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Lecture 13


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									INTRODUCTION TO COMPUTER                                          LECTURE NO
A network is a way to connect computers together so that they can communicate,
exchange information, and pool resources.
Uses of Network
A network provide tremendous benefits. Four of the most compelling benefits
 Allowing simultaneous access to critical programs and data
 Allowing people to share peripheral deices, such as printers and scanners
 Streamlining personal communication with e-mail
 Making the backup process easier
Simultaneous Access
It is a fact business computing that multiple employees, using a computer
network, often need access to the same data at the same time. Without a network
that enables file-sharing employees keep separate copies of data on different
hard disks, and universally updating the data becomes very difficult. It is also
true that most office workers use the same programs. One solution t purchasing
separate copies of applications for every wo4rker is to use network versions of
programs. A network version of a software application is also a more efficient
use of hard disk space because many users can access a single shared copy
instead of storing separate copies on each user’s hard disk.
Shared Peripheral Devices
Perhaps the best incentive of r small business to link computers in a network is to
share peripheral devices, especially expensive ones such as laser printers, large
hard disks, and scanners. Sharing a laser printer on a network makes the cost
much less prohibitive. By using a process called spooling, multiple users can
send multiple print jobs to a printer.
Personal Communication
One of the most far-reaching applications of data communication in electronics
mail (e-mail) a system of exchanging written messages (and increasingly, voice
and video messages) through a network. E-mail in something of a cross between
the postal system and a telephone answering system. In an e-mail system, each
user has a unique address. To send someone an e-mail message, you enter the
person’s e-mail address and then type the message. When you are finished, the
massage is sent to the e-mail address. The next time that user accesses the e-mail
system; it reports that mail has arrived. Some systems notify the recipient as ea h
message arrives by flashing a message on the computer screen or beeping. After
reading the message, the recipient can save it, delete it, forward it o someone
else, or respond by sending back a reply massage.
Easier Backup
In business, data is extremely valuable, so making sure that employees back up
their data is critical. One way to address this problem is to keep all valuable data
on a shared storage device that employee’s access through a network. Often he
person managing the network has the responsibility of making regular backups
INTRODUCTION TO COMPUTER                                       LECTURE NO
of the data on the shared storage device from a single, central location. Network
backup software is also available that enables backups to be made of files stored
on employees’ hard drives.
To understand the different types of networks and how they operate, it is
important to know something about how networks can be structured. First, there
are two main types of networks, distinguished mainly by geography: local area
networks (LANs) and wide area networks (WANs).
Local Area Networks
A network of computers located relatively near each other and connected by a
cable (or a small radio transmitter) is a local area network (LAN). A LAN can
consist of just two or three PCs connected together to share resources, or it can
include several hundred computers of different kinds. Any network that exists
within a single building, or even a group of adjacent buildings, is considered a
LAN. A LAN permits all the computers connected to it to share hardware,
software, and data. The most commonly shared resources are disk storage
devices and printers.
Connecting Networks
It is often helpful to connect different LANs. For example, two different
departments in a large business may each have its own LAN, but if there is
enough need for data communication between the departments, then it may be
necessary to create link between the two LANs. To understand how this can be
accomplished, you must firs know that, on a network, data is sent in small
groups called packets. A packet is a group of bits that includes a header,
payload, and control elements that are transmitted together.
Each LAN is governed by a protocol, which is a set of rules and formats for
sending and receiving data. If two LANs are built around the same
communication rules, then they can be connected with a bridge or a router. A
bridge is a relatively simple device that looks at the information in each packet
header and rebroadcasts data that is traveling from on LAN t another. A router is
a more complicated device that stores the addressing information of each
computer on each LAN and uses this information to act like an electronic post
office, sorting data and sending it along the most expedient route to its
Wide Area Networks
Typically, a wide area network (WAN) is two or more LANs connected together,
generally across a wide geographical area using high-speed or dedicated
telephone lines. For example, a company7 many have its corporate headquarters
and manufacturing facility in one city and its marketing office in another. Each
site needs resources, data, and programs locally, but it also needs to share data
with the other site. To accomplish this feat of data communication, the company
can attach a router each LAN to create a WAN. The Internet is the ultimate WAN
because it connects many thousands of computer and LANs around the world.
INTRODUCTION TO COMPUTER                                         LECTURE NO
File Server Networks
Many networks include not only nodes but also a central computer with a large
hard disk that is user of shared storage. This computer is known as the file
server, network server, or simply, server. Files and programs used by more than
one user (at different nodes) are generally kept on the server.
One relatively simple implementation of a network with nodes and a file server
is a file server network. This is a hierarchical arrangement in which each node
can have access to the files on the server but not necessarily to files on other
Client/Server Networks
One popular type of server-based network is client/server computing, a
hierarchical strategy in which individual computers share the processing and
storage workload with a central server. This type of arrangement requires
specialized software for both the individual node and the network server.
Client/server software is valuable to large, modern organizations because it
distributes processing and storage workloads among resources efficiently. This
means that users get the information they need faster.
Peer-to-Peer Computing
A third arrangement is a peer-to-peer network, in which all nodes on the
network have equal relationships to all others, and all have similar types of
software. Typically, each node has access to at least some of the resources on all
other nodes, so the relationship is nonhierarchical.
Peer-to-peer LANs are commonly set up in small organizations (fewer than 50
employees) or in schools, where the primary benefit of a network is shared
storage, printers, and enhanced communication. Where large databases are use,
LANs are more likely to include client/server relationships.
Topology---the physical layout of the cables that connect the nodes of the
network. There are three basic topologies: bus, star, and ring. Network designers
consider a number of factors in determining which topology, or combination of
topologies, to use.
The Bus Topology
A bus network, like the bus of a computer itself, is a single conduit to which all
the network nodes and peripheral devices are attached. Nodes on one type of
bus network, Ethernet, transmit data at any time, regardless of any data being
sent by other nodes. If one set of data happens to collide with another set of data
transmitted by other nodes---the is, if two nodes try to send data at the same
time----each node waits a small, random amount of time and then attempts to
retransmit the data.
The Star Topology
A star network places a hub in the center of the network nodes. Groups of data
are routed through the central hub to their destinations. This scheme has an

INTRODUCTION TO COMPUTER                                          LECTURE NO
advantage in that the hub monitors traffic and prevents collisions, and a broken
connection does not affect the rest of the network. If you lose the hub, however,
the entire network goes down.
The Ring Topology
The ring topology connects the nodes of the network in a circular chain in which
each node is connected to the next. The final node in the chain connects to the firs
to complete the ring. With this methodology, each node examines data that is
sent through the ring. If the at is not addressed to the node examining it, that
node passes it along to the next node in the ring. The ring topology has a
substantial advantage over the bus topology. There’s no danger of collisions
because data lays flows in one direction. One drawback to the ring, however, is
that if a connection is broken, the entire network goes down.


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