Computer Networks

Transmission Media

Transmission Medium

Physical path b/w transmitter and receiver Exists in two forms

Guided – Wire, Optical Fiber Un-Guided – Wireless

Characteristics and quality determined by medium & signal In Guided: Medium is more important In Unguided: Bandwidth produced by antenna is more important

Key concerns are data rate and distance

Design Factors

Number of design factors related to transmission medium and signal determine data rate and distance: Bandwidth

Greater the bandwidth, higher data rates could be achieved (if other factors remain constant)

Transmission Impairments Impairments like attenuation limit the distance

Interference E.g. EMI Competing signals in overlapping frequency bands may distort or

wipe out a signal Number of Receivers

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

Provide a Transfer Path from one device to another

Include Twisted Pair Cable Coaxial Cable Fiber Optic Cable

Twisted Pair Cable

Least Expensive Easy to install Disadvantage:

Works in short range Support low data rate (at least now this is not the case)

Most widely used guided transmission medium Physically, twisted pair cable consists of two insulated copper wires

arranged in a regular spiral pattern ‘Pair’ is a single communication link

Number of pairs could be wrapped together in a tough shield Twisting the cable

Reduces crosstalk interference among adjacent pairs In bundled pairs

Different pairs have separate twist length Copper thickness is 0.4 – 0.9mm

Twisted Pair Cable

Twisted Pair Cable – Applications Telephone network

Subscriber loop; house and local exchange Within Buildings

Private Branch Exchanges (PBX) 64Kbps

Local Area Networks Traditionally 10Mbps but now 100Mbps and

1Gbps are also common

Twisted Pair Cable – Transmission Characteristics Can transmit both analog & digital signal

For analog requires amplifier every 5 – 6 KM For Digital requires repeater every 2 – 3 KM

Limited Distance Limited Bandwidth (Traditionally 1MHz)

Now improved up to 200MHz Limited Data rate (Traditionally 10Mbps)

Now supports Gbps Susceptible to interference & noise

Twisted Pair Cable – Types

Twisted Pair Cable comes in two varieties Unshielded Twisted Pair Cable (UTP)

Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference

Shielded Twisted Pair Cable (STP) 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 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 Coupling of signal from one pair to another Occurs when transmit signal entering the link

couples back to receiving pair Near transmitted signal is picked up by near

receiving pair At connector level From neighbor pair

Coaxial Cable

Versatile Medium Television Distribution

Ariel to TV Cable to TV

Previously used for long distance telephone transmission Now replaced by Fiber Optic Theoretically, can carry up to 10,000 calls simultaneously

Also used for Short Distance Computer links

LAN

Coaxial Cable

Coaxial Cable

Coaxial Cable – Transmission Characteristics 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 Less susceptible to interference and crosstalk

Analog Signals Amplify every few kilometers Usable spectrum: up to 500MHz

Digital Signals Repeater every 1KM

Categories of Coaxial Cable

Coaxial Cable Connectors

Quiz

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) What is the reason behind it?

Assignment 2

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

Thin (2–125µm), flexible guided medium uses optical ray to transmit data

Offer greater capacity Data rates of several Gbps

Smaller size and weight Lower attenuation Electromagnetic isolation Support longer distances

Repeaters required after 10s of KM

Optical Fiber – Parts

Three concentric sections: Core

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

Cladding Less refractive Glass/plastic coating around core Optical property different than core Reflect the light back into the core that tries to escape

Jacket Protects against moisture, abrasion, crushing and other damages May be bundling a number of fibers

Optical Fiber – Parts

Optical Fiber – Parts

Optical Fiber – Operation

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: 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

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 Act as wave guide for 1014 to 1015 Hz frequencies

Portions of infrared and visible spectrum Light rays created through LED or ILD Light Emitting Diode (LED)

Cheaper Wider operating temp range Last longer

Injection Laser Diode (ILD) More efficient Greater data rate

Fiber Optic – Pros

Greater Capacity 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

10s of KM at least

Fiber Optic – Cons

Installation! Maintenance is also difficult Cost Specialized Equipment and operating

Personnel Uni-directional Propagation

Light from one side can travel in a fiber Solution: Two fibers could be used

Fiber Optic – Applications

Long haul trunks 1500 KM, 20 – 60 thousand voice channels

Metropolitan trunks 12 KM, 100,000 voice channels

Rural exchange trunks 400 – 60 KM, 5000 channels

Subscriber loop Replacing STP/UTP and Coaxial

LAN

Fiber Optic – Transmission Modes Two modes of light propagation

Multimode Step Index Graded-index

Single mode Different modes operate on fiber bearing

different characteristics

Fiber Optic – Transmission Modes

Fiber Optic – Transmission Modes Multimode

Multiple beams from source to destination Two types

Step Index Graded Index

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

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

Fiber Optic – Transmission Modes Multimode: Graded Index

Density of core varies from center to edges Center is more dense while density increases towards edges

Smooth change in density reflect rays back smoothly Low Distortion

Fiber Optic – Transmission Modes Single Mode

Uses fiber like step index Density of core is constant

Very small diameter fiber 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

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

Radio 1GHz – 40GHz

Microwaves 3x1011 – 2x1014 Hz

Infrared

Unguided Media

Electromagnetic Spectrum for Wireless Communication

Antenna

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 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

Two Common types Omni-Directional Antenna

An antenna which radiate power equally in all directions Usually not possible Isotropic antenna is assumed which radiate power

equally in all directions Actual radiation pattern for the isotropic antenna is a

sphere with antenna at the center Directional Antenna

Further Different Types Parabolic Reflective Antenna Highly Directional Antenna etc

Types of Antenna

Directional Antenna (Cont) Parabolic

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

Measure of Efficiency of Antenna Measure of Directionality of Antenna

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

Signal radiated from antenna travels along one of three routes Ground ware Propagation Sky Wave Propagation Line of Sight Propagation

Wireless Propagation

Frequency Bands Vs Propagation and Use

Wireless Propagation

Ground Wave Propagation Follows almost the contour of earth Uses frequencies up to 2MHz Several factors involved in such movement

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

Sky Propagation Used for amateur radio Signal from earth-based antenna is reflected from the

ionized layer of the upper atmosphere (ionosphere) back down to earth 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

Line of Sight Propagation 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 Microwaves Bent due to refraction

Microwave Communication

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 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 Long haul telecommunication service

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 To connect two buildings in the same city To have wireless internet connection from some

ISP

Microwave – Terrestrial Communication: Transmission Characteristics Most common band for long-haul

telecommunications are 4 – 6GHz Congested

11GHz band is in use now For Short Range (Connecting Two Buildings)

22GHz band is utilized Attenuation is not problem in short distances

Microwave – Satellite

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 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 Match occurs at height of 35,863Km of equator

Microwave – Common Satellite Configuration

Microwave – Limitation in Satellite Two satellites using same frequency band

will interfere with each other if they come closer

To avoid 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 Television Distribution Long Distant Telephone

Transmission Private Business Networks

VSAT (Very Small Aperture Terminal)

Microwave – Satellite Transmission Characteristics Optimum Frequency range

1 – 10GHz Remember: Low frequency for longer distances, higher

attenuate Below 1GHz, lot of noises from natural sources

Most P-t-P satellites today use 5.925 – 6.425GHz for Upload and 3.7 – 4.2GHz for download Combination is called 4/6 Band

Transmission and Reception frequencies differ Otherwise interference will occur

Microwave – Satellite Transmission Characteristics 4/6GHz Band is in optimum zone but got saturated Other bands with 1 – 10GHz are not available

because of interferences Usually terrestrial devices operate on those

12/14GHz band is developed 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 Main difference in Microwave and Radio is

that Microwave is usually directional while radio is omni-directional

Radio Doesn’t require dish-shaped antennas Doesn’t need antennas to be mounted accurately

Radio – Application

“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 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 Ideal for broadcast transmission

Impairments are usually caused by Multi-path

Infrared Communication

Requires LOS May use reflected surface in the absence of LOS

Infrared cannot transfer through walls Microwave can!

More secure Communication in closed environments could not

be hacked from outside No Licensing required since no frequency

allocation issue

Problems

Free Space Loss Multi-path Refraction

Free Space Loss

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

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

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

What is IEEE802.x Write about IEEE Focus on 802 Standards defined under 802 Umbrella