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

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 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

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, 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 institutes  became JANET IPSS – International Packet Switched System  1 st 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

Inter-networking By early 80s, several different networks  ARPAnet, SERCnet, UUCP, Tymenet, AOL… Gateways linked some networks Difficult to transfer data/information across Solution – TCP/IP  protocol sits ‘on-top’ of different networks  allows seamless integration

Layers of Protocol Break networking into series of ‘layers’ Application – Web browser, 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

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

Application Layer Application specific data Application layer protocols  HTTP: world wide web  FTP: File Transfer Protocol  SMTP: Send message to :

Transport Layer (TCP) Take message from application layer Break message into packets Put sequence numbers on packets Pass to Networking layer Re-transmit faulty packets Put received packets in correct order Discard duplicate packets

Network Layer (IP) Takes data from Transport Layer Provide unique global addressing Deal with routing ‘Unreliable’  Best efforts at getting data through  Rely on upper layer to ensure relability Connectionless protocol  Send packets on without establishing prior connection

Data Link Layer Takes data from Network layer Prepares for transmission on physical layer Depends on medium  Modem: Speed etc.  Ethernet: Ethernet addresses, collision avoidance

Physical Layer Cables, Plugs & Sockets Voltages, Frequencies etc

Review of Lecture Early data communications Circuit switching The modem – Data over phone lines Packet switched networks ARPAnet, SERCnet Internet protocols World wide Web