Transcript Computer Communications - School of Informatics

Computer Communications
An Introduction
Nigel Topham
Rm 2502, JCMB
[email protected]
Office hours: Tuesdays 1-2pm, semester 2
CS3 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
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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
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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."
- Western Union internal memo, 1876
• “I think there is a world market for maybe five computers.”
- Thomas Watson, chairman of IBM, 1943
• “Computers in the future may weigh no more than 1.5 tons.”
- Popular Mechanics, forecasting the relentless march of science, 1949
• “There is no reason anyone would want a computer in their home.”
- Ken Olson, president, chairman and founder of DEC, 1977
• “640K ought to be enough for anybody”
- Bill Gates, 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”
- Nigel Topham, 2006
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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
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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:
– Characteristics, architectures, standards
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Syllabus (continued)
• Inter-networks:
– Characteristics, architectures, standards
• Case studies:
– Couple; which could be, for example, factory automation and electronic mail
• Real world issues:
– Internet, OSI, social implications, local experience
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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
(not eduni.inf.ug3 as written in handout)
– Office hours:
» Tuesdays 1-2pm throughout this semester, JCMB rm 2502
• Un-assessed exercises suggested to provoke thought and discussion
• Two assessed practical exercises
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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 mark
Preliminary practical
Main Practical
Set of short pencil &
paper exercises
Implement components of a
standard connectionoriented protocol
20%
80%
Promulgated
23rd Jan 2006
13th Feb 2006
Deadline
10th Feb 2006
17th Mar 2006
Assignment
Weighting
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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
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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!
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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
» But we need awareness of Cat5 versus Cat3 wiring schemes
– Not interested in implementation of congestion control protocols
» But we need awareness of the fundamental congestion control algorithms
– Similar approach taken to each concept we encounter
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Most Students do Survive this Course 
• Typical spread of marks for an undergraduate course
Computer Communications
Coursework Mark Distribution
Computer Communications
Exam Mark Distribution
2004-5
2003-4
2004-05
0.18
0.35
0.16
Fraction of candidates
Fraction of candidates
0.3
0.25
0.2
0.15
0.1
0.05
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
0
10
20
30
40
50
60
70
80
90
Decile
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0
10
20
30
40
50
60
70
80
90
Decile
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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
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How? (or, what problems must we solve?)
• How:
– Overcoming the problems posed by communication systems
» Identifying each entity
» Finding a route between communicating entities
» Coping with distance and delays (a.k.a. latency)
» Immunity to noise and transmission errors
– Requires rules to connect different computer systems
– Requires a communications architecture
» Hardware transmits bits
» Entities exchange Content (data with an agreed interpretation)
– Key concept is that of a protocol
» Usually based on internationally-agreed standards
» Conformance to standards enables diverse systems to interact reliably
» Think of an example where lack of standardisation causes problems…
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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:
» Informing us about correctness, delay, throughput, security and cost
– and How:
» I.e. definitions of the required protocols
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Communication is Everywhere
• Fundamental requirement of most systems:
– Machines, computers in our case
– Biological systems:
» Information, emotions, cooperative processes …
• Examples (not computers):
– Local - sounds (mouth), signs (body, writing)
– Wider area - sounds (drumming), signs (smoke, electronic beacons)
– Persistent – language, writing, painting
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Diverse forms of Communication
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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 . . .
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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:
» HTTP is an obvious example
» What about email, remote login, file services?
– Try to describe other protocols you may be aware of that are not intended
specifically for computer communications, e.g.
» Diplomacy, interacting with strangers/friends/family,…
» Organising a formal meeting, writing a formal letter, voting…
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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
» Mobile voice, image, video, networking
» New products and services; new markets and societal evolution
– Immobile wireless communications is also extremely important
» Reduced installation costs
» Pervasive networked appliances
» Emergence of WiMAX in 2006-07
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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
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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
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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
Computer communication is error-prone…
"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
Sometimes humans have similar problems
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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
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