CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 19 ppt

Furthermore, the transmission of light on one fiber in a cable does not generate inter-ference that disturbs transmission on any other fiber, which means that fiber does not have the proble

Fiber-optic patch panels, as shown in Figure 3-31, are similar to the patch panels used

with copper cable These panels increase the flexibility of an optical network by

allow-ing quick changes to the connection of devices like switches or routers with various

available fiber runs (cable links)

Figure 3-31 Fiber-Optic Patch Panels

Signals and Noise in Optical Fiber

Fiber-optic cable is not affected by the sources of external noise that cause problems

on copper media Why? Because external light cannot enter the fiber except at the

transmitter end The cladding is covered by a buffer and an outer jacket that stops

light from entering or leaving the cable

Furthermore, the transmission of light on one fiber in a cable does not generate

inter-ference that disturbs transmission on any other fiber, which means that fiber does not

have the problem with crosstalk that copper media does In fact, the quality of fiber-optic

links is so good that the recent standards for Gigabit and 10-Gigabit Ethernet specify

transmission distances that far exceed the traditional 2-kilometer reach of the original

Ethernet (You learn more about the Ethernet technologies in Chapter 6, “Ethernet

Technologies and Ethernet Switching”.) Fiber-optic transmission allows the Ethernet

protocol to be used on metropolitan-area networks (MANs) and WANs

Although fiber is the best of all the transmission media at carrying large amounts of

data over long distances, fiber is not without problems When light travels through

fiber, some of the light energy is lost The farther a light signal travels through a fiber,

the more it loses strength This attenuation of the signal is due to several factors involving the nature of fiber itself The most important factor is scattering The scat-tering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy, as shown in Figure 3-32

Figure 3-32 Scattering

Absorptionis another cause of light energy loss When a light ray strikes some types of chemical impurities in a fiber, the impurities absorb part of the ray’s energy This light energy is converted to a small amount of heat energy Absorption makes the light signal

a little dimmer

Another factor that causes attenuation of the light signal is manufacturing irregularities

or roughness in the core-to-cladding boundary Power is lost from the light signal as a result of the less than perfect total internal reflection in that rough area of the fiber If there are any microscopic imperfections in the thickness or symmetry of the fiber, it cuts down on total internal reflection, and some light energy is absorbed by the cladding

Dispersion of a light flash limits transmission distances on a fiber Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber, as shown in Figure 3-33

Figure 3-33 Dispersion

Graded index multimode fiber is designed to compensate for the different distances the various modes of light have to travel in the large diameter core Single-mode fiber does not have the problem of multiple paths that the light signal can follow Chromatic

dispersion, however, is a characteristic of both multimode and single-mode fiber Some

wavelengths of light travel at slightly different speeds through glass than do other

wave-lengths This discrepancy causes chromatic dispersion That is why a prism separates

the wavelengths of light Ideally, an LED or laser light source emits light of just one

frequency Then, chromatic dispersion is not a problem

Unfortunately, lasers and, especially, LEDs generate a range of wavelengths so chromatic

dispersion limits the distance you can transmit on a fiber If you try to transmit a signal

too far, what started as a bright pulse of light energy is spread out, separated, and dim

when it reaches the receiver The receiver is not able to distinguish a 1 from a 0

Installation, Care, and Testing of Optical Fiber

A major cause of too much attenuation in fiber-optic cable is improper installation If

the fiber is stretched or curved too tightly, it can cause tiny fissures (cracks) in the core

that scatter the light rays Bending the fiber in too tight a curve can change the incident

angle of light rays striking the core-to-cladding boundary Then, the ray’s incident angle

becomes less than the critical angle for total internal reflection Instead of reflecting

around the bend, some light rays refract into the cladding and are lost

There are two types of bending:

■ Macrobending—A macrobend is a bend you can see When you bend fiber, you

can cause some of the light rays to exceed the critical angle, allowing light to leak out of the core and into the cladding When light is in the cladding, it cannot easily get back into the core; it then leaks out through the buffer, as shown in Figure 3-34

■ Microbending—Microbending produces the same effect as macrobending; it

causes the light to exceed the critical angle and leak out of the core, as shown in Figure 3-34 It occurs on a microscopic scale and is not visible to the eye

Figure 3-34 Macrobending and Microbending

Microbend

Macrobend

NOTE

Microbending can also be caused by extreme temperature swings in installed cable when the differ-ent materials in the cable structure expand and contract at different rates This expansion and con-traction causes the fiber to be squeezed

or stretched, which causes microbending.

To prevent fiber bends that are too sharp, fiber is usually pulled through a type of

installed pipe called interducting The interducting is much stiffer than fiber and

can-not be bent so sharply that the fiber inside the interducting has too tight a curve The interducting protects the fiber, makes it easier to pull the fiber, and ensures that the bending radius (curve limit) of the fiber is not exceeded

When the fiber has been pulled, the ends of the fiber must be cleaved (cut) and prop-erly polished to ensure that the ends are smooth Figure 3-35 illustrates the problems with improper fiber end face finishes, and Figure 3-36 illustrates the proper fiber end face polishing techniques

Figure 3-35 Fiber End Face Finishes

Figure 3-36 Fiber End Face Polishing Techniques

Flat Finish

Air Gap

Flat finish causes light to be reflected back into the fiber due to a step in the refractive index caused by the glass-air-glass interface.

Angled Finish

Angled Surfaces Touch

Angle polish connectors cause the reflection to exit the core and dissipate in the cladding.

Round Finish

Physical Contact

Physcial Contact (PC)finish minimizes back-reflection because of the very small refractive index discontinuity.

Ultra-Fine Round Finish

Physical Contact

Ultra polish connector finish uses several grades of polishing film to achieve an ultra-smooth surface.

Polishing Firm

Polishing Buyshing with Fiber and Connector Installed

Figure Eight Motion

Glass Plate Fiber Being Polished

A microscope or test instrument with a built-in magnifier is used to examine the end of

the fiber and verify that it is properly polished and shaped Then, the connector is

care-fully attached to the fiber end Improperly installed connectors, improper splices, or

the splicing of two cables with different core sizes dramatically reduces the strength of

a light signal Figure 3-37 illustrates the splicing of a 62.5 micron fiber to a 50 micron

fiber

Figure 3-37 Splicing of Different Types of Fiber

After the fiber-optic cable and connectors are installed, the connectors and the ends of

the fibers must be kept spotlessly clean The ends of the fibers should be covered with

protective covers to prevent damage to the fiber ends When these covers are removed

prior to connecting the fiber to a port on a switch or router, the fiber ends must be

cleaned Clean the fiber ends with lint-free lens tissue moistened with pure isopropyl

alcohol The fiber ports on a switch or router should also be kept covered when not in

use and cleaned with lens tissue and isopropyl alcohol before a connection is made

Dirty ends on a fiber cause a big drop in the amount of light that reaches the receiver

All these factors, scattering, absorption, dispersion, improper installation, and dirty

fiber ends diminish the strength of the light signal and are referred to as fiber noise

Before using a fiber-optic cable, it must be tested to ensure that enough light actually

reaches the receiver for it to detect the 0s (off) and 1s (on) in the signal

When a fiber-optic link is being planned, the amount of signal power loss that can be

tolerated must be calculated This tolerance is referred to as the optical link loss

bud-get It is like your monthly financial budbud-get After all your expenses (attenuations) are

subtracted from your initial income, enough money must be left to get you through the

month

Thedecibel (dB) is the unit used to measure the amount of power loss It tells what

percent of the power that leaves the transmitter actually enters the receiver

Testing fiber links is extremely important, and records of the results of these tests must

be kept Several types of fiber-optic test equipment are used Two of the most important

instruments are Optical Loss Meters and Optical Time Domain Reflectometers (OTDRs)

Light Lost Light Lost

62.5 Micron 50 Micron

These meters both test optical cable to ensure that the cable meets the TIA standards for fiber They also test to verify that the link power loss does not fall below the optical link loss budget OTDRs can provide a lot of detailed diagnostic information about a fiber link and can be used to troubleshoot a link when problems occur

Wireless Communications

Wireless signals are electromagnetic waves that can travel through the vacuum of outer space or through a medium such as air No physical copper-based or fiber-optic medium

is necessary for wireless signals, which makes utilizing wireless signals a very versatile way to build a network Wireless transmissions can cover large distances by using high-frequency signals Each signal uses a different frequency measured in hertz so that they remain unique from one another

Wireless technologies have been around for many years Satellite TV, AM/FM radio, cellular phones, remote-control devices, radar, alarm systems, weather radios, cordless phones, and retail scanners are integrated into everyday life Today, wireless technolo-gies are a fundamental part of business and personal life

Wireless Data Communications

The radio spectrum is the part of the electromagnetic spectrum used to transmit voice, video, and data It uses frequencies from 3 kilohertz (kHz) to 300 gigahertz (GHz) This section considers only the part of the radio spectrum that supports wireless data transmission

Many different types of wireless data communications exist, as illustrated in Figure 3-38

Lab Activity Fiber Optic Purchase

In this lab, you are introduced to the variety and prices of cabling and related components in the market This lab looks specifically at fiber-optic patch cables and bulk fiber cable

Figure 3-38 Wireless Data Networks

Each type of wireless data communication has its advantages and drawbacks, as

follows:

■ Infrared (IR)—Very high data rates and lower cost, but very short distance.

■ Narrowband—Low data rates and medium cost Requires a license and covers

a limited distance

■ Spread spectrum—Medium cost and high data rates Limited to campus coverage

Cisco Aironet products are spread spectrum

■ Broadband personal communications service (PCS)—Low data rates, medium

cost, and citywide coverage Sprint is an exception; Sprint PCS provides nation-wide and international coverage

■ Circuit and packet data (cellular data and Cellular Digital Packet Data

[CDPD])—Low data rates, high packet fees, and national coverage.

■ Satellite—Low data rates, high cost, and nationwide or worldwide coverage.

Wireless Signal

When a signal is transmitted in a data format, you must consider the following three

parameters:

■ How fast—What data rate can be achieved?

■ How far—How far can wireless LAN (WLAN) units be placed apart and still get

the maximum data rate?

■ How many—How many users can exist without slowing the data rate?

54 Mbps

11 Mbps

4 Mbps

2 Mbps

1 Mbps

56 kbps 19.6 kbps 9.6 kbps

Infrared Wireless LANs

Spread Spectrum Wireless LANs

Broadband PCS

Circuit and Packet Data Cellular, CDPD

Satellite

Narrowband PCS

Narrowband Wireless LANs

Coverage Area

These parameters all relate to the ability to receive a good signal as far away as possible Increasing the amount of data requires the use of more frequency spectra or a different method of placing the data on the radio frequency (RF) signal

RF efficiency is affected by the following three factors, as shown in Figure 3-39:

■ Type of modulation used—More complex modulation techniques provide

greater throughput

■ Distance—The farther the signal must be transmitted, the weaker the signal

becomes

■ Noise—Electronic noise and barriers negatively affect RF.

Figure 3-39 Factors Affecting RF Efficiency

The following sections discuss these three factors in greater detail

Modulation Modulation is the process by which the amplitude, frequency, or phase of an RF or light wave is altered to transmit data The characteristics of the carrier wave instanta-neously are varied by another modulating waveform Modulation blends a data signal (text, voice, and so on) into a carrier for transmission over a network

The most common methods of modulation are as follows (see Figure 3-40):

■ Amplitude modulation (AM)—Modulates the height of the carrier wave

■ Frequency modulation (FM)—Modulates the frequency of the wave

■ Phase modulation (PM)—Modulates the polarity (phase) of the wave

Signal Strength

Noise Level

Figure 3-40 Modulation

Effects of Distance on a Signal

As a receiver moves farther from a transmitter, the signal gets weaker, and the

differ-ence between the signal and noise becomes less Eventually, the signal cannot be

distin-guished from the noise, and loss of communication occurs The amount of compression

(or modulation scheme) at which the signal is transmitted determines the amount of

signal needed to be heard through the noise As transmission, or modulation schemes

(compression), becomes more complex and data rates increase, immunity to noise

less-ens Therefore, the distance is reduced

Effects of Noise on a Signal

Electronic noise and barriers negatively affect RF efficiency An exact transmission

dis-tance for WLAN products cannot be provided without going to the site and actually

testing the environment Walls with internal metal structures, for example, greatly

limit RF transmission range

Radio Frequency Bands

Most radio frequencies are licensed by government agencies, such as the Federal

Com-munications Commission (FCC) in the United States To broadcast over these frequencies,

you need to have a license and to pay a fee

Unlicensed frequency bands are easier to implement and cost less over time because

they do not require licenses Three unlicensed bands exist, as illustrated in Figure 3-41:

■ 900 megahertz (MHz)—The 900-MHz band carries cordless and cellular phones.

■ 2.4 gigahertz (GHz)—The 802.11b standard, the most widely deployed wireless

standard, operates in the 2.4-GHz unlicensed radio band, delivering a maximum data rate of 11 Mbps

Digital Signal AM FM PM

NOTE

To be received cor-rectly, complex modu-lation schemes require optimal signal-to-noise ratios (more signal with less noise) If there is noise on the channel, the line speed

is reduced Noise, speed, and distance are all interrelated.

■ 5 GHz—Recently, the FCC opened up the 5-GHz band for unlicensed use by

high-speed data communications devices Cisco has acquired 5-GHz technology and uses this frequency in new products, such as Cisco Aironet 1200 series, which

is dual band, delivering support for both 2.4 GHz (802.11b) and 5 GHz (802.11a) standards The 802.11a standard can deliver a maximum data rate of 54 Mbps

Figure 3-41 Unlicensed Frequency Bands

A relationship exists between the frequency and the amount of data that can be sent The concept is like that of a pipe The wider the bandwidth is, the more frequencies are available The wider the spectrum is, the higher the data rate can be transmitted The amount of spectrum available determines the data rate

Because the 900-MHz band supports cellular phones and other consumer products, the band has become overcrowded As a result, users often experience interference or cannot access the network As a benefit, 900 MHz offers longer range (for the same gain antennas) than 2.4 GHz The drawback of 900 MHz is that the fastest, most reli-able data rate is only 1 Mbps because of its limited frequency range

The 2.4-GHz frequency range is much wider than 900 MHz, allowing higher data rates with a reliable range of up to 25 miles The Cisco Aironet 340 Wireless LAN Series can deliver 11-Mbps throughput because it operates in the 2.4-GHz frequency Cisco has acquired 5-GHz technology and will deliver products for the 5-GHz frequency range because its wider bandwidth allows for faster throughput of data The Cisco Aironet 5 GHz 54 Mbps Wireless LAN client adapter is an IEEE 802.11a-compliant CardBus adapter that operates in the UNII-1 and UNII-2 bands The client adapter

FM Broadcast Television Short-Wave Radio

AM Broadcast Audio

Cellular (840 MHz) Narrowband PCS (1.9 GHz)Infrared Wireless LAN

Extremely Low VeryLow LowMedium HighVeryHighUltraHigh SuperHigh Infrared VisibleLight Ultra-Violet X-Rays

902-928 MHz

26 MHz

5 GHz (IEEE 802.11) HyperLAN HyperLAN2

2.4 - 2.4835 GHz 83.5 MHz (IEEE 802.11)