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CS 453 Computer Networks

CS 453 Computer Networks. Lecture 4 Layer 1 – Physical Layer. Data Communications Growth. A little more that 25 years ago The IBM PC had a clock speed of less than 5 MHz Networking technology (specifically ARPANET) ran at 56Kbps Today PC clock speeds run up to 4 GHz

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CS 453 Computer Networks

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  1. CS 453Computer Networks Lecture 4 Layer 1 – Physical Layer

  2. Data Communications Growth • A little more that 25 years ago • The IBM PC had a clock speed of less than 5 MHz • Networking technology (specifically ARPANET) ran at 56Kbps • Today • PC clock speeds run up to 4 GHz • High speed networks run at a max of 10 Gbps • In comparison in about 25 years • CPU clock speed improved by a factor of 800 • Communications speeds improved by a factor of 178,000

  3. Data Communications Growth • During the same time • Communications error rates dropped from about 1 error per 10,000 bits • To near zero • Due to a large extent to Fiber Optics

  4. A Brief History • The idea of guiding light has been around for a while • Tyndall’s Water Fountain • Early 20th century – glass tubes for projecting images from hard to reach places • Medical images, equipment

  5. A brief History • 1950s Kapany did early work that lead to optical fibers • Fiberscope – use of fibers for internal medical examinations • There was a strong interest in using fiber optics for communications • Light attenuation to great

  6. A Brief History • Many believed that light attenuation was due to principles of physics • 1960s Kao and Hockham theorized that attenuation was due to impurities in the glass • Kao and Hockham suggested that optical fiber could be used for telecommunications if … • Attenuation could be made less than 20 dB/km

  7. A Brief History • 1970 Researchers at Corning Glass Works developed an optical fiber … • With 17 dB/km light attenuation • A few years later they developed fiber with 4 dB/km attenuation

  8. A Brief History • For more on the history of fiber optics • http://en.wikipedia.org/wiki/Fiber_optics#History • http://www.sff.net/people/Jeff.Hecht/history.html • http://www.fiber-optics.info/fiber-history.htm

  9. Fiber Optics • A waveguide for propagating light along its length

  10. Fiber Optics • Fiber Optics as a data communication medium is based on a principle of physics • The principle of refraction • When light passes the boundary from one medium to another – • It is refracted --- i.e. it bends • Recall looking at a coin in the bottom of a clear pool of water • Most noticeable with prisms, magnifying lens, etc.

  11. Fiber Optics • Light passing a boundary between, for example, glass and air at an angle A will be refracted (bent) to angle B. • Beyond a certain angle all of the light will be refracted back into the original media (glass) • That “certain angle” is dependent on characteristics of the media on both sides of the boundary – • Refraction Index

  12. Fiber Optics • Refraction of light at the Glass (silica)/Air boundary From Tanenbaum (2003) pg. 94

  13. Fiber Optics From Wikipedia (http://en.wikipedia.org/wiki/Fiber_optics)

  14. Fiber Optics • Incredibly high bandwidth • Data rates (theoretical) greater that 50,000 Gbps • Very low light attenuation

  15. Fiber Optics • Long distances without attenuation • 1 Gbps data rates common • 10 Gbps available and economically feasible – major trunks • 40 Gbps – currently possible • Fiber optics can achieve much higher data rates • Limited by transceiver electronics

  16. Fiber Optics • Fiber Optic cable includes • A core – made of glass – about 50 microns in diameter for multimode or 10 microns for single mode • Cladding – usually also glass but with a lower refraction index • This keeps the light trapped in the cable • A sheath – plastic outer jacket of the fiber cable • Often “packaged” in multi-fiber cables… • But always in pairs

  17. Fiber Optics • Multimode Fiber • Multiple wavelengths of light • Thicker core (50 microns) • Cheaper • Single Mode • Small diameter core • Propagates light in a straight line • Longer distances • More expensive fiber, end equipment

  18. Fiber Optics • Interconnecting Fiber • Termination in connectors • Plug into “patch panels” • Connectors up to 20% light attenuation • Mechanical Splice • Cut fibers, polish ends and connect in sleeves • Requires skill – with skill about 5 minutes per splice • Fusion – welding • Expensive equipment • Very little attenuation

  19. Fiber Optic Network • A fiber optic link must have – • The medium – fiber • A light emitter • LED • Semiconductor laser • A receiver • Fiber is unidirectional • Must use in pairs • Fiber Interface • Convert light to electrical signal and electrical signal to light

  20. Fiber Optic Networks • Fiber connector information • http://www.fiber-optics.info/articles/connector-care.htm

  21. Fiber Optics • Fiber Networks • Popular for long distance links • Used in LANs and high performance applications • Fiber connections must be point to point • Cannot use broadcast technology • Like Bus topology • So, how do we connect many computers with a fiber network

  22. Fiber Optic Network • Long Distance Link • Router to Router • Routers hand off to individual computers • …or to computers on LAN • LANs • Pass Taps • Active Repeater • Takes incoming light converts to electrical signal… • Converts electrical signal to light and sends

  23. Fiber Optic Networks • Remember that we could squeeze all of the bandwidth out of fiber optics • So, how do we get more of the bandwidth • Wave Division Multiplexing (WDM) • Remember that emitter diodes can be tunable – to different wavelengths of light • Suppose – • You take multiple input channels • Tune each to a different wavelength of light on its own fiber () • Then combine them on one fiber….

  24. Fiber Optic Networks • …each  is split out to a different fiber at the receiving end • From Tanenbaum (2003) pg. 139

  25. Fiber Optic Networks • …that’s Wave Division Multiplexing (WDM) • …its Layer 1 – protocol independent • So, how much • 96 10Gbps channels on a fiber pair

  26. Fiber Optics Networks • DWDM – Dense Wave Division Multiplexing • Very small channel separation • Large number of channels • See • http://www.cisco.com/univercd/cc/td/doc/product/mels/dwdm/dwdm_fns.htm

  27. Fiber Optic Networks • Optical Carrier Levels - OC • Used on SONET Networks • Units of measure measurement for data rates on fiber optic links • One OC roughly corresponds to 52 Mbps • More on this later

  28. Fiber vs. Copper • Fiber has much higher bandwidth • Very low signal attenuation relative to copper • Repeaters needed after long distance – • 50 km for fiber vs. 5 km for copper* • Light weight • One km of 1000 pair copper twisted pair = more than 17,000 lbs. • One km of 1 fiber pair = about 220 lbs. • 1 fiber pair can carry more data than 1000 copper twisted pair cables From Tanenbaum (2003)

  29. Fiber vs. Copper • Security • Copper leaks • Fiber does not leak • Fiber deployment requires more advanced skill • Fiber sensitive to damage

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