Chapter 15: Data Transmission

1. Chapter 15:Data Transmission Business Data Communications, 6e

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

3. Analog and Digital Waveforms

4. Periodic Signal Characteristics • Peak Amplitude (A) • Maximum signal value (strength), 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

5. s(t) = (4/)  (sin (2ft) + (1/3) sin (2(3f)t)) Frequency Domain Concepts

6. 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

7. 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

8. Analog Signaling

9. 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

10. Voice Signals

11. 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

12. Digital Signals

13. 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

14. Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate • bandwidth • physical environment • number of receivers • impairments

15. 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)

16. 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

17. 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 - Change in signal speed due to atmospheric conditions • Thermal Noise- White noise, arises from thermal activity of devices

18. 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, 5e

19. 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

20. Shannon Equation • C = B log2 (1 + SNR) • B = Bandwidth • C= Channel capacity (in bits per second) • SNR = Signal-to-noise ratio