03-DataTransmission.ppt

Data and Computer
Communications
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 3 – Data Transmission

Transmission Terminology
 data transmission occurs between a
transmitter & receiver via some medium
 guided medium
 eg. twisted pair, coaxial cable, optical fiber
 unguided / wireless medium
 eg. air, water, vacuum

 direct link
 no intermediate devices
 point-to-point
 direct link
 only 2 devices share link
 multi-point
 more than two devices share the link

 simplex
 one direction
• eg. television
 half duplex
 either direction, but only one way at a time
• eg. police radio
 full duplex
 both directions at the same time
• eg. telephone

Frequency, Spectrum and
Bandwidth
 time domain concepts
 analog signal
• various in a smooth way over time
 digital signal
• maintains a constant level then changes to another
constant level
 periodic signal
• pattern repeated over time
 aperiodic signal
• pattern not repeated over time

Sine Wave
 peak amplitude (A)
 maximum strength of signal
 volts
 frequency (f)
 rate of change of signal
 Hertz (Hz) or cycles per second
 period = time for one repetition (T)
 T = 1/f
 phase ()
 relative position in time

Varying Sine Waves
s(t) = A sin(2ft +)

Wavelength ()
 is distance occupied by one cycle
 between two points of corresponding
phase in two consecutive cycles
 assuming signal velocity v have  = vT
 or equivalently f = v
 especially when v=c
 c = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts
 signal are made up of many frequencies
 components are sine waves
 Fourier analysis can shown that any signal
is made up of component sine waves
 can plot frequency domain functions

Addition of
Frequency
Components
(T=1/f)
 c is sum of f & 3f

Frequency
Domain
Representations
 freq domain func of
Fig 3.4c
 freq domain func of
single square pulse

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

Data Rate and Bandwidth
 any transmission system has a limited band of
frequencies
 this limits the data rate that can be carried
 square have infinite components and hence
bandwidth
 but most energy in first few components
 limited bandwidth increases distortion
 have a direct relationship between data rate &
bandwidth

Analog and Digital Data
Transmission
 data
 entities that convey meaning
 signals & signalling
 electric or electromagnetic representations of
data, physically propagates along medium
 transmission
 communication of data by propagation and
processing of signals

Audio Signals
 freq range 20Hz-20kHz (speech 100Hz-7kHz)
 easily converted into electromagnetic signals
 varying volume converted to varying voltage
 can limit frequency range for voice channel to
300-3400Hz

Video Signals
 USA - 483 lines per frame, at frames per sec
 have 525 lines but 42 lost during vertical retrace
 525 lines x 30 scans = 15750 lines per sec
 63.5s per line
 11s for retrace, so 52.5 s per video line
 max frequency if line alternates black and white
 horizontal resolution is about 450 lines giving
225 cycles of wave in 52.5 s
 max frequency of 4.2MHz

Digital Data
 as generated by computers etc.
 has two dc components
 bandwidth depends on data rate

Advantages & Disadvantages
of Digital Signals
 cheaper
 less susceptible to noise
 but greater attenuation
 digital now preferred choice

Transmission Impairments
 signal received may differ from signal
transmitted causing:
 analog - degradation of signal quality
 digital - bit errors
 most significant impairments are
 attenuation and attenuation distortion
 delay distortion
 noise

Attenuation
 where signal strength falls off with distance
 depends on medium
 received signal strength must be:
 strong enough to be detected
 sufficiently higher than noise to receive without error
 so increase strength using amplifiers/repeaters
 is also an increasing function of frequency
 so equalize attenuation across band of
frequencies used
 eg. using loading coils or amplifiers

Delay Distortion
 only occurs in guided media
 propagation velocity varies with frequency
 hence various frequency components
arrive at different times
 particularly critical for digital data
 since parts of one bit spill over into others
 causing intersymbol interference

Noise
 additional signals inserted between
transmitter and receiver
 thermal
 due to thermal agitation of electrons
 uniformly distributed
 white noise
 intermodulation
 signals that are the sum and difference of
original frequencies sharing a medium

Noise
 crosstalk
 a signal from one line is picked up by another
 impulse
 irregular pulses or spikes
• eg. external electromagnetic interference
 short duration
 high amplitude
 a minor annoyance for analog signals
 but a major source of error in digital data
• a noise spike could corrupt many bits

Channel Capacity
 max possible data rate on comms channel
 is a function of
 data rate - in bits per second
 bandwidth - in cycles per second or Hertz
 noise - on comms link
 error rate - of corrupted bits
 limitations due to physical properties
 want most efficient use of capacity

Nyquist Bandwidth
 consider noise free channels
 if rate of signal transmission is 2B then can carry
signal with frequencies no greater than B
 ie. given bandwidth B, highest signal rate is 2B
 for binary signals, 2B bps needs bandwidth B Hz
 can increase rate by using M signal levels
 Nyquist Formula is: C = 2B log2M
 so increase rate by increasing signals
 at cost of receiver complexity
 limited by noise & other impairments

Shannon Capacity Formula
 consider relation of data rate, noise & error rate
 faster data rate shortens each bit so bursts of noise
affects more bits
 given noise level, higher rates means higher errors
 Shannon developed formula relating these to
signal to noise ratio (in decibels)
 SNRdb
=10 log10 (signal/noise)
 Capacity C=B log2(1+SNR)
 theoretical maximum capacity
 get lower in practise

Summary
 looked at data transmission issues
 frequency, spectrum & bandwidth
 analog vs digital signals
 transmission impairments

03-DataTransmission.ppt

More Related Content

Similar to 03-DataTransmission.ppt (20)

More from Praches1 (7)

Recently uploaded (20)

03-DataTransmission.ppt