Chapter 15: Data Transmission Business Data Communications, 5e

2 Electromagnetic Signals Analog Signal –signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal Digital Signal –signal intensity maintains a constant level for some period of time and then changes to another constant level

Business Data Communications, 5e3 Analog Sine Wave

Business Data Communications, 5e4 Digital Square Wave

Business Data Communications, 5e5 Periodic Signal Characteristics Peak Amplitude (A) –Maximum signal value, measured in volts Frequency (f) –Repetition rate –Measured in cycles per second or Hertz (Hz) Period (T) –Amount of time it takes for one repetition, T=1/f Phase (  ) –Relative position in time, measured in degrees

Business Data Communications, 5e6 s(t) = (4/  )  (sin (2  ft) + (1/3) sin (2  (3f)t))

Business Data Communications, 5e7 Frequency Domain Concepts Spectrum of a signal is the range of frequencies that it contains Absolute bandwidth of a signal is the width of the spectrum Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found The greater the bandwidth, the higher the information-carrying capacity of the signal

Business Data Communications, 5e8 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

Business Data Communications, 5e9 Analog Signaling

Business Data Communications, 5e10 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

Business Data Communications, 5e11 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

Business Data Communications, 5e12 Digital Signaling

Business Data Communications, 5e13 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 forms: ASCII, UTF

Business Data Communications, 5e14 Transmission Media Physical path between transmitter and receiver (“channel”) Design factors affecting data rate –bandwidth –physical environment –number of receivers –impairments

Business Data Communications, 5e15 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)

Business Data Communications, 5e16 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

Business Data Communications, 5e17 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 Thermal Noise

Business Data Communications, 5e18 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

Business Data Communications, 5e19 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

Business Data Communications, 5e20 Shannon Equation C = B log 2 (1 + SNR) –B = Bandwidth –C= Channel –SNR = Signal-to-noise ratio

Business Data Communications, 5e21