Transcript PH19510 Lecture 12

Chaos, Communication
and Consciousness
Module PH19510
Lecture 12
Data Networks Across the
World
Overview of Lecture
Early data communications
 Circuit switching
 The modem – Data over phone lines
 Packet switched networks
 ARPAnet, SERCnet
 Internet protocols
 World Wide Web
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Early Data Communications
1793 Chappe tower network
 1850s onward – Telegraph network
 1889 Automatic switching of telephones
 1940 George Stibitz used teletype to send
instructions & receive back data.
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Circuit Switching
Dedicated connection between nodes
 Established for duration of
communications
 Guaranteed capacity
 Inefficient use of resources
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The Modem
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Modulate & Demodulate
Convert 1s and 0s into
sounds for transmission
over telephone line
Originally acoustic
coupling
Must be in audio
(400Hz-4Khz) range
Encode 1 & 0 as tones
Nowadays up to 56kbps
over conventional
phone system
Computer Networking in the 1960s
& 70s
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Remote users
connected to central
facilities
Several terminals
multiplexed over
dedicated phone line
Multiplexer shares
connection between
users
Minicomputer/
mainframe
Terminals
Direct
Connection
Multiplexer
Packet Switching - 1965
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3 key ideas
 Decentralised
network, multiple routes
between nodes
 Information split into message blocks
(packets)
 Delivery of packets by store & forward
Efficient use of resources
 Reliable
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What’s in a packet ?
Source Address (IP)
 Destination Address (IP)
 Length of packet in bytes
 Total number of packets in message
 Sequence Number
 Checksum
 Data
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ARPANET (USA)
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Advanced Projects Research Agency (USA)
First Packet switched Network, Q4, 1969
4 Nodes
 University
of California, Los Angeles
 Stanford Research Institute
 University of California, Santa Barbara
 University of Utah
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Interface Message Processors (IMPs) deal
with packet switching.
IMPs connected to mainframe/minis at each
node.
ARPAnet in 1977
X.25 networks
Simlar principles to ARPAnet
 1974 SERCnet
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 linked
UK universities & research institutes
 became JANET
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IPSS – International Packet Switched
System
 1st
international PSS
 US, Canada, Europe, Australia
Request For Comment (RFC)
Building networks is a collaborative
enterprise.
 RFC mechanism came out of ARPA
 Serve as mechanism for dissemination for
information
 Some become adopted as standards
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Inter-networking
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By early 80s, several different networks
 ARPAnet,
SERCnet, UUCP, Tymenet, AOL…
Gateways linked some networks
 Difficult to transfer data/information across
 Solution – TCP/IP
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 protocol
sits ‘on-top’ of different networks
 allows seamless integration
Layers of Protocol
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Break networking into series of ‘layers’
Application – Web browser, email etc
Transport – Get messages from host to host
(TCP)
Network – Get packets across single network
(IP)
Data Link – Protocol over single link in network
(UDP)
Physical – Cables, connectors
Seamless networking – from
NOAA
Growth of the net
Map of
the
Internet
- 2005
The World Wide Web
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Service running on the
internet
1990 First implementation
 Tim
Berners-Lee, CERN
 Browser – World Wide Web
 Server – info.cern.ch
 Pages – CERN Telephone
book
Review of Lecture
Early data communications
 Circuit switching
 The modem – Data over phone lines
 Packet switched networks
 ARPAnet, SERCnet
 Internet protocols
 World wide Web
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