Lecture 4 - Transmission Media

Design Factors Determining Data Rate and Distance

Key Factors

• Bandwidth: Higher bandwidth gives higher data rate
• Transmission impairments: Impairments, such as attenuation, limit the distance
• Interference: Overlapping frequency bands can distort or wipe out a signal
• Number of receivers: More receivers introduces more attenuation

Electromagnetic Spectrum for Telecommunications

Point-to-Point Transmission Characteristics of Guided Media

Comparison Table

Guided Transmission Media Structure

Physical Composition

Twisted Pair

• Structure: Separately insulated copper wires twisted together
• Configuration: Often "bundled" into cables
• Installation: Usually installed in building during construction
• Twist length: Regular spiral pattern to reduce interference

Coaxial Cable

• Inner conductor: Solid metal
• Insulation: Separates inner and outer conductors
• Outer conductor: Braided shield
• Covering: Protective padding

Optical Fiber

• Core: Glass or plastic center
• Cladding: Surrounds the core
• Buffer coating: Protective layer
• Outer sheath: Final protective covering
• Light source: Laser or light emitting diode
• Size advantage: Small size and weight

Twisted Pair

Overview

• Definition: Least expensive and most widely used guided transmission medium
• Structure: Two insulated copper wires arranged in regular spiral pattern
• Function: Wire pair acts as single communication link
• Bundling: Pairs are bundled together into cables

Applications

• Telephone network: Most commonly used
• Building communications: Within buildings
• Digital signaling: Most common medium for digital signals
• Ethernet: Commonly used within buildings for LAN supporting PCs
• Data rates:
  • Ethernet: 100 Mbps to 1 Gbps typically
  • Emerging technology: 10 Gbps
  • Long-distance: 4 Mbps or more

Electromagnetic Interference Reduction

• Twisting purpose: Reduces electromagnetic interference
• Principle: Regular spiral pattern cancels out interference

Unshielded and Shielded Twisted Pair Types

U/UTP or UTP: Unshielded Twisted Pairs

• Structure: One or more twisted-pair cables in thermoplastic jacket
• Shielding: No electromagnetic shielding
• Example: Ordinary telephone wire
• Vulnerability: Subject to external electromagnetic interference (EMI)
• Cost factor: Tighter twisting = higher transmission rate = greater cost per meter

F/UTP or FTP: Foiled with Unshielded Twisted Pairs

• Features: Overall foil shield wrapped around unshielded twisted pairs
• Drain wire: Included for grounding
• Protection: Extra protection against EMI and radio frequency interference (RFI)
• Function: Drain wire redirects unwanted noise to ground when correctly connected

S/UTP or STP: Shielded with Unshielded Twisted Pairs

• Shielding: Metal braid or sheathing
• Advantages:
  • Reduces interference
  • Better performance at higher data rates
• Disadvantage: More expensive

SF/UTP: Shielded and Foiled with Unshielded Twisted Pairs

• Features: Both overall braid shield and foil shield with unshielded twisted pairs
• Protection: Effective protection from EMI both from and into the cable
• Grounding: Much better grounding due to additional braid

U/FTP: Unshielded with Foiled Twisted Pairs

• Overall shielding: No overall shielding
• Individual pairs: Wrapped in foil screen
• Protection: Some protection from EMI and crosstalk from adjacent pairs and other cables

F/FTP: Foiled with Foiled Twisted Pairs

• Features: Overall foil shield with individually foil tape shielded twisted pairs
• Similar to: F/UTP cables with addition of foil shield around each twisted pair
• Purpose: Greater protection from crosstalk, RFI, and EMI

S/FTP: Shielded with Foiled Twisted Pairs

• Structure: Individual twisted pairs wrapped in foil tape before overall flexible braid screen
• Crosstalk reduction: Additional foil reduces crosstalk from adjacent pairs
• Grounding: Braid provides better grounding

SF/FTP: Shielded and Foiled with Foiled Twisted Pairs

• Maximum protection: From RFI/EMI, crosstalk, and alien crosstalk
• Features: Overall braid shield and foil shield with individually foil tape screened pairs
• Best protection: Highest level of protection from interference and better grounding

Twisted Pair Categories and Classes

Key Performance Metrics

• Insertion loss: Loss of signal when traveling in and out of a component
• NEXT: Near End Cross Talk (expressed in dB)
• ACR: Attenuation to Crosstalk Ratio (ratio of received signal strength to crosstalk amount)

Near-End Crosstalk (NEXT) Details

• Definition: Coupling of signal from one pair of conductors to another
• Location: Metal pins in connector or wire pairs in cable
• Occurrence: When transmit signal couples back to receive conductor pair at same end
• Measurement: Greater NEXT loss magnitudes = less crosstalk noise
• System impact: Affects signal quality between transmit and receive systems

Coaxial Cable

Overview

• Advantage: Can be used over longer distances and support more stations than twisted pair
• Structure: Hollow outer cylindrical conductor surrounding single inner wire conductor
• Versatility: Used in wide variety of applications
• Applications: TV distribution, long distance telephone transmission, LANs

Transmission Characteristics

• Frequency characteristics: Superior to twisted pair
• Performance limitations: Limited by attenuation and noise

Analog Signals

• Amplifiers: Needed every few kilometers (closer for higher frequency)
• Usable spectrum: Extends up to 500MHz

Digital Signals

• Repeaters: Required every 1km (closer for higher data rates)

Optical Fiber

Overview

• Definition: Thin flexible medium capable of guiding an optical ray
• Materials: Various glasses and plastics
• Structure: Cylindrical shape with three sections (core, cladding, jacket)
• Applications: Widely used in long distance telecommunications
• Popularity: Performance, price and advantages have made it popular

Benefits

• Greater capacity: Data rates of hundreds of Gbps over tens of kilometers demonstrated
• Smaller size and lighter weight:
  • Considerably thinner than coaxial or twisted pair cable
  • Reduces structural support requirements
• Lower attenuation: Less signal loss over distance
• Electromagnetic isolation:
  • Not vulnerable to interference, impulse noise, or crosstalk
  • High degree of security from eavesdropping
• Greater repeater spacing: Longer distances between repeaters
• Lower cost and fewer sources of error

Optical Communication System

• Components:
  • Electronic interface (input)
  • LED or laser light source
  • Optical fiber medium
  • Detector (light sensor)
  • Electronic interface (output)
• Process:
  • E/O Conversion: Electrical to optical
  • O/E Conversion: Optical to electrical
• Signal: Lightwave pulses through fiber

Optical Fiber Transmission Modes

Step-index Multimode

• Characteristics: Input pulse spreads to wider output pulse
• Usage: Basic multimode transmission

Graded-index Multimode

• Characteristics: Less pulse spreading than step-index
• Improvement: Better pulse preservation

Single Mode

• Characteristics: Minimal pulse spreading
• Performance: Best signal integrity
• Usage: Long distance, high-speed applications

Frequency Utilization for Fiber Applications

WDM = Wavelength Division Multiplexing

Wireless Transmission Frequencies

Frequency Ranges and Applications

1GHz to 40GHz

• Classification: Microwave frequencies
• Characteristics: Highly directional beams possible
• Usage:
  • Point-to-point transmissions
  • Satellite communications

30MHz to 1GHz

• Classification: Radio range
• Characteristics: Suitable for omnidirectional applications

3 x 10¹¹ to 2 x 10¹⁴ Hz

• Classification: Infrared portion of spectrum
• Applications: Local point-to-point and multipoint applications within confined areas

Antennas

Definition and Function

• Definition: Electrical conductor or system of conductors used to radiate or collect electromagnetic energy
• Transmission process: Radio frequency electrical energy from transmitter converted to electromagnetic energy and radiated
• Reception process: Electromagnetic signal intersects antenna
• Bidirectional: Same antenna can be used for both transmission and reception

Radiation Patterns

• Definition: Graphical representation of radiation properties as function of space coordinates

Pattern Types

• Omnidirectional patterns: Radiates equally in all horizontal directions
• Directional patterns: Focuses energy in specific direction

Antenna Types

Parabolic Antenna

• Structure: Uses parabolic reflector with cross-sectional shape of parabola
• Function: Directs radio waves using reflective property
• Design: Source at focus reflects off parabola to create parallel waves

Isotropic Antenna

• Theoretical concept: Point in space that radiates power equally in all directions
• Reality: Does not perform equally well in all directions
• Pattern: Actual radiation pattern is sphere with antenna at center

Antenna Gain ($G_{\text{dB}}$)

• Definition: Measure of directionality of an antenna
• Calculation: Power output in particular direction versus isotropic antenna
• Measurement: Expressed in decibels (dB)
• Trade-off: Increased power in given direction at expense of other directions
• Relationship: Effective area related to physical size and shape

Formula:

G_dB = 10 log (P2/P1)

Where:

• G = antenna gain
• P1 = Power out of antenna of interest
• P2 = Power out of reference antenna

Terrestrial Microwave Transmission System (TMTS)

Physical Characteristics

• Most common type: Parabolic "dish"
• Typical size: About 3 meters in diameter
• Mounting: Fixed rigidly
• Function: Focuses narrow beam for line-of-sight transmission
• Location: Usually at substantial heights above ground level
• Long-distance: Series of microwave relay towers used

Applications

• Long haul telecommunications: Alternative to coaxial cable or optical fiber
• Transmission types: Both voice and TV transmission
• Advantages: Fewer repeaters required
• Requirement: Line-of-sight transmission
• Frequency range: 1-40GHz (higher frequencies = higher data rates)
• Main losses: Distance, rainfall, and interference

Typical Digital Microwave Performance

Satellite Microwave Transmission System (SMTS)

Overview

• Function: Uses satellites for broadcasting and receiving signals
• Orbit: Satellites in geostationary orbit (36,000 km above earth)
• Components: Receiving antenna, transponder, and transmitting antenna
• Role: Communication satellite acts as microwave relay station
• Purpose: Links two or more ground-based microwave transmitter/receivers (earth stations)

Operation

• Process: Receives transmissions on one frequency band, amplifies/repeats signal, transmits on another frequency
• Channels: Frequency bands called transponder channels

Communication Configurations

Point-to-Point Link

• Structure: Single transmitter to satellite to single receiver
• Links: Uplink and downlink

Broadcast Link

• Structure: Single transmitter to satellite to multiple receivers
• Distribution: Multiple downlinks from single uplink

Applications

Television Distribution

• Process: Programs transmitted to satellite then broadcast to multiple stations
• Distribution: Stations distribute programs to individual viewers
• Direct Broadcast Satellite (DBS): Transmits video signals directly to home users

Long-Distance Telephone Transmission

• Optimization: Optimum medium for high-usage international trunks

Private Business Networks

• Capacity division: Satellite providers divide capacity into channels
• Leasing: Channels leased to individual business users

Global Positioning (GPS)

• Function: Satellite-based positioning system

VSAT (Very Small Aperture Terminal)

• Equipment: Subscriber stations with low-cost VSAT antennas
• Capacity sharing: Earth stations share satellite transmission capacity
• Hub station: Central station for message exchange and relay
• Configuration: Hub can exchange with each subscriber and relay between subscribers

Terrestrial vs Satellite Microwave Comparison

Transmission Characteristics for Satellite

Optimum Frequency Range

• Best range: 1 - 10 GHz
• Below 1 GHz: Significant noise from natural sources
• Above 10 GHz: Severe attenuation by atmospheric absorption and precipitation

Frequency Bands

4/6-GHz Band (Now Saturated)

• Uplink: 5.925 to 6.425 GHz (earth to satellite)
• Downlink: 3.7 to 4.2 GHz (satellite to earth)

12/14-GHz Band (Developed Due to Saturation)

• Uplink: 14 to 14.5 GHz
• Downlink: 11.7 to 12.2 GHz

Broadcast Radio

Characteristics

• Directional comparison: Broadcast radio is omnidirectional; microwave is directional
• Frequency definition: Term "radio" encompasses 3kHz to 300GHz
• Broadcast range: 30MHz - 1GHz

Coverage

• FM radio: Part of broadcast band
• Television: UHF and VHF bands
• Data networking applications: Various implementations

Limitations

• Line of sight: Limited to line-of-sight transmission
• Multipath interference: Suffers from reflections off land, water, and man-made objects

Infrared

Characteristics

• Technology: Uses transceivers that modulate noncoherent infrared light
• Line of sight requirement: Transceivers must be within line of sight (direct or via reflection)
• Penetration limitation: Does not penetrate walls
• Licensing: No licensing required
• Frequency allocation: No frequency allocation issues

Wireless Propagation Modes

Frequency Bands and Propagation

• GW: Ground Wave
• LOS: Line of sight

Ground Wave Propagation (Below 2 MHz)

• Characteristic: Follows contour of earth
• Range: Can propagate distances well over visual horizon
• Frequency limit: Up to about 2MHz
• Example: AM radio (best known example)
• Mechanism: Wave follows earth's surface

Sky Wave Propagation (2 to 30 MHz)

• Usage: Amateur radio and international broadcasts
• Mechanism: Signal reflects from ionized layer of upper atmosphere back to earth
• Multiple hops: Signals can bounce back and forth between ionosphere and earth's surface
• Range: Can achieve very long distances through multiple reflections

Line-of-Sight (LOS) Propagation (Above 30 MHz)

• Frequency range: Above 30 MHz
• Requirement: Direct line-of-sight path between transmitter and receiver
• Limitation: Ground and sky wave propagation modes don't operate above 30MHz
• Applications: Most modern wireless communications

Line-of-Sight Transmission Issues

Refraction

• Cause: Velocity of electromagnetic wave varies with medium density
• Speed reference: 3 x 10⁸ m/s in vacuum, less in other media
• Speed change: Occurs when moving between media of different densities
• Index of refraction:
  • Sine of angle of incidence ÷ sine of angle of refraction
  • Equal to ratio of respective velocities in two media
  • Varies with wavelength
• Atmospheric effect:
  • Gradual bending occurs
  • Density decreases with height
  • Results in bending of radio waves toward earth

Transmission Impairments

Free Space Loss

• Effect: Loss of signal with distance
• Characteristic: Fundamental limitation of wireless transmission

Atmospheric Absorption

• Causes: Water vapor and oxygen absorption
• Impact: Signal strength reduction

Multipath

• Problem: Multiple interfering signals from reflections
• Sources: Buildings, terrain, atmospheric layers
• Effect: Signal distortion and fading

Examples of Multipath Interference

Microwave Line of Sight

• Issue: Reflections from terrain and obstacles
• Impact: Signal arrives via multiple paths with different delays

Mobile Radio

• Issue: Reflections from buildings, vehicles, terrain
• Impact: Rapid signal variations and fading