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Part II

Part II. Signals, Media, And Data Transmission. Can be very primitive or very advanced From physics Energy Electromagnetic wave propagation From mathematics Coding theory. Transmission of Information. Copper wire Possibilities Twisted pair Coaxial cable Optical fiber Flexible

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Part II

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  1. Part II Signals, Media, And Data Transmission

  2. Can be very primitive or very advanced From physics Energy Electromagnetic wave propagation From mathematics Coding theory Transmission of Information

  3. Copper wire Possibilities Twisted pair Coaxial cable Optical fiber Flexible Light “stays in” Air / space Used for electromagnetic transmission Transmission Media

  4. Twisted Pair

  5. cross section core cladding protective coating two propagation modes >

  6. Why does fiber have more bandwidth than coaxial cable? Bits are more crowded, not faster. One second

  7. Electromagnetic Spectrum

  8. ISM (Industrial/Scientific/Medical) Band Transmitters using these bands do not require government licensing. One band is allocated worldwide: 2.400-2.484 GHz. In addition, in the US and Canada, bands also exist from 902-928 MHz and from 5.725-5.850 GHz. These bands are used for cordless telephones, garage door openers, wireless hi-fi speakers, security gates, etc.

  9. Electric current Audible sounds Omni-directional electromagnetic waves Radio Frequency (RF) Light Infrared Forms of Energy Used To Transmit Data

  10. Directional electromagnetic waves Point-to-point satellite channel Microwave Laser beam Forms of Energy Used to Transmit Data (continued)

  11. Geosynchronous Earth Orbit (GEO) Low Earth Orbit (LEO) Array needed Types of Satellites

  12. Satellite Contain several transponders. Properties: 1. Longer delay 2. Broadcast in nature 3. Bad security 4. Deployment is fast downlink channel uplink channel

  13. Kepler’s Law Near the surface of the earth, the period is about 90 minutes. Communication satellites at such low altitudes are problematic because they are within sight of any given ground station for only a short time interval. Earth

  14. Geosynchronous satellite However, at an altitude of approximately 36,000 km above the equator, the satellite period is 24 hours, so it revolves at the same rate as the earth under it. Having the satellite fixed in the sky is extremely desirable, because otherwise an expensive steerable antenna would be needed to track it. With current technology, it is unwise to have satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference. So there are only 180 geosynchronous satellites in the sky at once.

  15. Geosynchronous satellite Fortunately, satellites using different parts of the spectrum do not compete, so each of the 180 possible satellites could have several data streams going up and down simultaneously. Alternately, two or more satellites could occupy one orbit slot if they operate at different frequencies. To prevent total chaos in the sky, there have been international agreements about who may use which orbit slots and frequencies.

  16. Low-Orbit Satellites Motorola’s Iridium Project (77 LOS original, later revised to 66) Operate in the L band, at 1.6 GHz, making it possible to communicate with a satellite using a small battery-powered device. bankrupted

  17. Propagation delay Time required for signal to travel across media Example: electromagnetic radiation travels through space at the speed of light (c = 3x108 meters per second) Bandwidth How much information can be carried per second Two Important Physical LimitsOf a Transmission System

  18. Network hardware encodes information for transmission Two types of encoding Analog (amount of energy proportional to value of item) Digital (two forms of energy to encode 0 and 1) (most) computer networks use the latter Transmission of Data

  19. Medium Copper wire Energy form Electric current Encoding Negative voltage encodes 1 Positive voltage encodes 0 Example Digital Encoding

  20. Known as waveform diagram X-axis corresponds to time Y-axis corresponds to voltage Illustration Of Digital Encoding

  21. Several organizations produce networking standards IEEE ITU EIA Hardware that adheres to standard --interoperable Encoding Details

  22. Example use Connection to keyboard / mouse Serial port on PC Specified by EIA Voltage is +15 or –15 Cable limited to ~50 feet Latest EIA standard is RS-422 (ITU standard is V.24) Uses asynchronous communication The RS-232C Standard

  23. Sender and receiver must agree on Number of bits per character Duration of each bit Receiver Does not know when a character will arrive May wait forever To ensure meaningful exchange send Start bit before character One or more stop bits after character Asynchronous Communication

  24. Start bit Same as 0 Not part of data Stop bit Same as 1 Follows data Illustration of RS-232

  25. Determined by baud rate (number of signals per second) Typical baud rates: 9.6 Kbaud, 14.4 Kbaud, 28.8 Kbaud Sender and receiver must agree a priori Receiver samples signal Disagreement results in framing error Duration of a Bit in RS-232C

  26. Desirable in practice Requires each side to have transmitter and receiver Called full duplex Two-Way Communication

  27. Transmitter on one side connected to receiver on other Separate wires needed to carry current in each direction Common ground wire DB-25 connector used Pin 2 is transmit Pin 3 is receive Pin 7 is ground Illustration Of Full-Duplex Communication

  28. It’s an ugly world Electrical energy dissipates as it travels along Wires have resistance, capacitance, and inductance which distort signals Magnetic or electrical interference distorts signals Distortion can result in loss or misinterpretation Electrical Transmission(The Bad News)

  29. In practice Distortion can be much worse than illustrated Illustration of DistortedSignal for a Single Bit

  30. RS-232 hardware must handle minor distortions Take multiple samples per bit Tolerate less than full voltage Cannot use electrical current for long-distance transmission Consequences

  31. Important fact: an oscillating signal travels farther than direct current For long-distance communication Send a sine wave (called a carrier wave) Change (modulate) the carrier to encode date Note: modulated carrier technique used for radio and television Long-Distance Communication

  32. Carrier Usually a sine wave Oscillates continuously Frequency of carrier fixed Illustration of a Carrier

  33. Amplitude modulation (used in AM radio) Frequency modulation (used in FM radio) Phase shift modulation (used for data) Types of Modulation

  34. Strength of signal encodes 0 or 1 One cycle of wave needed for each bit Data rate limited by carrier bandwidth Illustration ofAmplitude Modulation

  35. Frequency Modulation (frequency shift keying)

  36. Change in phase encodes K bits Data rate higher than carrier bandwidth Illustration ofPhase-Shift Modulation

  37. Section of wave is omitted at phase shift Data bits determine size of omitted section Phase-Shift Example

  38. constellation patterns Diagrams, which show the legal combinations of amplitude and phase, are called constellation patterns. Each high-speed modem standard has its own constellation pattern and can talk only to other modems that use the same one (although most modems cam emulate all the slower ones). 3 bits/baud modulation (A combination of AM and PM)

  39. constellation patterns QAM (Quadrature Amplitude Modulation) Used to transmit 9600 bps over a 2400-band line. A 4 bits/baud modulation (ITU V.32 9600 bps modem standard)

  40. Hardware device Used for long-distance communication Contains separate circuitry for Modulation of outgoing signal Demodulation of incoming signal Name abbreviates modulator / demodulator Modem

  41. One modem at each end Separate wires carry signals in each direction Modulator on one modem connects to demodulator on other Illustration of ModemsUsed Over a Long Distance

  42. Conventional Use four wires Transmit modulated electrical wave Optical Use glass fibers Transmit modulated light Wireless Use air / space Transmit modulated RF wave Types of Modems

  43. Dialup Use voice telephone system Transmit modulated audio tone Note: in practice, a dialup modem uses multiple tones simultaneously Types of Modems(continued)

  44. Modem can Dial Answer Carrier is audio tone Illustration of Dialup Modem

  45. Full-duplex modem Provides 2-way communication Allows simultaneous transmission Uses four wires Half-duplex modem Does provide 2-way communication Transmits in one direction at any time Uses two wires Modem Terminology

  46. PCM (Pulse Code Modulation) sampling and quantization Sampling is the periodic measurement of the signal every T seconds. These periodic measurements are called samples. Quantization is the approximation of the possible values of the samples by a finite set of (binary) values.

  47. PCM (Pulse Code Modulation) Nyquist's sampling theorem A signal with maximum frequency fmax can be recovered exactly from samples that are measured more frequently than 2fmax every second.

  48. PCM (Pulse Code Modulation) digitization of audio: (1) telephone voice (~4000 Hz) 8000 samples per second, every sample 8 bits=64kbps (DPCM: differential PCM, only encode the differences between samples) (Predictive encoding) (Delta Modulation: use only 1 bit to mean a difference of +1 or -1) (2) compact discs (~20KHz) 41000 samples per second, encoded in 16 bits, two channels =1.3Mbps

  49. PCM (Pulse Code Modulation) Delta modulation

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