data transmission

1. Data Transmission: Concepts and
Terminology

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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
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3. 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
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4. Transmission Terminology
 Simplex transmission
 one direction
• eg. television
 Half-duplex transmission
 either direction, but only one way at a time
• eg. police radio (walkie-talkie: push-to-talk and
release-to-listen)
 Full-duplex transmission
 both directions at the same time
• eg. telephone
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5. 5
Frequency (f)
 Frequency is the number of occurrences of a repeating
event per unit of time. The number of waves that pass a
fixed point in unit time
 Frequency is measured in units of hertz (Hz) which is
equal to one occurrence of a repeating event per
second.

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Wavelength (λ)
 is the distance occupied by one cycle
 assuming signal velocity v, then  = vT
 or equivalently f = v, since T=1/f
 for the special case when v=c
 c = 3*108 m/s (speed of light in free space)
 c=λf
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Spectrum and 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
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Acoustic Spectrum
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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
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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
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Digital Data
 as generated by computers etc.
 has two dc components
 bandwidth depends on data rate
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Data is an entity, which conveys some
meaning. On the other hand, the signal is a
representation of data in some electric,
electromagnetic or optical form.

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Analog Signals
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Digital signals
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Advantages and disadvantages of digital
signals
 cheaper
 less susceptible to noise
 but greater attenuation
 digital now preferred choice
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Transmission Impairments (బలహీనత)
 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
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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
 It is also an increasing function of frequency
 so equalize attenuation across band of frequencies used
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Delay distortion
 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
intersymbol interference (ISI) is a form of distortion of a
signal in which one symbol interferes with subsequent
symbols.
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Noise
 Additional unwanted signals inserted
between transmitter and receiver
 Thermal
 due to thermal agitation of electrons
 uniformly distributed
 white noise
 Interference from other users in a multi-user
environment (e.g., mobile environment)
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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
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Noise: example
0
1
+5V
-5V
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Data-rate
Data rate: is the rate, in bits per second (bps), at
which data can be communicated
kbps
m
T
duration
bit
R
Rate
data
b
50
sec
02
.
0
1
1
1





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Channel Capacity
 Channel Capacity: max possible rate at which data
can be transmitted over a given communication
path, under given conditions
 Channel capacity is a function of :
data rate - in bits per second [bps]
bandwidth - in Hertz [Hz]
noise - on communication link
error rate - the rate at which errors occur, reception of 1
when 0 is transmitted, and visa versa
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