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Networks, the internet & the webNumber systems, bits and bytes, ASCII,
Unicode Digitising Multimedia, colours, Algorithms. Turing machines, Von Neumann architecture, Fetch execute cyclesSecurity
Value – what’s been covered
Networks
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You’ve got …
Physical Networks LAN 'local area networks'– could be a
building or more MAN 'metropolitan area networks' (Kentish
MAN)Logical Networks Within a business or a university or ...
– IntranetsAnd you’ve got the InternetAnd you’ve got wireless networks
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What is the Internet
It’s a network of networks, a global super-network – and it’s not just any such – Overall, it works in a specific kind of way and it is really global.
As we know it, it was invented 30 years ago and deployed 20 years ago.
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The Internet before the Web
Email – STMP (Simple Mail Transfer Protocol)(actually predates the internet)
Usenet (bulletin boards) Gopher (something like a search engine) ...
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IP addresses (Internet protocol)
All networked computers have IP addresses
129.12.185.83
The IP address of the computer I was using when I originally made this slide
IP = “Internet Protocol”
IP addresses
A unique number that refers to a device on a network
What have IP addresses?Web-sites, servers, your pc, the printer in the hallway ... (some are static – they stay the same, some are dynamic, ...)
IP addresses are used over LANs as well as over the Internet
www.cs.kent.ac.ukresolves to 129.12.4.58
www.kent.ac.ukresolves to 129.12.200.17
www.gre.ac.ukresolves to 193.60.49.86
www.whitehouse.govresolves to ... 84.53.134.9 and 84.53.134.14 [among others]
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The Internet
A model representing roughly how it works:Not the Telephone service
the Postal service!The technical term is “packet switching”. Imagine you wanted to
send an encyclopaedia to someone by post, but you could only put a sheet or two of paper in an envelope. You’d need to ‘tear’ the encyclopaedia up in a reasonable way, note down the order that the sheets needed to be in and send a sheet or two in each envelope. The at the other end everything would need to be put together again. An even better analogy – POSTCARDS.
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What the Internet isn’t
It is NOT like the telephone service –In the telephone service there’s a single dedicated
connection all across the network(s) between the two ends of the conversation. This uses a lot of network resources.
Although it makes use often of telephone technology On the Internet, there’s no connection from one end to
the other and parts of what’s crossing the Internet may have followed different routes
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Types of networks
Circuit switching – like the original telephone networks
Packet switching – like the internet Broadcast – like radio, television and the
Ethernet
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The TCP/IP
TCP/IP – Transmission Control Protocol over IP
Using TCP, computers can exchange messages (data) in packets across an inter-network, with the assurance that these packets can be delivered reliably, in-order (if something has gone wrong, likely to know about it) with flow control
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What the WWW runs on:
Web servers Browsers HTML http URLs The Internet
Tim Berners-Lee
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So, what is the www?
“An internet-based hypermedia initiative for global sharing” http://www.w3.org/People/Berners-Lee/Overview.html
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What users see:Domain names
www.yahoo.com www.cs.kent.ac.uk www.kent.ac.uk en.wikipedia.org ftp.internic.net
Are easy to use Are easy to
remember May reflect something
about the organisation
Are not as easily handled by machines as numbers are
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Domain hierarchy
.uk.ac
.kent.cs
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Top-Level Domains (TLDs)
Generic Top-Level Domains
.com .org .net .edu .int .mil
Country Code Top Level Domains
.uk .fr .de .us .jp .au .ca
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URLs (Universal Resource Locator)
http://en.wikipedia.org/wiki/URL
host 'path'
'scheme'
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Domain name system (DNS)
Connects Domain names with IP addresses
129.12.185.83
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Protocols
Communication protocols specify how computers can talk to each other.
Networks need agreed upon conventions of how to communicate. Protocols set down these procedures, often by specifying the format of a message and how errors are to be handled.
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http = hyper text transfer protocol It sets down how two computers (‘client’ ‘server’) can
have a ‘conversation’ General Pattern – request / response
request: “I’d like you to give me X”response:
“Here” – sending X
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Layers
Many different protocols may be involved in a complex communication across a network.
HTTP hypertext transfer protocol (application layer) TCP transmission control protocol (transport layer) IP internet protocol (network layer) PPP - point-to-point protocol (data-link layer)
Number systems, bits and bytes
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Number systems
Base "ten" – uses ten symbols = Decimalcould be written with using our usual numerals
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Base "two" – uses two symbols = BinaryThese symbols could be almost anythingCould be written with zeros and ones –
0 and 1
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Between Machines & end-users
Machines use binary representations internally
(It's fast, convenient, easy direct and useful)
For end users, machines are designed to convert to the standard representation (0,1,2,3,4,5,6,7,8,9)
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For computers
Computers (internally) represent everything as binary (eg as 0 and 1)
Everything means everything – text, sound, images, computer programmes …
If a computer is used to search through a database and find all those people whose name begins with the letter "G", the most basic representation of the letter G is as some sequence of 0s and 1s
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Bits For Symbols
Bit stands for binary digit.– A bit is single item of binary information –
We can think of a bit as a single zero or a single one (in some forms of computer memory – it could the presence or absence of an electrical charge at a specific location – it could be the presence or absence of a magnetic charge at a location
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Hexadecimal – 'hex'
Base 16Relatively easy for computers
Comparatively easy for us (to read, write, remember, use)
Symbols: 0 1 2 3 4 5 6 7 8 9 a b c d e f
e stands for fourteen
f stands for fifteen
10 stands for sixteen
ff stands for two hundred fifty five
You've seen this with colours
in CSS
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Text: The minimum needed for representing English and numbers Lower-case letters Upper-case letters Some punctuation Blank spaces Frequent symbols
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ASCII
American Standard Code for Information Interchange
ASCII was 7 bits –an eighth bit was used for various purposes
it became clear that was not enough bits “Extended” ASCII with 8 bits was developed Eight bits produce 256 symbols which is more
than enough for English and for many European languages
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ASCII
Representing 128 different elements
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Representation
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Representation
Demo!
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Bytes & Unicode
IBM gave the 8-bit sequences a special name, byte, and adopted it as a standard unit for computer memory.
Bytes are still the standard unit or memory
The ultimate and complete solution is the representation, called Unicode.It has 8, 16 and 32 bit versions.
It can handle all languages.
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What we need for a global internet
The symbols of all written languages The symbols of basic and useful symbol
systems – Math symbols– IPA "the international phonetic alphabet"
Enough space left over to grow€ (new symbols get added)
a unified method of representing all of these together is needed and it is Unicode
Digitising Multimedia
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The subtractive primaries (pigments)
Ink combines to black (dark
brown)
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Colour-printing
Printers create colour by subtracting. Colour printers use Cyan Magenta Yellow blacK CMYK
Although black can be achieved by adding colours together, in fact, it’s four colour printing with black as a separate colour)
Reflectance
CMYK
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The additive primary colours
Light combines to white
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How monitors work
Monitors create colour by adding light. They use red, green
and blue light. RGB White is achieved by
adding red and blue and green together (shining a light on a wall)
Luminance
black
redwhite
green
blue
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Putting them together(oversimplified)
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Two Kinds of Computer Graphics
Raster (‘bit maps’) – pixel by pixel description of the image – array (1-bit, 8-bit (usual), 24 or 32)
Vector – mathematically described images ('line from a to b' | 'a line through point a with slope c') described as ‘resolution independent graphics’
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More about kinds of graphics
Paint packages –
create bitmapped images
another term: raster graphics
Scanning images create bitmaps as well. Other example: photographs in digital formats, air-brushed images
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More about kinds of graphics
Draw packages – create vector graphics
lines, rectangles, circles mathematical / geometric objects
Resolution independent
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File compression
Get rid of redundant information – file size gets smaller without loss of information. (restore the information at a later time if needed – for example, a long strip of solid colour, could be stored as a sequence of bits or stored in a more efficient manner
Other ways of compressing file information, loses information but information that we don’t notice being gone.
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Two types of file compression
“lossless” – same amount of information, smaller file size (eliminate redundancy)
“lossy” – loss of information but we may not notice or care
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GIFs
Especially effective at dealing with large horizontal bands of a single colour
256-colour (8-bit) images
LosslessGraphics Interchange Format, from Compuserve (patented)
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JPEG (aka .jpg)
“Lossy” – it loses information, but if this loss is barely perceptible or not perceptible at all, a great significant reduction in file size can be achieved.
24-bit (16.7 million colours)
Joint Photographics Experts Group
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jpeg vs gif
Jpeg for photos and other continuous toned images
Gifs for large swatches of single colours
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Computer speak:Hexadecimal
Hexadecimal numbers use a base 16.Decimal use base 10 (‘deci’ means 10; ‘hex’ means 6 – ‘hexadeci’ 16)
Binary use base 2.
Why base 16? Human readable and useable. Easy to convert to binary (which is what machines read
and use)
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Hexadecimal colours
HTML colours are specified with three pairs of hex digits, preceded by a hash sign. Each pair of digits represents the quantity of red, green & blue that make up the colour. Thus:– #FFFFFF refers to white– #000000 refers to black– #FF0000 refers to red
There are lookup tables!
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Anti-aliasing
Apparent smoothing out of the edges of images is called:Anti-aliasing Anti-aliasing
The 'jaggies'
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Anti-aliasing (2)
Anti-aliasing – blend hues of the colours
Many graphics programs allow you to choose whether images should be anti-aliased
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Dithering
Dithering can be used as a general way to create illusions of colours and shades – let’s it seem as if there are more colours or shade than there really are‘application dither’ occurs when an application
(PhotoShop, PaintShopPro … ) creates an image in a format like gif png-8 from a original with more colours
‘browser dither’ occurs when a web-browser using an 8-bit colour display (256 colours) shows an image with more colours.
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To see for yourself:
Blow up something a fairly great amount. Eventually the underlying discontinuous range becomes visible.
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Quick terminology review
Anti-aliasing – a way to create the illusion of a sharp edge (where in fact its actually jagged) by taking an number of intermediate colours
Dithering – a way to use a limited palate of colours to create a wider range of colours and tones (dithering is also a term in sound technology for a similar phenomenon)
Algorithms
Turing machines Von Neumann Architecture Fetch execute cycle
Historical perspectives – Turing's machine
Alan Turing – developed the abstract notion of a "Turing machine":– NOT a physical machine, but a mathematical
model of one type of very simple abstract calculating device
– He proved that there were Turing machines (Universal machines) that were capable of calculating anything and everything that any Turing machine could calculate (given enough time and enough memory)
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The upshot of this result of Turing's:
Very simple devices can be designed that are as powerful computers as can be (if we don't have to worry about how much time it will take or how much memory is involved)
The practical lesson – make things that have larger and more efficient memory and faster speeds and we can do anything that a computer can do
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Historical perspectives: von Neumann's architecture
Von Neumann added in a number of ideas that made digital computing practical and his ideas remain those of most digital programming to this day
Key among those ideas:– Programmes themselves are really just like
any other piece of data (0s and 1s) at least from the standpoint of the computer
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Von Neumann architecture(simplified)
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
Von Neumann architecture
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
Accumulator
Stores the program and the data
It has specific and discrete locations – each (generally) 1 byte
Each location has a specific address
These addresses store values
The total size of memory locations is limited
Diagram of computer memory-
01010101 00101010 00111010 11101001
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Each memory location is one byte size
Von Neumann architecture(simplified)
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
The Control Unit:
Executes instructions
Von Neumann architecture(simplified)
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
The ALU:
Does the logic and maths
Devices for both input and output
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
Hard diskFloppy disk
Input devices
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
MouseKeyboardScanner
Output devices
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Memory
Input Output
ControlUnit
ArithmeticLogicUnit
MonitorPrinter
Speakers
The fetch/execute cycle.
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Illustration of a single ADD 2000 2080 4000 instruction
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Results depend on the contents of the memory locations referenced in the instruction.
The Program counter
This keeps track of what step needs to be executed next. It uses the address of the next instruction as its way of keeping track. This is called the program counter.
Some instructions change the Program counter (branch and jump instructions)
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What is an algorithm?
An algorithm is an ordered set of unambiguous, executable steps that defines a terminating process Brookshear Computer Science
A step-by-step procedure for solving a problem http://images.rbs.org/appendices/d_glossary_geometric.shtml
A finite set of well-defined rules for the solution of a problem in a finite number of steps. http://www.sabc.co.za/manual/ibm/9agloss.htm
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Five properties of algorithms
Input-specified Output-specified Definiteness Effectiveness Finiteness
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Fluency with IT by Lawrence Snyder
Specifying the input and output
Input – what is going to be changed, used, transformed by the process
Output – the data that carries the results at the end of the process
What type (of data, of …) How much
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Alphabetise and file your company's correspondence Input: A stack of folders. Each folder
contains a letter to your company. At the moment, there is no order to the folders.
Output: The same folders in the stack, but in alphabetical order
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This is an example of sorting –
Sorting is a very common algorithmic process for computers. (Here we have alphabetical sorting for example. With any of these, the efficiency of the process is important, but that's for later for now ...)
Security
Encryption/Decryption Terminology
Encryption: Transform representation so it is no longer understandable
Cryptosystem: A combination of encryption and decryption methods
Cleartext or Plaintext: Information before encryption
Cipher text: Information in encrypted form
One-way cipher: Encryption system that cannot be easily reversed (used for passwords)
Decryption: Reversing encryption process
Symmetric keys
Use the same key to encode and decodePlaintext message
Key(plainText) -> codedMessage
Key(codeMessage) ->(plainText)
Plain textsell-northern-rock-
buy-gold
A cipher
Ciphertextfryy-abegurea-ebpx-
ohl-tbyq
A cipher
Plain textsell-northern-rock-buy-gold
Substitution cyphers
An effective cypher method is a substitution cypherOne letter is substituted for another, in a systematic way.
More sophisticated examples
18th and 19th Century military codes leading up to the Enigma code and other military codes during the second World War
Different letters substituting for different letters depending on complex patterns
Difficult but not impossible to decypher
Breaking the Code
Longer text is easier to decode
– Notice what bit sequences show up frequently
– Knowledge of most frequent letters in the cleartext language
Smarter byte-for-byte substitutions
– Group more than two bytes
– Be sure not to exchange the key over unsecured connection
Managing keys becomes a problem
Keeping keys straight may not be an issue, but a secure way of “a new kid on the block” getting keys is a problem - (lots of couriers?)
Public Key encryptionAsymmetric key
There are two keys – say, KeyOne and KeyTwo
What one encodes the other decodes, and vice-versa
KeyOne(plainText) -> codedMessage1
KeyTwo(codedMessage1) -> plainText
Public Key Cryptosystems
People who want to receive information securely publish a key that senders should use to encrypt messages
Key is chosen so only receiver can decode
Legal issues and computing
IPR = Intellectual property rights
The most common forms of IPR Copyright Patent Trade mark
IP as a public good
Intellectual property is what economists call a 'public good'
A public good isHard to produce, taking time, skill,
effort, understanding, originalityCheap to reproduce once created
This presents a problem ('the public goods problem') = WHY do the original production at all
Property v. IntellectualRights property rights Can be sold, traded,
assigned, inherited, divided
Continue until sold, traded, …
Can be sold, traded, assigned, inherited, divided
Eventually go away (after some duration, which differs for the type of IPR and for the jurisidiction and for lots of other technical reasons)
Copyright
You can copyright the way something is expressed, not the idea itselfEinstein (who in fact was working at a patent office at the time) couldn't copyright the principles of relativity Though he would have a copyright for the actual words he used in the article he wrote
Copyright
Act covers:– original literary, dramatic, musical or artistic works,– sound recordings, films, broadcasts or cable
programmes, and– the typographical arrangement of published editions. – Computer programs are treated as literary works
“Works” involve skill, labour, judgment– Must be original or new – Must be recorded in some non-transient form
Copyright
Exists whenever a “work” is created– No special action needed by author
Exists world-wide May be transferred (sold, leased, donated) to
others Persists for:
– Literary works etc., 50 years after death of author,– Recordings, films, etc., 50 years after release– Typographical arrangements, 25 years after
publication
Patents
Gives owner a legal monopoly over the exploitation of an invention.
Patents may be obtained on inventions that are– capable of industrial application– novel or new (not previously disclosed)– involve an inventive step– non-obvious
Patents
Patent application process can be time consuming (<5 years).
Not world-wide: need to apply for patents in different jurisdictions– Patent law and what can be patented varies
between countries May be transferred (sold, leased, donated)
to others May be expensive to protect.
