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