hypertext documents can be accessed using the web. Unlike the WWW,
intranets are available only to user within the company network system. Intranet
is a very common service in large networked organizations.
o Extranet : Although similar to intranet, extranet provides selected users from
outside the organization to access data from the internal network. Extranets are
commonly used by suppliers to provide data to company clients.
3.5. Types of computer network
Computer Network is a collection of computers and terminal devices connected by a
communication system. The use of computer network is to share resources like file, device,
printer, scanner, and program. Topology refers to the way in which multiple devices are
interconnected via communication links.
There are different types of network based on the geographical area covered by a network: -
Local area Network (LAN)
Metropolitan Area Network (MAN)
wide Area Network (WAN) and
Personal Area Network (PAN)
A local area network is a network confined to a small area like a building. Most LAN has
communication stations that are physically linked by a cable.
Advantages of LAN
Speed
Cost
Security
Personal Area Network (PAN)
A PAN is a network that is used for communicating among computers and computer devices
(including telephones) in close proximity of around a few meters within a room It can be used
for communicating between the devices themselves, or for connecting to a larger network such
as the internet. PAN’s can be wired or wireless
A personal area network (PAN) is a computer network used for communication among computer
devices, including telephones and personal digital assistants, in proximity to an individual's
body. The devices may or may not belong to the person in question. The reach of a PAN is
typically a few meters.
According to Span of Control
Based on span of control computer networks can be classified into two major groups:
Centralized Network
Distributed Network
I) Centralized Network
Centralized network has one main CPU that processes all information requests and handles
communication. The main CPU in a centralized network is usually a mainframe or minicomputer
capable of handling the processing workload of many people simultaneously. People interface
with the host computer by using terminals and other input and output devices.
A terminal is a hardware device consisting of a keyboard and monitor. There are two types of
terminals:
Dumb Terminals
Intelligent Terminals
Terminals that have no storage or processing capabilities are called dumb terminals. Because
dumb terminals do not have any processing capabilities, they must be connected to a host
computer that can perform any processing functions necessary.
Intelligent terminals, on the other hand, do have limited storage and processing capabilities.
II) Distributed Network
Distributed network is a collection of workstations connected to each other, along with various
shared storage and input and output devices (for example, scanners and printers). In a distributed
network each workstation can handle some, if not all, of its own processing. Workstations in a
distributed network also maintain local information and software.
Distributed Network can be either
Client/Server, or
Peer-to-Peer
A Client/Server network is a distributed network in which many workstations (called Clients)
are connected to a central host computer (called the Server). The clients are the workstations in a
Client/Server network that maintain local software and information and do as much of the
processing as possible.
File ServerFile Server
Capabilities of the network
Growth of the network
Way the network is managed
Developing a sense of how to use the different topologies is a key to understanding the
capabilities of the different types of networks.
Before computers can share resources or perform other communication tasks they must be
connected. Most networks use cable to connect one computer to another. However, it is not as
simple as just plugging a computer into a cable connecting to other computers. Different types of
cable—combined with different network cards, network operating systems, and other
components—require different types of arrangements. To work well, a network topology takes
planning. For example, a particular topology can determine not only the type of cable used but
also how the cabling runs through floors, ceilings, and walls. Topology can also determine how
computers communicate on the network. Different topologies require different communication
methods, and these methods have a great influence on the network.
There are five basic types of computer topology: bus, star, ring and mesh
Bus topology: - In bus topology, all stations attach through appropriate interfacing hardware,
directly to a line as transmission medium, or bus. A transmission from any station propagates
the length of the medium in both directions and can be received by all other stations. The
message that is transmitted contains the address of the station and the data and each station
monitors the medium and copies packets addressed to itself. Because all stations share a common
transmission link, only one station can successfully transmit at a time, duo some form of medium
access control technique is needed to regulate access.
Ring topology: - In the ring topology the network consists of a set repeaters joined by point – to
point links in a closed loop. The links are unidirectional, that is, data are transmitted in one
direction only, and all oriented in the same way.
- It is very easy to extend
Disadvantages:
- Fault diagnosis and isolation is difficult
Hybrid Topology : By modifying and combining some of the characteristics of these “pure”
network topologies, “hybrid” topologies result that can often provide greater efficiency.
3. 6 Internet and Electronic communication
Internet is a wide area network, which is the network of networks. Internet has a lot of services.
Electronic- Mail (E-mail)
E-mail is one of the most widely used services on the internet, E-mail is easy to send, read, reply
to and manage, and it is fast and convenient. E-mail is also global, allowing ménages to pass
from Japan to the U.S in minutes, regardless or the time they were sent. It is much cheaper to
send an e-mail manage then a letter, or to make a long distance telephone call. Another
advantage of e-mail is, it is possible to send the same message to any number of individuals as
long as you know their e-mail address.
World Wide Web (WWW):- Is one of the newest Internet services. The WWW allows you to
combine text, a video, graphics, and even animation to make a document a viewed easy. Links
within WWW documents can take you quickly to other related documents. WWW is a set of
sites that you can go o for information.
To access the WWW, it is necessary that you run a WWW browser on your computer.
3. 7 Network Devices
3 .7.1 Network Interface Card
Network interface card (NICs,) also known as network adapters provide the interface between
cables, and computers. The cards are installed in an expansion slot in each computer and server
on the network.
Components that enable you to expand the network include:
Modem
Repeaters and Hubs
Bridges
Switches
Routers
Gateways
a) Modem
A modem is a device that makes it possible for computers to communicate over a telephone line.
When computers are too far apart to be joined by a standard computer cable, a modem can
enable communication between them. In a network environment, modems serve as a means of
communication between networks and as a way to connect to the world beyond the local
network.
Computers cannot simply be connected to each other over a telephone line, because computers
communicate by sending digital electronic pulses (electronic signals), and a telephone line can
send only analog waves (sound).
The modem at the sending end converts the computer's digital signals into analog waves and
transmits the analog waves onto the telephone line. A modem at the receiving end converts the
incoming analog signals back into digital signals for the receiving computer.
In other words, sending modem MOdulates digital signals into analog signals, and receiving
modem DEModulates analog signals back into digital signals.
Modems convert digital signals to analog waves, and convert analog waves to digital signals
b) Repeaters
Repeaters receive signals and retransmit them at their original strength and definition. This
increases the practical length of a cable. (If a cable is very long, the signal weakens and become
unrecognizable.) Installing a repeater between cable segments enables to travel farther.
