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Transmission Media

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Media

Basic function of mediacarry flow of information in form ofbits through a LAN

In a copper based network, bits will be electrical signals

In a fiber based network, bits will be light pulses

Media considered to be Layer 1 component of a LAN Physical path between transmitter and receiver

Wired and Wireless

Communication is in the form of electromagnetic waves

Characteristics and quality of data transmission are determinedby characteristics of medium and signal

In wired media, medium characteristics is more important,whereas in wireless media, signal characteristics is moreimportant

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Transmission Media

Physical path between transmitter and receiver

Wired and Wireless

Communication is in the form of electromagnetic waves Characteristics and quality of data transmission are determined

by characteristics of medium and signal

In wired media, medium characteristics is more important,

whereas in wireless media, signal characteristics is moreimportant

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Electromagnetic Spectrum

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Basic limitations

Attenuation

Delay Distortion

Noise Thermal/White Noise

Intermodulation Noise

Crosstalk

Echo

Impulse Noise

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Design Factors

for Transmission Media Bandwidth: All other factors remaining constant, the greater the

band-width of a signal, the higher the data rate that can be

achieved.

Transmission impairments. Limit the distance a signal can travel.

Interference: Competing signals in overlapping frequency bands

can distort or wipe out a signal.

Number of receivers: Each attachment introduces some

attenuation and distortion, limiting distance and/or data rate.

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Transmission media

Guided Unguided

Twisted-pair

cable

Coaxial

cable

Fiber-optic

cable

Conducted or guided media use a conductor such as a wire or a fiber optic cable to

move the signal from sender to receiver

Wireless or unguided media use radio waves of different frequencies and do not need

a wire or cable conductor to transmit signals

Classes of Transmission Media

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Wire Conductors

Wire types:single conductor, twisted pair, &shieldedmulticonductor bundles.

Large installed base.

Reasonable cost.

Relatively low bandwidth, however, recent LAN speedsin the 100 Mbps range have been achieved.

Susceptible to external interference.

Shielding can reduce external interference.

Can transmit both analog and digital signals. Amplifierrequired every 5 to 6 km for analog signals. For digitalsignals, repeaters required every 2 to 3 km.

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A metric for copper cables

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Wired - Twisted Pair

The oldest, least expensive, and most commonly used

media

Pair of insulated wires twisted together to reduce

susceptibility to interference : ex) capacitive coupling,crosstalk

Skin effect at high frequency

Up to 250 kHz analog and few Mbps digital signaling ( for

long-distance point-to-point signaling) Need repeater every 2-3 km (digital), and amplifier every

5-6 km (analog)

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Consists of two insulated copper wires arranged in a regular spiral pattern to

minimize the electromagnetic interference between adjacent pairs Often used at customer facilities and also over distances to carry voice as well

as data communications

Low frequency transmission medium

Telephone (subscriber loop: between house and local exchange)

High-speed (10 - 100 Mbps) LAN : token ring, fast - Ethernet

Twisted Pair

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Types of Twisted Pair

STP (shielded twisted pair)

the pair is wrapped with metallic foil or braid to insulate

the pair from electromagnetic interference UTP (unshielded twisted pair)

each wire is insulated with plastic wrap, but the pair is

encased in an outer covering

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Ratings of Twisted Pair Category 3

UTP cables and associated connecting hardware whosetransmission characteristics are specified up to 16 MHZ.

data rates of up to 16mbps are achievable

Category 4

UTP cables and associated connecting hardware whose

transmission characteristics are specified up to 20 MHz. Category 5

UTP cables and associated connecting hardware whosetransmission characteristics are specified up to 100 MHz.

data rates of up to 100mbps are achievable

more tightly twisted than Category 3 cables more expensive, but better performance

Category 5 enhanced, Cat 6, cat 7 Fast and giga-ethernet

STP More expensive, harder to work with

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Twisted Pair Advantages

Advantages

Inexpensive and readily available

Flexible and light weight

Easy to work with and install

Disvantages

Susceptibility to interference and noise

Attenuation problem

For analog, repeaters needed every 5-6km

For digital, repeaters needed every 2-3km

Relatively low bandwidth (MHz)

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Twisted-Pair Cable

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Effect of Noise on Parallel Lines

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Noise on Twisted-Pair Lines

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Unshielded Twisted-Pair Cable

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Shielded Twisted-Pair Cable

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Wired Transmission Media

Coaxial Cable Most versatile medium

=> LANs, Cable TV, Long-distance telephones, VCR-to-TVconnections

Noise immunity is good Very high channel capacity

=> few 100 MHz / few 100 Mbps

Need repeater/amplifier every few kilometer or so (about the sameas with twisted pair)

Has an inner conductor surrounded by a braided mesh

Both conductors share a common center axial, hence the termco-axial

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Coaxial cable

Signal and ground wire Solid center conductor running coaxially inside a solid

(usually braided) outer circular conductor.

Center conductor is shielded from external interferencesignals.

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Properties of coaxial cable

Better shielding allows for longer cables and higher transfer rates.

100 m cables

1 to 2 Gbps feasible (modulation used)

10 Mbps typical

Higher bandwidth

400 to 600Mhz up to 10,800 voice conversations

Can be tapped easily: stations easily added (pros and cons)

Much less susceptible to interference than twisted pair

Used for long haul routes by Phone Co. Mostly replaced now by optical fiber.

High attenuation rate makes it expensive over long distance

Bulky

Basebandvs. broadbandcoax.

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Wired Transmission Media

Optical Fiber Flexible, thin (few to few hundred m), very pure

glass/plastic fiber capable of conducting optical rays

Extremely high bandwidth : capable of 2 Gbps

Very high noise immunity, resistant to electromagneticinterference

Does not radiate energy/cause interference

Very light

Need repeaters only 10s or 100 km apart

Very difficult to tap : Better security but multipoint not easy

Require a light source with injection laser diode (ILD) orlight-emitting diodes (LED)

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Wired Transmission Media

Optical Fiber (Contd)

Need optical-electrical interface (more expensive than

electrical interface)

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Wired Transmission Media

Principle of optical fiber transmission: Based on the principle of totalinternal reflection

If >, medium B (water) has a higher optical density than medium A

(air) In case the index of refraction), if is less than a certain

critical angle, there is no refracted light i.e., all the light is reflected.This is what makes fiber optics work.

Optical Fiber

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Fiber optics & Physics 101

Refractive indexmaterial=

(Speed of light in vacuum)/(Speed of light inmaterial)

Light is bent as it passes through a surface wherethe refractive index changes. This bending depends

on the angle and refractive index. Frequency doesnot change, but because it slows down, the wavelength gets shorter, causing wave to bend.

In case of fiber optic media, refractive index of core

> refractive index of cladding thereby causinginternal reflection.

core

cladding

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plastic jacket glass or plastic

claddingfiber core

Fiber Optic Layers

consists of three concentric sections

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Modes of fiber Fiber consists of two parts: theglass coreandglass

claddingwith a lower refractive index. Light propagates in 1 of 3 ways depending on the type and

width of the core material.

Multimode stepped index fiber

Both core and cladding have different but uniform refractive

index. Relies on total internal reflection; Wide pulse width.

Multimode graded index fiber

Core has variable refractive index (light bends as it movesaway from core).

Narrow pulse width resulting in higher bit rate.

Singlemode fiber (> 100 Mbs)

Width of core diameter equal to a single wavelenth.

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Mode

Multimode Single mode

Step index Graded-index

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Fiber Optic Types

multimode step-index fiber

the reflective walls of the fiber move the light pulses tothe receiver

multimode graded-index fiber

acts to refract the light toward the center of the fiber byvariations in the density

single mode fiber the light is guided down the center of an extremely

narrow core

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Types of optical fiber Modes, bundles of light rays enter the fiber at a particular

angle

Single-mode

Also known as mono-mode

Only one mode propagates through fiber

Higher bandwidth than multi-mode

Longer cable runs than multi-mode

Lasers generate light signals

Used for inter-building connectivity

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Types of optical fiber

Multi-mode

Multiple modes propagate through fiber

Different angles mean different distances to travel

Transmissions arrive at different times

Modal dispersion LEDs as light source

Used for intra-building connectivity

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fiber optic multimode

step-index

fiber optic multimode

graded-index

fiber optic single mode

Fiber Optic Signals

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Optical Fiber Transmission Mode

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Fiber Optic

Advantages

greater capacity (bandwidth Gbps)

smaller size and lighter weight

lower attenuation

immunity to environmental interference

highly secure due to tap difficulty and lack of signal radiation

Disvantages

expensive over short distance

requires highly skilled installers

adding additional nodes is difficult

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Fiber Channel Requirements

Full duplex links with 2 fibers/link 100 Mbps800 Mbps Distances up to 10 km Small connectors high-capacity Greater connectivity than existing multidrop channels Broad availability Support for multiple cost/performance levels

Support for multiple existing interface command sets

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Components of anoptical transmission system

3 components

1. Light source

2. Transmission medium

3. The detector Light means a 1 bit, no light means a 0 bit.

Transmitter LED or injection laser diode.

Detector (photodiodeorphoto transistor) generates anelectrical pulse when light falls on it.

Unidirectional data transmission system.

Electrical signal to light signal and back again.

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Fiber cables

Multimode: diameter of core is ~50 microns.

About the same as a human hair.

Single mode: diameter of core 8-10 microns.

They can be connnected by connectors, or by splicing, or byfusion.

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Fiber vs. copper

Fiber (pros)

Higher bandwidth,

Lower attenuation,

Immune to electromagnetic noise and corrosive chemicals,

Thin and lightweight,

Security (does not leak light, difficult to tap).

Fiber (cons)

Not many skilled fiber engineers,

Inherently unidirectional,

Fiber interfaces are expensive.

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Wireless (Unguided Media)Transmission

transmission and reception are achieved by means of an

antenna

directional transmitting antenna puts out focused beam

transmitter and receiver must be aligned

omnidirectional

signal spreads out in all directions

can be received by many antennas

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The Radio Spectrum

Radio wave Wavelength l= c/f

Speed of light c=3x108

m/s

Frequency: f

ftAts 2cos)(

l

f

f = 900 MHz l = 33 cm

[V|U|S|E]HF = [Very|Ultra|Super|Extra] High Frequency

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Atmospheric Transmission Media

Infrared Transmission

Infrarednetworks use infrared light signals to transmitdata

Direct infrared transmissiondepends on transmitterand receiver remaining within line of sight

In indirect infrared transmission, signals can bounceoff of walls, ceilings, and any other objects in their path

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Atmospheric Transmission Media

RF Transmission

Radio frequency (RF)transmission relies on

signals broadcast over specific frequencies

Narrowband concentrates significant RF energy ata single frequency

Spread spectrumuses lower-level signalsdistributed over several frequencies simultaneously

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Infrared

For short-range communication

Remote controls for TVs, VCRs and stereos

IRD port

Indoor wireless LANs

Do not pass through solid walls

Better security and no interference (with a similar system in

adjacent rooms) No government license is needed

Cannot be used outdoors

Wireless Transmission

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Infrared

Transceivers must be within line of sight of each other (directlyor via reflection)

Unlike microwaves, infrared does not penetrate walls

Fairly low bandwidth (4 Mbps). Uses wavelengths between microwave and visible light.

Uses transmitters/receivers (transceivers) that modulatenoncoherent infrared light.

No frequency allocation issue since not regulated. Uses include local building connections, wireless LANs, and new

wireless peripherals.

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Infrared Waves

Short range communication.

e.g. Remotes on VCRs and TVs.

Directional.

Do not pass through walls.

Behaves more like visible light.

Can be used for LANs

indoors only.

Can just use visible unguided light (lasers).

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2GHz to 40GHz

Microwave

Highly directional Point to point

Satellite

30MHz to 1GHz

Omnidirectional

Broadcast radio

3 x 1011to 2 x 1014

Infrared

Wireless Transmission

Frequencies

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Wireless Transmission

Parabolic dish

Focused beam

Line of sight

Long haul telecommunications

Higher frequencies give higher data rates

Terrestrial Microwave

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Terrestrial Microwave

used for long-distance telephone service

uses radio frequency spectrum, from 2 to 40 Ghz

parabolic dish transmitter, mounted high used by common carriers as well as private networks

requires unobstructed line of sight between source and

receiver curvature of the earth requires stations (repeaters) ~30

miles apart

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Radio Transmission

Radio waves

Easy to generate, travel long distances, and penetrate buildingseasily.

Omnidirectional. Low frequencies

Pass through obstacles well,

Quick power drop off (e.g. 1/r3 in air).

High frequencies

Travel in straight lines and bounce off obstacles.

Absorbed by rain.

Subject to electrical interference

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Media: Broadcast Radio

Covers 30MHz to 1 GHz

Omindirectional

Enables mobilecommunication & computing!

Broadcast mechanisms: cellular radio, radio nets, & low-orbit

satellites. Low bandwidth.

Lack of security.

Susceptible to interference (primarily multipath interference).

Reallocation of limited frequencies may be required for wirelesscommunication growth.

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Microwave transmission

Microwave waves

Travel in straight lines and thus can be narrowly focused.

Easy to avoid interference with other microwaves. Parabolic antenna is used to concentrate the energy

(improves SNR).

More popular before fiber.

Waves do not pass through buildings.

Multiple towers used as repeaters.

M di T i l

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Media: TerrestrialMicrowave

High bandwidth (~45 Mbps).

No cabling between sites.

Clear line-of-sight required (30 miles).

Susceptible to radio interference. Attenuation increases with rainfall

Lack of security.

Up-front investment in towers & repeaters.

Low power used to minimize effects on people line of sight requirement

expensive towers and repeaters

subject to interference such as passing airplanes and rain

Propagation Types

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Propagation Types

Wi l T i i

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Satellite Microwave Satellite is relay station Satellite receives on one frequency, amplifies or repeats

signal and transmits on another frequency

Requires geo-stationary orbit Height of 35,784km

Optimum transmission in 1 - 10 GHz range;

Bandwidth of 100s MHz

Significant propagation delay (270 ms)

Application: Television, long distance telephone, Privatebusiness networks

Wireless Transmission

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dishdish

uplink station downlink station

satellite

transponder

22,300 miles

Satellite Transmission Process

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Satellite TransmissionApplications

television distribution

a network provides programming from a central location

direct broadcast satellite (DBS) long-distance telephone transmission

high-usage international trunks

private business networks

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Principal Satellite TransmissionBands

C band: 4(downlink) - 6(uplink) GHz

the first to be designated

Ku band: 12(downlink) -14(uplink) GHz rain interference is the major problem

Ka band: 19(downlink) - 29(uplink) GHz

equipment needed to use the band is still very expensive

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Physical media and theirapplications

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Physical Media

physical link:transmitted

data bit propagates across

link

guided media:

signals propagate in solid

media: copper, fiber

unguided media:

signals propagate freely e.g.,radio

Twisted Pair (TP) two insulated copper wires

Category 3: traditional

phone wires, 10 Mbps

ethernet

Category 5 TP: 100Mbps

ethernet

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Physical Media: coax, fiber

Coaxial cable: wire (signal carrier) within a

wire (shield)

baseband: single channel on

cable broadband: multiple channel

on cable

bidirectional

common use in 10MbsEthernet

Fiber optic cable: glass fiber carrying light pulses

high-speed operation:

100Mbps Ethernet

high-speed point-to-pointtransmission (e.g., 5 Gbps)

low error rate

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Physical media: radio

signal carried inelectromagnetic spectrum

no physical wire

bidirectional

propagation environmenteffects:

reflection

obstruction by objects

interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN(e.g., waveLAN) 2Mbps, 11Mbps, 54 Mbps

wide-area(e.g., cellular) e.g. CDPD, 10s Kbps

satellite up to 50Mbps channel (or

multiple smaller channels)

270 msec end-end delay geosynchronous versus

LEOS

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IEEE 1394 Fi i

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IEEE 1394 Firewire

28

AWG

22AWG

400 Mb/s over 4.5 m

IEEE 1394b

800, 1600, 3200 Mb/s over POF

3.2 Gb/s over glass fibre

100 Mb/s over UTP

Universal Serial Bus

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Universal Serial Bus

1.5, 12 or 480 Mb/s, up to 5 m,

cascade 5 devices up to 30 m

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Fiber vs Satellite

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Choosing the Right TransmissionMedia

Areas of high EMI or RFI

Corners and small spaces

Distance Security

Existing infrastructure

Growth

Media: Selection

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Media: SelectionCriteria

Cost (Initial, Expansion, & Maintenance) Speed (Data Rate & Response Time)

Availability

Expandability

Error Rates

Security

Distance (Geography & Number of Sites)

Environment

Application-Specific Constraints

Maintenance

Media: Selection

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Criteria

Media Advantages DisadvantagesTwisted PairWire

Inexpensive.Easy to install.Large installed base.

Fairly low bandwidth.Does not handle highfrequencies well.

Coaxial Cable Fairly inexpensive.

Fairly high bandwidth.

Bulky and somewhat

inflexible.Fiber OpticCable

Unaffected by noise.

Very high bandwidth.Expensive to install.

Satellite No line of sight needed.No cabling required.High bandwidth.

Leased channels.Initial equipment cost.Long delays (GEOS).

TerrestrialMicrowave

No cabling required.High bandwidth.

Line of sight required.Towers/repeaters needed.Initial equipment cost.

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From Signals to Packets

Analog Signal

Digital Signal

Bit Stream 0 0 1 0 1 1 1 0 0 0 1

Packets0100010101011100101010101011101110000001111010101110101010101101011010111001

Header/Body Header/Body Header/Body

ReceiverSenderPacket

Transmission

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Modulation Sender changes the nature of the signal in a way that the

receiver can recognize. Similar to radio: AM or FM

Digital transmission: encodes the values 0 or 1 in the signal.

It is also possible to encode multi-valued symbols

Amplitude modulation: change the strength of the signal,typically between on and off.

Sender and receiver agree on a rate

On means 1, Off means 0

Similar: frequency or phase modulation. Can also combine method modulation types.

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Amplitude and FrequencyModulation

0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 0

0 1 1 0 1 1 0 0 0 1

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Assumptions

We use two discrete signals, high and low, to encode 0

and 1

The transmission is synchronous, i.e., there is a clockused to sample the signal

In general, the duration of one bit is equal to one or more

clock ticks

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Encoding

Goal: Send bits from one node to another node on the

samephysical media

Problem: Specify a robustand efficientencodingscheme to achieve this goal

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Encoding Schemes

Non Return to Zero (NRZ)

Non Return to Zero Inverted (NRZI)

Manchester Encoding 4B/5B Encoding

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Modulation

Non-Return to Zero (NRZ)

Used by Synchronous Optical Network (SONET)

1=high signal, 0=low signal Long sequence of same bit cause difficulty

DC bias hard to detectlow and high detected by

difference from average voltage

Clock recovery difficult

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Show the NRZ encoding for thefollowing pattern

NRZ

clock

1000100011111001Bits

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Modulation

Non-Return to Zero Inverted (NRZI)

1=inversion of current value, 0=same value

No problem with string of 1s NRZ-like problem with string of 0s

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Show the NRZI encoding for thefollowing pattern

NRZI

clock

1000100011111001Bits

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Modulation

Manchester

Used by Ethernet

1=low to high transition, 0=high to low transition Transition for every bit simplifies clock recovery

Not very efficient

Doubles the number of transitions

Circuitry must run twice as fast

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Modulation

4b/5b

Used by FDDI

Uses 5bits to encode every 4bits Encoding ensures no more than 3 consecutive 0s

Uses NRZI to encode resulting sequence

16 data values, 3 special illegal values, 6 extra

values, 7 illegal values

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Chapter Summary

Information can be transmitted via analog or digitally

Both signals suffer attenuation

Throughput is the amount of data a medium can transmit during a givenperiod of time

Costs depend on many factors

Three specifications dictating networking media

Length of a network segment is limited due to attenuation

Connectors connect wire to the network device

Coaxial cable consists of central copper core surrounded by an insulatorand a sheath

In baseband transmission, digital signals are sent through direct currentpulse applied to the wire

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Chapter Summary

Twisted-pair cable consists of color-coded pairs of insulated copper wires,

twisted around each other and encased in plastic coating

The more twists per inch in a pair of wires, the more resistant to noise

STP cable consists of twisted pair wires individually insulated andsurrounded by a shielding

UTP cabling consists of one or more insulated wire pairs encased in a

plastic sheath

UTP comes in a variety of specifications

Fiber-optic cable contains one or several glass fibers in its core

On todays networks, fiber is used primarily as backbone cable

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Chapter Summary

Best practice for installing cable is to follow theTIA/EIA 568 (see structured cabling) specificationsand manufacturers recommendations

Wireless LANs can use radio frequency (RF) or

infrared transmission

Infrared transmission can be indirect or direct

RF transmission can be narrowband or spreadspectrum

To make correct media transmission choices, consider,throughput, cabling, noise resistance,security/flexibility, and plans for growth