Bussiness data communications 4e chapter 6

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 (φ): 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 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 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 and ✘ How 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 ✘ Irregular spikes, less predictable 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 ... 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... 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 ✘... design factors affecting data rate ✘ bandwidth ✘ physical environment ✘ number of receivers ✘ impairments Impairments and Capacity ✘ Impairments exist in all forms of data transmission ✘ Analog