ECIS469: Lecture 2

1. ECIS469: Lecture 2 Fundamentals of Networking

2. Data Communications • exchange of digital information between two devices using an electronic transmissionmedium

3. Converting Analog to Digital • Pulse Code Modulation (PCM) • Like getting a ticker quote every 10 minutes • Approximates the actual signal curve • In PCM • Measure the signal height every 1/8000th of a second • 8 bits used to report the height at each measurement • 8*8000=64,000 bits per second to provide approximation of analog signal • 64Kbps represents a single voice line in digital telecommunications

4. Pulse Code Modulation 1111 1111 Value transmitted 128 values This side Sampling Interval = 1/8000 second 128 values This side 0000 0000 Value transmitted

5. How about a CD? • Lasers etches lands and pits on the surface of a CD • Uses 16 bits to measure height of signal • Samples 44,100 times per second for each of two channels • 16*44100*2 = 176,000 bps • One hour of music requires 633Mb

6. Digital to Analog Conversion • Needed to transmit computer signals over telephone lines • Analog signal characteristics • Amplitude • Intensity of the wave (height) • Wavelength • Distance between comparable points on the wave • Frequency • Number of up and down cycles per second (Hz) • Phase • Relative state of the amplitude

7. Wave Characteristics Amplitude Wavelength

8. Amplitude Modulation

9. Frequency Modulation

10. Telecommunications System Minicomputer terminals Main frame Front-End Processor modems multiplexer Remote location

11. Multiplexing • Allows multiple signals to be sent over same medium at same time • Modes of multiplexing • Frequency Division (FDM) • Time Division (TDM)

12. Frequency Division Multiplexing X X X X X X X X X X Y Y Y Y Y Y Y Y Y Y Z Z Z Z Z Z Z Z Z Z • originally designed so multiple voice • streams could be placed on same telephone line • Multiple analog signals superimposed but on • different frequency spectra • Involves pair of multiplexers

13. Time Division Multiplexing X X X X Y Y Y Y X Y Z X Y Z X Y Z Z Z Z Z • Each signal allotted a time slot • Creates a composite stream with slots dedicated to data sources • If data source is not sending, slot goes unused – wasteful • Instead, use statistical TDM in which slots are dynamically allocated • If there is big demand, buffers are used.

14. Transmission Media • the physical path along which the data is carried • Types • twisted pair • coaxial • fiber optics and free space • satellite • terrestrial

15. Transmission Media • Twisted Pair • pair of wires twisted along entire length • usually copper with an insulating coat • Unshielded Twisted Pair (UTP) popular with LANs • CAT3 (voice) and CAT5 are common • CAT5 used for both voice and data • 100Mbs transmission speed • Limited segment length – signals needs regeneration every 100 meters

16. Transmission Media • Coaxial cable • thick insulated copper wire • Longer segment lengths • can carry up to 200 Mb/second • less interference due to shielding • Uses FDM to transmit 1000s of voice channels and 100s of TV channels • Not popular in LANS • More difficult to work with than UTP

17. Transmission Media • Fiber Optics cable • thousands of little fiber optic strands • May be glass or plastic • Thickness of a human hair • Inner core surrounded by glass (cladding) • Can be single mode or multimode • Single mode • Expensive, bigger capacity, long segment length • 8/125 • Multimode • Cheaper, less capacity • 62.5/125 • Data transmitted as pulses of light • 500 Kb/sec to several GB/sec

18. A typical optic fiber • Core made of silica and germania • Optic cladding is pure silica • Mix of different refractive indices allows for • total internal reflection

19. Point-to-point fiber optic system

20. Advantages of fiber optics • Nearly infinite capacity • Single fiber can carry 40000 telephone calls or 250 channels of television • High transmission rates at greater distances • Immune to interference and electricity • Does not corrode (being glass) • Smaller and lighter than coaxial or twisted pair • Extremely secure

21. Wireless Transmission • Directional • Focuses electromagnetic beam in direction of receiver • Terrestrial microwave • Satellite microwave • Omni directional • Spreads the electromagnetic signal in all directions • AM and FM radio • 3G networks • Smart watches

22. Terrestrial Microwave • Parabolic dish antenna sends signal to receiving dish • Line-of-sight • Typically on towers to avoid obstacles • Frequencies in the gigahertz range

23. What is a telecommunications satellite?

24. Telecommunications satellites • Space-based cluster of radio repeaters (called transponders) • Link • terrestrial radio transmitters to satellite receiver (uplink) • Satellite transmitters to terrestrial receivers (downlink)

25. Orbits • Mostly geostationary (GEO) • Circular orbit • 22,235 miles above earth • Fixed point above surface • Almost always a point on Equator • Must be separated by at least 4 degrees

26. Satellite services • Wide Area Broadcasting • Single transmitter to multiple receivers • Wide Area Report-Back • Multiple transmitters to a single receiver • Example VSATs (very small aperture terminals) • Also have microwave transmitters and receivers • Allows for spot-beam transmission (point- to-point data communications) • Can switch between beams upon request (Demand Assigned Multiple Access –DAMA) • Multi-beam satellites link widely dispersed mobile and fixed point users

27. Earth-based equipment • Original microwave transmitters and receivers were large installations • Dishes measuring 100 feet in diameter • Modern antennas about 3 feet in diameter

28. A Modern GEO satellite (IntelSat 900 series) • May have more than 72 separate microwave transponders • Each transponder handles multiple simultaneous users (protocol called Time Division Multiple Access) • Transponder consists of • Receiver tuned to frequency of uplink • Frequency shifter (to lower frequency to that of transmitter) • Power amplifier

29. IntelSat 902 (launched August 30, 2001)

30. Frequency ranges • Most transponders operate in 36MHz bandwidth • Use this bandwidth for • voice telephony (400 2-way channels/transponder) • Data communication (120Mbs) • TV and FM Radio

31. C-band, Ku-band, Ka-band • Most GEO satellites operate in the C-Band frequencies • Uplink at 6 GHz • Downlink at 4 GHz • Ku-band also used • Uplink at 14 GHz • Downlink at 11 GHz • Above bands best suited for minimal atmospheric attenuation • Few slots left… forcing companies to look at Ka band (uplink:30 GHZ , downlink: 20 GHz)

32. Companies on the forefront: Teledesic • Offer “Internet-in-the-Sky” • Main shareholders Craig McCaw and Bill Gates • McCaw also has taken over ICO Global Communications • Wanted Iridium but has backed out

33. Teledesic • Again, series of LEO satellites • 24 pole orbiting satellite rings, 15 degrees apart • 12 satellites in each ring (total = 288 LEO satellites) • Worldwide switching.. Satellites pass on data through laser • Will map IP packets on latitudes and longitudes .. Average will be 5 satellite hops in 75 ms • Supposed to start in 2002; offer 2Mbps Internet access from terminals starting at $1000 each • Postponed to 2005

34. Optical Transmission • Cutting edge • Uses modulated monochromatic light to carry data from transmitter to receiver • Optical wavelengths are suited for high rate broadband communications • Laser-based (up to 1000 times faster than coaxial)

35. Research Question for Next Class • What is Abilene?