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Chaos, Communication and ConsciousnessModule 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
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.
Circuit Switching
Dedicated connection between nodes Established for duration of
communications Guaranteed capacity Inefficient use of resources
The Modem
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 Remote users
connected to central facilities
Several terminals multiplexed over dedicated phone line
Multiplexer shares connection between users
Terminals
Multiplexer
Direct Connection
Minicomputer/mainframe
Packet Switching - 1965
3 key ideasDecentralised network, multiple routes between
nodes Information split into message blocks (packets)Delivery of packets by store & forward
Efficient use of resources Reliable
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
ARPANET (USA) 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
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
linked UK universities & research institutesbecame JANET
IPSS – International Packet Switched System1st international PSSUS, 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
Inter-networking
By early 80s, several different networksARPAnet, SERCnet, UUCP, Tymenet, AOL…
Gateways linked some networks Difficult to transfer data/information across Solution – TCP/IP
protocol sits ‘on-top’ of different networksallows seamless integration
Layers of Protocol
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
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