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  1. Internet Concepts Telecommunications & Basic Internet By Ralph B. Bisland, Jr. Title Slide CSS 404/504

  2. Basic model of how the Internet operates Client/Server Model: Consists of at least two computer systems: Client Computer System Server Computer System Client/Server Model

  3. Runs “locally” (probably on a PC) May have different versions for different platforms Initiates requests Example client command: Send me “file-a” Client Computer System

  4. Runs on remote computer that is permanently connected to the Internet Tasks Performed: Authorizes users Stores and manages data Responds to requests from clients Example server action: Here is the file you requested Server Computer System

  5. Client/Server Architecture Client Computer Server Computer Data Communication Link

  6. How computers communicate with each other (telecommunications) Can be either analog or digital Analog: A representation of communication using an analog device Digital: A representation of communication using a digital device Communication

  7. Any representation of data in which the amount of a substance or signal is proportional to the information represented. Examples: Watches with hour, minute, and second hands. Gasoline flowing through a punp. Analog Data

  8. Phonograph-Recording involved cutting groves into the record analogous to the vibrations that make sounds. Height of the “bumps” was proportional to the loudness of the sound. A needle traveled across the surface of the record and as it ran across the “bumps”, it vibrated a diaphragm, producing vibrations that humans perceive as sounds. Analog Communication: Example

  9. Early telephones were analog devices. Speaker caused an electronic current to be generated and transmitted over telephone line. The louder you talked, the more current was sent over the line. Analog Communication: Example

  10. Easy to understand how it works. The pitch of a human voice is an exact analog of the force a person applies to stretch their vocal cords. Advantage of Analog

  11. Impossible to produce an exact analog of all possible inputs. (All people do not sing the same way.) Media wears out. (Each time a record is played, part of the “bumps” are worn away). Inaccuracies arise because amplifiers are not perfect. Disadvantages

  12. Analog signals fade and must be re-amplified. Amplifications causes noise to (called distortion) to creep into the signal. The more times the signal is amplified, the more distortion is added. Disadvantages (ctd)

  13. Information is represented by specific numbers. These numbers are represented in binary. Data is transmitted with a series of 0’s and 1’s. Because computers are basically digital machines, digital communication works best with them (no conversion to analog is needed). Digital Communication

  14. Most telephone lines are analog lines. Since computers are digital machines, the digital signal must be converted to an analog for transmission. This is done by an A to D converter Specific device is called a modem modulate-demodulate. Process is transparent to end users. A To D Converters

  15. Example Of The Use Of Modems Communication Line Computer B Computer A Modem 1 Modem 2 Analog Data Digital Data Digital Data

  16. Early telecommunication: Morse Code Series of dots and dashes (Min=1, Max=6)Examples of Morse Code A: .- B: -... C: -.-. 0: ----- 1: .----- 2: ..--- 3: ...-- 5: ..... 6: -.... .: .-.-.- Morse Code

  17. Telegraphs were digital - either a dot or a dash Experienced telegraphers knew when a “word break” occurred. Telegraph operators “translated” message into Morse code. Receiving telegrapher decoded message for receiver Morse Code (ctd)

  18. Modern computer systems do not use Morse Code to transmit data over the Internet Use ASCII codes (8 binary bits = 1 byte) Each character has a specific number associated with it A = 0100 0001 = 65 (base 10) B = 0100 0010 = 66 (base 10) Digital representation of data ASCII Codes

  19. Advantage: Do not “wear out”. Since a number represents a value, the number does not “fade”. Disadvantage: Since most values are analog, data must be converted into a number by some A to D process. Advantages/Disadvantages

  20. Key to understanding how the Internet works Understanding telecommunications and the UNIX operating system Telecommunications: The electronic transmission of data (text, graphics, sound, and video) over any one of different communications channels, such as public telephone lines, private cables, microwave, or satellite. Networks connect a set of computer systems to each other. Telecommunications

  21. A network is a series of interconnected computer systems. Two types of networks Local Area Networks (LANs) Wide Area Networks (WANs) Types Of Networks

  22. Hardware, software, and communications equipment that allow computers to be connected within a limited area. Limited Area: Within a building or a set of buildings in close proximity. Analogy: An “in house” telephone system - don’t have to use all digits. LANs

  23. Same as a LAN except that the computers can be anywhere. Distance is not a problem. Difference is how the computers are connected. Largest WAN is the Internet. Analogy: A telephone system to call anywhere. WANs

  24. A series of interconnected LAN’s and WAN’s What Is The Internet? LAN#1 USM WAN#1 MILNET WAN#2 BITNET LAN#2 MSU LAN#3 Ole’ MS

  25. Because of costs, LAN technology differ in speed of transmission, encoding schemes for data. Various LAN technologies are incompatible LAN’s probably do not use modems to communicate between computers. LANs just plug a wire connecting one computer to another More About LAN’s

  26. Can not just plug two LAN’s together Operate over a limited distance May have its own specification for electrical signals (voltage and frequency) Technology for encoding data may be different More About LAN’s (ctd)

  27. WANs (or long haul networks) probably douse modems to connect computers together. WAN technologies are also incompatible. WAN and LAN technologies are incompatible. WANs use special purpose computers to resolve incompatibilities between different WANS. More about WANs

  28. A special purpose computer that connects two or more networks and routes data to its final destination. Analogy: An intermediate Post Office. A router forwards datagrams to another router, etc. until the datagram can be delivered to its final destination Routers connect “nodes” on networks Routers

  29. Since routers are computers, they have a finite amount of storage space in which to store queued messages. If the storage space is full (the router has too many messages to store), the messages can not fit in the storage spaces are thrown away. Routers (ctd)

  30. ARPA (Advanced Research Products Agency) allocated money to research the connectivity problem. Came up with a new approach called “internetwork”. internet: approach in general Internet: experimental prototype How To Connect Them All Together

  31. ARPANET: A large WAN (called a backbone) that tied many researcher’s computers together. Agreed upon a standard for packaging data to transmit to other computers. Software consisted of two parts: IP: Internet Protocol TCP: Transmission Control Protocol Produced a smooth seamless method of transmitting data. The Solution

  32. Example Of A Backbone N2 N1 R2 R1 N3 R6 R3 Internet N6 R5 R4 N4 N5

  33. An agreed upon set of rules on how something is to be done. Examples: Red and Green traffic lights Answering the telephone Transmitting data between computer systems Protocols

  34. IP: The basic rules of how computers communicate with each other Makes Internet appear to be one big network Data is packaged into fixed size bundles called packets (general term: datagrams) The format of the packet follows the IP protocol Internet computers must have IP software to code/decode data IP- Internet Protocol

  35. Every computer connected to the Internet must have a unique address IP Address Syntax: A quadruple set up in “dotted notation” Example IP addresses: = orca 1100 0011 0001 1111 0111 0111 1110 1100 = ocean All computers on the same network have the same prefix IP Addresses

  36. Most people can not remember IP address numbers Domain Name System (DNS): A distributed database that translates IP address numbers into names Example DNS entries: = = Synonyms: computer aliases = Domain Name System

  37. Networks usually do not dedicate a single wire for each pair of communicating computers Multiple computers share underlying hardware facilities Sharing causes delays Sharing is economical Sharing is accomplished by taking turns transmitting data Accomplished by packet switching How Data Is Transmitted

  38. Computer divides data to be transmitted into packets As packets are created, they are are placed on the network for transmission. Each packet contains a “header” To IP address From IP address Sequence Number Checksum total Other stuff Packet Switching

  39. As packets are received, they are placed in order and the computer distributes them to the designated “receiver”. All packets are not the same size, but there is a maximum size. Size depends upon the application - may be a large E-mail message or a single keystroke. Packet Switching (ctd)

  40. Insures that data is delivered correctly Works closely with IP Collectively called TCP/IP protocol suite TCP is also software to exists on most computers connected to the Internet TCP/Transmission Control Protocol

  41. Sometimes because of network volume, data is lost. Sometimes some of the packets are received and some are not. Because of network traffic, all packets in a transmission may not travel the same route to the receiver. How TCP Works

  42. If all packets are received and the checksum is correct an acknowledgment is sent back to the sending computer. If not, a message to retransmit is sent back to the sending computer Retransmission can result in duplicate packets being received If a duplicate packet is received, it is ignored How TCP Works (ctd)

  43. Most organizations that are very security “aware” Some organizations want the capability of the internet, but only for their employees. Solution: An intranet (applications restricted to a local closed LAN) A software “firewall” is created on the LAN allowing no packets in or out. Intranets

  44. Viruses E-mail Viruses Worms Trojan Horses Spyware Why Protect Your Computer?

  45. Virus: A small piece of computer code that is piggybacked onto a real program. It might attach itself to a spreadsheet, etc. Each time the spreadsheet is opened, the virus could spread to other programs. Viruses

  46. Early viruses were pieces of code that were attached to a common program (word processor or computer game). When the program started up, the virus loads itself into main memory and looks for other executable programs to attach itself to. If the program was copied to a floppy disk and loaded onto another computer, it spread. How Viruses Spread

  47. This technique worked for a while because the virus could write itself out to floppy disks. After a while programs became so sophisticated they could not fit on one floppy disk. Larger programs were now loaded onto CDs. CDs can not be written to. This virus spreading technique faded out. How Viruses Spread (ctd)

  48. To spread, a program that contains the virus must be started up. Virus creators needed a program that was always executed. The “boot sector” of a floppy disk or hard disk contains the first part of the operating system. It basically loads the operating system into memory. Viruses began infecting the boot sector program which is executed when the computer starts up. Boot Sector Viruses

  49. This was a fool proof method of infecting a computer. It was particularly bad on college campuses as students share computer resources. Executable program and boot sector viruses are not very threatening any more as operating systems have built in methods of eliminating this kind of virus. Boot Sector Viruses (ctd)

  50. People create viruses. They are a program designed to do something. Once written, the virus is “released”. The virus may be simply a nuisance (writing a message on your screen) or highly destructive (erasing files on your hard drive). Origins of Viruses