CS3 Computer Communications, Copyright © University of Edinburgh 2004 Computer Communications An Introduction Nigel Topham Rm 2502, JCMB [email protected].

CS3 Computer Communications, Copyright © University of Edinburgh 2004

Computer Communications

An Introduction

Nigel Topham

Rm 2502, JCMB

[email protected]

Office hours: Tuesdays 1-2pm, semester 2

2CS3 Computer Communications, Copyright © University of Edinburgh 2004

Polite requests…

• Please set mobile phones to silent

• Please mute the sound on your laptop

– Actually, you should not need to use it during the lecture

– Printed lecture notes are available and there is space to scribble notes

• Please interrupt at any point with questions


Overview

• This course introduces the fundamentals of computer communications

• Computer networking has seen phenomenal growth

– Almost entirely within last two decades

– In commerce, education, entertainment and the home

– The Internet connects hundreds of millions of people worldwide

– How does it work?

– What fuels its growth?

– How can it be used effectively?

• Communication is the act of sharing information

• Two ways to explore the science of communication

– Top down: from services to bit-level communication

– Bottom up: from basic concepts to higher-level concepts

– This course is mostly the latter, but not exclusively


Predicting Technology can be Tricky…

• "This 'telephone' has too many shortcomings to be seriously considered as a means of communication. The device is inherently of no value to us."

WesternUnioninternalmemo,1876

• “I think there is a world market for maybe five computers.” ThomasWatson,chairmanofIBM,1943

• “Computers in the future may weigh no more than 1.5 tons.” PopularMechanics,forecastingtherelentlessmarchofscience,1949

• “There is no reason anyone would want a computer in their home.” KenOlson,president,chairmanandfounderofDEC,1977

• “640K ought to be enough for anybody” BillGates,1981

• “100 million dollars is way too much to pay for Microsoft” IBM,1992

• “I can’t predict the future, but it will arrive sooner than you expect” NigelTopham,2006


Context

• No pre-requisites for this module, apart from . . .

– Basic understanding of how computers work

– Knowledge & experience of practical computer network usage

– Appreciation of human communication

– The will-power to read (sometimes arid) textbooks

– A desire to further your knowledge in computer communication

– A willingness to communicate

• This course is a pre-requisite for Inf4 Computer Networks

• The Inf4 module covers

– More recent developments

– Future trends

– Computer networks in greater depth


Syllabus

• Introduction and Overview:

– Motivation

– Information, time, space, protocols

• Information:

– Sharing information in distributed systems

• Time:

– Synchronising information transfer

• Space:

– Connectivity in distributed systems

• Message broadcast networks:

– Characteristics, architectures, standards

• Message switching networks:



Syllabus (continued)

• Inter-networks:


• Case studies:

– Couple; which could be, for example, factory automation and electronic mail

• Real world issues:

– Internet, OSI, social implications, local experience


Activities

• Usual two lectures per week

– Monday 11.10am – 12noon, in JCMB 6301

– Thursday 11.10am – 12noon, in JCMB 6301

• No formal tutorials

– Post questions to eduni.inf.course.com newsgroup

(noteduni.inf.ug3aswritteninhandout)

– Office hours:

» Tuesdays1-2pmthroughoutthissemester,JCMBrm2502

• Un-assessed exercises suggested to provoke thought and discussion

• Two assessed practical exercises


Assessment

• Examination

– 75% of course mark

– Past papers available from the School Archive or from Course web page

– www.informatics.ed.ac.uk/teaching/exam_papers

• Two assessed practical exercises

– 25% of course markPreliminary practical Main Practical

AssignmentSet of short pencil &

paper exercises

Implement components of a standard connection-oriented protocol

Weighting 20% 80%

Promulgated 23rd Jan 2006 13th Feb 2006

Deadline 10th Feb 2006 17th Mar 2006


Recommended Reading

• Background reading is ESSENTIAL

• Recommended reading

(**) Computer Networks (4e), Andrew S. Tanenbaum, Pearson, 2003

(**) Computer Networks and Internets (4e), Douglas Comer, Pearson, 1997

(*) Data and Computer Communications (7e), William Stallings, Pearson, 2003

Distributed Systems and Networks, William Buchanan, McGraw Hill, 2000

Data Communications, Computer Networks and Open Systems (4e), Fred Halsall, Addison-Wesley, 1996

Computer Networks: A Systems Approach, Larry Peterson & Bruce Davie, Morgan-Kaufman, 1996


Handouts

• Weekly summaries:

– Summary of lecture and previous Thursday lecture

– Recommended text reading for lectures

– Occasional informal exercises, which are not assessed

– Points to ponder

• Occasional specialist note

• Copy of slides

• Tips for a successful outcome

– Don’t rely on perfect one-way communication [ lecturer -> student ]

– Question assumptions

– Check statements of fact in textbooks

– Look for inconsistencies

– Respond . . . And communicate!


Surviving the Complexity of the Subject

• Computer communications is a complex subject

– Many distinct technologies with different nomenclature

– Many products with different proprietary names for similar products

– Internetworking, by definition, involves all such technologies

– There’s no single underlying theory

– There’s no single unified terminology

– There is a profusion of TLAs which can confuse and distract

• However, the principles are not difficult

– This course focuses on principles rather than details

– Not interested in details of how to wire up a network

» ButweneedawarenessofCat5versusCat3wiringschemes

– Not interested in implementation of congestion control protocols

» Butweneedawarenessofthefundamentalcongestioncontrolalgorithms

– Similar approach taken to each concept we encounter


Most Students do Survive this Course

• Typical spread of marks for an undergraduate course

Computer CommunicationsExam Mark Distribution

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 10 20 30 40 50 60 70 80 90 100

Decile

Fra

cti

on

of

ca

nd

ida

tes

2004-5 2003-4

Computer CommunicationsCoursework Mark Distribution

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 10 20 30 40 50 60 70 80 90

Decile

Fra

cti

on

of

ca

nd

ida

tes

2004-05


Who What When Where Why?

• Who are the communicating agents?

– Digital computers, performing digital communication

• What do they communicate?

– Any information that computers can handle

• When (i.e. all factors relating to time)

– Ordering and sequence of events, defined by protocols

– Rate and duration of events, defining performance

• Where are the communicating agents?

– Locality and connectivity

– Spatial aspects of communication

• Why:

– Perhaps the most interesting question

– Can identify positive (and negative) effects on human interaction

– Buried Infrastructure versus User Services


How? (or, what problems must we solve?)

• How:

– Overcoming the problems posed by communication systems

» Identifyingeachentity

» Findingaroutebetweencommunicatingentities

»Copingwithdistanceanddelays(a.k.a.latency)

» Immunitytonoiseandtransmissionerrors

– Requires rules to connect different computer systems

– Requires a communications architecture

»Hardwaretransmitsbits

» EntitiesexchangeContent(datawithanagreedinterpretation)

– Key concept is that of a protocol

»Usuallybasedoninternationally-agreedstandards

»Conformancetostandardsenablesdiversesystemstointeractreliably

» Thinkofanexamplewherelackofstandardisationcausesproblems…


Data, Information and Knowledge

• Other courses are concerned with the use of information

– We just move it

– Don’t care how it is interpreted by communicating entities

• When does data becomes information ?

• When does information becomes knowledge?

– We don’t care

• Being uncommitted we just refer to what is transmitted as information

• Information theory does have an impact though

• In this course we are most concerned with:

– Questions of When and Where:

» Informingusaboutcorrectness,delay,throughput,securityandcost

– and How:

» I.e.definitionsoftherequiredprotocols


Communication is Everywhere

• Fundamental requirement of most systems:

– Machines, computers in our case

– Biological systems:

» Information,emotions,cooperativeprocesses…

• Examples (not computers):

– Local - sounds (mouth), signs (body, writing)

– Wider area - sounds (drumming), signs (smoke, electronic beacons)

– Persistent – language, writing, painting


Diverse forms of Communication


Protocols

• This is the key concept.

– A protocol is a collection of agreed rules that allows the sharing of control and information between two or more entities

• The entities in our case are computers

• Concepts we shall discuss hold for entities in any system

• A protocol is the enabling mechanism – it must:

– Identify the communicating entities

– Define how a channel is established

– Define how data is encapsulated and-or encrypted

– Break large messages into manageable chunks

– Deal with transmission errors (detect and-or correct)

– Define a flow-control mechanism

– Define how a channel is relinquished

• Consider snail-mail from each of these protocol perspectives . . .


Use of protocols

• Consider these statements:

– Computers use protocols so that system components can be used to control and manipulate information

– Governments use protocols so that information can be used to control and manipulate people, other governments and the media

• Spend some time thinking about protocols for communication

– Make a list of all the communication protocols you are aware of, and identify their intended use:

»HTTPisanobviousexample

»Whataboutemail,remotelogin,fileservices?

– Try to describe other protocols you may be aware of that are not intended specifically for computer communications, e.g.

»Diplomacy,interactingwithstrangers/friends/family,…

»Organisingaformalmeeting,writingaformalletter,voting…


Communications is Evolving Rapidly

• A most exciting field right now:

– Many and varied opportunities

– Rapidly expanding technology and market

– Email, Internet, mobile phones,…

– Integration of mobile phone, PDA and DVB devices

– Interoperability of cellular and WiFi networking

• Mobility of communicating entities has profound effect

– Opens new opportunities

»Mobilevoice,image,video,networking

» Newproductsandservices;newmarketsandsocietalevolution

– Immobile wireless communications is also extremely important

» Reducedinstallationcosts

» Pervasivenetworkedappliances

» EmergenceofWiMAXin2006-07


Keeping up with Developments

• Recommended reading for the course includes:

– Mainstream, business and computing press carry items each day with some communications content

– If you don’t understand the references, keep them to the end of the course when they might make sense to you

• Follow the online lecture log at:

http://www.inf.ed.ac.uk/teaching/courses/com/log.html

– It will often contain web links to further information


Communications past and future

• Past:

– We need to look at some of the history of communications in this course to set context

– And to understand developments

• Future:

– We’ll be looking at some emerging technologies

– This is also the function of the Inf4 Computer Networking module


Discussion point for this week

• First of weekly suggestions usually suitable for group discussions:

– Look out for different examples of computer communications in your everyday life

– You’ll probably find some you had not thought of as such

– Some may not seem necessary – are they?

– Some may be necessary but missing at present

"I think if you know what you believe, it makes it a lot easier to answer questions … I can't answer your question." —In response to a question about whether he wished he could take back any of his answers in the first debate. Reynoldsburg, Ohio, Oct. 4, 2000

Computer communication is error-prone…

Sometimes humans have similar problems


Next time

• Next two lectures cover:

– Ways in which computer communications affects everyday life

– Review of communications media

– Impact of computer systems

– Some basic Information Theory

CS3 Computer Communications, Copyright © University of Edinburgh 2004 Computer Communications An Introduction Nigel Topham Rm 2502, JCMB [email protected].

Documents