1. CP2022- Lecture 21 CP2022 - Lecture 2 Computer communications 1

2. CP2022- Lecture 22 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?

3. CP2022- Lecture 23 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?

4. CP2022- Lecture 24 Computer Communication principles Computer communication is based on the principles outlined in Lecture 1: Standards Protocols Error control, redundancy and accuracy Channel Context Coding

5. CP2022- Lecture 25 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

6. CP2022- Lecture 26 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

7. CP2022- Lecture 27 A to D sampling

8. CP2022- Lecture 28 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

9. CP2022- Lecture 29 PSTN and Modem use

10. CP2022- Lecture 210 Capacity and Bandwidth Digital channels have a limit on the amount of data that can be transmitted CapacityThe channel’s Capacity On analogue channels similar restrictions exist BandwidthThis is known as the channel’s Bandwidth These terms are now used interchangeably

11. CP2022- Lecture 211 Multiplexing In many networks individual channels are used by more than one communication –different applications –different users –different data /media types multiplexingThis requires multiplexing The use of a single channel by more than one communication

12. CP2022- Lecture 212 Multiplexing (2)

13. CP2022- Lecture 213 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.

14. CP2022- Lecture 214 Protocols and error control Communication needs a structure this is aProtocol 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

15. CP2022- Lecture 215 Principles of protocol design Message format Error control Acknowledgements Time-outs and re-tries Sequence control Flow control Recovery Data transparency

16. CP2022- Lecture 216 Standards Communication and protocols are largely determined byStandards These are decided by expert groups, manufacturers, user groups, international committees etc. Use of standards simplifies the communication process

17. CP2022- Lecture 217 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

18. CP2022- Lecture 218 Internet and OSI communication models

19. CP2022- Lecture 219 Summary principlesComputer communication is based on the same principles as human communication analoguedigitalInformation can be analogue or digital capacityChannels have capacity multiplexedand can be multiplexed ProtocolsProtocols need to be defined StandardsStandards simplify the use of systems