CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 18 ppsx

The part of an optical fiber through which light rays travel is called the core of the fiber.. Likewise, once the rays have entered the fiber’s core, a limited number of optical paths exist

Figure 3-18 Refraction Example

Total Internal Reflection

A light ray that is being turned on and off to send data 1s and 0s into an optical fiber

must stay inside the fiber until it reaches the far end The ray must not refract into the

material wrapped around the outside of the fiber because such refraction causes the

loss of part of the ray’s light energy A design for the fiber that makes the outside

sur-face of the fiber act like a mirror to the light ray moving through the fiber must be

achieved If any light ray that tries to move out through the side of the fiber is reflected

back into the fiber at an angle that sends it towards the far end of the fiber, this is a

good pipe or wave guide for the light waves, as illustrated in Figure 3-19.

Figure 3-19 Total Internal Reflection

The laws of reflection and refraction tell how to design a fiber that guides the light waves

through the fiber with a minimum energy loss Two conditions are needed to cause

light rays in a fiber to be reflected back into the fiber with out any loss due to

refrac-tion These two conditions are as follows:

■ The core (the inside) of the optical fiber has to have a larger (a higher) index of

refraction than the material that surrounds it The material that surrounds the core of the fiber is called the cladding

■ The angle of incidence of the light ray is greater than the critical angle for the

core and its cladding

When both of these conditions are met, all the incident light in the fiber is reflected back

inside the fiber This condition is called total internal reflection, which is the foundation

on which optical fiber is constructed Total internal reflection causes the light rays in the

Cladding Light

Core

fiber to bounce off the core-cladding boundary and continue their journey toward the far end of the fiber The light follows something of a zigzag path through the core of the fiber

A fiber that meets the first condition (a core with a higher index of refraction than the cladding) can be easily created Also, the angle of incidence of the light rays that enter the core can be controlled Restricting two factors controls the angle of incidence:

■ The numerical aperture of the fiber The numerical aperture of a core is the range

of angles of incident light rays entering the fiber that are totally internally reflected, as illustrated in Figure 3-20

■ The paths (called the modes) that a light ray can follow when traveling down a fiber

Figure 3-20 Numerical Aperture

Controlling conditions 1 and 2 creates a fiber with total internal reflection, consequently giveing a light wave path that can be used for data communications, as shown in Figure 3-21

Figure 3-21 Light Wave Path

Numerical Aperture Axis

n1

n1

NA = Sin 0 = n 2 Ðn1

Where:

n 1 is cladding index

n 2 is core index

n2

0

n0

n1 Light must fall inside this angle

to be guided in the fiber core.

Fiber-Optic Cables

Fiber-optic cable is a networking medium that uses modulated light for data

transmis-sions through thin strands of glass Signals that represent data bits are converted into

beams of light It is important to recognize that while electricity is required to generate

and interpret the fiber-optic signals at the end devices, no electricity is in the cable itself

as there is with copper media In fact, fiber-optic cable components are very good

insu-lators Many characteristics of fiber-optic media are superior to copper

Every fiber-optic cable used for networking consists of two glass fibers encased in

sep-arate sheaths One fiber carries transmitted data from device A to device B; the second

fiber carries data from device B to device A A fiber for data goes in each direction,

similar to two one-way streets going in opposite directions This arrangement provides

a full-duplex communication link Just as copper twisted-pair uses separate wire pairs

to transmit (Tx) and receive (Rx), fiber-optic circuits use one fiber strand to transmit

and one to receive, as illustrated in Figure 3-22 Typically, these two fiber cables are in

a single outer jacket until they reach the point at which connectors are attached

Figure 3-22 Duplex Fiber

At this point, the two fiber cables are separated No need for twisting or shielding

exists because no light escapes when it is inside a fiber, which means no crosstalk issues

exist with fiber It is common to see multiple fiber pairs encased in the same cable This

arrangement allows a single cable to be run between data closets, floors, or buildings

One cable can contain 2, 4, 8, 12, 24, 48, or more separate fibers With copper, one

UTP cable has to be pulled for each circuit Fiber can carry many more bits per second

and carry them farther than copper can

As illustrated in Figure 3-23 and Figure 3-24, five parts typically make up each

fiber-optic cable:

■ The core

■ The cladding

■ A buffer

■ A strengthening material

■ An outer jacket

6N

6N 4N 4N

Figure 3-23 Five Elements of a Fiber-Optic Cable

Figure 3-24 Cross Section Showing the Five Elements of a Fiber-Optic Cable

The core is the light transmission element at the center of the optical fiber, and all the light signals travel through the core This core is typically glass made from a combina-tion of silica (silicon dioxide) and other elements Surrounding the core is the cladding, also made of silica but with a lower index of refraction than the core Light rays travel-ing through the fiber core reflect off this core-to-claddtravel-ing interface where the core and cladding meet, which keeps light in the core as it travels down the fiber

Surrounding the cladding is a buffer material, usually plastic, that helps shield the core and cladding from damage

The strengthening material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it The material used is often Kevlar, the same material used to produce bulletproof vests The final element, the outer jacket, surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants This outer jacket composition can vary depending on the cable usage

Jacket (Typically PVC)

Strengthening Material (Aramid Yarn)

Buffer Cladding Core

Jacket Aramid Yarn

Buffer Cladding

Core

The part of an optical fiber through which light rays travel is called the core of the fiber

Light rays cannot enter the core of an optical fiber at all angles The rays can enter the

core only if their angle is inside the fiber’s numerical aperture Likewise, once the rays

have entered the fiber’s core, a limited number of optical paths exist that a light ray can

follow through the fiber These optical paths are called modes If the diameter of a fiber’s

core is large enough so that many paths exist that light can take as it passes through

the fiber, the fiber is called multimodefiber Single-modefiber has a much smaller core

that allows light rays to travel along only one path (one mode) inside the fiber Figure 3-25

illustrates the differences between multimode and single-mode fibers

Figure 3-25 Single-Mode Versus Multimode

Table 3-3 compares the features of single-mode and multimode fiber

The next two sections cover the two basic types of optical fiber, multimode and

single-mode, in more detail

Table 3-3 Features of Single-Mode and Multimode Fiber

Core Features Small core (10 microns

or less)

Larger core than single-mode cable (50 or 62.5 microns or greater)

Dispersion

Characteristics

Less dispersion Allows greater dispersion and,

therefore, loss of signal

Distance

Characteristics

Suited for long-distance applications (up to 3 kilo-meters [9,842 feet])

Used for long-distance application, but shorter than single-mode (up

to 2 kilometers [6,560 feet])

Light Source Uses lasers as the light source

often within campus back-bones for distances of several thousand meters

Uses LEDs as the light source, often within LANs or distances

of a couple hundred meters within a campus network

Single-Mode Polymeric

Coating

Glass Cladding

125 Microns d/a

Glass Core = 5.8 Microns Requires Very Straight Paths

Multimode Coating

Glass Cladding

125 Microns d/a

Glass Core = 6 0 microns Multiple Paths-Sloppy

Multimode Fiber

Multimode fiber allows multiple modes (paths) of light to propagate through the fiber-optic core, as compared to single-mode fiber, which allows only one mode Multiple modes of light propagating through fiber might travel different distances, depending

on their entry angles This angle causes them to arrive at the destination (receiving end of the cable) at slightly different times—a phenomenon called modal dispersion

Multimode uses a type of glass, called graded index glass, which has a lower index of

refraction towards the outer edge of the core This glass causes the light to slow down when passing through the center of the core and accelerate when passing through the outer areas of the core, ensuring that all modes of light reach the end at approximately the same time This design is used because a light ray following a mode that goes straight down the center of the core does not have to go as far as a ray following a mode that bounces around in the fiber All rays should arrive at the end of the fiber together Then, the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse

A standard multimode fiber-optic cable (the most common type of fiber-optic cable used

in LANs) uses an optical fiber with either a 62.5- or a 50-micron core and a 125-micron diameter cladding This cable is commonly designated as 62.5/125 or 50/125 micron optical fiber A micron is one millionth of a meter Because the diameter of the cladding

is considerably larger than the wavelength of the light being transmitted, the light bounces around (reflects) inside the core as it is propagated along the transmission line Infrared light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) are usually the light source used with multimode fiber LEDs are a little cheaper to build and require somewhat less safety concerns than lasers However, LEDs cannot transmit light over cable as far as the lasers Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6560 feet) Multimode fiber is mainly used in LAN applications including backbone cabling

Lab Activity Optical Fiber Purchase

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

Single-Mode Fiber

Single-mode fiber uses only one mode of light to propagate through the fiber-optic

core In single-mode fiber-optic cabling, the core is much smaller than in multimode

The single-mode core is 8 to 10 microns in diameter Nine micron cores are the most

common A 9/125 marking on a single-mode fiber’s jacket indicates that the core fiber

has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter

The size of the core in single-mode fiber leaves very little room for light to bounce

around Furthermore, a very focused infrared laser is used as the light source in

single-mode fiber The ray of light it generates enters the core at a 90-degree angle As a result

the data carrying light ray pulses in single-mode fiber are essentially transmitted in a

straight line right down the middle of the core, as shown in Figure 3-26 This greatly

increases both the speed and the distance that data can be transmitted

Figure 3-26 Single-Mode Fiber

Because of its design, single-mode fiber is capable of higher rates of data transmission

(bandwidth) and greater cable run distances than multimode fiber Single-mode fiber

can carry LAN data up to 3000 meters Multimode is only capable of up to 2000 meters

Lasers and single-mode fibers are more expensive than LEDs and multimode fiber

Because of these characteristics, single-mode fiber is often used for interbuilding

con-nectivity or WANs (for example, telephone company network connections)

Figure 3-27 compares the relative sizes of the core and cladding for both types of

fiber-optic cable in different sectional views The much smaller and more refined fiber core

in mode fiber, although it entails more manufacturing costs, is the reason

single-mode has a higher bandwidth and cable run distance than multisingle-mode fiber

Figure 3-27 Single-Mode and Multimode Fiber

WARNING

Be aware that the laser light used with single-mode has a longer wavelength than can be seen, and

it is so strong that it can seriously damage eyes Never look at the end of a fiber that

is connected to a device

at its far end Never look into the transmit port on a NIC, switch,

or router There is nothing that can be seen anyway Remem-ber to keep protective covers over the ends

of fiber and inserted into the fiber-optic ports of switches and routers Be very careful!

Multimode

100/140 Microns

Multimode

62.5/125 Microns

Multimode

50/125 Microns

Single-Mode

10/125 Microns

The following summarizes the features of fiber-optic cables:

■ Speed and throughput—More than 1 Gbps

■ Average cost per node—Expensive

■ Media and connector size—Small

■ Maximum cable length—More than 10 kilometers (km) for single mode; up to

2 km for multimode

Cable Designs

As illustrated in Figure 3-28, two basic cable designs exist:

■ Loose-tube

■ Tight-buffered

Figure 3-28 Loose-Tube Versus Tight-Buffer Construction

Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding The main practical difference between the two designs is the applications for which they are used Loose-tube cable is primarily used for outside-building installations, while tight-buffered cable is used inside outside-buildings Most of the fiber used in LANs is tight-buffered multimode cable

Loose-Tube Fiber

Tight-Buffered Fiber

Other Optical Networking Components

Most of the data sent over a LAN is in the form of electrical signals But, optical fiber

links use light to send data So, something is needed to convert the electricity to light

and, at the other end of the fiber, convert the light back to electricity This situation

means that two devices are always required: a transmitter and a receiver

In addition to transmitters and receivers, this section also discusses different types of

fiber-optic cable connectors as well as devices used in optical networking

Transmitters

The transmitter receives data to be transmitted from switches and routers This data

is in the form of electrical signals The transmitter converts the electronic signals into

their equivalent light pulses Two types of light sources encode and transmit the data

through the cable These two sources are

■ Light emitting diodes (LEDs)—An LED produces infrared light with wavelengths

of either 850 or 1310 nm These are used with multimode fiber in LANs Lenses are used to focus the infrared light on the end of the fiber LEDs have fewer safety concerns

■ Light amplification by stimulated emission radiation (Laser)—Laser is a light

source producing a thin beam of intense infrared light usually with wavelengths

of 1310 or 1550 nm Lasers are used with single-mode fiber over the longer dis-tances involved in WANs or campus backbones Exercise extra care to prevent eye injury

Each light source can be lighted and darkened quickly to send data 1s and 0s at a high

number of bits per second

Receiver

At the other end of the optical fiber from the transmitter is the receiver It functions

something like the photoelectric cell in a solar-powered calculator When light strikes

the receiver, it produces electricity The first job of the receiver is to detect a light pulse

that arrives from the fiber Then the receiver converts the light pulse back into the

orig-inal electrical signal that first entered the transmitter at the far end of the fiber Now

the signal is again in the form of voltage changes The signal is ready to be sent over

copper wire into any receiving electronic device such as a computer, switch, or router

The semiconductor devices that are usually used as receivers with fiber-optic links are

called p-intrinsic-n diodes (PIN photodiodes)

PIN photodiodes are manufactured to be sensitive to the particular wavelength of light

(850, 1310, or 1550 nm) generated by the transmitter at the far end of the fiber When

struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces

an electric current of the proper voltage for the network It instantly stops producing the voltage when no light strikes the PIN photodiode This process generates the voltage changes that represent the data 1s and 0s on a copper cable

Connectors

Connectors are attached to the fiber ends so that the fibers can be connected to the ports

on the transmitter and receiver The most common type of connector used with multi-mode fiber is the subscriber connector (SC ), as shown in Figure 3-29 On single-multi-mode fiber, the straight tip (ST) connector, as shown in Figure 3-30, is frequently used With

SC and ST, there is one connector for each fiber Newer connectors combine the send and receive fibers into one modular connector, comparable in size to an RJ-45, to save space

Figure 3-29 SC Connector

Figure 3-30 ST Connector

Optical Amplifiers and Fiber Patch Panels

In addition to the transmitters, receivers, connectors, and fibers that are always required

on an optical network, you can sometimes see several other devices on an optical fiber network

Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings Then the restored signals can be sent on along the journey to the receiver at the far end of the fiber