Transcript Networks

Networks
Learning Objectives
Explain the client-server model of networked computers.
Give examples of applications which use the client-server model.
Describe what is meant by the World Wide Web (WWW) and the
Internet
Explain how hardware is used to support the Internet: networks,
routers, gateways, servers.
Explain how communication systems are used to support the
Internet: The Public Service Telephone Network (PSTN), dedicated
lines, cell phone network
Explain the benefits and drawbacks of using copper cable, fibreoptic cabling, radio waves, microwaves, satellites.
Show understanding of bit streaming (both real-time and ondemand)
Show understanding of the importance of bit rates/broadband speed
on bit streaming.
Stand-alone machines
A computer:
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cannot communicate with and is not
connected to other computers.
with all devices connected directly to it.
Disadvantages of stand-alone
machines
Users cannot easily share information
Every computer needs to be connected to
a printer.
With these problems in mind, it was
realised at an early stage that connecting
these computers together would overcome
many of these disadvantages.
Servers and Workstations / Terminals
Workstations / Terminals:
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A computer connected to other computers.
Servers:
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A computer that 'serves‘ and store files for the
workstations.
They are often more powerful than a standard
computer because they need to store a vast
amount of data.
Communications software
Necessary to allow computers to
communicate.
Computer Networks
Advantages
A user can access their files from any workstation /
terminal.
Files and software can be shared by users.
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Also some software can installed on all workstations from the
server which makes installation quicker and easier.
Users can communicate with others on the network.
Individual workstations do not need a printer, one high
quality printer can now be shared by everyone, thus
cutting costs.
Networks provide security:
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A user must have the correct Password and User ID in order to
be able to access the information on the network.
Computer Networks
Disadvantages
Networks can be expensive to set up:
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They often involve taking up floors and ceilings to lay
hundreds of metres of cables.
The Server needs to be a powerful computer,
which often means that it is expensive.
Networks are vulnerable to security problems:
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Hackers, disgruntled employees or even competitors
might try to break into the system to read or damage
crucial information.
Much effort is spent preventing unauthorised access
to data and software.
Computer Networks
Disadvantages
If the main Server breaks down, then the whole
system becomes useless and no-one can carry
on working.
Because networks are often complicated, they
need expensive expert staff to look after them.
As the number of users increase on the
network, the performance of the system can be
affected and things start to slow down.
World Wide Web / Internet
The Internet is worldwide and intended to be accessed
by anybody.
A global network connecting millions of computers.
The best “known” network.
More than 100 countries are linked into exchanges of
data, news and opinions.
The World Wide Web, or just "the Web," as ordinary
people call it, is a subset of the Internet. The Web
consists of pages that can be accessed using a Web
browser.
The Internet is the actual network of networks where all
the information resides.
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Things like Telnet, FTP, Internet gaming, Internet Relay Chat
(IRC), and e-mail are all part of the Internet, but are not part of
the World Wide Web.
Advantages
Convenience:

accessible at home, work, or through a wireless
device
Easy to contact other people no matter where
they are.
Access to other cultures
Access to a vast amount of information
Can be used Night or Day - it never stops
Can be used to learn (or teach)
Makes it possible to Telework
Possible to buy and sell things online
Advantages
Can present their work more neatly
Pupils can share ideas
Less waste of paper
Disadvantages
Must have a home computer
Transmission of viruses
Must have Internet / Modem
Must remember passwords / usernames
Pupils can copy off others
Disadvantages
Undesirable sites may be accessed
Have to pay for internet access
May receive unsolicited e-mails from
undesirable characters
Health reasons
Demand on equipment
Can waste time exploring unreliable sites
Can get distracted playing games
Transmission Media
(cables)
Presented in order of performance
(increasing expense, speed of communication, level of traffic that can be
managed, difficulty and usable length of installation and decreasing
susceptibility to interference)
http://members.tripod.com/barhoush_2/cabling.htm
http://fcit.usf.edu/network/chap4/chap4.htm
http://www.ecst.csuchico.edu/~sim/547/Old547/notes/NOTE4.htm
Copper - UTP & Shielded TP
Unshielded Twisted Pair (UTP)
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Two conductors, enclosed in a
plastic sheath, twisted together.
Shielded Twisted Pair
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Has a grounded screen around the
conductors to shield the cable from
extraneous electromagnetic
radiation.
Used in pre-existing telephone networks.
Changing current denotes different signals.
Copper - Coaxial cable
A central conductor enclosed in a plastic sheath
surrounded by a copper sheath which is
surrounded by a plastic coating.
Baseband coaxial:
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Single copper braid shield.
Broadband coaxial:
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Two shields
1 - aluminium & 1 - copper.
Copper screen
conductor
Central conductor
Plastic insulators
Used in pre-existing telephone networks.
Changing current denotes different signals.
Optical Fiber
- Used as backbones (fast route) for LANs
or in WANs
Contains many optical fibers:
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Glass or plastic fibers designed
to guide light along them.
Signals transmitted as light
pulses so travel at the speed of
light with the highest bandwidth
currently possible.
Separate fibres can be used for
separate signals.
Wireless Communication
Radio signals:
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Satellite communications.
Microwave signals:
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Communication must be a straight line from sender to
receiver.
Wireless communication allows for mobile
communication.
what-when-how.com/data-communications-andnetworking/communication-media-data-communicationsand-networking
Routers
LAN
Router
Public
network
Router
LAN
Routers
Connect different types of network
together.
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Stores logical IP addresses and can deal with
complex networks where there is more than
one possible path to a destination by
calculating the optimum route at the time.
i.e. Directs packets across a packet switched
network.

Is software based as it uses its own operating
system to make its decisions.
Routers
Directs messages down different routes
according to their desired destinations.
Use network addresses and addresses of other
routers to create a route between two networks so
must keep tables of addresses.
 These tables are copied between routers using
routing information protocol (RIP).
 Often used to connects LANs & WANs e.g. connect a
LAN to the internet using one IP address.
Operates at all layers up to the transport layer of OSI.
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Routing Algorithm
1. Router receives a network level packet.
2. Network destination address is checked.
3. Packet is sent to the appropriate
network.
Gateway
A gateway is a network point/router that acts as an
entrance to another network.
On the Internet, a node/router or stopping point can be
either a gateway node or a host (end-point) node/router.
Both the computers of Internet users and the computers
that serve pages to users are host nodes.
The computers that control traffic within your company's
network or at your local Internet service provider (ISP)
are gateway nodes/routers.
In the network for an enterprise, a computer server
acting as a gateway node is often also acting as a proxy
server and a firewall server.
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http://en.wikipedia.org/wiki/Default_gateway
Proxy Server
www.igcseict.info/theory/4/hware/index.html
A proxy server is a computer setup to share a
resource, usually an Internet connection.
Other computers can request a web page via the
proxy server. The proxy server will then get the page
using its Internet connection, and pass it back to the
computer who asked for it.
Proxy servers are often used instead of router since
additional software can be easily installed on the
computer such as anti-virus, web filtering etc.
Bit / Baud Rate
The speed at which a given amount of
data can be transmitted.

The number of bits that can be sent in one
second is known as the BIT RATE.
The units used to measure the bit rate are BAUD.
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1 baud = 1 bit per second.
Transmitting Pictures / Graphics
Pictures take up a lot of memory because each
pixel must be defined.
High resolution (quality) means more pixels and
more colours.
There is a limit to the amount of information that
can be transmitted in a given time down the
phone line.
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Choice 1:
Have as much detail as possible and accept that it will take a
long time to transmit it.
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Choice 2:
Limit the resolution (quality) of the picture, meaning that there
is less information to send so that the message will be sent
more quickly.
Transmitting Text
Text files contain a relatively small amount
of data (compared to pictures / graphics / video)
because each character only takes one
byte.
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Text files are not time sensitive.
They can still be read even if the bit / baud rate is
slow and they get sent pretty quickly anyway due
to their relatively small size.
Transmitting Video
Large amount of data which must be
transmitted in a standard time frame.
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A video cannot be run (watched) without a
high bit / baud rate.
Importance of Bit / Baud Rate
More important the more data there is.

Due to the increasing time required for transfer.
Different volumes per second mean that different
transfer rates are appropriate.
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Videos demand a high bit / baud rate
Contain a large amount of data.
Must be transmitted in a standard time frame.
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Pictures take a long time without a high bit / baud
rate.
Large amount of data because each pixel must be defined.
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Text doesn’t need a high bit / baud rate.
Small amount of data.
Not time sensitive.
Bit Streaming
Multimedia that is constantly received by and
presented to an end-user while being delivered
by a provider.
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Video on demand:
Systems which allow users to select and watch/listen to video
or audio content when they choose to, rather than having to
watch at a specific broadcast time.
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https://en.wikipedia.org/wiki/Video_on_demand
Real-time:
Refers to content delivered live over the Internet.
Requires a form of source media (e.g. a video camera, an audio
interface, screen capture software) and an encoder to digitize
the content, a media publisher, and a content delivery
network to distribute and deliver the content.
https://en.wikipedia.org/wiki/Streaming_media
Plenary
What are the advantages and
disadvantages of using the Internet?
Advantages
Allows communication.
Pupils can share ideas.
Large amounts of educational material.
Allows students to access information to
help with studies and to enjoy their
learning.
Allows student to learn at own speed / at
own times.
Disadvantages
Many sites do not give accurate information.
Many sites give access to inappropriate material
(downloading music / ordering drugs / watching
pornography).
Health and Safety problems.
Sites available to buy coursework / helps to
cheat in examinations.
May allow hacker into your system / viruses
prevalent.
Disadvantages
Undesirable sites may be accessed
Have to pay for internet access
May receive unsolicited e-mails from
undesirable characters
Health reasons
Demand on equipment
Can waste time exploring unreliable sites
Can get distracted playing games
Plenary
Explain the part played in network
systems by:
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Switches
Routers
Bridges
Modems
Plenary
Switches
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A device that allows many terminals all to use the
same communications line at different times.
Routers
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Connects different types of network together.
Software at a node (on the network).
Which directs messages down different routes.
According to their desired destination.
Plenary
Bridges
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Links two LANs (which may or may not be similar).
Uses address information in packets.
Has the ability to learn the layouts of the networks.
Can control access from one part of the network to
the other.
Modems
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Necessary if communication link is analogue.
Converts digital signals to analogue for
transmission.
Plenary
Explain the importance of bit rates to
communication.
Importance of Bit / Baud Rate
More important the more data there is.
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Due to the increasing time required for transfer.
Different volumes per second mean that different
transfer rates are appropriate.

Videos demand a high bit / baud rate
Contain a large amount of data.
Must be transmitted in a standard time frame.

Pictures take a long time without a high bit / baud
rate.
Large amount of data because each pixel must be defined.
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Text doesn’t need a high bit / baud rate.
Small amount of data.
Not time sensitive.