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CP2022 - Lecture 2 Computer communications 1

CP2022 - Lecture 2 Computer communications 1. Introduction. Information technology The convergence of computer systems and communications technologies Now includes The Internet and World Wide Web PCs, mini-computers, mainframes The telephone network, ISDN, other networks

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CP2022 - Lecture 2 Computer communications 1

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  1. CP2022 - Lecture 2 Computer communications 1 CP2022- Lecture 2

  2. Introduction • Information technology • The convergence of computer systems and communications technologies • Now includes • The Internet and World Wide Web • PCs, mini-computers, mainframes • The telephone network, ISDN, other networks • How does it all work? CP2022- Lecture 2

  3. Electronic communication fundamentals • What distinguishes new technology from old? • How is a system evaluated for effectiveness? • Which system is best to use in a particular application? • Which method is most cost effective? CP2022- Lecture 2

  4. Computer Communication principles Computer communication is based on the principles outlined in Lecture 1: • Standards • Protocols • Error control, redundancy and accuracy • Channel • Context • Coding CP2022- Lecture 2

  5. Analogue and digital signals • Traditional systems (telephone, radio, TV) are all analogue • Computers use digital signals • Analogue = Continuous signals - Infinite levels • Digital = Discrete signals - Discrete levels CP2022- Lecture 2

  6. Analogue to digital conversion • To use analogue channels (telephone, radio) digital signals need conversion • Digital channels need only format changes • For example, voltage change and/or message structuring • Analogue input (e.g. video signals) require A to D conversion • Analogue output (e.g.. sound) requires D to A conversion CP2022- Lecture 2

  7. A to D sampling CP2022- Lecture 2

  8. Using the telephone system POTS (Plain Old Telephone System) or PSTN (Public Switched Telephone Network) • Analogue system (at the end-points) • designed for voice • limited capacity for data • needs D-A/A-D conversion • done by Modems • MODEM - MOdulator/DEModulator CP2022- Lecture 2

  9. PSTN and Modem use CP2022- Lecture 2

  10. Capacity and Bandwidth • Digital channels have a limit on the amount of data that can be transmitted • The channel’s Capacity • On analogue channels similar restrictions exist • This is known as the channel’s Bandwidth • These terms are now used interchangeably CP2022- Lecture 2

  11. Multiplexing • In many networks individual channels are used by more than one communication • different applications • different users • different data /media types • This requires multiplexing The use of a single channel by more than one communication CP2022- Lecture 2

  12. Multiplexing (2) CP2022- Lecture 2

  13. Multiplexing (3) • Frequency Division Multiplexing • Channel divided by frequency • Limited by total bandwidth • Time Division Multiplexing • Channel divide in time • Each use allotted a slot • 1,2,3,4,1,2,3,4,etc. CP2022- Lecture 2

  14. Broadband CP2022- Lecture 2

  15. Broadband technology • Uses a carrier to modulate digital signal over telephone line in addition to voice • Effectively a form of FDM • Works over an analogue PSTN line • Computer has a digital link • Telephone has an analogue link CP2022- Lecture 2

  16. Communication needs a structure this is a Protocol A protocol needs to satisfy 8 principles (see next slide) Protocols Provide framework for the communication Allow error control mechanisms to be used Monitor the communication Provide communication of all data types Protocols and error control CP2022- Lecture 2

  17. Principles of protocol design • Message format • Error control • Acknowledgements • Time-outs and re-tries • Sequence control • Flow control • Recovery • Data transparency CP2022- Lecture 2

  18. Standards • Communication and protocols are largely determined by Standards • These are decided by expert groups, manufacturers, user groups, international committees etc. • Use of standards simplifies the communication process CP2022- Lecture 2

  19. Standard models • Standard models are used to simplify the design of communication systems • The Internet uses a four layer model (or five including the physical layer) • OSI is a seven layer model developed by the ISO and ITU-T • International Standards Organisation • International Telecommunications Union CP2022- Lecture 2

  20. Internet and OSI communication models CP2022- Lecture 2

  21. Summary • Computer communication is based on the same principles as human communication • Information can be analogue or digital • Channels have capacity • and can be multiplexed • Protocols need to be defined • Standards simplify the use of systems CP2022- Lecture 2

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