Transcript File - telecommunication and networking

Computer Networks
Transmission Media
Transmission Medium
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Physical path b/w transmitter and receiver
Exists in two forms
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Characteristics and quality determined by medium & signal
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Guided – Wire, Optical Fiber
Un-Guided – Wireless
In Guided: Medium is more important
In Unguided: Bandwidth produced by antenna is more important
Key concerns are data rate and distance
Design Factors
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Number of design factors related to transmission medium and
signal determine data rate and distance:
 Bandwidth
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Transmission Impairments
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Impairments like attenuation limit the distance
Interference
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Greater the bandwidth, higher data rates could be achieved (if other
factors remain constant)
E.g. EMI
Competing signals in overlapping frequency bands may distort or
wipe out a signal
Number of Receivers
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In Guided Media
Link may be P-to-P or multipoint
In multipoint, more the number of attachments, more
attenuation/distortion, limiting the distance/data rate
Electromagnetic Spectrum for
Telecommunication
Guided Media
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Provide a Transfer Path from one device to
another
Include
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Twisted Pair Cable
Coaxial Cable
Fiber Optic Cable
Twisted Pair Cable
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Least Expensive
Easy to install
Disadvantage:
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Most widely used guided transmission medium
Physically, twisted pair cable consists of two insulated copper wires
arranged in a regular spiral pattern
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Reduces crosstalk interference among adjacent pairs
In bundled pairs
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‘Pair’ is a single communication link
Number of pairs could be wrapped together in a tough shield
Twisting the cable
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Works in short range
Support low data rate (at least now this is not the case)
Different pairs have separate twist length
Copper thickness is 0.4 – 0.9mm
Twisted Pair Cable
Twisted Pair Cable – Applications
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Telephone network
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Subscriber loop; house and local exchange
Within Buildings
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Private Branch Exchanges (PBX)
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64Kbps
Local Area Networks
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Traditionally 10Mbps but now 100Mbps and
1Gbps are also common
Twisted Pair Cable – Transmission
Characteristics
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Can transmit both analog & digital signal
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Limited Distance
Limited Bandwidth (Traditionally 1MHz)
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Now improved up to 200MHz
Limited Data rate (Traditionally 10Mbps)
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For analog requires amplifier every 5 – 6 KM
For Digital requires repeater every 2 – 3 KM
Now supports Gbps
Susceptible to interference & noise
Twisted Pair Cable – Types
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Twisted Pair Cable comes in two varieties
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Unshielded Twisted Pair Cable (UTP)
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Shielded Twisted Pair Cable (STP)
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Ordinary telephone wire
Cheapest
Easiest to install
Suffers from external EM interference
Metal braid or sheathing that reduces interference
More expensive
Hard to handle (thick, heavy)
Read Variety of Twisted Pair Categories e.g. Cat
3, Cat 4, Cat 5 etc
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Find out difference at characteristics, physical and
operational levels
Twisted Pair Cable – Types
Twisted Pair Connectors (For LAN)
Near End Crosstalk in Twisted Pair Cable
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Coupling of signal from one pair to another
Occurs when transmit signal entering the link
couples back to receiving pair
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Near transmitted signal is picked up by near
receiving pair
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At connector level
From neighbor pair
Coaxial Cable
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Versatile Medium
Television Distribution
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Previously used for long distance telephone
transmission
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Ariel to TV
Cable to TV
Now replaced by Fiber Optic
Theoretically, can carry up to 10,000 calls simultaneously
Also used for
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Short Distance Computer links
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LAN
Coaxial Cable
Coaxial Cable
Coaxial Cable – Transmission
Characteristics
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Can transmit both Digital and Analog Signals
Have superior frequency characteristics than twisted
pair so could be used for high frequencies and data
rates
Shielded, Concentric Construction
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Analog Signals
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Less susceptible to interference and crosstalk
Amplify every few kilometers
Usable spectrum: up to 500MHz
Digital Signals
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Repeater every 1KM
Categories of Coaxial Cable
Coaxial Cable Connectors
Quiz
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Although you may feel that coaxial has
several advantages over twisted pair, yet
twisted pair is getting more popularity in
different types of installations particularly
networks (specially LAN)
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What is the reason behind it?
Assignment 2
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How to connect two PC using point-to-point and multipoint
configuration
 Write in brief and focus on your own experience/problems faced
How to make up straight, cross and console/roll over cables
 Write the configuration/scheme you followed
 No need to submit cables but bring those on submission day so
that cables could be tested
What are the differences/improvements made in different
categories of twisted pair cables
 SUBMIT it as a brief report
Optical Fiber
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Thin (2–125µm), flexible guided medium uses
optical ray to transmit data
Offer greater capacity
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Data rates of several Gbps
Smaller size and weight
Lower attenuation
Electromagnetic isolation
Support longer distances
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Repeaters required after 10s of KM
Optical Fiber – Parts
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Three concentric sections:
 Core
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Cladding
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Inner most section
More dense than cladding
One or more very thin (width of hair, 8–100µm) strands/fiber made of
glass/plastic
Very pure material
Less refractive Glass/plastic coating around core
Optical property different than core
Reflect the light back into the core that tries to escape
Jacket
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Protects against moisture, abrasion, crushing and other damages
May be bundling a number of fibers
Optical Fiber – Parts
Optical Fiber – Parts
Optical Fiber – Operation
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The light travels into the core, produced by
Light Emitting Diode (LED) of ILD (Injection
Laser Diode)
While passing through the core, if the light
gets out of core, cladding around the core
reflects it back inside the core
Wavelength Multiplexing:
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Lights differ in wavelength
Different lights could be sent in a single fiber and
could be distinguished distinctly at the receiver
Fiber Optic – Operation
Total Internal Reflection
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Light that is reflected back from the edge of the
medium it is traveling through; When light rays travel
at an angle greater than the "critical" angle, which is
determined by the medium, the light reflects back
into the medium. If less than the critical angle (more
perpendicular), the light is refracted out of the
medium and lost to the outside
The reflection that occurs when light, in a higher
refractive-index medium, strikes an interface, with a
medium with a lower refractive index, at an angle of
incidence (with respect to the normal) greater than
the critical angle
Total Internal Reflection
Fiber Optic – Transmission Characteristics
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Act as wave guide for 1014 to 1015 Hz frequencies
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Light rays created through LED or ILD
Light Emitting Diode (LED)
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Portions of infrared and visible spectrum
Cheaper
Wider operating temp range
Last longer
Injection Laser Diode (ILD)
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More efficient
Greater data rate
Fiber Optic – Pros
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Greater Capacity
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Hundreds of Gbps
Smaller size & weight
Less expensive for long length installations
Lower attenuation
Low power requirements
Non-Flammable (no short-circuit hazards)
Electromagnetic Isolation
Greater Repeater Spacing
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10s of KM at least
Fiber Optic – Cons
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Installation!
Maintenance is also difficult
Cost
Specialized Equipment and operating
Personnel
Uni-directional Propagation
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Light from one side can travel in a fiber
Solution: Two fibers could be used
Fiber Optic – Applications
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Long haul trunks
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Metropolitan trunks
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400 – 60 KM, 5000 channels
Subscriber loop
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12 KM, 100,000 voice channels
Rural exchange trunks
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1500 KM, 20 – 60 thousand voice channels
Replacing STP/UTP and Coaxial
LAN
Fiber Optic – Transmission Modes
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Two modes of light propagation
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Multimode
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Step Index
Graded-index
Single mode
Different modes operate on fiber bearing
different characteristics
Fiber Optic – Transmission Modes
Fiber Optic – Transmission Modes
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Multimode
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Multiple beams from source to destination
Two types
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Step Index
Graded Index
Fiber Optic – Transmission Modes
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Multimode: Step Index
 Density of core is constant from center to edges
 Abrupt changes due to sudden density change with cladding
 Rays which hit will less than critical angle penetrate (although
very less in number)
 Other rays are reflected back
 Different rays have different angles of reflections in a single core
 Beams with small angle of incidence would face more bounces till
it reaches the other end
 Distortion and attenuation problems
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Sudden/abrupt change of direction due to total internal reflection
Today used only by POF (Plastic Optic Fiber)
Fiber Optic – Transmission Modes
Fiber Optic – Transmission Modes
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Multimode: Graded Index
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Density of core varies from center to edges
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Center is more dense while density increases towards edges
Smooth change in density reflect rays back smoothly
Low Distortion
Fiber Optic – Transmission Modes
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Single Mode
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Uses fiber like step index
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Very small diameter fiber
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Density of core is constant
Approximately the size of wavelength of light which will travel
across it
Employs highly focused light
Fiber Optic – Transmission Modes
Fiber Optic – Connectors
Un-Guided Media
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Unguided media transport electromagnetic waves
without using a physical conductor
Usually referred to as Wireless Communication
Three ranges of frequencies are of our interest
30MHz – 1GHz
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1GHz – 40GHz
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Radio
Microwaves
3x1011 – 2x1014 Hz
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Infrared
Unguided Media
Electromagnetic Spectrum for Wireless
Communication
Antenna
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For unguided media, transmission and reception are
achieved by means of antenna
Antenna is an electrical conductor/system of
conductors used either for radiating electromagnetic
energy or for collecting electromagnetic energy
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For transmission, conversion of electromagnetic energy
into radiation for traveling in surroundings and vice versa in
reception
Both transmission and reception is normally done by same
antenna in two-way communication
Types of Antennas
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Two Common types
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Omni-Directional Antenna
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An antenna which radiate power equally in all directions
Usually not possible
Isotropic antenna is assumed which radiate power
equally in all directions
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Actual radiation pattern for the isotropic antenna is a
sphere with antenna at the center
Directional Antenna
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Further Different Types
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Parabolic Reflective Antenna
Highly Directional Antenna etc
Types of Antenna
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Directional Antenna (Cont)
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Parabolic
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Uses parabolic dish
All the points on the dish are equidistant from a single point
known as FOCUS of the parabola
If a source of electromagnetic energy is placed at the focus
(considering paraboloid as reflecting surface) the waves will
bounce back to the axis of paraboloid
Larger the diameter of antenna, more tightly directional is
the beam
On reception, all the waves that fall on the paraboloid are
concentrated at focus
Types of Directional Antenna
Antenna Gain
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Measure of Efficiency of Antenna
Measure of Directionality of Antenna
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More an antenna is directional towards its target,
more would be its gain
It is one of the yardstick to select antennas
for different purposes
Wireless Propagation
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Signal radiated from antenna travels along
one of three routes
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Ground ware Propagation
Sky Wave Propagation
Line of Sight Propagation
Wireless Propagation
Frequency Bands Vs Propagation and Use
Wireless Propagation
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Ground Wave Propagation
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Follows almost the contour of earth
Uses frequencies up to 2MHz
Several factors involved in such movement
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Electromagnetic wave induces current in the earth
surface, causes the wave-front to tilt downward and
travel over the earth curvature
These waves are scattered by atmosphere in
such a way that they do not penetrate the upper
atmosphere
Wireless Propagation
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Sky Propagation
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Used for amateur radio
Signal from earth-based antenna is reflected from the
ionized layer of the upper atmosphere (ionosphere) back
down to earth
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Happens due to Refraction: Change in the density/medium
while the wave travel from earth to the height
Signal can take many bounces while moving from
transmitter to receiver
This causes the signal to be picked up even after
thousands of kilometers from the transmitter (ideally)
Wireless Propagation
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Line of Sight Propagation
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Above 30MHz, neither ground nor sky wave
propagation mode operate
Communication takes place on Line of Sight basis
High frequency signal is not reflected by the
ionosphere so signal can travel from an earth
station to satellite
For ground based communication, both the
antennas must be within Effective LOS
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Microwaves Bent due to refraction
Microwave Communication
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Microwave communication takes place somewhere in 1 – 40GHz
band of electromagnetic spectrum
Keep in mind that bigger the frequency used, higher would be the
bandwidth and potentially higher data rates would be offered
But also notice that bigger frequencies have to face more
attenuation problems and are more prone to several types of
interferences
Assignment of frequency band is strictly regulated to be used for
different purposes
Two General Types of Microwave Communication
 Terrestrial Microwave
 Satellite Microwave
Microwave – Terrestrial Communication
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Usually uses parabolic dish antenna or other
directional antennas
Sending & Receiving antennas are rigidly fixed and
focused towards each-other to use a narrow beam
in LOS transmission
Antennas are usually fixed at heights to extend
range b/w them and to avoid obstacles
These point-to-point links may be cascaded for
multiple times for prolonged communication links
Microwave – Terrestrial Communication:
Application
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Long haul telecommunication service
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Alternative to coaxial and fiber since require less
repeaters and is easy to install but requires line of
sight
May be used as Short Haul
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To connect two buildings in the same city
To have wireless internet connection from some
ISP
Microwave – Terrestrial Communication:
Transmission Characteristics
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Most common band for long-haul
telecommunications are 4 – 6GHz
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Congested
11GHz band is in use now
For Short Range (Connecting Two Buildings)
22GHz band is utilized
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Attenuation is not problem in short distances
Microwave – Satellite
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Satellite is a microwave relay station
Links two or more ground stations
Satellite receives transmission on one frequency,
may amplify, and transmits on another frequency
For effective functionality, a satellite is required to
remain stationary w.r.t its position over earth
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In other case, it will lose the line of sight to its earth stations
To accomplish this goal, satellite must have a
rotation period equal to earth
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Match occurs at height of 35,863Km of equator
Microwave – Common Satellite
Configuration
Microwave – Limitation in Satellite
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Two satellites using same frequency band
will interfere with each other if they come
closer
To avoid
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40 spacing b/w satellites is required in 4/6GHz
Band (Measured from earth)
30 spacing is required in 12/14GHz Band
Microwave – Satellite Applications
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Television Distribution
Long Distant Telephone
Transmission
Private Business Networks
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VSAT (Very Small Aperture
Terminal)
Microwave – Satellite Transmission
Characteristics
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Optimum Frequency range
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1 – 10GHz
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Most P-t-P satellites today use 5.925 – 6.425GHz
for Upload and 3.7 – 4.2GHz for download
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Remember: Low frequency for longer distances, higher
attenuate
Below 1GHz, lot of noises from natural sources
Combination is called 4/6 Band
Transmission and Reception frequencies differ
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Otherwise interference will occur
Microwave – Satellite Transmission
Characteristics
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4/6GHz Band is in optimum zone but got saturated
Other bands with 1 – 10GHz are not available
because of interferences
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12/14GHz band is developed
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Usually terrestrial devices operate on those
Uplink: 14 – 14.5GHz
Downlink: 11.7 – 12.2GHz
It is expected that 12/14GHz band will also saturate
shortly so 20/30GHz band is proposed
Broadcast Radio Communication
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Main difference in Microwave and Radio is
that Microwave is usually directional while
radio is omni-directional
Radio
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Doesn’t require dish-shaped antennas
Doesn’t need antennas to be mounted accurately
Radio – Application
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“Radio” term illustrate frequencies from 3KHz
to 300GHz in general
Broadcast Radio is an informal term to cover
FM, VHF and part of UHF i.e. 30MHz to
1GHz
Broadcast Radio – Transmission
Characteristics
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30MHz band is transparent to Ionosphere
LOS is required for communication
Frequency used is less than employed by
microwaves so the signal faces less
attenuation
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Ideal for broadcast transmission
Impairments are usually caused by
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Multi-path
Infrared Communication
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Requires LOS
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Infrared cannot transfer through walls
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Microwave can!
More secure
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May use reflected surface in the absence of LOS
Communication in closed environments could not
be hacked from outside
No Licensing required since no frequency
allocation issue
Problems
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Free Space Loss
Multi-path
Refraction
Free Space Loss
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Signal disperses with distance
Signal becomes weaker as distance b/w
antennas increase
For satellite it is the main cause of signal loss
Multi-Path
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For Wireless Communication, LOS is
preferred and mostly required
In other cases like mobile telephony,
obstacles are there
Signals can be reflected back from such
obstacles and receiver may receive multiple
copies of same signals with varying delays
In extreme cases, there may be no direct
signal
Multi-Path
Refraction
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Radio Waves refract/bent while traveling
through atmosphere
Caused by changes in speed of signal with
altitude or by spatial changes in atmospheric
conditions
Speed of signal increases with altitude but
bents downwards
Assignment 3
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What is IEEE802.x
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Write about IEEE
Focus on 802
Standards defined under 802 Umbrella