TOPIC: Computer Networks
Communication Channels
Data communication forms an integral part of the computer world. Using this
technology, computers and other devices (such as printers and storage devices) can
communicate via communication channels. A communication channel is the path
that facilitates the communication and is composed of one or more transmission
media. These media can be either physical or wireless.
Physical Communication Channels
Physical communication channels, which are specialized cables (e.g. twisted-pair,
coaxial or fiber-optic cables) are used to connect computers and their peripherals to
form computer networks.
NETWORKS
A computer network is made up of a number of connected computers each with
their own processor, for example a number of connected PCs. Networks are popular
because they provide a number of users with access to resources (e.g. data files,
printers, software). Therefore, a network allows computing resources to be used
more efficiently between a
group of users.
In some computer networks, a device is connected directly to another device by a
dedicated communication channel, giving those devices sole use of that channel.
These are called point-to-point transmission computer networks. In a multipoint transmission computer network, one channel is used to connect three or
more devices. In general, a multi-point configuration uses a communication channel
more efficiently than a point-to-point configuration. It reduces the amount of intercabling needed thus lowering costs. A point-to-point configuration, on the other
hand, is very useful in systems that require high-speed response.
Internet
A global network connecting millions of computers. More than 100 countries are
linked into exchanges of data, news and opinions.
It is possible to gain access through a commercial Internet Service Provider (ISP).
The Internet is not synonymous with World Wide Web. The Internet uses a capability
known as the World Wide Web. World Wide Web is a system of universally
accepted standards of storing, retrieving, formatting, and displaying information in
a networked environment. Information is stored and displayed as electronic “pages”
that contain text, graphics, animations, sound and video, called web pages. These
web pages can be linked electronically to other web pages, regardless of where they
are located and viewed by any computer. The web can serve as the foundation for
new kinds of information systems, product and services. All of the web pages
maintained by an organization or individual are called a web site.
Intranet
A network based on TCP/IP protocols (an internet) belonging to an organization,
usually a corporation, accessible only by the organization's members, employees, or
others with authorization. An intranet's Web sites look and act just like any other
Web sites, but the firewall surrounding an intranet fends off unauthorized access.
Like the Internet itself, intranets are used to share information. Secure intranets are
now the fastest-growing segment of the Internet because they are much less
expensive to build and manage than private networks based on proprietary
protocols.
Extranet
A buzzword that refers to an intranet that is partially accessible to authorized
outsiders. Whereas an intranet resides behind a firewall and is accessible only to
people who are members of the same company or organization, an extranet
provides various levels of accessibility to outsiders. You can access an extranet only
if you have a valid username and password, and your identity determines which
parts of the extranet you can view.
Extranets are becoming a very popular means for business partners to exchange
information.
NETWORK CLASSIFICATIONS
Local Area Network (LANs)
A local area network (LAN) is a network of computers located in a single building, a
single site, an office suite or a group of buildings. It connects devices that are
confined to a relatively small geographical area. The parts of the network are linked
by computer cable rather than via telecommunications lines.
LAN Characteristics
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Flexibility – Many different
hardware devices e.g. plotters,
printers and computers can be
attached to a LAN.
Speed – high speed data transfer
Reliability
Hardware and software sharing
Transparent interface – network
access for users should not be
complicated.
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Adaptability – can accommodate a
variety of hardware and can be
reconfigured easily.
Access to other LANs and WANs
Security
Centralized Management
Private ownership of the LAN
Wide-Area Network (WAN)
A computer network that spans a relatively large geographical area. Typically, a
WAN consists of two or more local-area networks (LANs). Computers connected to a
wide-area network are often connected through public networks, such as the
telephone system. They can also be connected through leased lines or satellites. The
largest WAN in existence is the Internet.
At each end of the leased line, a router connects to the LAN on one side and a hub
within the WAN on the other. Network protocols including TCP/IP deliver transport
and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM
and Frame relay are often used by service providers to deliver the links that are
used in WANs.
Metropolitan-Area Networks (MANs)
A data network designed for a campus, town or city. They typically use wireless
infrastructure or optical fiber connections to link their sites. For instance a
university or college may have a MAN that joins together many of their local area
networks (LANs) situated around site of a fraction of a square kilometer. Then from
their MAN they could have several wide area network (WAN) links to other
universities or the Internet.
Benefits of Networks
Speed - Networks provide a very rapid method for sharing and transferring files.
Without a network, files are shared by copying them to floppy disks, then carrying
or sending the disks from one computer to another. This method of transferring files
(referred to as sneaker-net) is very time-consuming.
Cost - Networkable versions of many popular software programs are available at
considerable savings when compared to buying individually licensed copies. Besides
monetary savings, sharing a program on a network allows for easier upgrading of
the program. The changes have to be done only once, on the file server, instead of on
all the individual workstations.
Security - Files and programs on a network can be designated as "copy inhibit," so
that you do not have to worry about illegal copying of programs. Also, passwords
can be established for specific directories to restrict access to authorized users.
Centralized Software Management - One of the greatest benefits of installing a
network is the fact that all of the software can be loaded on one computer (the file
server). This eliminates that need to spend time and energy installing updates and
tracking files on independent computers throughout the building.
Resource Sharing - Sharing resources is another area in which a network exceeds
stand-alone computers. Most schools cannot afford enough laser printers, fax
machines, modems, scanners, and CD-ROM players for each computer. However, if
these or similar peripherals are added to a network, they can be shared by many
users.
Electronic Mail - The presence of a network provides the hardware necessary to
install an e-mail system. E-mail aids in personal and professional communication,
and it facilitates the dissemination of general information. Electronic mail on a LAN
can enable persons to communicate. If the LAN is connected to the Internet, persons
can communicate with others throughout the world.
Flexible Access - Networks allow users to access their files from computers
throughout the network. Persons can also work cooperatively through the network.
Workgroup Computing - Workgroup software (such as Microsoft BackOffice)
allows many users to work on a document or project concurrently. For example,
educators located at various schools within a county could simultaneously
contribute their ideas about new curriculum standards to the same document and
spreadsheets.
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Sharing Software
Sharing Data
Sharing System resources
Security and backup – Using
security resources of a network,
software can be safeguarded by
installing passwords, trustee rights
and file attributes.
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Easy maintenance and upgrades
Disadvantages of Installing a Network
Expensive to Install - Although a network will generally save money over time, the
initial costs of installation can be prohibitive. Cables, network cards, and software
are expensive, and the installation may require the services of a technician.
Requires Administrative Time - Proper maintenance of a network requires
considerable time and expertise.
File Server May Fail - Although a file server is no more susceptible to failure than
any other computer, when the files server "goes down," the entire network may
come to a halt. When this happens, the entire network may lose access to necessary
programs and files.
Cables May Break - The topology refers to the various configurations of cables.
Some of the configurations are designed to minimize the inconvenience of a broken
cable; with other configurations, one broken cable can stop the entire network.
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There is an increased risk of data corruption. Since many users will be using
the system, there is a greater chance of data being corrupted or tampered with.
There is a greater risk from viruses, because they are usually spread between
the computers.
Network Topology is the configuration or physical arrangement of the devices or
nodes. i.e. The layout of the computers and devices on a network. The 3 main
topologies are ring, star and bus.
Bus
A bus network is a network architecture in which there is a single central cable to
which all devices are attached. The central cable is called a bus. The bus transmits
data in both directions. Only one device can transmit at a time. When a sending
device transmits data, the address of the receiving device is included with the
transmission so that the data is routed to the appropriate receiving device. It is easy
to add/remove devices from a bus network. It is also an inexpensive topology.
Failure of one device does not affect another device. The network will fail if the bus
(central cable) fails.
Advantages of a Bus Topology
 Easy to connect a computer or peripheral to a linear bus.
 Typically the cheapest topology to implement
 Failure of one station does not affect others
Disadvantages of a Bus Topology
 Entire network shuts down if there is a break in the main cable.
 Terminators are required at both ends of the backbone cable.
 Difficulty in identifying the problem if the entire network shuts down.
 Performance degrades as additional computers are added
Ring
A ring network is a topology where each device is connected to two others, so as to
create a ring or closed
loop. Data transmitted
on a ring network travels
in one direction on the
ring from device to device
until it reaches its
destination. If a device fails, devices before it are not affected, it is the devices after it
that are affected.
Advantages of Ring Network
 Growth of the system has minimal impact on performance
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All stations have equal access
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Each node on the ring acts as a repeater (booster of the signal), allowing
ring networks to span greater distances than other physical topologies.
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Because data travels in one-direction high speeds of transmission of data
are possible.
Disadvantages of Ring Network
 Often the most expensive topology
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Failure of one computer may impact others
Star
A star network topology, in its simplest form, consists of one central, or hub
computer, which acts as a router to transmit messages. All devices are connected to
the central computer (hub). All data passes through the hub. If a device fails, there is
no effect on the network, only if the hub fails will the network be affected.
Advantages of Star Network
 Easy to implement and extend, even in large networks
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Well suited for temporary networks (quick setup)
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The failure of a non-central node will not have major effects on the
functionality of the network.
Disadvantages of Star Network
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Limited cable length and number of stations
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Maintenance costs may be higher in the long run
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Failure of the central node (hub) can disable the entire network
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Central hub can be a bottleneck.
Other topologies include mesh and extended star.
Tutorial Questions
1. Differentiate between the internet, intranet and extranet.
2. With the aid of diagrams differentiate between the bus, ring and star networks.
3. Identify two advantages and two disadvantages of the bus, ring and star
networks.
4. Which network will fail if the central cable breaks?