Transcript Chapter 16 - Communications

Communications
C.S. French
Chapter 16
communications
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Two possible communications
setups.
 Point
to point transmission
– two computers connected together
– Not so common now.
 Computer
Network
– interconnection of many computers.
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Using the Telephone system.
 Circuit
switching
– When a caller dials a number the exchanges
involved make the connection between the two
points by means of switches.
– The caller does not share the long distance line
Sligo
exchange
communications
Dublin
exchange
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Data transmission

Sound is carried along the wires as an analog
signal
– gradual voltage variations of amplitude and frequency.

Digital voltage variations must be converted to
analog for transmission over telephone lines.
– This is done with a MODEM

MODulator/DEModulator
modem
communications
modem
High frequency
Low frequency
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Data Transmission rates.





A data transmission link is normally called a ‘channel’
The capacity of a channel to carry data is related to the range of
frequencies with which it is possible to transmit data.
Capacity is normally measured in ‘bits per second’ which is also
known as ‘baud’.
Channels with higher bandwidth (range of frequencies) are called
broadband or wideband and have greater transmission capacities.
Approximate transmission baud rates:
– Twisted pair cables: ~10K Baud
– Coaxial cables:
~100MBaud
– Optical fibre:
~5GBaud
 optical fibre uses short pulses of light instead of voltages to transmit
binary data.
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Advantages of using DSL over
a modem.
 Faster
data transfer rates
– 10 times the capacity of a regular phone line.
– Can use your phone at the same time as the Internet.
 “Always
on”
– When your computer is on, you are connected to the
Internet.
– No need to dial up every time you want access..
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Multiplexers

If a communications channel is being under-utilised it is
possible to share the line using a device called a
multiplexer.
– data being transmitted may be divided into packets.
– each packet has the name of the sender and the destination added
to it.
– the multiplexer sends these packets seperately.
– The multiplxer may be transmitting several messages
simultaneously and can mix up the different packets.
– the demultiplexer at the other end directs the packets to their
correct destinations.
– this ‘packet switching’ technique is also used by telephone systems
to pack many phone conversations into a single wideband channel
(eg an optical fibre cable or a satelite link)
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
Computer
Networks
Computer Network
–

Distributed system
–

an inter connected set of two or more computers.
an computer network which appears to act as a single computer.
LAN - Local Area Network
–
–
–
a network covering a single building or set of adjacent buildings.
computers are connected together using a wideband connection such as
coaxial cable.
some computers on the network may be dedicated to special tasks
(servers)
•
•
•

storage of central data (file server)
control of printed output (print server)
communication with other systems (communications server)
WAN - Wide Area Network
–
–
communications
a network covering more than one distant sites.
may consist of LANs interconnected by broadband channels.
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Advantages of Networks

Central Storage
– Sharing of resources, information and expertise.



equipment (eg printers)
central data storage
experts available on the network.
– Personal data is available from any machine on the network


The provision of local facilities without the loss of central
control.
Improved communication within an organisation
– shared data.
– electronic mail.
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Data transmission techniques.
(not required for 1st years)
 Different
methods are used to send data
from one computer to another.
 Two techniques are:
– Collision detection
– Token passing
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Collision detection
(not required for 1st years)






Each computer ‘listens’ to the network to see if it is free.
If it is, it transmits the data (a packet with the source and destination
addresses attached)
It ‘listens’ again to see if it has clashed with any other data
transmissions.
If it has, it waits a random time and retransmits.
Ethernet uses this technique.
Advantages:
– computers can be removed from the network without effecting the
operation.
– it is very fast at moderate traffic levels.

Disadvantages:
– transmission rates deteriorate dramatically at high traffic levels because of
the number of collisions and retransmissions.
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Token passing
(not required for 1st years)



A ‘token’ (a certain binary code) is passed from one computer to the
next.
If there is no data attached to the token a computer may attach some
data and send it on to the next computer.
If there is data attached the computer will check the destination
address to see if it is for this computer:
– if it is not, it is just sent onto the next computer.
– if it is the data is read, the token freed and then sent onto the next
computer.

Advantages:
– transmission rates do not significantly deteriorate with high traffic.

Disadvantages:
– if one computer breaks down the network will not operate.
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Network Topologies
(not required for 1st years)
 Star
 Loop
 Multidrop
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Star Networks
(not required for 1st years)

Advantages:
– Messages do not need to wait for a free ‘token’.
– No ‘collision’ problem.

Disadvantages:
– Requires a very high performance ‘hub’ for routing information.
– A separate cable must be laid down from the hub for each
computer on the network.


Not currently very popular for LANs.
Asynchronous Transfer Mode network (ATM)
– high performance network
– used for high traffic


communications
eg as a telephone exchange
hub is called an ATM switch.
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Ring Network
(not required for 1st years)
 Uses
a ‘token’ system.
 eg IBM’s ‘token ring’ network.
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Multidrop Network
(not required for 1st years)
 Collision
detection must be used for data
transmission.
 eg: Ethernet (as in Sligo RTC)
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The Internet




A worldwide network of voluntary ‘service providers’ located
at points called ‘nodes’.
Each node in the network may be connected permanently to
several other nodes with wideband connections.
At the node is a device called a ‘router’ that is capable of
receiving messages and sending them further if required.
This network of nodes is known as the ‘backbone’ of the
network (Physically it resembles a net)
Athlone IT
GMIT
IT Sligo
communications
Node
Broadband
connection
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The World Wide Web






The WWW is a huge set of documents stored on computers connected
to the Internet interlinked by inserting phrases or images in documents
that contain hidden information specifying the location of other
documents on the Internet. (Hyperlinks)
These documents are described in a language called HyperText
Markup Language (HTML) which describes the layout of text and
images within the documents.
The documents may be viewed using programs called browsers which
are capable of interpreting HTML and displaying the documents.
HTML has become more advanced recently and now allows the use
and submitting of forms, embedding of sophisticated programs (Java
or Javascript) and the definition of data structures (XML).
More sophisticated data (e.g. audio, video or multimedia) require extra
programs referred to as ‘plug-ins’.
The Internet servers that store pages can also create pages on the fly by
using ‘server side’ programs to generate the necessary HTML to
describe the page. (eg. Yahoo mail, Google etc)
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Service Providers
 Each
service provider (node) has one or more
‘servers’.
 A server is a computer used for managing the
connection to users:
– live transfer of data to and from the user to the network.
– holding of data until the user logs in.
 The
user may connect to the server in the
following ways:
– ‘dial up’ connection (modem, ISDN e.g Ireland On Line)
– LAN/WAN connection (if there is a node on your network e.g. IT
Sligo)
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Services on the Internet
 Electronic Mail
– ability to send and receive text based messages (but may also
include other media)
 File Transfer
– ability to see files on servers and download them to your own
computer.
– Also as attachments to email messages
 World Wide Web
– use of special programs called browsers that can read specially
prepared information files (‘rich’ text and graphics) on servers.
– these files can contain ‘hyperlinks’ that can automatically connect
you to other documents on other servers.
 Network News and Newsgroups
– ‘bulletin board’ where you can post notices and read others.
– Email user groups where you can post letters which will be
distributed to all members.
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Uses




Customer Support – helping a customer with a product.
Electronic Shopping
Banking
Marketing
– Information on Products and Services
– Selling the Organisation PR

Product Distribution
– for software based products (programs and data)
– Entertainment, programs, education etc




Information Services
Publishing - electronic books and journals
Virtual Corporations (Teleworking)
– people working together on a single project at different locations.
Special Interest Groups
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Teleworking
 Often
known as Computer Supported
Collaborative Working (CSCW)
– people working together on a single project at different locations
 Working Aids
– Email, file transfer
– live voice communication
– ‘video-conferencing’

you can see the person you are talking to on your screen.
– application sharing

you can see the program the other person is working on
(eg.AutoCAD) and also be allowed control that application.
– ‘whiteboarding’

a shared piece of computer screen on which both people can write or
sketch.
– Shared document editing
– Shared Calendars
– Web based project management
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