Data and Computer Communications Data Transmission Data Transmission
- Slides: 40
Data and Computer Communications Data Transmission
Data Transmission What we've got here is failure to communicate. Paul Newman in Cool Hand Luke
Data Transmission The successful transmission of data depends on two factors: • quality of the signal being transmitted • characteristics of the transmission medium
Transmission Terminology Data transmission occurs between transmitter and receiver over some transmission medium. Communication is in the form of electromagnetic waves. Guided media twisted pair, coaxial cable, optical fiber Unguided media (wireless) air, vacuum, seawater
Transmission Terminology Direct link • no intermediate devices Point-to-point • direct link • only 2 devices share link Multi-point • more than two devices share the link
Transmission Terminology Ø Simplex l signals transmitted in one direction • eg. Television Ø Half duplex l both stations transmit, but only one at a time • eg. police radio Ø Full duplex l simultaneous transmissions • eg. telephone
Frequency, Spectrum and Bandwidth Time Domain Concepts Ø analog signal • signal intensity varies smoothly with no breaks Ø digital signal • signal intensity maintains a constant level and then abruptly changes to another level Ø periodic signal • signal pattern repeats over time Ø aperiodic signal • pattern not repeated over time
Analog and Digital Signals
Periodic Signals
Sine Wave (periodic continuous signal) Ø peak amplitude (A) l l Ø frequency (f) l l Ø maximum strength of signal typically measured in volts rate at which the signal repeats Hertz (Hz) or cycles per second period (T) is the amount of time for one repetition T = 1/f phase ( ) l relative position in time within a single period of signal
Varying Sine Waves s(t) = A sin(2 ft + )
Wavelength ( ) the wavelength of a signal is the distance occupied by a single cycle can also be stated as the distance between two points of corresponding phase of two consecutive cycles especially when v=c • c = 3*108 ms-1 (speed of light in free space) assuming signal velocity v, then the wavelength is related to the period as = v. T or equivalently f = v
Frequency Domain Concepts Ø signals are made up of many frequencies Ø components are sine waves Ø Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoid Ø can plot frequency domain functions
Addition of Frequency Components (T=1/f) c is sum of f & 3 f
Frequency Domain Representations frequency domain function of Fig 3. 4 c Ø frequency domain function of single square pulse Ø
Spectrum & Bandwidth spectrum • range of frequencies contained in signal absolute bandwidth • width of spectrum effective bandwidth • often just bandwidth • narrow band of frequencies containing most energy dc component • component of zero frequency
Signal with dc Component
Data Rate and Bandwidth any transmission system has a limited band of frequencies limiting bandwidth creates distortions this limits the data rate that can be carried on the transmission medium most energy in first few components square waves have infinite components and hence an infinite bandwidth There is a direct relationship between data rate and bandwidth.
Analog and Digital Data Transmission Ø data l entities that convey information Ø signals l electric or electromagnetic representations of data Ø signaling l physically propagates along a medium Ø transmission l communication of data by propagation and processing of signals
Acoustic Spectrum (Analog)
Analog and Digital Transmission
Digital Data Examples: Text IRA Character strings
Advantages & Disadvantages of Digital Signals cheaper less susceptible to noise interference suffer more from attenuation digital now preferred choice
Audio Signals frequency range of typical speech is 100 Hz-7 k. Hz Ø easily converted into electromagnetic signals Ø varying volume converted to varying voltage Ø can limit frequency range for voice channel to 300 -3400 Hz Ø
to produce a video signal a TV camera is used Ø USA standard is 483 lines per frame, at a rate of 30 complete frames per second Ø Video Signals l actual standard is 525 lines but 42 lost during vertical retrace horizontal scanning frequency is 525 lines x 30 scans = 15750 lines per second Ø max frequency if line alternates black and white Ø max frequency of 4. 2 MHz Ø
Conversion of PC Input to Digital Signal
Analog Signals
Digital Signals
Analog and Digital Transmission
Transmission Impairments Ø signal received may differ from signal transmitted causing: l l analog - degradation of signal quality digital - bit errors Ø most significant impairments are l l l attenuation and attenuation distortion delay distortion noise
Equalize attenuation across the band of frequencies used by using loading coils or amplifiers. Received signal strength must be: • strong enough to be detected • sufficiently higher than noise to be received without error Strength can be increased using amplifiers or repeaters. ATTENUATION Ø signal strength falls off with distance over any transmission medium Ø varies with frequency
Attenuation Distortion
Delay Distortion Ø occurs because propagation velocity of a signal through a guided medium varies with frequency Ø various frequency components arrive at different times resulting in phase shifts between the frequencies Ø particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference
Noise unwanted signals inserted between transmitter and receiver is the major limiting factor in communications system performance
Categories of Noise Thermal noise • due to thermal agitation of electrons • uniformly distributed across bandwidths • referred to as white noise Intermodulation noise • produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium • effect is to produce signals at a frequency that is the sum or difference of the two original frequencies
Categories of Noise Crosstalk: l l Impulse Noise: l l caused by external electromagnetic interferences noncontinuous, consisting of irregular pulses or spikes short duration and high amplitude minor annoyance for analog signals but a major source of error in digital data a signal from one line is picked up by another can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals
Channel Capacity Maximum rate at which data can be transmitted over a given communications channel under given conditions data rate bandwidth noise in cycles average in bits per second noise level second or Hertz over path error rate of corrupted bits main limitations constraint due to on physical achieving properties efficiency is noise
Nyquist Bandwidth In the case of a channel that is noise free: Ø if rate of signal transmission is 2 B then carry signal with frequencies no greater than B l given bandwidth B, highest signal rate is 2 B for binary signals, 2 B bps needs bandwidth B Hz Ø can increase rate by using M signal levels Ø Nyquist Formula is: C = 2 B log 2 M Ø data rate can be increased by increasing signals Ø l l however this increases burden on receiver noise & other impairments limit the value of M
Shannon Capacity Formula Ø considering the relation of data rate, noise and error rate: l l faster data rate shortens each bit so bursts of noise corrupts more bits given noise level, higher rates mean higher errors Shannon developed formula relating these to signal to noise ratio (in decibels) Ø SNRdb=10 log 10 (signal/noise) Ø capacity C = B log 2(1+SNR) Ø l l theoretical maximum capacity get much lower rates in practice
Summary Ø transmission concepts and terminology l guided/unguided media Ø frequency, spectrum and bandwidth Ø analog vs. digital signals Ø data rate and bandwidth relationship Ø transmission impairments l attenuation/delay distortion/noise Ø channel capacity l Nyquist/Shannon
- Data and computer communications 10th edition
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