Chapter 2: PCs and Networking
Computers:
The basic
tools essential for e-Commerce are computers and networks. The computers are
either workstations of individual office workers, or servers wherein large
databases and other information resides. The Operating System or OS is
the most basic program within a computer. An OS manages the resources of the
computer system in a fair, efficient and secure way. Resources include memory
(main as well as secondary), peripherals such as printers, Input-Output devices
and the Central Processing Unit (CPU).
In the days of
mainframe computers, each machine would have its own OS which would have come
from the manufacturer. Some of the most common OS, that we have today are DOS, Windows
and many flavors of Unix.
The Disk
Operating System or DOS:
It was first
released by IBM in August 1981 as PC-DOS. Microsoft’s MS-DOS was also developed
around the same time and soon MS-DOS became the standard OS shipped with IBM
Personal Computers.
Windows OS:
Development of
the Windows OS was started by Microsoft in the early 1980s. The objective was
to develop an easy-to-use graphical interface with drop-down menus, mouse
support and multi-tasking.
Windows 2000
Professional:
It was designed
to replace Windows 95, Windows 98, and Windows NT Workstation, with added major
improvements in reliability, ease of use, Internet compatibility, and support
for mobile computing.
Windows XP
(experience):
It was launched
in October 2000. While Windows XP Professional built on the foundation of
Windows 2000 to improve reliability, security and performance.
Network Topologies:
There are
different topologies in which computers can be connected to one another over
networks. Following are the types of topologies –
- 1. Bus Technology
- 2. Ring Topology
- 3. Star Topology
- 4. Mesh Topology
- 5. Tree Topology
Bus Technology:
The topology of
the bus network is shown in Fig. This topology is commonly used to build LANs.
Every node connected on a bus network is allowed to receive every transmission
on that network. The main problem faced in implementing this topology is the
fact that only one communication channel exists to serve the entire network. As
a result, if this channel fails, then the whole network will go out of
operation.
Ring Topology:
The ring
topology is another popular topology used for configuring networks. As shown in
the Fig. 3.2, the data in a ring network flows in a circular fashion.
Ref: JavaTpointMostly data
flows in one direction, with one node receiving the transmission and relaying
it to the next node in the ring. Here too, there is a single channel to connect
the nodes. In the event of a channel failure between two nodes, the entire
network goes down. Network suppliers sometimes develop ring networks with two
rings so that in case of a single channel failure, the network continues to
function. This topology too is more commonly deployed in LANs.
Star Topology:
The topology of the star network normally used in WANs, is depicted in Fig. 3.3. At the center of a star network is the hub through which all traffic is routed. As a result, in the event of the failure of the hub computer, the network too will fail.
Mesh Topology:
The mesh topology, which is shown
in Fig. 3.4, has been used more frequently in recent years.
Due to the multiple number of paths
between nodes, the reliability of the network is improved. However, this
improvement comes for a price, and mesh networks are much more expensive as
compared to networks based on other topologies.
Tree Topology:
The tree, or a
hierarchical network topology is one of the simpler and more common topologies
found today. Figure 3.5 shows an example of a tree network. Reliability
problems can arise in this configuration due to the control exercised by the
topmost node in the ‘tree’. This topology too is used to set up WANs.
Due to the multiple number of paths
between nodes, the reliability of the network is improved. However, this
improvement comes for a price, and mesh networks are much more expensive as
compared to networks based on other topologies.
Tree Topology:
The tree, or a
hierarchical network topology is one of the simpler and more common topologies
found today. Figure shows an example of a tree network. Reliability
problems can arise in this configuration due to the control exercised by the
topmost node in the ‘tree’. This topology too is used to set up WANs.
Fig. Ref: Tutorial Point
Communication Media:
There are a large variety of
communication media that are used to interconnect computers. Following are the most
common transmission media used.
1. Twisted
Pair
2. Coaxial
Cable
3. Optical
Fibers
4. Satellite
communication
Twisted Pair:
Twisted pair is a physical media made up of a pair of
cables twisted with each other. A twisted pair cable is cheap as compared to
other transmission media. Installation of the twisted pair cable is easy, and
it is a lightweight cable. The frequency range for twisted pair cable is from 0
to 3.5KHz.
A twisted pair consists of two insulated copper wires
arranged in a regular spiral pattern.
The degree of reduction in noise interference is
determined by the number of turns per foot. Increasing the number of turns per
foot decreases noise interference.
Fig Ref: JavaTpoint
Advantages Of Unshielded Twisted
Pair:
- It
is cheap.
- Installation
of the unshielded twisted pair is easy.
- It
can be used for high-speed LAN.
Disadvantage:
- This
cable can only be used for shorter distances because of attenuation.
Co-axial :
Another very
commonly used transmission media is the co-axial cable. Co-axial cables contain
a very thick copper wire at the center. This wire is surrounded by insulating
material, which, in turn, is encased in a cylindrical conductor. The conductor
is again wrapped in protective plastic. These cables are therefore able to
provide higher bandwidth (1–2 Gbps on short distances) with superior noise
immunity.
Fig Ref: JavaTpoint
Advantages Of Coaxial cable:
The data can be
transmitted at high speed.
It has better
shielding as compared to twisted pair cable.
It provides
higher bandwidth.
Disadvantages
Of Coaxial cable:
It is more
expensive as compared to twisted pair cable.
If any fault
occurs in the cable causes the failure in the entire network.
They are made
from ultra-thin fibers of glass. Information is sent on optical fibers in the
form of light pulses, which when detected, generate electric pulses. Fiber
optics can be used for LANs as well as for long distance transmissions. While
supporting much higher bandwidths than copper, it is also much more reliable,
has relatively lower attenuation and is much more difficult to tap.
Fig Ref: JavaTpoint
Basic elements of Fibre optic cable:
AD
- Core: The
optical fiber consists of a narrow strand of glass or plastic known as a
core. A core is a light transmission area of the fiber. The more the area
of the core, the more light will be transmitted into the fiber.
- Cladding: The concentric layer of glass is known as cladding. The
main functionality of the cladding is to provide the lower refractive
index at the core interface as to cause the reflection within the core so
that the light waves are transmitted through the fiber.
- Jacket: The
protective coating consisting of plastic is known as a jacket. The main
purpose of a jacket is to preserve the fiber strength, absorb shock and
extra fiber protection.
Satellite communication:
provides reliable data transmission over a network of
a large number of geographically distributed sites. Not only textual data, but
images, voice and video are also transmitted over satellite networks. An
essentially broadcast medium, communication can be achieved in two ways—either
directly between end-users, or via a central or master station, which relays
the contents of the communication to the destination. Satellite communication
has been used in sectors such as television broadcasting and public telecommunications.
Satellites used for communications are almost
exclusively in the geostationary orbit, located at 36,000 km above the equator.
Satellites are launched and operated by organizations like Intelsat, which
offer data communication speeds ranging from 1200 bps (bits per second) to
several Mbps.
The former are in the C-band which operates at lower
frequencies, while high data rates are possible at very high frequencies in the
gigahertz (GHz) range in what is known as the Ku-band. In between, the extended
C-band offers data communication at 64 Kbps or higher rates. Regional and
national satellites have also been launched. Fig. 3.7 A model of satellite
communication. The Indian satellites are known as INSAT. They enable
communication in C-band, extended C-band and the Ku-band.
VSAT or Very Small Aperture
Terminal:
This is an end-user equipment used to receive and
send data, images, voice and video over the satellite network. With a typical
antenna size of 1.2 to 2.4 meters, a VSAT gives full access to a network which
may comprise hundreds or even thousands of nodes.
The interactive nature of a VSAT allows two-way
communication from remote locations in the same manner as the terrestrial telephone
network. VSATs are more reliable than normal leased lines as there is no
question of digging or damage to the cables.
In general, fixed, commercial VSAT systems use
satellite transponders operating at C-band (uplink 6 GHz and downlink 4 GHz) or
Ku-band (uplink 14 GHz and downlink 11 or 12 Ghz).
VSAT networks are generally set up in one of the
following three configurations:
Point-to-point networks: It provide two-way communications
between two VSATs located at remote sites.
Star networks: It provide multi-point
communications between a Master Earth Station (MES) or ‘hub’ and VSATs located
at multiple remote sites.
Mesh networks: It provide direct
communications between multiple VSATs located at different sites on the network.
In the Star configuration, communication is
‘double-hop’, i.e. all communications are routed through the MES or hub
whereas, in the case of point-to-point and mesh networks ‘single-hop’
communications is achieved without the need for going via the hub station.
These communication channels are not permanent and are established only for the
duration of the call.
Access Schemes:
The various access schemes in use
are as follows:
1.
Frequency Division Multiple Access (FDMA)—users
share the transponder by prior allocation of individual channels. Single
Channel Per Carrier (SCPC) is an FDMA scheme in which the input data stream is
used to modulate an RF (radio frequency) carrier and assign dedicated carrier
frequency to each client.
2.
Time Division Multiple Access (TDMA)—each user
is assigned the full bandwidth of the channel for a short period, which is then
made available to another user the next period and so on. TDMA techniques are
used by mesh networks.
3.
Code Division Multiple Access (CDMA)—transmitted
signals are ‘spread’ over a bandwidth in excess of the data signal by combining
with a code signal. The codes allow individual codes to be distinguished from
each other and thereby allow sharing of a common frequency band.
4. Frequency
Time Division Multiple Access (FTDMA)—allows maximum utilization of available
bandwidth through a combination of FDMA and TDMA.
References
1.
E-Commerce The Cutting Edge of Business Second
Edition by KAMLESH K BAJAJ & DEBJANI NAG [Tata McGraw-Hill Publishing
Company Limited]
2.
Tanenbaum, Andrew S., ‘Computer Networks’ Fourth
edition, 2003
5.
www.tutorialpoint