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CS 453

CS 453. Computer Networking Data and Computer Communications. Networking Standards. So why can all of these devices from different vendors in different countries interoperate? Standards Lack of standards – a big problem – most of the time – consider wireless networking Standards can be –

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CS 453

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  1. CS 453 Computer Networking Data and Computer Communications

  2. Networking Standards • So why can all of these devices from different vendors in different countries interoperate? • Standards • Lack of standards – a big problem – most of the time – consider wireless networking • Standards can be – • Per company • Per country • International • More concrete, specific than reference models

  3. Networking Standards • Usually, • starts with an identified problem domain • The a proposed or draft standard • A review and comment process • Adoption of the standards

  4. Standards Organizations • Telecommunications • ITU-T (previously known as CCITT) • About 200 government members • About 500 industry members • Networking (International Standards • ISO (International Standards Organization) • Members are national standards body • US = ANSI, GB=BSI, France=AFNOR, Germany=DIN • 89 or so countries

  5. Standards Organizations • NIST (National Institute of Standards and Technology) • U.S. Dept. of Commerce • Primarily concerned with government standards, except DoD • IEEE

  6. IEEE Standards From Tanenbaum, 2003

  7. Internet Standards • The Internet Society (ISOC) • The Internet Architecture Board (IAB) • The Internet Engineering Task Force (IETF) • The Internet Research Task Force (IRTF) • ICANN • Internet Corporation for Assigned Names and Numbers

  8. Layer 1 – The Physical Layer • The physical layer is concern with, literally, the physical connections between machines that allows them to communicate • This layer is concerned with physical properties of communications devices --- voltages, timing, etc. • Coaxial cable, twisted pair, fiber,…

  9. Layer 1 – the Physical Layer • A channel is a physical path between two machines • It can use several different forms of media

  10. A brief theoretical analysis of data communications • Ever wonder why we talk about data rates being limited by distance or the length of a cable? • Several factors can limit the bandwidth and, therefore, the data rate of a communications channel • This is limited by the amplitude of the signal

  11. Layer 1 – The Physical Layer • Bandwidth is a characteristic of the communication medium • Using wire as an example, these include • the length of the wire – longer wire - more attenuation • The gauge of the wire – the thinner the wire the more attenuation • Heat, bend radii*,etc. • All transmission media attenuate different signal frequencies at different amounts… • With higher frequencies attenuated more than lower frequencies • Bandwidth (Hz) is the highest frequency at which at least 50% of power gets through

  12. Physical Layer - Bandwidth • A bit of trivia (sort of) • Traditionally phone wire has a bandwidth of about 1 Mhz for short runs of wire • Telephone companies used low-pass filters to cap the bandwidth at around 3100 Hz • OK for human speech, not much else • Limits communications data rates (more later) • Why did they do this? • So how did we ever get 38.4Kbps, 56Kbps?

  13. Physical Layer -Bandwidth • Nyquist at AT&T in 1920s theorized that any communications channel has a maximum possible data rate… • That is a function of the bandwidth (Hz) of the channel media… • … in the case of a perfect signal (no noise)

  14. Physical Layer • Nyquist’s theorem – • Max data rate (in bps) = 2(Hz)Log22 • Where Hz is the signal bandwidth of the communication medium • With a pure signal – no noise

  15. Physical Layer • …but what about noise… • …there’s always noise • Noise is measured as the Signal to Noise ratio S/N • Usually reported in decibels (dB) • dB = 10*Log10(S/N) • So S/N = 10 … dB = 10 • .. S/N = 1000 …dB = 30

  16. Physical Layer • Shannon (1948) introduced noise to Nyquist’s equation… • Max Data Rate (in bps) = Hz*Log2(1+S/N) • Where Hz is the bandwidth of the medium • S = signal power • N = noise power

  17. Physical Layer • So what does this mean? • That with an analog phone line… • …with very little noise (say 30 dB S/N) • You should never be able to transmit more than about … • 30,000 bps • Have you ever seen a 56Kbps modem?

  18. Classes of Transmission Media • Guided Transmission Media • Twisted pair • Coaxial Cable • Fiber Optics • Wireless Transmission Media • Radio • Microwave • Iaser/infrared/millimeter wave • Satellite

  19. Thought Experiment • Imagine that you have a massive amount of data that you need to regularly transfer to and store at an off-site storage location • You have to establish a system for transmitting the data to the storage location and you have to options- • Build a fiber optic network from your location to the off-site location… or • Have the data routinely transported by truck

  20. Thought Experiment • So, your question is – • What is the bandwidth (data rate) of the highway?

  21. Thought Experiment • Some parameters for your problem • You have about 200 Terabytes (~1012) of data to transmit every day • A magnetic tape will hold about 200 Gbytes of data • Your 200 Tbytes will fit on about 1000 magtapes • 1000 magtapes will fit in a large box, which will fit in the back of a car • The storage location is one hour by car from your system location

  22. Thought Experiment • So, what is the bandwidth of the highway? • Suppose you choose to build the network • How long will it take you to transmit your 200 Tbytes of data… • On a 100 Mbs network? • On a 1 Gbps network?

  23. Guided Transmission Media • Twisted Pair • Cable contains mutual pairs of wire • Each pair is twisted together • Wire acts as an antennae – radiating its signal • Wires twisted together tend to cancel out the radiated signals. • So less cross-talk from other signals… • … less noise… • Greater effective bandwidth in the medium • Don’t untwist the conductor pairs!

  24. Twisted Pair Cable • Two types • UTP – Unshielded Twisted Pair • Multiple twisted pairs of conductors in an insulation jacket • STP – Shielded Twisted Pair • Just like UTP except the bundle of conductors are wrapped in a metal braided jacket, then an out insulation jacket • Used by IBM, primarily for Token Ring (802.5), and few other things • Not popular outside of IBM systems

  25. Twisted Pair Cable • Category 3 (Cat3) • Early network cabling • 3 to 4 twists per foot • Up to 16MHz • Low speed applications • Still used for structured telephone wiring • Category 5 (Cat5) • 3 to 4 twists per inch • Reduces cross-talk noise • Up to 100 Mhz • Widely used

  26. Twisted Pair Cabling • Category 5e (Cat5e) • More twists per inch • Can be used for Gigabit Ethernet using all four pairs • Category 6 (Cat6) • Supports Gigabit Ethernet using four pairs • GigE at higher speeds than Cat5e • Category 7 (Cat7) – still in development • Different connector • 1.2 GHz bandwidth

  27. Twisted Pair • See • http://www.infocellar.com/networks/cables/twisted-pair-cables.htm • Note TIA-568 B

  28. Coaxial Cable • Coax – two common types • 50 ohm coax – intended for digital communications • 75 ohm coax – intended for analog communications, cable TV • With cable Internet service 75 is often used for digital communications

  29. Coaxial Cable • Coax – center conductor surrounded by a relatively large insulator… • Encased in a braided conductor… • … the outside insulation jacket From Tanenbaum, 2003

  30. Coaxial Cable • Very good noise immunity • High bandwidth – around 1GHz • Bandwidth sensitive to quality of terminations

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