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CP2022 - Lecture 2 Computer communications 1 CP2022- Lecture 2 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 • How does it all work? CP2022- Lecture 2 2 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 3 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 4 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 Digital signal Analogue signal CP2022- Lecture 2 5 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 6 A to D sampling Volts Sampling levels Time Sampling interval CP2022- Lecture 2 7 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 8 PSTN and Modem use Digital link Computer Digital link Computer PSTN Modem Modem Analogue links CP2022- Lecture 2 9 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 10 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 11 Multiplexing (2) A B Multiplexer A,B,C,D C D CP2022- Lecture 2 12 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 13 Protocols and error control • 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 CP2022- Lecture 2 14 Principles of protocol design • • • • • • • • Message format Error control Acknowledgements Time-outs and re-tries Sequence control Flow control Recovery Data transparency CP2022- Lecture 2 15 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 16 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 17 Internet and OSI communication models Internet OSI Application Application services Presentation Session Transmission control Internet protocol Transport Network Network access Data link Physical CP2022- Lecture 2 18 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 19