Transmission Media

1
COE 341 Data Computer Communications
(T061)Dr. Radwan E. Abdel-Aal

• Chapter 4
• Transmission Media

2
Agenda

• Overview
• Guided Transmission Media
• Twisted Pair
• Coaxial Cable
• Optical Fiber
• Wireless Transmission
• Antennas
• Terrestrial Microwaves
• Satellite Microwaves
• Broadcast Radio
• Infrared

3
Overview

• Media
• Guided wire or fiber
• Unguided - wireless
• Transmission characteristics and quality
  determined by
• Signal
• Medium
• For guided, the medium is more important
• For unguided, the bandwidth provided by the
  antenna is more important

4
Design Issues

• Key communication objectives are
• High data rate
• Low error rate
• Long distance
• Bandwidth economy Tradeoff
• - Want larger BW for higher data rates C ? B
• - But limited by economy Larger BW is costly
  e.g. Coaxial Vs TP
• Transmission impairments
• Attenuation Twisted Pair gt Cable gt Fiber (best)
• Interference
• Worse with unguided (the medium is shared!)
  
• Number of receivers
• In multi-point links of guided media
• More connected receivers introduce more
  attenuation

5
The Electromagnetic Spectrum
Microwaves
Light
10 GHz
100 MHz
10 KHz
6
Standard Multiplier Prefixes 1-18 to 1018
exa- E 1018 1,000,000,000,000,000,000 peta- P
1015 1,000,000,000,000,000 tera- T 1012
1,000,000,000,000 giga- G 109 1,000,000,000
mega- M 106 1,000,000 kilo- K 103 1,000
milli- m 10-3 0.001 micro- 10-6 0.000,001
nano- n 10-9 0.000,000,001 pico- p 10-12
0.000,000,000,001 femto- f 10-15
0.000,000,000,000,001 atto- a 10-18
0.000,000,000,000,000,001
7
Electromagnetic Spectrum
Ultra violet, X-Rays, Gamma-Rays
Used for Communications
8
Study of Transmission Media

• Physical description
• Main applications
• Main transmission characteristics

9
Guided Transmission Media

• Twisted Pair
• Coaxial cable
• Optical fiber

10
Transmission Characteristics of Guided Media
Overview

• Same delay
• (except with loading)

Lower Attenuation
Fewer Repeaters Required
Larger Operating Frequencies
11
Twisted Pair (TP)
Effect of loading with coils in series at
intervals
But attenuation rises rapidly Outside this narrow
band. No good for ASDL which tries to get 1 MHz
BW from the TP line!
Flat and low attenuation Over the telephone
voice band (300-3400 Hz) (Passive Equalizer)
12
UTP Cables
unshielded
13
Twisted Pair - Applications

• Most commonly used guided medium
• Telephone network (Analog Signaling)
• Between houses and the local exchange (subscriber
  loop)
• Originally designed for analog signaling.
• Digital data transmitted using modems at low
  data rates
• Within buildings (short distances) (Digital
  Signaling)
• To private branch exchange (PBX) (64 Kbps)
• For local area networks (LAN) (10-100Mbps)
• Example
• 10BaseT Unshielded Twisted Pair, 10 Mbps,100m
  range

Digital signal travels in its base band i.e.
without modulating a carrier (short distances)
14
Twisted Pair - Pros and ConsCompared to other
guided media

• Pros
• Low cost
• Easy to work with (pull, terminate, etc.)
• Cons
• Limited bandwidth
• Limited data rate
• Large Attenuation
• Limited distance range
• Susceptible to interference and noise (exposed
  construction)

15
Twisted Pair - Transmission Characteristics

• Analog Transmission
• For analog signals only
• Amplifiers every 5km to 6km
• Bandwidth up to 1 MHz (several voice channels)
  ADSL
• Digital Transmission
• For either analog or digital signals (carrying
  digital data)
• Repeaters every 2km or 3km
• Data rates up to few Mbps (1Gbps very short
  distance)
• Impairments
• Attenuation A strong function in frequency
  (? Distortion, need for equalization)
• EM Interference Crosstalk, Impulse noise, Mains
  interference, etc.

16
Attenuation in Guided Media
Thinner Wires
17
Ways to reduce EM interference
WK 7

• Shielding the TP with a metallic braid or
  sheathing
• Twisting also reduces low frequency interference
• Different twisting lengths for adjacent pairs
  help reduce crosstalk

18
STP Metal Shield
19
Unshielded (UTP) and Shielded (STP)

• Unshielded Twisted Pair (UTP)
• Ordinary telephone wire Abundantly available in
  buildings
• Cheapest
• Easiest to install
• Suffers from external EM interference
• Shielded Twisted Pair (STP)
• Shielded with foil, metal braid or sheathing
• Reduces interference
• Reduces attenuation at higher frequencies
  (increases BW)
• ? Better Performance
• Increased data rates used
• Increased distances covered
• ? But becomes
• More expensive
• Harder to handle (thicker, heavier)

20
TP Categories EIA-568-A Standard (1995)
(cabling of commercial buildings for data)

• Cat 3 Unshielded (UTP)
• Up to 16MHz
• Voice grade
• In most office buildings
• Twist length of 7.5 cm to 10 cm
• Cat 5 Unshielded (UTP)
• Up to 100MHz
• Data grade
• Pre-installed now in many new office buildings
• Twist length 0.6 cm to 0.85 cm
• (Tighter twisting increases cost but improves
  performance)
• Newer, shielded twisted pair (150 W STP)
• Up to 300MHz

21
Near End Crosstalk (NEXT)

• Coupling of signal from one wire pair to another
• Coupling takes place when a transmitted signal
  entering a pair couples into an adjacent
  receiving pair at the same end
• i.e. near transmitted signal is picked up by near
  receiving pair

Transmitted Power, P1
Disturbing pair
Coupled Received Power, P2
Disturbed pair
NEXT Attenuation 10 log P1/P2 dBs The larger
the smaller the crosstalk (The better the
performance)
NEXT attenuation is a desirable attenuation-
The larger the better!
22
Transmission Properties for Shielded Unshielded
TP
Undesirable Attenuation- Smaller is better
Desirable Attenuation- Larger is better!
23
Newer Twisted Pair Categories and Classes
UTP Unshielded Twisted Pair
FTP Foil Twisted Pair
SSTP Shielded-Screen Twisted Pair
24
Coaxial Cable
Physical Description
1 - 2.5 cm
Designed for operation over a wider frequency
range
25
Physical Description
26
Coaxial Cable Applications

• Most versatile medium
• Television distribution (FDM Broadband)
• Cable TV (CATV) 100s of TV channels over 10s
  Kms
• Long distance telephone transmission
• Can carry 10s of thousands of voice channels
  simultaneously (though FDM multiplexing)
  (Broadband)
• Now facing competition from optical fibers and
  terrestrial microwave links
• Local area networks, e.g. Thickwire Ethernet
  cable
• (10Base5) 10 Mbps, Baseband signal, 500m segment

(5 time TP distance)
27
Coaxial Cable - Transmission CharacteristicsImpr
ovements over TP

• Extended frequency range
• Up to 500 MHz
• Reduced EM interference and crosstalk
• Due to enclosed concentric construction
• EM fields terminate within cable and do not stray
  outside causing interference
• Remaining limitations
• Attenuation
• Thermal and inter modulation noise (FDM)

28
Attenuation in Guided Media
29
Coaxial Cable - Transmission Characteristics

• Analog Transmission
• Amplifiers every few kms
• Closer amplifier spacing for higher operating
  frequencies
• Digital Transmission
• Repeater every 1km
• Closer repeater spacing for higher data rates

30
Optical Fiber

• A thin (2-125 mm) flexible strand of glass or
  plastic
• Light entering at one end travels confined within
  the fiber until it leaves it at the other end
• As fiber bends around corners, the light remains
  within the fiber through multiple internal
  reflections
• Lowest losses (attenuation) with ultra pure
  fused silica glass but
  expensive and more difficult to manufacture
• Reasonable losses with multi-component glass and
  with plastic

Quality, Cost, Difficulty of
Handling
Attenuation (Loss)
Pure Glass
Multi-component Glass
Plastic
31
Optical Fiber Construction

• An optical fiber consists of three main parts
• Core
• A narrow cylindrical strand of glass/plastic,
  with refractive index n1
• Cladding
• A tube surrounding each core, with refractive
  index n2
• The core must have a higher refractive index than
  the cladding to keep the light beam trapped
  inside n1 gt n2
• Protective outer jacket
• Protects against moisture, abrasion, and crushing

Important Each core surrounded by a cladding
32
Reflection and Refraction

• At a boundary between a denser (n1) and a rarer
  (n2) medium, n1 gt n2 (e.g. water-air, optical
  fiber core-cladding) a ray of light will be
  refracted or reflected depending on the incidence
  angle

Increasing Incidence angle,
?1
Angles With the Normal
?2
rarer
v2 c/n2
n2
denser
?1
?2
n1
?critical
?1
n1 gt n2
v1 c/n1
Total internal reflection
Critical angle
Refraction
33
Optical Fiber
Refraction at boundary for .
Escaping light is absorbed in jacket
?i lt
?critical
n2
Rarer
n1
Denser
Denser
n1
Rarer
?i
Total Internal Reflection at core-cladding
boundary for
?i gt
?critical
n1 gt n2
34
Attenuation in Guided Media
Larger Frequency
35
Optical Fiber - Benefits

• Greater capacity
• Fiber 100s of Gbps over 10s of Kms
• Cable 100s of Mbps over 1s of Kms
• Twisted pair 100s of Mbps over 10s of meters
• Lower/more uniform attenuation (Fig. 4.3c)
• An order of magnitude lower
• Relatively constant over a larger range of
  frequencies
• Electromagnetic isolation
• Not affected by external EM fields
• No interference, impulse noise, crosstalk
• Does not radiate
• Not a source of interference
• Difficult to tap (data security)

With careful selection of operating band
36
Optical Fiber Benefits, Contd.

• Greater repeater spacing Lower cost, Fewer Units
• Fiber 10-100s of Kms
• Cable, Twisted pair 1s Kms
• Smaller size and weight
• An order of magnitude thinner for same capacity
• Useful in cramped places
• Reduced cost of digging in populated areas
• Reduced cost of support structures

37
Optical Fiber - Applications

• Long-haul trunks
• Telephone traffic over long routes between
  cities, and undersea
• Fiber Microwave now replacing coaxial cable
• ? 1500 km, Up to 60,000 voice channels
• Metropolitan trunks
• Joining exchanges inside large cities
• ? 12 km, Up to 100,000 voice channels
• Rural exchange trunks
• Joining exchanges of towns and villages
• ? 40-160 km, Up to 5,000 voice channels
• Subscriber loops
• Joining subscribers to exchange
• Fiber replacing TP, allowing all types of data
• LANs, Example
• 10BaseF 10 Mbps, 2000 meter segment

Exchange
City
City
Main Exchange
38
Optical Fiber - Transmission Characteristics

• Acts as a wave guide for light (1014 to 1015 Hz)
• Covers portions of infrared and visible spectrum
• Transmission Modes

Multimode
Single Mode
Graded Index
Step Index
39
Optical Fiber Transmission Modes
Dispersion Spread in ray arrival time
Refraction
Shallow reflection
Deep reflection
n2
n1
Large Spread
Core
Cladding
2 ways
Curved path n is not uniform- decreasing
Smaller

• v c/n
• n1 is made lower away from centerthis speeds up
  deeper rays
• and compensates for their larger distances,
  arrive together with shallower rays

Smallest

• Smaller spread ? Narrower pulses
• Higher data rates supported

40
Optical Fiber Transmission modes

• Spread of received light pulse in time
  (dispersion) is bad
• Causes inter-symbol interference ? bit errors
  (similar to delay distortion)
• Limits usable data rate and usable transmission
  distance
• Caused by propagation through multiple
  reflections at different angles of incidence
• Dispersion increases with
• Larger distance traveled
• Thicker fibers with step index
• Less focused sources
• Can be reduced by
• Limiting the distance
• Thinner fibers and a highly focused light source
• ? Single mode (in the limit) High data rates,
  very long distances
• Or Graded-index multimode thicker fibers The
  half-way (lower cost) solution

41
Optical Fiber Transmission System Light Source
Fiber Light Detector
Light Sources

• Light Emitting Diode (LED)
• Incoherent light- More dispersion ? Lower data
  rates
• Low cost
• Wider operating temp range
• Longer life
• Injection Laser Diode (ILD)
• Coherent light- Less dispersion ? Higher data
  rate
• More efficient
• Faster switching ? Higher data rate

42
Optical Fiber Wavelength Division Multiplexing
(WDM)

• A form of FDM (Channels sharing the medium by
  occupying different frequency bands)
• Multiple light beams at different light
  frequencies (wavelengths) transmitted on the same
  fiber
• Each beam forms a separate communication channel
• Separated at destination by filters
• Example
• 256 channels
• _at_ 40 Gbps each
• ? 10 Tbps total data rate

WDM
43
Optical Fiber Four Transmission bands (windows)
in the Infrared (IR) region

• Band selection is a system decision based on
• Attenuation of the fiber
• Properties of the light sources
• Properties of the light receivers

S
L
C
Bandwidth, THz
33
12
4
7
Note l in fiber v/f (c/n)/f (c/f)/n l in
vacuum/n i.e. l in fiber lt l in vacuum
44
Wireless Transmission

• Free-space is the transmission medium
• Need efficient radiators, called antennas
• Signal fed from transmission line (wireline) and
  radiated it into free-space (wireless)
• Popular applications
• Radio TV broadcast
• Cellular Communications
• Microwave Links
• Wireless Networks

45
Wireless Transmission Frequency Ranges

• Radio 30 MHz to 1 GHz
• Omni directional
• Broadcast radio
• Microwaves 1 GHz to 40 GHz
• Highly directional beams
• Point to point (Terrestrial)
• Satellite
• Infrared Light 0.3 THz to 20 THz (below light)
• Localized communications (confined spaces)

46
Antennas

• Electrical conductor (or system of conductors)
  used to radiate / collect electromagnetic energy
  into/from surrounding space
• Transmission
• Radio frequency electrical energy from
  transmitter
• Converted into electromagnetic energy
• Radiated into surrounding space
• Reception
• Electromagnetic energy impinging on antenna
• Converted to radio frequency electrical energy
• Fed to receiver
• Same antenna often used for both TX and RX in
  2-way communication systems

47
Radiation Pattern

• Power radiated in all directions, but usually not
  with the same efficiency
• Isotropic antenna
• A hypothetical point source in space
• (Small dimensions relative to l)
• Radiates equally in all directions
  A spherical
  radiation pattern
• Used as a reference for other antennae
• Directional Antenna
• Concentrates radiation in a given desired
  direction hence
  point-to-point, line of sight
• communications
• Gives antenna gain in that direction
• relative to isotropic for both TX and RX
• Larger dimensions relative to l ? Greater
  directivity

Radiation Patterns
Isotropic
Directional
48
Parabolic Reflective Antenna
WK 8

• Used for terrestrial and satellite microwave
• Source placed at the focal point will produce
  waves that get reflected from parabola parallel
  to the parabola axis
• Creates a (theoretically) parallel beam of
  light/sound/radio that does not spread (disperse)
  in space
• In practice, some divergence (dispersion) occurs,
  because source at focus has a finite size (not
  exactly a point!)
• On reception, only signal from the axis direction
  is concentrated at focus, where detector is
  placed. Signals from other directions miss
  the focus ? negligible O/P
• The larger the antenna
• (in wavelengths) the better
• the directionality ? so, using
• Higher frequency is advantageous

Parabola
Focus
49
Parabolic Reflective Antenna
WK 8
Axis
50
Antenna Gain, G

• A measure of antenna directionality
• Power output of the antenna in a particular
  direction compared to that produced by a perfect
  isotropic antenna
• Can be expressed in decibels (dB, dBi) (i
  relative to isotropic)
• Increased power radiated in one direction causes
  less power radiated in another direction (Total
  power is fixed)
• Effective area Ae
• Related to size and shape of antenna
• Determines the antenna gain,
• Ae is the effective area

51
Antenna Gain, G Effective Areas

• An isotropic antenna has a gain G 1 (0 dBi)
• i.e.
• A parabolic antenna has
• Substituting we get
• Gain in dBi 10 log G
• Important Gains apply to both TX and RX antennas

A Actual Area p r2
52
Propagation Attenuation

• As signal propagates in space, its power drops
  with distance according to the inverse square law

While with a guided medium, signal drops
exponentially with distance giving larger
attenuation and lower repeater spacing
d distance in ls
i.e. loss in signal power over distance traveled,
d

• Show that L increases by 6 dBs for every
  doubling of distance d.
• For guided medium, corresponding attenuation a
  d dBs, a in dBs/km

A disadvantage for operating at higher frequency?
53
Microwave Link Calculations
TX-RX Net attenuation, A L G1 G2, dBs S
dBm P dBm A dBs
54
Terrestrial Microwave

• Parabolic dish
• Focused beam (with antenna gain)
• Line of sight requirement
• Beam should not be obstructed
• Curvature of earth limits maximum range ? Use
  relays to increase range (multi-hop link)
• Link performance sensitive to antenna alignment
• Applications
• Long haul telecommunications
  Many
  voice/data channels over long distances between
  large cities, possibly through intermediate
  relays Competes with
  coaxial cable and fiber
• Short wireless links between buildings
• CCTV links
• Wireless links between LANs in close-by buildings
• Cellular Telephony

55
Terrestrial Microwave Transmission Properties

• 1 - 40 GHz
• Higher f Advantages
• Larger bandwidth, B ? higher data rate (Table
  4.6)
• Smaller l ? smaller (lighter, cheaper) antenna
  for a required antenna gain (see gain eqn.)
• But Higher f ? larger attenuation due propagation
  and absorption by rain
• So,
• Long-haul links (long distances) operate at lower
  frequencies (4-6 GHz,11 GHz) to avoid large
  attenuation
• Short links between close-by buildings operate at
  higher frequencies (e.g. 22 GHz) (Attenuation is
  not a big problem for the short distances,
  smaller antenna size)

56
Satellite Microwave

• Satellite acts as a relay station
• for the link
• Satellite receives on one frequency (uplink),
  amplifies or repeats signal and re-transmits it
  on another frequency (downlink)
• Spatial angular separation (e.g. 3?) to avoid
  interference from neighboring TXs
• Require a geo-stationary orbit (satellite rotates
  at the same speed of earth rotation, so appears
  stationary)
• Height 35,784km (long link, large transmission
  delays)
• Applications
• Television direct broadcasting
• Long distance telephony
• Private business networks linking multiple
  company sites worldwide

57
a. Satellite Point to Point Link
Relay
Downlink
Uplink
Earth curvature Obstructs line of sight for large
distances
58
b. Satellite Broadcast Link
Direct Broadcasting Satellite
59
Transmission Characteristics

• 1-10 GHz
• Frequency Trade offs
• Lower frequencies More noise and interference
• Higher frequencies Larger rain attenuation, but
  smaller antennas
• Downlink/Uplink frequencies recently going
  higher
  4/6 GHz ? 12/14 ? 20/30
  (better receivers becoming
  available)
• Delay 0.25 s ? noticeable for telephony
• Inherently a broadcasting facility

60
Broadcast Radio 30 MHz 1 GHz

• Omni directional (no need for antenna
  directionality horizontally)
• No dishes
• No line of sight requirement
• No antenna alignment requirement/problems
• Applications
• FM radio
• UHF and VHF television
• Choice of frequency range
• Reflections from ionosphere lt 30 MHz -1 GHz lt
  Rain
• Propagation attenuation
• Lower than for Microwaves (as l is larger)
• Problems caused by omni directionality
  Interference due to
• multi-path reflections
• e.g. TV ghost images

61
Multi-Path effects of omni-directionality
Omni-Directional TV Broadcasting Antenna
TV ghost images
62
Infrared

• Data Modulates a non coherent infrared light
• Relies on line of sight (or reflections through
  walls or ceiling)
• Blocked by walls (unlike microwaves)
• No licensing required for frequency allocation
• Applications
• TV remote control
• Wireless LAN within a room