Networks 2
Gabriela Ochoa
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 1
Content
• How is Internet connected?
– Internet backbone
– Internet service providers (ISP)
– How to connect a home computer to the Internet?
• How do networks work?
– Packet switching
– Open systems
– Protocols
BBC WebWise: Introductory video: What is Internet?
http://www.bbc.co.uk/webwise/courses/internet-basics/lessons/internet-basics
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Page 1
Lecture Networks 2/Slide 2
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How is Internet connected?
• Internet backbone
• Internet service providers (ISP)
• How to connect a home computer to the
Internet?
1. Phone modem
2. Digital subscriber line (DSL)
3. Cable modem
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 3
The Internet backbone
• A set of high capacity data routes that carry Internet
traffic (high data transfer rates: Mbps, Gbps)
• Analogy with human backbone or spine
• Provided by various companies (per regions) such as
– US: AT&, Verizon, NTT, etc.
– UK: British Telecom
– China: China Telecom, China Unicom
– Germany: Deutsche Telekom AG
• Use special optical cables
• Note: There is no really central network, Internet is
redundant
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 4
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Kbps, Mbps, Gbps
• The data rate of a computer network connection is
normally measured in units of bits per second (bps)
• Network equipment makers rate their products using
related, larger units of Kbps, Mbps and Gbps.
– Kilobit per second (Kbps): 1000 bps
– Megabit per second (Mbps): 1000 Kbps or one million
bps (10002 bps)
– Gigabit per second (Gbps): 1000 Mbps or one million
Kbps or one billion bps (10003 bps)
What is a bit?
•
•
•
Basic unit of information in computing and digital communications
One of two values (0,1) physically implemented with a two-state device
Contraction of binary digit
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 5
The internet backbone: BT
• http://www.bt.net/info/maps.shtml
•
•
© 2014 University of Stirling
Core topology of the BT
IP Network within
Europe, and its
connectivity to the USA.
BT also offers IP
services globally through
strategic supply
arrangements.
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 6
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Internet backbone: NTT network
•
•
•
Ranked among the top 5 networks worldwide
World’s largest transpacific network
http://www.ntt.net/about/network-map.cfm
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 7
Internet service providers (ISP)
• Company or organization that offers users access to the
Internet and related services.
• Most ISP are telephone companies or other
telecommunications providers
• ISPs connect directly to the Internet backbone, or
connect to larger ISPs with a connection to the backbone
Top ISP in the UK:
http://www.uswitch.com/broadband/?gclid=COXHv4Po3rkCFTLMtAodqjYAAA
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 8
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How to connect a home computer to Internet?
Three main technologies available to connect a home
computer to the Internet
1. Phone modem converts computer data into an
analog audio signal for transfer over a telephone
line, and then a modem at the destination converts
it back again into data
2. Digital subscriber line (DSL) uses regular copper
phone lines to transfer digital data to and from
the phone company’s central office
3. Cable modem uses the same line that your cable
TV signals come in on to transfer the data back
and forth
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 9
Phone modem
• Use the telephone system, which is already in place
• Modem: stands modulator/demodulator
• A phone modem a converts computer data into an analog audio signal
for transfer over the telephone line, a modem at the end converts it
back into data again
• Establish a telephone connection between your home computer and a
computer that is permanently connected to Internet
• ISP provides the right to call to one or several (preferably local)
computers set for this purpose
Data transfer limited to that of analogue
voice communication: 64 kbps
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 10
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Digital subscriber line (DSL)
• Still uses the telephone system (copper lines) without preventing
preventing normal use of the telephone line
• BUT, treats data as digital rather than analogue
• High speed connection
• Most telephone companies today offer DSL
• The telephone company, may become your ISP, or sells the use of
its lines to a third-party ISP.
• No need to “dial in”. An active connection is maintained between
your home an a computer at the ISP.
• Your home should be close to the DSL central office.
• The farther away you get from the central office, the weaker the
signal becomes.
What is the difference between analogue and digital signals?
http://www.m2m.ecs.soton.ac.uk/Wc9992beb18704.htm
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 11
Cable modem
• Many people get their TV signal from cable television
• CATV provides a clearer picture and more channels
• CATV can also provide a high connection to the Internet
• Uses either
– Coaxial cables (radio frequency signals)
– Fibre-optic cables (light pulses)
• All nodes for cable subscribers in a neighbourhood connect to a
cable company's central office, known as the "head end.“
• The cable company then connects to the Internet using either:
fibre optic cable or digital satellite and microwave transmission
• Like DSL, cable provides a continuous connection with an ISP
Optical fibre
cable
© 2014 University of Stirling
Coaxial
Cable
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Lecture Networks 2/Slide 12
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Broadband
A connection in which transfer speeds are faster than 1
Mbps (up to 100 Mbps or more)
• DSL connections and cable modems are broadband
connections
• Debate between the DSL and cable modem communities
continues, to see have the larger market share
• The speed for downloads (getting data from the
Internet to your home computer) may not be the same as
uploads (sending data from your home computer to the
Internet)
• Most traffic for home users consists of downloads.
Therefore, companies devote more speed to downloads.
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 13
Computer network hierarchy
POP (Point of Presence) : place for local users to access to the
organization network (e.g. through a local phone or dedicated line)
NAP (Network Access Point): centralized point where Internet traffic is
exchanged or shared. Serve to tie ISPs
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 14
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How do networks work?
• Communication requires that devices agree on the format of the
data
• Software is required to organise and structure users’ data
• Protocol: set of rules defining format of the data. Prepare
information to be transmitted via hardware (encoding/decoding)
Users’ Software
Protocol
software
Hardware
© 2014 University of Stirling
Users’ Software
‘virtual’ links
physical link
Protocol
software
Hardware
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 15
Packet switching
Messages are divided into fixed-sized, numbered packets; packets
are individually routed to their destination, then reassembled
Packet: A unit of data sent across a network
Router: A network device that directs a packet between networks toward
its final destination
http://computer.howstuffworks.com/internet/basics/internet-infrastructure3.htm
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 16
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Open systems
A logical progression...
Proprietary system
A system that uses technologies kept private by a
particular commercial vendor
Interoperability
The ability of software and hardware on multiple
machines and from multiple commercial vendors to
communicate
Open systems
Systems based on a common model of network
architecture and a suite of protocols used in its
implementation
17
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 17
Open systems
Open Systems Interconnection
Reference Model
A seven-layer logical break down
of network interaction to facilitate
communication standards
Each layer deals with a particular
aspect of network communication
The layers of the OSI Reference Model
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Page 9
Lecture Networks 2/Slide 18
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Summary
• How is Internet connected?
–Backbone, Internet providers
–How to connect a home computer to Internet
• How do networks work?
– Packet switching
–Open systems and protocols
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
Lecture Networks 2/Slide 19
Networks - References
• N. Dell & J. Lewis, Computer Science Illuminated, Chapters 15-16
• H.L Capron & J.A. Jhonson, Computers Tools for an information
Age, Chapters 7-8
• How Stuff Works - Internet
Infrastructure:http://computer.howstuffworks.com/internetinfrastructure.htm
© 2014 University of Stirling
CSCU9B1 Essential Skills for the Information Age
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Lecture Networks 2/Slide 20
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