Computer Networks Transmission Media Transmission Medium n n

Computer Networks Transmission Media

Transmission Medium n n Physical path b/w transmitter and receiver Exists in two forms q q n Characteristics and quality determined by medium & signal q q n 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 n Number of design factors related to transmission medium and signal determine data rate and distance: q Bandwidth n q Transmission Impairments n q Impairments like attenuation limit the distance Interference n n q 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 n n n 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 n n Provide a Transfer Path from one device to another Include q q q Twisted Pair Cable Coaxial Cable Fiber Optic Cable

Twisted Pair Cable n n n Least Expensive Easy to install Disadvantage: q q n n Most widely used guided transmission medium Physically, twisted pair cable consists of two insulated copper wires arranged in a regular spiral pattern q n n Reduces crosstalk interference among adjacent pairs In bundled pairs q n ‘Pair’ is a single communication link Number of pairs could be wrapped together in a tough shield Twisting the cable q n 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. 9 mm

Twisted Pair Cable

Twisted Pair Cable – Applications n Telephone network q n Subscriber loop; house and local exchange Within Buildings q Private Branch Exchanges (PBX) n n 64 Kbps Local Area Networks q Traditionally 10 Mbps but now 100 Mbps and 1 Gbps are also common

Twisted Pair Cable – Transmission Characteristics n Can transmit both analog & digital signal q q n n Limited Distance Limited Bandwidth (Traditionally 1 MHz) q n Now improved up to 200 MHz Limited Data rate (Traditionally 10 Mbps) q n 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 n Twisted Pair Cable comes in two varieties q Unshielded Twisted Pair Cable (UTP) n n q Shielded Twisted Pair Cable (STP) n n 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 q 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 n n Coupling of signal from one pair to another Occurs when transmit signal entering the link couples back to receiving pair q Near transmitted signal is picked up by near receiving pair n n At connector level From neighbor pair

Coaxial Cable n n Versatile Medium Television Distribution q q n Previously used for long distance telephone transmission q q n Ariel to TV Cable to TV Now replaced by Fiber Optic Theoretically, can carry up to 10, 000 calls simultaneously Also used for q Short Distance Computer links n LAN

Coaxial Cable

Coaxial Cable

Coaxial Cable – Transmission Characteristics n n n 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 q n Analog Signals q q n Less susceptible to interference and crosstalk Amplify every few kilometers Usable spectrum: up to 500 MHz Digital Signals q Repeater every 1 KM

Categories of Coaxial Cable

Coaxial Cable Connectors

Quiz n 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) q What is the reason behind it?

Assignment 2 n n n How to connect two PC using point-to-point and multipoint configuration q Write in brief and focus on your own experience/problems faced How to make up straight, cross and console/roll over cables q Write the configuration/scheme you followed q 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 q SUBMIT it as a brief report

Optical Fiber n n Thin (2– 125µm), flexible guided medium uses optical ray to transmit data Offer greater capacity q n n Data rates of several Gbps Smaller size and weight Lower attenuation Electromagnetic isolation Support longer distances q Repeaters required after 10 s of KM

Optical Fiber – Parts n Three concentric sections: q Core n n q Cladding n n n q 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 n n Protects against moisture, abrasion, crushing and other damages May be bundling a number of fibers

Optical Fiber – Parts

Optical Fiber – Parts

Optical Fiber – Operation n 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: q q 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 n n 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 n Act as wave guide for 1014 to 1015 Hz frequencies q n n Light rays created through LED or ILD Light Emitting Diode (LED) q q q n Portions of infrared and visible spectrum Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) q q More efficient Greater data rate

Fiber Optic – Pros n Greater Capacity q n n n n 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 q 10 s of KM at least

Fiber Optic – Cons n n n Installation! Maintenance is also difficult Cost Specialized Equipment and operating Personnel Uni-directional Propagation q q Light from one side can travel in a fiber Solution: Two fibers could be used

Fiber Optic – Applications n Long haul trunks q n Metropolitan trunks q n 400 – 60 KM, 5000 channels Subscriber loop q n 12 KM, 100, 000 voice channels Rural exchange trunks q n 1500 KM, 20 – 60 thousand voice channels Replacing STP/UTP and Coaxial LAN

Fiber Optic – Transmission Modes n Two modes of light propagation q Multimode n n q n Step Index Graded-index Single mode Different modes operate on fiber bearing different characteristics

Fiber Optic – Transmission Modes

Fiber Optic – Transmission Modes n Multimode q n Multiple beams from source to destination Two types q q Step Index Graded Index

Fiber Optic – Transmission Modes n Multimode: Step Index q Density of core is constant from center to edges q Abrupt changes due to sudden density change with cladding q Rays which hit will less than critical angle penetrate (although very less in number) q Other rays are reflected back q Different rays have different angles of reflections in a single core q Beams with small angle of incidence would face more bounces till it reaches the other end q Distortion and attenuation problems n q 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 n Multimode: Graded Index q Density of core varies from center to edges n q q Center is more dense while density increases towards edges Smooth change in density reflect rays back smoothly Low Distortion

Fiber Optic – Transmission Modes n Single Mode q Uses fiber like step index n q Very small diameter fiber n q 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 n n Unguided media transport electromagnetic waves without using a physical conductor Usually referred to as Wireless Communication Three ranges of frequencies are of our interest 30 MHz – 1 GHz q n 1 GHz – 40 GHz q n Radio Microwaves 3 x 1011 – 2 x 1014 Hz q Infrared

Unguided Media

Electromagnetic Spectrum for Wireless Communication

Antenna n n 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 q q 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 n Two Common types q Omni-Directional Antenna n n n An antenna which radiate power equally in all directions Usually not possible Isotropic antenna is assumed which radiate power equally in all directions q q Actual radiation pattern for the isotropic antenna is a sphere with antenna at the center Directional Antenna n Further Different Types q q Parabolic Reflective Antenna Highly Directional Antenna etc

Types of Antenna q Directional Antenna (Cont) n Parabolic q q q 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 n n Measure of Efficiency of Antenna Measure of Directionality of Antenna q n 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 n Signal radiated from antenna travels along one of three routes q q q Ground ware Propagation Sky Wave Propagation Line of Sight Propagation

Wireless Propagation

Frequency Bands Vs Propagation and Use

Wireless Propagation n Ground Wave Propagation q q q Follows almost the contour of earth Uses frequencies up to 2 MHz Several factors involved in such movement n q 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 n Sky Propagation q q Used for amateur radio Signal from earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth n q q 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 n Line of Sight Propagation q q Above 30 MHz, 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 n Microwaves Bent due to refraction

Microwave Communication n n Microwave communication takes place somewhere in 1 – 40 GHz 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 q Terrestrial Microwave q Satellite Microwave

Microwave – Terrestrial Communication n n 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 n Long haul telecommunication service q n 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 q q To connect two buildings in the same city To have wireless internet connection from some ISP

Microwave – Terrestrial Communication: Transmission Characteristics n Most common band for long-haul telecommunications are 4 – 6 GHz q n n Congested 11 GHz band is in use now For Short Range (Connecting Two Buildings) 22 GHz band is utilized q Attenuation is not problem in short distances

Microwave – Satellite n n 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 q n 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 q Match occurs at height of 35, 863 Km of equator

Microwave – Common Satellite Configuration

Microwave – Limitation in Satellite n n Two satellites using same frequency band will interfere with each other if they come closer To avoid q q 40 spacing b/w satellites is required in 4/6 GHz Band (Measured from earth) 30 spacing is required in 12/14 GHz Band

Microwave – Satellite Applications n n n Television Distribution Long Distant Telephone Transmission Private Business Networks q VSAT (Very Small Aperture Terminal)

Microwave – Satellite Transmission Characteristics n Optimum Frequency range q 1 – 10 GHz n n q Most P-t-P satellites today use 5. 925 – 6. 425 GHz for Upload and 3. 7 – 4. 2 GHz for download n q Remember: Low frequency for longer distances, higher attenuate Below 1 GHz, lot of noises from natural sources Combination is called 4/6 Band Transmission and Reception frequencies differ n Otherwise interference will occur

Microwave – Satellite Transmission Characteristics n n 4/6 GHz Band is in optimum zone but got saturated Other bands with 1 – 10 GHz are not available because of interferences q n 12/14 GHz band is developed q q n Usually terrestrial devices operate on those Uplink: 14 – 14. 5 GHz Downlink: 11. 7 – 12. 2 GHz It is expected that 12/14 GHz band will also saturate shortly so 20/30 GHz band is proposed

Broadcast Radio Communication n n Main difference in Microwave and Radio is that Microwave is usually directional while radio is omni-directional Radio q q Doesn’t require dish-shaped antennas Doesn’t need antennas to be mounted accurately

Radio – Application n n “Radio” term illustrate frequencies from 3 KHz to 300 GHz in general Broadcast Radio is an informal term to cover FM, VHF and part of UHF i. e. 30 MHz to 1 GHz

Broadcast Radio – Transmission Characteristics n n n 30 MHz band is transparent to Ionosphere LOS is required for communication Frequency used is less than employed by microwaves so the signal faces less attenuation q n Ideal for broadcast transmission Impairments are usually caused by q Multi-path

Infrared Communication n Requires LOS q n Infrared cannot transfer through walls q n Microwave can! More secure q n 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 n n n Free Space Loss Multi-path Refraction

Free Space Loss n n n 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 n n 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 n 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 n What is IEEE 802. x q q q Write about IEEE Focus on 802 Standards defined under 802 Umbrella