Chapter 6: Data Transmission Business Data Communications, 4e

Electromagnetic Signals 8Function of time 8Analog (varies smoothly over time) 8Digital (constant level over time, followed by a change to another level) 8Function of frequency (more important) 8Spectrum (range of frequencies) 8Bandwidth (width of the spectrum)

Periodic Signal Characteristics 8Amplitude (A): signal value, measured in volts 8Frequency (f): repetition rate, cycles per second or Hertz 8Period (T): amount of time it takes for one repetition, T=1/f  Phase (  ): relative position in time, measured in degrees

Bandwidth 8Width of the spectrum of frequencies that can be transmitted 8if spectrum=300 to 3400Hz, bandwidth=3100Hz 8Greater bandwidth leads to greater costs 8Limited bandwidth leads to distortion

Why Study Analog in a Data Comm Class? 8Much of our data begins in analog form; must understand it in order to properly convert it 8Telephone system is primarily analog rather than digital (designed to carry voice signals) 8Low-cost, ubiquitous transmission medium 8If we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively

Data vs Signals 8Analog data 8Voice 8Images 8Digital data 8Text 8Digitized voice or images

time (sec) amplitude (volts) 1 cycle frequency (hertz) = cycles per second phase difference Analog Signaling 8represented by sine waves

Voice/Audio Analog Signals 8Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage 8Human voice has frequency components ranging from 20Hz to 20kHz 8For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

Image/Video: Analog Data to Analog Signals 8Image is scanned in lines; each line is displayed with varying levels of intensity 8Requires approximately 4Mhz of analog bandwidth 8Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

Digital Signaling 8represented by square waves or pulses time (sec) amplitude (volts) 1 cycle frequency (hertz) = cycles per second

Digital Text Signals 8Transmission of electronic pulses representing the binary digits 1 and 0 8How do we represent letters, numbers, characters in binary form? 8Earliest example: Morse code (dots and dashes) 8Most common current form: ASCII

Digital Image Signals 8Analog facsimile 8similar to video scanning 8Digital facsimile, bitmapped graphics 8uses pixelization 8Object-oriented graphics 8image represented using library of objects 8e.g. Postscript, TIFF

Pixelization and Binary Representation 8Used in digital fax, bitmapped graphics 1-bit code:

Transmission Media 8the physical path between transmitter and receiver (“channel”) 8design factors affecting data rate 8bandwidth 8physical environment 8number of receivers 8impairments

Impairments and Capacity 8Impairments exist in all forms of data transmission 8Analog signal impairments result in random modifications that impair signal quality 8Digital signal impairments result in bit errors (1s and 0s transposed)

Transmission Impairments: Guided Media 8Attenuation 8loss of signal strength over distance 8Attenuation Distortion 8different losses at different frequencies 8Delay Distortion 8different speeds for different frequencies 8Noise 8distortions of signal caused by interference

Transmission Impairments: Unguided (Wireless) Media 8Free-Space Loss 8Signals disperse with distance 8Atmospheric Absorption 8Water vapor and oxygen contribute to signal loss 8Multipath 8Obstacles reflect signal creating multiple copies 8Refraction 8Noise

Types of Noise 8Thermal (aka “white noise”) 8Uniformly distributed, cannot be eliminated 8Intermodulation 8When different frequencies collide (creating “harmonics”) 8Crosstalk 8Overlap of signals 8Impulse noise 8Irregular spikes, less predictable

Channel Capacity 8The rate at which data can be transmitted over a given path, under given conditions 8Four concepts 8Data rate 8Bandwidth 8Noise 8Error rate

Shannon Equation 8C = B log 2 (1 + SNR) 8B = Bandwidth 8C= Channel 8SNR = Signal-to-noise ratio