Compsci Exam Study Notes.docx

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Western University
Computer Science
Computer Science 1011A/B
Andrew Roberts

Compsci Study Notes Internet Success Factors 1. Size (things that help make the Internet bigger) 2. Speed (things that help make the Internet faster) 3. Reliability (things that help make the Internet more reliable) 4. Security (things that help make the Internet more secure) 1. Size - Some of the aspects of the size of the Internet: – Number of computers connected to the Internet (exponential growth occurring by 2.28)  More content available  Internet available in more locations – Size of computer memory and other storage media  “Moore’s Law”: Amount of info a computer can store has been doubling every couple of years  More memory at the same price o Ram o Disk memory o USB – Co-evolutionary impact on software  Software now has : bigger programs, with more capacity, complex user interfaces, and access to other comps on internet  Hardware now has: greater push to make comps bigger and greater incentive to connect comps  More memory means software can be bigger and do more  More computers on internet means more interest on software that connects to other computers 2. Speed – internet has been getting faster - - Processing Speeds are faster o Clock cycles per second have been getting 10 times faster/8 years - Data Transfer speeds are faster 3. Reliability - Less line noise (noise on electronic cables that altered data) due to improved infrastructure - Improved communication protocols for data transfer - Internetz is always on and available 4. Security - More secure o Commercial transactions occur (due to cryptography) Networks (can be wireless or networks) - Abstract network (made of edges and nodes) o Edges: ( data is sent bit by bit over edges)  Cables  Wireless transmission (short distance)  Satellite Links (takes a long time) o Nodes: (data is relayed from node to node by computers)  Computers Arpanet (1971): o 18 nodes (computers) o 22 edges (land based communication cables) Internet by Land - Optical Fiber Cables: o Carries long distance internet traffic (speed of light) o Usually underground Internet by Sea - Optical fibers on bottom of ocean o Majority of cable is protection (only tiny portion is fiber optic cables) o Two major sea accidents: Alexandria (NA, Europe, South Asia) + Mombasa (most of africa) Internet by Air - Phone connects to cellular tower which then uses internet cables to send/receive data - Phone accesses wifi which uses internet cables to send/receive data Host Name: (no backslash or http) Symbolic Internet Protocol (IP) Address: (com is own domain and so is www) Numeric Internet Protocol (IP) Address: - Expressed as 4 numbers (components) each between 0 and 255 inclusive - 256 choices for each component (256^4 for the whole thing) Domain Name Servers: Correspondence between symbolic and numeric addresses held by special Internet computers - Can communicate with others (receives symbolic IP address and looks up/sends back numeric IP address) - Has table associating numeric and symbolic IP addresses Router: computer customized to quickly route messages from one machine to another on the Internet - Often has many cables going in, many going out - Routers don’t have numeric IP addresses Internet Process: 1. Type in URL 2. Computer separates info and sends symbolic IP address to DNS 3. DNS sends back corresponding numeric IP address 4. Computer/Browser receives numeric IP address and composes message of specific info wanted to be opened and sends it to the ISP 5. ISP looks at address and directs info to appropriate computer with info on it (based on numeric IP address) 6. Computer sends request for specific web page on its web server program 7. Contents retrieved 8. Internet message composed and sent back to original computer via the same method (may use different computers though) 9. Original Computer receives info and opens page! History Behind the Internet 1940’s - Manchester mark I: Kilburn and Williams o First non-prototype comp you could store a program on - Women were many of the programmers of original computers (also most of original human comps?) 1950’s (only one program at a time) - Big businesses and government use comps o Complex calculations o Data processing - Computer input via: o Punched cards o Early typewriter based terminals - Computer output via: o Punched cards o Paper tape 1960’s (more than one program) –Fernando Corbato - ARPA takes off (advanced research projects agency – US department of defence) - Many people work off of one computer - Types of networks (Paul Baran, 1964) o Centralized can be destroyed taking out central node (bad) o Decentralized can be cut into two or more pieces by taking out nodes (better but still bad) o Distributed are still in one piece if several nodes are taken out (best) - Packet Switching: sending blocks of data that are the same size by: 1. Breaking up the data into blocks 2. Labeling blocks with serial numbers 3. Sending blocks separately 4. Receiver reassembles blocks - ARPA Net (1969) o Funded by ARPA o 4 nodes (UCLA, SRI – Stanford, UCSB – U. of California, U. of Utah) o Compatibility problem: comps could only send to comps by same manufacturer o Compatability Solution: standardized format of message (IMPS interface message processors) 1970’s/80’s - TCP: inter-networking protocol that allow messages to be forwarded from one to another (forerunner of IP) o Vincent Cerf, and Robert Kahn (fathers of internet) - IP (internetworking protocol) splits from TCP - Domain name servers introduced (DNS) - Computers and their networks grow substantially (more socially) 1990 (Dot com bubble/ Internet Bubble) - Can access data on other internet nodes via FTP (file transfer protocol) - 1991, Tim Berners Lee announces the WWW (World Wide Web) o Merges info retrieval and hypertext o Allows access of data by existing protocols (FTP, NNTP, HTTP) and a gateway - 1993, Mosaic we browser developed (precursor of things like Firefox) - Internet begins to dominate other networks (speed and reliability) URL’s (uniform resource locators) - o http: protocol to be used o Symbolic IP address o faculty/Andrews: name of resource HTML (Hypertext Markup Language) - syntax for documents - allow embedding of URL’s and images 21’st Century - networks other than internet are decommissioned - most traffic on internet is web traffic (messages sending and receiving from users) Internet statistics - China has most users - India has smallest percent of pop using internet - USA has largest percent of pop using internet - Iran has highest growth rate while America has the lowest - Most internet traffic now occurs through mobile devices Encoding - Two notions of encoding: 1. Basic encoding of info as a sequence of bits  Always necessary 2. Encryption of sensitive info  Optional  To ensure info is readable only by intended recipient Bits and Bytes - Bit: binary digit that can have a value of 0 or 1 o Everything on internet is represented in bits - Bit Pattern o 2^n possible patterns of n bits ( ex. 2 bits = 4 bit patterns, or 3 bits = 8 bit patterns) o Code value: assigning certain amounts of bits to represent something else (ex. The four points of a compass has a code value of 2 (2^2= 4))  Context (the position, file they are in and the bits before them) is required (00 can be north or john Lennon) o 2^n > k represents amount of minimum number of bits needed to represent k items - Byte: A sequence of 8 bits (smallest unit of memory stored or transferred) Interpretation of Bit Patterns - 1011010 o [(0*1) + (2*1) + (4*0) + (8*1) + (16*1) + (32*0) + (64*1)] = 90 o Universal and easier to talk about in decimals such as 90 (benefits) - Conventional interpretation of bit patterns o The 2^n bit patterns of length n are interpreted as the numbers 0 to 2^n – 1 ( length 2= 2^2- 1, =3, while length 3 = 2^3-1 =7) o The value of each component is therefore from 0 to 255 Files - Consists of: 1. Data (sequence of bytes) 2. Metadata (file name, date last modified, access permissions) - Simplest kind of file is text ASCII (American standard Code for Information Exchange) -finalized in 1960’s - Different fonts have different ASCII code numbers -one single representation of text (connect any machine to another) - English text is represented in 2^7 bits (since there are at least 74 characters) Important ASCII Codes 8-backspace, 9- tab, 10- line feed, 13- carriage return, 27- escape, 32- space, 127-delete Encoding ISO-8859-1 - ASCII only supported 128 characters - Encoding ISO-8859-1 covered all languages except for Asian and European symbols Unicode: an attempt to cover every major script - Standard system used in all recent applications - Assigns point between 0 and 1,114,111 to character UTF-8 - 1byte = 128 code points - 2 bytes = 2048 code points - 3 bytes = 65536 code points - A completely ASCII file IS a Unicode file in UTF-8 encoding - Example, euro symbol (unavailable) is 8364 ASCII, ISO-8859-1, Unicode are all international standards which represent text in sequences of bytes Bases - Our number system is in base 10 - Base 5 numbers: 0,1,2,3,4 (5,6,7,8,9 have no use) o Last digit: # of 1’s (1) o Second last digit is the number of 5’s (5 in base ten = 10 in base five) o Third last digit is number of 25’s (25 in base ten = 100 in base five) o Fourth last digit is number of 125’s (125 in base ten = 1000 in base five) o Thus 1332 (base five) = (2 + 15 + 75 + 125 = 217 in base ten) - Base 2 numbers: 0, 1 o Last digit: # of 1’s (1) o Second last digit is the number of 2’s (2 in base ten = 10 in base two) o Third last digit is number of 4’s (4 in base ten = 100 in base two) o Fourth last digit is number of 8’s (8 in base ten = 1000 in base two) o Thus 0110 (base two) = ( 0 + 2 + 4 + 0 = 6 in base ten) Rule: If B is base then: – The number b is written as 10 in base b – The number b2 is written as 100 in base b – The second-last digit is the “number of (b)s” – The third-last digit is the “number of (b2)s” – The number b3 is written as 1000 in base b – The fourth-last digit is the “number of (b3)s”, e.c – The number b4 is written as 10000 in base b, etc. Programming HTML (Hypertext Markup Language) - Devised by Tim Berners Lee - Language for describing layout of hypertext documents - HTML Files contain: o Normal text o Markup
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