hssn2-03-transmissionmedia

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Prepared by: KL First Prepared on: 13-12-04 Last Modified on: 24/07/2005

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Copyright 2004 Asia Pacific Institute of Information Technology

Hardware and Software Systems and Networks 2

CT024-3-2

Level-2


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CT014-1 Hardware, Systems Software and Networks Chapter 2 2

Topic & Structure of the lesson

Guided Media

Twisted-Pair Cable

Coaxial Cable

Fiber-Optical Cable

Unguided Media: Wireless

Radio Waves

Microwaves

Infrared


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Key Terms you must be able to

use

Angle of incidence

BNC connector

Cladding Core

Critical angle

Electromagnetic spectrum

Fiber-optic cable

Ground propagation

Guided media

Horn antenna

Infrared wave IrDA port

Line-of-sight propagation

Microwave

MT-RJ

Multimode graded-index

fiber

If you have mastered this topic, you should be able to use the following terms

correctly in your assignments and exams:-


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Key Terms you must be able to

use

Multimode graded-indexfiber

Multimode step-index fiber

Omnidirectional antenna

Optical fiber Parabolic dish antenna

Radio wave

Reflection

RG number

RJ45 Shielded twisted-pair (STP)

Single-mode fiber

Sky propagation

Straight-tip (ST) connector

Subscriber channel (SC)

connector Transmission media

Twisted-pair cable

Unguided medium

Unidirectional antenna

Unshielded twisted-pair(UTP)

Wireless communication


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Transmission medium and physical layer

Transmission mediaare actually located below

the physical layer and directly controlled by the

physical layer.

Signals are transmitted from one device toanother in the form of electromagnetic energy

Transmission media can be divided into two

broad categories: Guided mediainclude twisted-pair cable, coaxial

cable, and fiber-optic cable.

Unguidedmedium is usually air


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Transmission medium and physical layer


7/84CT014-1 Hardware, Systems Software and Networks Chapter 2 7

Classes of transmission media



Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

Guided Media



Twisted-Pair Cable

Its frequency range is suitable fortransmitting both dataand voice.

A twisted pair consists of two conductors(normally copper), each with its ownplastic insulation, twisted together

Interference(noise) and crosstalkmay

affect both wires and create unwantedsignals.

If the two wires are parallel, the effectofthese unwanted signals is not the same inboth wires.



Twisted-Pair Cable

Twisting makes it probable that both wires

are equally affectedby external influences.

The number of twists per unit of length(e.g., inch) determines the quality of the

cable; more twists mean better quality.



Twisted-pair cable



Unshielded versus Shielded Twisted-

Pair

The most common twisted-pair cable used

in communications is referred to as

unshielded twisted-pair (UTP).

Shielded twisted-pair (STP)cable has a

metal foil or braided-meshcovering that

encases each pair of insulated conductors.

Metal casing improves the quality of cable

by preventing the penetration of noise or

crosstalk, it is bulkier and more expensive.



UTP and STP



Categories of UTP

The Electronic Industries Association (EIA)

has developed standards to classify UTP

cable into seven categories

Categories are determined by cable

quality, with 1 as the lowest and 7 as the

highest.



Categories of unshielded twisted-pair cables

Category Bandwidth Data Rate Digital/Analog Use

1 very low < 100 kbps Analog Telephone

2 < 2 MHz 2 Mbps Analog/digital T-1 lines

3 16 MHz 10 Mbps Digital LANs



6 (draft) 200 MHz 200 Mbps Digital LANs

7 (draft) 600 MHz 600 Mbps Digital LANs



UTP connector



UTP connector & Performance

The most common UTP connector is RJ45

(RJ-Registered Jack)

The RJ45 is a keyed connector, meaningthe connector can be insertedin only one

way.

Measure the performanceof twisted-paircable is to compare attenuationversus

frequencyand distance.



UTP performance


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Coaxial cable

Coaxial cable (or coax) carries signals of higherfrequencyranges than twisted-pair cable.

Coax has central coreconductor of solid(copper) enclosed in an insulating sheath,which is, in turn, encased in an outer conductorof metal foil, braid, or a combination of the two.

The outer metallic wrapping serves both as ashield against noiseand as the second

conductor, which completes the circuit. This outer conductor is also enclosed in an

insulating sheath, and the whole cable isprotected by a plastic cover.


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Coaxial cable


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Coaxial cable

Coaxial cable are categorized by their

radio government(RG) ratings.

RG number denotes a unique set ofphysical specifications, including the wire

gaugeof the inner conductor, the

thicknessand type of the inner insulator,

the construction of the shield, and the size

and type of the outer casing.


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Categories of coaxial cables

Category Impedance Use

RG-59 75 Cable TV

RG-58 50 Thin Ethernet

RG-11 50 Thick Ethernet


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Coaxial Cable Connectors

To connect coaxial cable to devices, needcoaxial connectors.

The most common type of connector used todayis the Bayone-Neill-Concelman (BNC).

The BNC connector is used to connect the endof the cable to a device, such as a TV set.

The BNC T connectoris used in Ethernetnetworks to branch out a cable for connection to

a computer or other devices. The BNC terminatoris used at the end of the

cable to prevent the reflectionof the signal.


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BNC connectors


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Coaxial Cable Performance

The attenuationis much higher in coaxialcables than in twisted-pair cable.

Although coaxial cable has a much higher

bandwidth, the signal weakensrapidly andneeds the frequent use of repeaters.

Coaxial started in analog telephone

networks where a single coaxial networkcould carry 10,000 voice signals.

It can carry digital dataup to 600 Mbps.


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Coaxial cable performance


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Coaxial Application

Cable TV networks used coaxial cables.

The recent cable TV providers replaced most ofthe network with fiber-opticcable.

Hybridnetworks use coaxial cable only at thenetwork boundaries, near the consumerpremises.

10Base-2, or Thin Ethernet, uses RG-58 coaxial

cable with BNC connectors to transmit data at10 Mbpswith a range of 185 m.

Thick Ethernet, uses RG-11 (thick coaxial cable)to transmit 10 Mbpswith a range of 5000 m.


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Fiber-Optic Cable

A fiber-optic cable is made of glass or

plasticand transmits signals in the form of

light.

Light travels in a straightline as long as it

is moving through a single uniform

substance.

If a ray of light traveling through one

substance suddenly enters another (more

or less dense), the ray change direction.


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Bending of light ray


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Bending of light ray

The above figure shows, if the angle of

incidence is less thenthe critical angle, the ray

refractsand moves closer to the surface.

If the angle of incidence is equal to the criticalangle, the light bends along the interface.

If the angle is greater than the critical angle, the

ray reflects(make a turn) and travels again inthe denser substance.


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Optical fiber


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Opticalfiber

Optical fibers use reflectionto guide light

through a channel.

A class or plastic coreis surrounded by acladdingof less dense glass or plastic.

The difference in densityof the two

materials must be such that a beam of

light moving through the core is reflected

off the cladding instead of being refracted

into it.


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Propagation Modes

Current technology support two modes

(multimodeand single mode) for

propagating light along optical channels,

each requiring fiber with different physical

characteristics.

Multimode can be implemented in two

form:

Step-index

Graded-index


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Propagation modes


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Multimode, step-index fiber

Multimode is so named because multiple beamsfroma light source move through the core in different path.

In multimode step-index fiber, the density of the coreremains constant from the centre to the edges.

A beam of lightmoves through this constant densityina straight line until it reaches the interface of the coreand the cladding.

At the interface, there is an sudden change to a lower

density that altersthe angelof the beams motion. The term step indexrefers to the suddenness of thischange.


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Multimode graded-index fiber

It decreases this distortionof the signal

through the cable.

The word indexhere refers to the index ofrefractionwhich is related to density.

Density is highest at the center of the core

and decrease gradually to its lowest at the

edge.


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Single Mode

Single-mode uses step-index fiberand highlyfocused source of light that limitsbeams to asmall range of angles.

The single-mode fiber itself is manufacturedwith a much smaller diameterthan that ofmultimode fiber, and with substantially lowerdensity(index of refraction).

The decrease in density results in a criticalangle that is close enough to 900to make thepropagation of beams almost horizontal.


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Modes


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Fiber Sizes

Optical fibersare defined by the ratioof

the diameter of their core to the diameter

of their cladding, both expressed in

micrometers.


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Type Core Cladding Mode

50/125 50 125 Multimode, graded-index

62.5/125 62.5 125 Multimode, graded-index

100/125 100 125 Multimode, graded-index

7/125 7 125 Single-mode

Fiber types


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Fiber construction


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Cable Composition

The outer jacket is made of either PVC or

Teflon.

Inside the jacket are Kevlar strands to

strengthen the cable. Kevlar is a strong material used in the fabrication

of bulletproof vests.

Below the Kevlar is another plastic coating tocushion the fiber.

The fiber is at the center of the cable, and it

consists of cladding and core.


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Fiber-optic cable connectors


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Fiber-Optic Cable Connectors

Fiber-optic cable use three different type of

connectors.

subscriber channel (SC) connectoris used in

cable TV. It is uses a push/pull locking system. straight-tip (ST) connectoris used for

connecting cable to networking devices. It uses

a bayonet locking system and is more reliable

than SC.

MT-RJ is a new connector with the same size as

RJ45.


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Optical fiber performance


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Fiber-optic Performance

Attenuationis flatter than in the case of

twisted-pair cable and coaxial cable.

The performance is such that we need

fewerrepeaterswhen we use fiber-optic

cable.


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Fiber-optic Application

Fiber-optic cable is often found in

backbonenetworks because its wide

bandwidth is cost-effective.

Wave-division multiplexing(WDM) can

transfer data at a rate of 1600 Gbps.

Some cable TV companies use a

combination of optical fiber and coaxial

cable, thus creating a hybrid network.


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Advantages of Optical Fiber

Higher bandwidthCan support

dramatically higher bandwidths than either

twisted-pair or coaxial cable.

Less signal attenuationtransmission

distance is significantly greater than that of

other guided media. A signal can run for

50 km without requiring regeneration. Weneed repeaters every 5 km for coaxial or

twisted-pair cable.


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Advantages of Optical Fiber

Immunity to electromagnetic interference

Electromagnetic noise cannot affect fiber-optic

cables.

Resistance to corrosive materialsGlass ismore resistant to corrosive materials than

copper.

Light weightMuch lighter than copper cables.

More immune to tappingMore immune to

tapping than copper cables. Copper cables

create antennas that can easily be tapped.


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U id d M di Wi l


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Radio Waves

Microwaves

Infrared



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Unguided media transport electromagnetic

waves without using a physical conductor;

often referred to as wireless

communication.

Signals are normally broadcast through air

and thus are available to anyone who has

a device capable of receiving them.

Electromagnetic spectrum for wireless


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Electromagnetic spectrum for wireless

communication


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Unguided Signal

Unguided signals can travel from the

source to destination in several ways.

Ground propagation

Sky propagation

Line-of-sight propagation


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Propagation methods


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Propagationmethods

In ground propagation, radio waves travel

through the lowest portion of the

atmosphere, hugging the earth.

These low-frequency signals emanate in

all directions from the transmitting antenna

and follow the curvature of the planet.

Distance depends on the amount of power

in the signal: The greater the power, the

greater the distance.


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Propagationmethods

Sky propagation, higher-frequency radio

waves radiate upward into the ionosphere

(the layer of atmosphere where particles

exist as ions) where they are reflectedback to earth.

This type of transmission allows for

greater distances with lower power output.


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Propagationmethods

In line-of-sight propagation, very high-

frequency signals are transmitted in

straight lines directly from antenna to

antenna.

Antennas must be directional, facing each

other, and either tall enough or close

enough together not to be affected by thecurvature of the earth.


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Bands

The electromagnetic spectrumdefined as

radio waves and microwaves is divided

into eight ranges. Called bands, each

regulated by government authorities.

These bands are rates from very low

frequency (VLF) to extremely high

frequency (EHF).

B d


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Bands

Band Range Propagation Application

VLF 330 KHz Ground Long-range radio navigation

LF 30300 KHz GroundRadio beacons and

navigational locators

MF 300 KHz

3 MHz Sky AM radio

HF 330 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF 30300 MHzSky and

line-of-sight

VHF TV,

FM radio

UHF 300 MHz3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF 330 GHz Line-of-sight Satellite communication

EHF 30300 GHz Line-of-sight Long-range radio navigation


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Wireless transmission waves


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Radio Waves

Electromagnetic waves ranging in frequencies between3 KHz and 1 GHz are normally call radio waves.

Wave ranging in frequencies between 1 and 300 GHzare called microwaves.

Radio waves use omnidirectional. When an antennatransmits radio waves, they are propagated in alldirections.

Radio waves that propagate in the sky mode, can travellong distances such as AM radio.

The disadvantageof radio waves is transmitted by oneantenna are susceptible to interferenceby anotherantenna that may send signals using the samefrequency or band.


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Omnidirectional antennas


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Application of Radio waves

The omnidirectional characteristics of

radio waves make them useful for

multicasting, in which there is one sender

but many receivers.

AM and FM radio, television, cordless

phones, and paging are examples of

multicasting.

R di


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Radio wavesare used for multicast

communications, such as radio and

television, and paging systems.

Note:

Radio waves


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Microwaves

Electromagneticwaves having frequenciesbetween 1 and 300 GHzare calledmicrowaves.

Microwaves are unidirectionalwhich ispropagation is line-of-sight.

When an antenna transmits microwave

waves, they can be narrowly focused. The advantage of antennas can be aligned

without interferingwith another pair ofaligned antennas.


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Disadvantage of Microwaves

The curvatureof the earth as well asother blocking obstacles do not allow twoshort towersto communicate using

microwaves . Repeatersare often needed for long-

distance communication.

Very high-frequencymicrowaves cannotpenetrate walls.

The microwave bandis relatively wide,almost 299 GHz.


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Unidirectional Antennas

Microwaves need unidirectional antennas

that send out signals in one direction.

Two type of antennas used

Parabalic dish

Horn


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Parabolic Dish

A parabolic dish antenna is based on thegeometryof a parabola.

Every line parallel to the line of symmetry

(line of sight) reflects off the curve atangles such that all the lines intersect in acommon point called the focus.

The parabolic disk works as a funnel,catching a wide range of waves anddirecting them to the common point.


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Horn Antenna

Looks like a gigantic scoop.

Outgoing transmissions are broadcast up

a stream (resembling a handle) and

deflected outward in a series of narrow

parallel beams by the curved head.

Received transmissions are collected by

the scooped shape of the horn.

Unidirectional antennas


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Unidirectional antennas

Microwaves


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Microwaves are used for unicast

communication such as cellular

telephones, satel l i te networks, and

wireless LANs.

Note:

Microwaves


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Infrared (IrDA)

Infrared signals, with frequencies from

(300 GHz to 400 THz, can be used for

short-range communication.

Infrared signals, having high frequencies,

cannot penetrate walls.

The IrDA standard originally defined a

data rate of 75 Kbps for distance up to 8

m. The recent standard defines a data rate

of 4 Mbps.


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Advantages and Disadvantages of Infrared

The advantages of infrared

A short-range communicationsystem in one room

cannot be affected by another system in the next

room.

When we use our infrared remote control, we do not

interferewith the use of the remote by our neighbors.

The disadvantages of infrared

It is useless for long rangecommunication. Can not use outside a building because the suns

rays contain infrared waves that can interferewith the

communication.

Infrared (IrDA)


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I nf rared signalscan be used for short-

range communication in a closed area

using line-of-sight propagation.

Note:

Infrared (IrDA)


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Quick Review Questions

Refer to tutorial question.


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Summary of Transmission Media

Transmission media lie below the physical layer.

A guided medium provides a physical conduitfrom one device to another.

Twisted-pair cable, coaxial cable, and opticalfiber are the most popular types of guidedmedia.

Twisted-pair cable consists of two insulated

copper wires twisted together. Twisting allowseach wire to have approximately the same noiseenvironment.

Twisted-pair cable is used in telephone lines for

voice and data communications.

Summary of Main Teaching


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Summary of Main Teaching

Points

Coaxial cable has the following layers (startingfrom the centre): a metallic rod-shaped innerconductor, an insulator covering the rod, a

metallic outer conductor (shield), an insulatorcovering the shield, and a plastic cover.

Coaxial cable is used in cable TV networks andtraditional Ethernet LANs.

Fiber-optic cable are composed of a glass orplastic inner core surrounded by cladding, allencased in an outside jacket.



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Fiber-optic cables carry data signals in the form

of light. The signal is propagated along the inner

core by reflection.

Fiber-optic transmission is becomingincreasingly popular due to its noise resistance,

low attenuation, and high-bandwidth capabilities.

Signal propagation in optical fibers can be

multimode (multiple beam from a light source) or

single-mode (essentially one beam from a light

source).


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In multimode step-index propagation, the core

density is constant and the light beam charges

direction suddenly at the interface between the

core and cladding. In multimode graded-index propagation, the core

density decreases with distance from the centre.

This causes a curving of the light beams.

Fiber-optic cable is used in backbone networks,

cable TV networks, and Fast Ethernet networks.


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Unguided media (usually air) transport

electromagnetic waves without the use of a

physical conductor.

Wireless data are transmitted through groundpropagation, sky propagation, and line-of-sight

propagation.

Wireless data can be classified as radio waves,

microwaves, or infrared waves.

Radio waves are omnidirectional. The radio

wave band is under government regulation.

S f


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Microwaves are unidirectional;propagation is line of sight. Microwavesare used for cellular phone, satellite, and

wireless LAN communications. The parabolic disk antenna and the horn

antenna are used for transmission andreception of microwaves.

Infrared waves are used for short-rangecommunications such as those between aPC and a peripheral device.

Q i d A S i


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Q & A

Question and Answer Session

N t S i


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Next Session

Circuit Switching and Telephone Network

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