Business data communications 5e by stallings chapter 15

Chapter 15: Data Transmission Business Data Communications, 5e 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 Commun ications, 5e Analog Sine Wave Business Data Commun ications, 5e Digital Square Wave Business Data Commun ications, 5e 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 (φ) – Relative position in time, measured in degrees Business Data Commun ications, 5e s(t) = (4/π) × (sin (2πft) + (1/3) sin (2π(3f)t)) Business Data Commun ications, 5e 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 Commun ications, 5e 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 Commun ications, 5e Analog Signaling Business Data Commun ications, 5e 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 Commun ications, 5e 10 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 Commun ications, 5e 11 Digital Signaling Business Data Commun ications, 5e 12 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 forms: ASCII, UTF Business Data Commun ications, 5e 13 Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate – – – – bandwidth physical environment number of receivers impairments Business Data Commun ications, 5e 14 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 Commun ications, 5e 15 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 Commun ications, 5e 16 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 Commun ications, 5e 17 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 Commun ications, 5e 18 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 Commun ications, 5e 19 Shannon Equation • C = B log2 (1 + SNR) – B = Bandwidth – C= Channel – SNR = Signal-to-noise ratio Business Data Commun ications, 5e 20 Business Data Commun ications, 5e 21 ... changes to another constant level Business Data Commun ications, 5e Analog Sine Wave Business Data Commun ications, 5e Digital Square Wave Business Data Commun ications, 5e Periodic Signal Characteristics... rate Business Data Commun ications, 5e 19 Shannon Equation • C = B log2 (1 + SNR) – B = Bandwidth – C= Channel – SNR = Signal-to-noise ratio Business Data Commun ications, 5e 20 Business Data. .. greater costs • Limited bandwidth leads to distortion Business Data Commun ications, 5e Analog Signaling Business Data Commun ications, 5e Voice/Audio Analog Signals • Easily converted from sound