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COMPUTER NETWORKS
UNIT - II
Transmission Media
• Guided - wire
• Unguided - wireless
• Characteristics and quality determined by
medium and signal
• For guided, the medium is more important
• For unguided, the bandwidth produced by the
antenna is more important
• Key concerns are data rate and distance
Design Factors
• Bandwidth
– Higher bandwidth gives higher data rate
• Transmission impairments
– Attenuation
• Interference
• Number of receivers
– In guided media
– More receivers (multi-point) introduce more
attenuation
Guided Transmission Media
• Twisted Pair
• Coaxial cable
• Optical fiber
Transmission Characteristics of Guided
Media
Frequency
Range
Typical
Attenuation
Typical
Delay
Repeater
Spacing
Twisted pair
(with loading)
0 to 3.5 kHz
0.2 dB/km @
1 kHz
50 µs/km
2 km
Twisted pairs
(multi-pair
cables)
Coaxial cable
0 to 1 MHz
0.7 dB/km @
1 kHz
5 µs/km
2 km
0 to 500 MHz
7 dB/km @ 10
MHz
4 µs/km
1 to 9 km
Optical fiber
186 to 370
THz
0.2 to 0.5
dB/km
5 µs/km
40 km
Twisted Pair
Twisted Pair - Applications
• Most common medium
• Telephone network
– Between house and local exchange (subscriber
loop)
• Within buildings
– To private branch exchange (PBX)
• For local area networks (LAN)
– 10Mbps or 100Mbps
Twisted Pair - Pros and Cons
•
•
•
•
Cheap
Easy to work with
Low data rate
Short range
Twisted Pair - Transmission
Characteristics
• Analog
– Amplifiers every 5km to 6km
• Digital
– Use either analog or digital signals
– repeater every 2km or 3km
•
•
•
•
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100MHz)
Susceptible to interference and noise
Near End Crosstalk
• Coupling of signal from one pair to another
• Coupling takes place when transmit signal
entering the link couples back to receiving pair
• i.e. near transmitted signal is picked up by
near receiving pair
Unshielded and Shielded TP
• Unshielded Twisted Pair (UTP)
– Ordinary telephone wire
– Cheapest
– Easiest to install
– Suffers from external EM interference
• Shielded Twisted Pair (STP)
– Metal braid or sheathing that reduces interference
– More expensive
– Harder to handle (thick, heavy)
UTP Categories
• Cat 3
– up to 16MHz
– Voice grade found in most offices
– Twist length of 7.5 cm to 10 cm
• Cat 4
– up to 20 MHz
• Cat 5
– up to 100MHz
– Commonly pre-installed in new office buildings
– Twist length 0.6 cm to 0.85 cm
• Cat 5E (Enhanced) –see tables
• Cat 6
• Cat 7
Comparison of Shielded and
Unshielded Twisted Pair
Attenuation (dB per 100 m)
Frequency
(MHz)
Category 3
UTP
Category 5
UTP
1
2.6
2.0
4
5.6
16
13.1
150-ohm
STP
Near-end Crosstalk (dB)
Category 3
UTP
Category 5
UTP
150-ohm
STP
1.1
41
62
58
4.1
2.2
32
53
58
8.2
4.4
23
44
50.4
25
—
10.4
6.2
—
41
47.5
100
—
22.0
12.3
—
32
38.5
300
—
21.4
—
—
—
31.3
Twisted Pair Categories and Classes
Category 3
Class C
Category 5
Class D
Bandwidth
16 MHz
100 MHz
Cable Type
UTP
Link Cost
(Cat 5 =1)
0.7
Category
5E
Category 6
Class E
Category 7
Class F
100 MHz
200 MHz
600 MHz
UTP/FTP
UTP/FTP
UTP/FTP
SSTP
1
1.2
1.5
2.2
Coaxial Cable
Coaxial Cable Applications
• Most versatile medium
• Television distribution
– Ariel to TV
– Cable TV
• Long distance telephone transmission
– Can carry 10,000 voice calls simultaneously
– Being replaced by fiber optic
• Short distance computer systems links
• Local area networks
Coaxial Cable - Transmission
Characteristics
• Analog
– Amplifiers every few km
– Closer if higher frequency
– Up to 500MHz
• Digital
– Repeater every 1km
– Closer for higher data rates
Optical Fiber
Optical Fiber - Benefits
• Greater capacity
– Data rates of hundreds of Gbps
•
•
•
•
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
– 10s of km at least
Optical Fiber - Applications
•
•
•
•
•
Long-haul trunks
Metropolitan trunks
Rural exchange trunks
Subscriber loops
LANs
Optical Fiber - Transmission
Characteristics
• Act as wave guide for 1014 to 1015 Hz
– Portions of infrared and visible spectrum
• Light Emitting Diode (LED)
– Cheaper
– Wider operating temp range
– Last longer
• Injection Laser Diode (ILD)
– More efficient
– Greater data rate
• Wavelength Division Multiplexing
Optical Fiber Transmission Modes
Frequency Utilization for Fiber
Applications
Wavelength (in
vacuum) range
(nm)
Frequency
range (THz)
820 to 900
366 to 333
1280 to 1350
234 to 222
1528 to 1561
1561 to 1620
Band
label
Fiber type
Application
Multimode
LAN
S
Single mode
Various
196 to 192
C
Single mode
WDM
185 to 192
L
Single mode
WDM
Attenuation in Guided Media
Adv. & Disadv. Of Optical Fiber
•
1.
2.
3.
4.
5.
•
1.
2.
3.
Advantages
Higher Bandwidth
Less Signal Attenuation
Immunity to electromagnetic Interference
Resistance to corrosive materials
Light Weight
Disadvantages
Require expertise in Installation and Maintenance
Unidirectional Light propagation
Expensive
Wireless Transmission Frequencies
• 3KHz to 1GHz
– Omnidirectional
– Radio Waves
– Broadcast radio
• 1GHz to 300GHz
–
–
–
–
Microwave
Highly directional
Point to point
Satellite
• 300GHz to 400THz
– Infrared
– Local
Electromagnetic Spectrum
Antennas
• Electrical conductor (or system of..) used to radiate
electromagnetic energy or collect electromagnetic energy
• Transmission
–
–
–
–
Radio frequency energy from transmitter
Converted to electromagnetic energy
By antenna
Radiated into surrounding environment
• Reception
– Electromagnetic energy impinging on antenna
– Converted to radio frequency electrical energy
– Fed to receiver
• Same antenna often used for both
Radiation Pattern
• Power radiated in all directions
• Not same performance in all directions
• Isotropic antenna is (theoretical) point in
space
– Radiates in all directions equally
– Gives spherical radiation pattern
Parabolic Reflective Antenna
• Used for terrestrial and satellite microwave
• Source placed at focus will produce waves reflected from
parabola in parallel to axis
– Creates (theoretical) parallel beam of light/sound/radio
• On reception, signal is concentrated at focus, where detector
is placed
Parabolic Reflective Antenna
Antenna Gain
• Measure of directionality of antenna
• Power output in particular direction compared
with that produced by isotropic antenna
• Measured in decibels (dB)
• Results in loss in power in another direction
• Effective area relates to size and shape
– Related to gain
Terrestrial Microwave
•
•
•
•
•
Parabolic dish
Focused beam
Line of sight
Long haul telecommunications
Higher frequencies give higher data rates
Satellite Microwave
• Satellite is relay station
• Satellite receives on one frequency, amplifies or
repeats signal and transmits on another
frequency
• Requires geo-stationary orbit
– Height of 35,784km
• Television
• Long distance telephone
• Private business networks
Satellite Point to Point Link
Satellite Broadcast Link
Broadcast Radio
•
•
•
•
•
Omnidirectional- signals sent in all diections.
FM radio
UHF and VHF television
Line of sight
Suffers from multipath interference
– Reflections
Infrared
•
•
•
•
Modulate noncoherent infrared light
Line of sight (or reflection)
Blocked by walls
e.g. TV remote control
Wireless Propagation
• Signal travels along three routes
– Ground wave
• Follows contour of earth
• Up to 2MHz
• AM radio
– Sky wave
• Amateur radio, BBC world service, Voice of America
• Signal reflected from ionosphere layer of upper atmosphere
• (Actually refracted)
– Line of sight
• Above 30Mhz
• May be further than optical line of sight due to refraction
Ground Wave Propagation
Sky Wave Propagation
Line of Sight Propagation
Refraction
• Velocity of electromagnetic wave is a function of density of material
– ~3 x 108 m/s in vacuum, less in anything else
• As wave moves from one medium to another, its speed changes
– Causes bending of direction of wave at boundary
– Towards more dense medium
• Index of refraction (refractive index) is
– Sin(angle of incidence)/sin(angle of refraction)
– Varies with wavelength
• May cause sudden change of direction at transition between media
• May cause gradual bending if medium density is varying
– Density of atmosphere decreases with height
– Results in bending towards earth of radio waves
Optical and Radio Horizons
Line of Sight Transmission
• Free space loss
– Signal disperses with distance
– Greater for lower frequencies (longer wavelengths)
• Atmospheric Absorption
–
–
–
–
Water vapour and oxygen absorb radio signals
Water greatest at 22GHz, less below 15GHz
Oxygen greater at 60GHz, less below 30GHz
Rain and fog scatter radio waves
–
–
–
–
Better to get line of sight if possible
Signal can be reflected causing multiple copies to be received
May be no direct signal at all
May reinforce or cancel direct signal
• Multipath
• Refraction
– May result in partial or total loss of signal at receiver
Free
Space
Loss
Multipath Interference
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
Protocol
Rules
.
.
.
Protocol
Rules
.
.
.
Message
Transmission Medium
Receiver
Sender
Data Communication and Computer Networks 1303330
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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
• A pair of nodes connected together via
dedicated link.
PC
PC
Link
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• Number of node connected and share a single
link.
PC
PC
PC
Link
Server
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Transmission Terminology
• 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
• Simplex: one direction only.
Remote Control
TV
• Always one side sender and another side
receiver.
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• Half-Duplex: two-way alternate.
Walki-Talki
In different time
• Each side maybe sender or receiver but
not a same time.
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• Duplex: two-way concurrent.
Computer network
At same time
Mobile Network
• Each side sender and receiver at same time.
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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
Analogue & Digital Signals
Periodic
Signals
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 +)
Propagation Speed
• Speed of travelling Signal
• Depends on medium and frequency of signal
• In vacuum, light is propagated with a speed of
3*108m/s.
• That speed is lower in air and even lower in
cable.
Wavelength ()
• is distance a simple signal can travel in one period.
• Dependent on frequency and medium.
• between two points of corresponding phase in two
consecutive cycles
• assuming signal velocity v have  = vT
• Wavelength=Propagation Speed * period
• or equivalently f = v
 especially when v=c
 c = 3*108 ms-1 (speed of light in free space)
Frequency Domain Concepts
• A single frequency signal is not useful in Data
communication.
• Composite signal are made up of many
frequencies components are sine waves
• Relationship between amplitude and
frequency, is called frequency domain plot.
• 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
single square pulse
1. Message: data.
2. Sender: The device that send the message.
3. Receiver: The device that receive the message.
4. Transmission Medium: The physical path
between sender and receiver, the message
travel.
5. Protocol: Is a set of rules that governs data
communication. It represents an agreement
between the communicating devices. Without a
protocol, two devices may be connected but not
communicating.
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1. Delivery: The system must deliver data to the correct
destination.
2. Accuracy:


Data delivered accurately.
Altered data which left uncorrected are unusable.
3. Timelines:
The system must deliver data in timely manner without delay
(real-time).
4. Jitter:
Jitter refers to the variation in the packet arrival time. It
is the uneven delay in the delivery of audio or video
packets.
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Simplified Communications Model Diagram
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
Simplified Data Communications
Model
Acoustic Spectrum (Analog)
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 3003400Hz
Data
• Analog:
Continuous values within some interval.
e.g. sound, video.
• Digital:
Discrete values.
e.g. text, integers.
Analog Signals
Analog
Transmission
• Analog signal transmitted without regard
to content.
• May be analog or digital data.
• Attenuated over distance .
• Use amplifiers to boost signal.
• Also amplifies noise.
Digital Signals
Digital Transmission
• Concerned with content.
• Integrity endangered by noise,
attenuation etc..
• Repeaters used.
• Repeater receives signal.
• Extracts bit pattern.
• Retransmits.
• Attenuation is overcome.
• Noise is not amplified.
Advantages & Disadvantages
of Digital Signals
•
•
•
•
cheaper
less susceptible to noise
but greater attenuation
digital now preferred choice
Advantages of Digital
Transmission
• Digital technology:
 Low cost LSI/VLSI technology.
• Data integrity:
– Longer distances over lower quality lines.
• Capacity utilization:
 High bandwidth links economical.
 High degree of multiplexing easier with digital
techniques.
• Security & Privacy:
 Encryption.
• Integration:
 Can treat analog and digital data similarly.
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
• Means loss of energy in propagation due to the
resistance of medium.
• Energy is measured in decibel(dB).
• Decibel measues the relative strength of two
signals or one signal at two different points.
• dB=10 log10 P2/P1
Attenuation
Signal strength falls off with distance.
Depends on medium.
Received signal strength:
 must be enough to be detected.
 must be sufficiently higher than noise to be received
without error.
Attenuation is an increasing function of frequency.
• increase strength using amplifiers/ repeaters
Distortion
• Means that Signal changes its Form and shape.
• only occurs in guided media
• composite signal made up by different frequency
Signals and various frequency components arrive at
different times
• propagation velocity varies with frequency
• Difference in delay may create a difference in phase,
Therefore, a Shape of Signal is changed.
• 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.
• Induced: comes from different sources like
motors an other appliances
• Thermal: The random motion of electrons in wire
which creates an extra signal, not originally sent
by Transmitter
• Intermodulation: Signals that are the sum and
difference of original frequencies sharing a
medium.
Noise (2)
• Crosstalk:
– A signal from one line is picked up by
another.
• Impulse:
– Irregular pulses or spikes.
– e.g. External electromagnetic interference.
– Short duration.
– High amplitude.
Channel
Capacity
• Data rate:
 In bits per second.
 Rate at which data can be communicated.
 Depends on three factors:
•
Bandwidth available
•
Level of Signals
•
Quality of the Channel
• Bandwidth:
 In cycles per second of Hertz.
 Constrained by transmitter and medium.
Nyquist Bandwidth( Noise Less Channel)
• consider noise free channels
• if rate of signal transmission is 2B then can
carry signal with frequencies no greater than B
(Bandwidth)
– 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
Nyquist Bandwidth( Noise Less Channel)
• In no. of Signal Level (M) is 2. They can easily
distinguish by receiver (either 0 or 1)
• But if no. of signal level is 64. then it’s very difficult to
distinguish by receiver.
• So Increasing the levels of a signal may reduce the
reliability of the system.
• so increase rate by increasing signals
– at cost of receiver complexity
– limited by noise & other impairments
Shannon Capacity Formula (Noisy Channel)
• 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