networks practicle

8/20/2019 networks practicle

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Practical no. 1

AIM: - Introduction to transmission media

The transmission medium is the physical path between the transmitter and the receiver ina data transmission system. Accordingly, the quality and characteristics of transmission

are determined by the characteristics of both the signal and the medium.ClassifcationA wide variety of media are available; however they all fall into two classes:

• Guided !ounded"

• #nguided$n both cases transmission ta%es place in the form of signals of one %ind or another such

as analog, digital and light pulse.

Guided Transmission MediaBy far the most common media employed for data transmission are the

guided ones. In such transmission media, the data signal is guidedalong a solid medium. That is, it is conned in a specic transmission

pathway. The transmission capacity is often expressed in terms of

either data rate or bandwidth. The bandwidth refers to the range of

frequencies that the medium can accommodate. The transmission

capacity depends critically on the distance and whether the medium is

point-to-point or multipoint. Copper twisted pair T!", coaxial cable and

optical ber are the most widely used guided transmission media. In

what follows, we will see each one of them in detail.

Twisted pair (TP)This is the most commonly used and the least e&pensive guided transmission medium. As

shown in fig '.(, it loo%s li%e a telephone wire and consists of two insulated copper wiresarranged in a regular spiral pattern.

Basic Structure of Twisted Pair (TP)

The wire pair acts as a single communication lin%. $n practice, a number of such pairs are bundled together into a cable by wrapping them in a tough protective sheath as shown infig '.'.


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Twisted Pair (TP) cableThe twists in the wire pairs are an important part of the electrical characteristics of T)cable. Twists reduce the cable*s sensitivity to outside electromagnetic interference +$"and the degree to which the cables radiate radio frequency signals. -emember that the

frequencies at which A/s operate fall into the range of radio signals. $f T) cable is

insufficiently twisted, it can function as an antenna and radiate significant amounts of radio signals that can interfere with local broadcast reception equipment. !esides, thetwisting reduces 0rosstal% interference signal lea%age" between ad1acent pairs in a cable.

Crosstalk occurs when signals from one line mix into another line2ence, neighboring pairs in a bundle typically have somewhat different twist lengths tominimi3e the 0rosstal% interference. Twist length varies from ' to 4 inches. 5ire si3e for

T)6s is measured in a unit called A5G American 5ire Gauge" based on its diameter, the

si3e being inverse to the rating. 7or e&ample, a '' A5G cable is thic%er than a '8 A5G

cable. T)6s of wire si3e '' and '8 A5G are most commonly used for data transmission.+ach wire in a T) cable is color9coded. range, blue, green, brown and white colors are

very often used. The white is used for the alternate wire in each pair. FurtherClassifcation o TP Twisted pair T)" can be further classified into two, namely:• #nshielded T) #T)"

• hielded T) T)"Unshielded Twisted-Pair (UTP)

#T) is the most popular type of twisted9pair cable and is fast becoming the most popular A/ cabling. The ma&imum cable length segment is (


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megabits per second". $t consists of four twisted9pairs.

• 0ategory

=

This category certifies #T) cable for data transmissions up to (< bps. $t

consists of four twisted9pairs with three twists per foot.

• 0ategory8 This category certifies #T) cable for data transmissions up to (4 bps. $tconsists of four twisted9pairs.

• 0ategory

This category certifies #T) cable for data transmissions up to (


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connector and jack

Coaxial Cable: Coaxial cable is a type of cable that is used by cable T and that is common for data communications. It carries signals ofhigher frequency ranges higher bandwidth" than twisted-pair cable. Itis also much less susceptible to interference $/I" and Crosstal0 and,more resistant to attenuation than twisted pair. %ttenuation is the lossof signal strength which begins to occur as the signal traels furtheralong a copper cable.

Attenuation causes signals to deteriorate

ome types of coa& have heavy mesh shields and center conductors to enhance thesecharacteristics and to e&tend the distances that signals can be transmitted reliably. As

shown in fig '.>, instead of having two wires, coa& has a central core conductor of solid

or stranded wire usually copper" enclosed in an insulating sheath, which is, in turn,encased in an outer conductor of metal foil, braid, or a combination of the two also

usually copper".

Coaxial cable showing arious la!ersAs you can see in this diagram, this cable is called coa&ial or coax for short" becausetwo conductors the center and outer mesh" share a 0ommon A&is. A typical coa&ial

cable has the following components:

• Center conductor Core". This conductor usually consists of a fairly heavy, solidyet fle&ible wire; stranded wires can also be used. $f the core is solid, it is usually copper.

olid conductors are preferred for permanent wiring, but stranded conductors ma%e thecable more fle&ible and easier to connect to equipment. The center conductor carries the

electronic signals which ma%e up the data.


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• Insulation layer. Also called a dielectric layer, this layer provides electricalinsulation and %eeps the inner and outer mesh" conductors in precise coa&ial

relationship. The conducting core and the wire mesh must always be separated from eachother. $f they touch, the cable will e&perience a short, and noise or stray signals on the

mesh will flow onto the copper wire. This will destroy the data.

• 1uter conductor or shield. This layer shields the inner conductor from outsideelectrical interference. The shield can consist of braided wires, metal foil, or a

combination of both. !ecause of this shield, coa& is highly resistant to electrical magnetic

interference +$".

• 2ac0et or sheath. A durable plastic or Teflon 1ac%et coats the cable to preventdamage.

(

"#$%& coaxial showing stranded wire and the solid co''er cores

Applications

0oa&ial cable can be used to transmit both analog and digital signals. $t can be effectively

used at higher frequencies and transmission rates. ome of the areas where it can be used

are:

• Television ?istribution9distribution of TC signals to homes 0able TC"

• ong distance Telephone Transmission• hort -un 0omputer in%s9high speed $D channels

• ocal area /etwor%ing2owever, 0oa& is sub1ect to the following constraints:

• Attenuation over long distances9repeaters may be required

• Thermal noise specially at higher frequencies• $nter9modulation noise during multiple&ed transmission

• 0oa& can be bul%y and hence difficult to wor% with.

• ore e&pensive than T)

Tpes o Coax

A wide variety of coa& cable is available. Eou must use cable that e&actly matches therequirements of a particular type of networ%. 0oa& cables vary in a measurement %nown

as the impedance measured in a unit called the ohm", which is an indication of thecable*s resistance to current flow. The specifications of a given cabling standard indicate

the required impedance of the cable. There are three types of coa&, namely:

• -G9> or Thinnet (

< ohms


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• -G9>D(( or Thic%net (

Therefore, because of thic0et6s ability to support data transfer oer

longer distances, it is sometimes used as a bac0bone to connect

seeral smaller Thinnet-based networ0s. % deice called a transceier

connects the Thinnet coaxial to the larger Thic0net coaxial cable.

Thicknet cable transceier with detail of a am'ire ta' 'iercing the core

A transceiver designed for Thic%net +thernet includes a connector %nown as a vampire

tap or a piercing tap to ma%e the actual physical connection to Thic%net core. Thisconnector is pierced through the insulating layer and ma%es direct contact with the

conducting core. 0onnection from the transceiver to the networ% adapter card is madeusing a transceiver cable drop cable" to connect to the attachment unit interface A#$"

port connector on the card. An A#$ port connector for Thic%net is also %nown as a7igital Intel 8erox ?$H" connector after the three companies that developed it and itsrelated standards, or as a ?!9( connector.


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As a general rule, the thic%er the cable, the more difficult it is to wor% with. Thin cable is

fle&ible, easy to install, and relatively ine&pensive. Thic% cable does not bend easily and

is, therefore, harder to install. This is a consideration when an installation calls for pullingcable through tight spaces such as conduits and troughs. Thic% cable is more e&pensive

than thin cable, but will carry a signal farther.

Coax Cabling Components

9 Connection hardware

!oth Thinnet and Thic%net use connection components, %nown as a !/0 !ritish /aval0onnector", to ma%e the connections between the cable and the computers. There are

several important components in the !/0 family, including the following:

• The B:C cable connector

The !/0 cable connector shown in fig '.(' is either soldered or crimped to the end of acable.

BC cable connector9 The B:C T connector

This connector, as shown in fig '.(=, 1oins the networ% interface card in the computer to

the networ% cable.

BC T connector

; 9 The B:C barrel connector

< This connector is used to 1oin two lengths of Thinnet cable to ma%e one longerlength.


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BC barrel connector

4 9 The B:C terminator

A !/0 terminator closes each end of the bus cable to absorb stray signals. 5ithout !/0

terminators, a bus networ% will not function.

BC terminator

Coaxial Considerations

0onsider these coa&ial capabilities when ma%ing a decision on the type of cabling to use.

#se coa&ial cable if you need:

( • A medium that will transmit voice, video, and data

' • To transmit data longer distances than less e&pensive cabling can transmit

= • A familiar technology that offers reasonable data security

The Fiber !ptic Cable:

7rom basic physics, we note that when light travels from one medium of to another medium of different density, there will be a change in the speed of propagation, which

results in change of direction, a phenomenon %nown as refraction.

"efraction occurs when light traels from one medium to another*hen light traels into a dense medium, the angle of refraction

becomes less than the angle of incidence and when light traels into a

less dense medium, the angle of refraction will be greater than the


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angle of incidence. *hen the angle of refraction becomes =ust aboe

>3 degrees the refracted ray will be totally re?ected at the boundary

into the second medium and will not leae the medium. This

phenomenon is 0nown as total internal re?ection. +o ber optic ber

uses this phenomenon to guide light pulses through it. @iber optic cableis composed of a number of such bers.

*iber$o'tic cable@iber optic cables utiliAe light waes to transmit data through a thin

glass or plastic ber. The structure of a typical ber optic cable is

shown in g ;.4. The parts of the cable are as follows

The light conductor is a ery ne ber core. lass is the most common

material, allowing signals to be transmitted for seeral 0ilometers

without being refreshed. !lastic is used in some circumstances as it is

easy to install, but plastic cables allow only short cable runs. $ach

glass strand passes signals in only one direction, so a cable consists of

two strands in separate =ac0ets. 1ne strand transmits and one

receies.

The cladding is a glass layer that surrounds the optical ber core. The

optical characteristics of the cladding re?ect light bac0 to the core,

ensuring that little of the light signal is lost.

% sheath or =ac0et protects the cable from damage. % single sheath

can be used to bundle multiple core&cladding bers into a multi-ber

cable.

The light signals on ber optic cables are generated either by light

emitting diodes D$7s" or by in=ection laser diodes ID7s", which are

similar to D$7s but produce laser light. The purity of laser light is

desirable, increasing both data rates and transmission distance.

+ignals are receied by photodiodes, solid-state deices that detectariations in light intensity. The interface deices required to operate

with ber optic cable are more expensie than those required for

copper cable. The higher cost is the result of seeral factors, including

cost of the components and tighter design characteristics because


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ber optic cables generally are operated at high data rates. The cost of

ber optic cable installation, howeer, is trending downward.

@iber optic cables hae many desirable characteristics. Because the

bers are small in diameter, a cable of a gien siAe can contain more

bers than copper wire pairs. Because ber optic cables use lightpulses instead of electrical signals, they oEer ery high bandwidth.

Bandwidths of 433 megabits million bits per second" are

commonplace, and bandwidths in the gigabit billion bit" per second

range are aailable.

Because the signal in a ber optic cable consists of light pulses, the

signal cannot be aEected by electromagnetic interference. :or can the

cables radiate radio frequency noise. 1ptical bers are, therefore,

suitable for use in the noisiest and most sensitie enironments.

Because these cables radiate no electromagnetic energy, it is

impossible to intercept the data signal with electronic eaesdropping

equipment. @iber optic transmissions are extremely secure.

Installation of ber optic cable requires greater s0ill than is necessary

to install most copper cables. Cables must not be bent too sharply, and

connectors must be installed by s0illed technicians using special tools.

Foweer, new connector technologies hae simplied installation and

reduced cost.

Fere are some adantages of ber optic cable

9 ery high bandwidth.

9 Immunity to $/IG ber optic cables can be used in enironments that

ma0e wire cables

unusable.

9 :o radio frequency emissionsG signals on ber optic cables cannot

interfere with nearby electronic deices and cannot be detected by

conentional electronic eaesdropping techniques.

"n#uided Transmission Media

#nguided transmission media is data signals that ?ow through the air.

They are not guided or bound to a channel to follow. The type of wae

propagation classies them. $F Propa#ation

There are three types of -7 radio frequency" propagation:

• Ground 5ave

• $onospheric


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• ine of ight "

round wae propagation follows the curvature of the +arth. Ground waves havecarrier frequencies up to ' 23. A radio is an e&ample of ground wave propagation.

Ionospheric propagation bounces off of the +arth*s $onospheric layer in the upperatmosphere. $t is sometimes called double hop propagation. $t operates in the frequency

range of =< 9 > 23. !ecause it depends on the +arth*s ionosphere, it changes with theweather and time of day. The signal bounces off of the ionosphere and bac% to earth. 2am

radios operate in this range.

Dine of sight propagation transmits e&actly in the line of sight. The receive stationmust be in the view of the transmit station. $t is sometimes called space waves or

troposphere propagation. $t is limited by the curvature of the +arth for ground9based

stations (


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The frequency spectrum operates from < 23 ?0" to gamma rays (

8-Hays43

4

#ltra-iolet Dight.' x 43

4'

isible Dight5.< x 43

45

Infrared Dight< x 43

44

$F@ - $xtremely Figh @requencies "

Hadar

+F@ - +uper Figh @requencies < FA +atellite K/icrowaes

#F@ - #ltra Figh @requencies


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depending on the local geography. Typically the line of sight due to the +arth*s curvature

is only < %m to the hori3onI -epeater stations must be placed so the data signal can hop,

s%ip and 1ump across the country.

icrowaves operate at high operating frequencies of = to (< G23. This allows them tocarry large quantities of data due to their large bandwidth.

Advantages:

a. They require no right of way acquisition between towers.

b. They can carry high quantities of information due to their high operating frequencies.

c. ow cost land purchase: each tower occupies only a small area.

d. 2igh frequencyDshort wavelength signals require small antennae.

?isadvantages:

a. Attenuation by solid ob1ects: birds, rain, snow and fog.

b. -eflected from flat surfaces li%e water and metal.

c. ?iffracted split" around solid ob1ects.

d. -efracted by atmosphere, thus causing beam to be pro1ected away from receiver.%atellite

atellites are transponders units that receive on one frequency and retransmit on another"

that are set in geostationary orbits directly over the equator. These geostationary orbitsare =4,


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The uplin% is the transmitter of data to the satellite. The downlin% is the receiver of data.#plin%s and downlin%s are also called +arth stations because they are located on the

+arth. The footprint is the JshadowJ that the satellite can transmit to, the shadow being

the area that can receive the satellite*s transmitted signal.

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