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Download Chapter 6: Data Transmission
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Chapter 6: Data Transmission Business Data Communications, 4e Electromagnetic Signals Function of time Analog (varies smoothly over time) Digital (constant level over time, followed by a change to another level) Function of frequency (more important) Spectrum (range of frequencies) Bandwidth (width of the spectrum) Periodic Signal Characteristics Amplitude (A): signal value, measured in volts Frequency (f): repetition rate, cycles per second or Hertz Period (T): amount of time it takes for one repetition, T=1/f Phase (f): relative position in time, measured in degrees Bandwidth Width of the spectrum of frequencies that can be transmitted if spectrum=300 to 3400Hz, bandwidth=3100Hz Greater bandwidth leads to greater costs Limited bandwidth leads to distortion Why Study Analog in a Data Comm Class? Much of our data begins in analog form; must understand it in order to properly convert it Telephone system is primarily analog rather than digital (designed to carry voice signals) Low-cost, ubiquitous transmission medium If we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively Data vs Signals Analog data Voice Images Digital data Text Digitized voice or images amplitude (volts) Analog Signaling 1 cycle represented by sine waves phase difference time (sec) frequency (hertz) = cycles per second Voice/Audio Analog Signals Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage Human voice has frequency components ranging from 20Hz to 20kHz For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz Image/Video: Analog Data to Analog Signals Image is scanned in lines; each line is displayed with varying levels of intensity Requires approximately 4Mhz of analog bandwidth Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal amplitude (volts) Digital Signaling 1 cycle represented by square waves or pulses time (sec) frequency (hertz) = cycles per second Digital Text Signals Transmission of electronic pulses representing the binary digits 1 and 0 How do we represent letters, numbers, characters in binary form? Earliest example: Morse code (dots and dashes) Most common current form: ASCII Digital Image Signals Analog facsimile similar to video scanning Digital facsimile, bitmapped graphics uses pixelization Object-oriented graphics image represented using library of objects e.g. Postscript, TIFF Pixelization and Binary Representation Used in digital fax, bitmapped graphics 1-bit code: 00000000 00111100 01110110 01111110 01111000 01111110 00111100 00000000 Transmission Media the physical path between transmitter and receiver (“channel”) design factors affecting data rate bandwidth physical environment number of receivers impairments Impairments and Capacity Impairments exist in all forms of data transmission Analog signal impairments result in random modifications that impair signal quality Digital signal impairments result in bit errors (1s and 0s transposed) Transmission Impairments: Guided Media Attenuation loss of signal strength over distance Attenuation Distortion different losses at different frequencies Delay Distortion different speeds for different frequencies Noise distortions of signal caused by interference Transmission Impairments: Unguided (Wireless) Media Free-Space Loss Signals disperse with distance Atmospheric Absorption Water vapor and oxygen contribute to signal loss Multipath Obstacles reflect signal creating multiple copies Refraction Noise Types of Noise Thermal (aka “white noise”) Uniformly distributed, cannot be eliminated Intermodulation When different frequencies collide (creating “harmonics”) Crosstalk Overlap of signals Impulse noise Channel Capacity The rate at which data can be transmitted over a given path, under given conditions Four concepts Data rate Bandwidth Noise Error rate Shannon Equation C = B log2 (1 + SNR) B = Bandwidth C= Channel SNR = Signal-to-noise ratio
