chapter 15: data transmission

Chapter 15:Data Transmission

Business Data Communications, 5e


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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


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Analog Sine Wave


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Digital Square Wave


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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


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s(t) = (4/) (sin (2ft) + (1/3) sin (2(3f)t))


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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


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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


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Analog Signaling


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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


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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


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Digital Signaling


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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


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Transmission Media

• Physical path between transmitter and receiver (“channel”)

• Design factors affecting data rate– bandwidth– physical environment– number of receivers– impairments


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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)


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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


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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


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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


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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


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Shannon Equation

• C = B log2 (1 + SNR)– B = Bandwidth – C= Channel– SNR = Signal-to-noise ratio


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chapter 15: data transmission

Documents

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