Module 4: Cable Testing (Overview) / Frequency-Based Cable Testing (Core)- Waves

Cable Testing
Overview
Networking media is the backbone of a network. Networking media is literally and physically the backbone of a network. Inferior quality of network cabling results in network failures and unreliable performance. Copper, optical fiber, and wireless networking media all require testing to ensure that they meet strict specification guidelines. These tests involve certain electrical and mathematical concepts and terms such as signal, wave, frequency, and noise. These terms will help students understand networks, cables, and cable testing.


The first lesson in this module will provide some basic definitions to help students understand the cable testing concepts presented in the second lesson.

The second lesson of this module describes issues related to cable testing for physical layer connectivity in LANs. In order for the LAN to function properly, the physical layer medium should meet the industry standard specifications.

Attenuation, which is signal deterioration, and noise, which is signal interference, can cause problems in networks because the data sent may be interpreted incorrectly or not recognized at all after it has been received. Proper termination of cable connectors and proper cable installation are important. If standards are followed during installations, repairs, and changes, attenuation and noise levels should be minimized.

After a cable has been installed, a cable certification meter can verify that the installation meets TIA/EIA specifications. This module also describes some important tests that are performed.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.

Students who complete this module should be able to perform the following tasks:

• Differentiate between sine waves and square waves
• Define and calculate exponents and logarithms
• Define and calculate decibels
• Define basic terminology related to time, frequency, and noise
• Differentiate between digital bandwidth and analog bandwidth
• Compare and contrast noise levels on various types of cabling
• Define and describe the affects of attenuation and impedance mismatch
• Define crosstalk, near-end crosstalk, far-end crosstalk, and power sum near-end crosstalk
• Describe how twisted pairs help reduce noise
• Describe the ten copper cable tests defined in TIA/EIA-568-B
• Describe the difference between Category 5 and Category 6 cable

4.1 Frequency-Based Cable Testing (Core)
 
Waves
4.1.1 This lesson provides definitions that relate to frequency-based cable testing. This page defines waves.


A wave is energy that travels from one place to another. There are many types of waves, but all can be described with similar vocabulary.

It is helpful to think of waves as disturbances. A bucket of water that is completely still does not have waves since there are no disturbances. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide.

Ocean waves can be described in terms of their height, or amplitude, which could be measured in meters. They can also be described in terms of how frequently the waves reach the shore, which relates to period and frequency. The period of the waves is the amount of time between each wave, measured in seconds. The frequency is the number of waves that reach the shore each second, measured in hertz (Hz). 1 Hz is equal to 1 wave per second, or 1 cycle per second. To experiment with these concepts, adjust the amplitude and frequency in Figure .

Networking professionals are specifically interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves. The amplitude of an electrical signal still represents height, but it is measured in volts (V) instead of meters (m). The period is the amount of time that it takes to complete 1 cycle. This is measured in seconds. The frequency is the number of complete cycles per second. This is measured in Hz.

If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are an important part of electrical signals because they are the basis of digital transmission. The pattern of the pulses represents the value of the data being transmitted.

The next page will introduce sine waves and square waves.

Module 3 Summary

Summary
This page summarizes the topics discussed in this module.


Copper cable carries information using electrical current. The electrical specifications of a cable determines the kind of signal a particular cable can transmit, the speed at which the signal is transmitted and the distance the signal will travel.

An understanding of the following electrical concepts is helpful when working with computer networks:

• Voltage – the pressure that moves electrons through a circuit from one place to another

• Resistance – opposition to the flow of electrons and why a signal becomes degraded as it travels along the conduit

• Current – flow of charges created when electrons move

• Circuits – a closed loop through which an electrical current flows

Circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. A multimeter is used to measure voltage, current, resistance, and other electrical quantities expressed in numeric form.

Coaxial cable, unshielded twisted pair (UTP) and shielded twisted pair (STP) are types of copper cables that can be used in a network to provide different capabilities. Twisted-pair cable can be configured for straight through, crossover, or rollover signaling. These terms refer to the individual wire connections, or pinouts, from one end to the other end of the cable. A straight-through cable is used to connect unlike devices such as a switch and a PC. A crossover cable is used to connect similar devices such as two switches. A rollover cable is used to connect a PC to the console port of a router. Different pinouts are required because the transmit and receive pins are in different locations on each of these devices.

Optical fiber is the most frequently used medium for the longer, high-bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Light energy is used to transmit large amounts of data securely over relatively long distances The light signal carried by a fiber is produced by a transmitter that converts an electrical signal into a light signal. The receiver converts the light that arrives at the far end of the cable back to the original electrical signal.

Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive.

The part of an optical fiber through which light rays travel is called the core of the fiber. Surrounding the core is the cladding. Its function is to reflect the signal back towards the core. Surrounding the cladding is a buffer material that helps shield the core and cladding from damage. A strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar. The final element is the outer jacket that surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants.

The laws of reflection and refraction are used to design fiber media that guides the light waves through the fiber with minimum energy and signal loss. Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Because of its design, single-mode fiber is capable of higher rates of data transmission and greater cable run distances than multimode fiber.

Fiber is described as immune to noise because it is not affected by external noise or noise from other cables. Light confined in one fiber has no way of inducing light in another fiber. Attenuation of a light signal becomes a problem over long cables especially if sections of cable are connected at patch panels or spliced.

Both copper and fiber media require that devices remains stationary permitting moves only within the limits of the media. Wireless technology removes these restraints. Understanding the regulations and standards that apply to wireless technology will ensure that deployed networks will be interoperable and in compliance with IEEE 802.11 standards for WLANs.

A wireless network may consist of as few as two devices. The wireless equivalent of a peer-to-peer network where end-user devices connect directly is referred to as an ad-hoc wireless topology. To solve compatibility problems among devices, an infrastructure mode topology can be set up using an access point (AP) to act as a central hub for the WLAN. Wireless communication uses three types of frames: control, management, and data frames. To avoid collisions on the shared radio frequency media WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

WLAN authentication is a Layer 2 process that authenticates the device, not the user. Association, performed after authentication, permits a client to use the services of the access point to transfer data.

Signals and noise on a WLAN / Wireless security

Signals and noise on a WLAN
3.3.6 This page discusses how signals and noise can affect a WLAN.


On a wired Ethernet network, it is usually a simple process to diagnose the cause of interference. When using RF technology many kinds of interference must be taken into consideration.

Narrowband is the opposite of spread spectrum technology. As the name implies narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. Actually diagnosing the cause of narrowband interference can be a costly and time-consuming experience. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive.

All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering. In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.

Generally the RF signal will not be affected by even the most extreme weather conditions. However, fog or very high moisture conditions can and do affect wireless networks. Lightning can also charge the atmosphere and alter the path of a transmitted signal.

The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic dish antenna that concentrates the signal will increase the transmission range.

In a SOHO environment most access points will utilize twin omnidirectional antennae that transmit the signal in all directions thereby reducing the range of communication.

The next page describes WLANs security.

Wireless security
3.3.7 This page will explain how wireless security can be achieved.


Where wireless networks exist there is little security. This has been a problem from the earliest days of WLANs. Currently, many administrators are weak in implementing effective security practices.

A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose. Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.

• EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest authentication type, which is very similar to CHAP password protection on a wired network.

• LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP keys, and supports mutual authentication.

• User authentication – Allows only authorized users to connect, send and receive data over the wireless network.

• Encryption – Provides encryption services further protecting the data from intruders.

• Data authentication – Ensures the integrity of the data, authenticating source and destination devices.

VPN technology effectively closes the wireless network since an unrestricted WLAN will automatically forward traffic between nodes that appear to be on the same wireless network. WLANs often extend outside the perimeter of the home or office in which they are installed and without security intruders may infiltrate the network with little effort. Conversely it takes minimal effort on the part of the network administrator to provide low-level security to the WLAN.

This page concludes the lesson. The next page will summarize the main points from the module.

How wireless LANs communicate / Authentication and association / The radio wave and microwave spectrums

How wireless LANs communicate
3.3.3 This page explains the communication process of a WLAN.


After establishing connectivity to the WLAN, a node will pass frames in the same manner as on any other 802.x network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, management, and data. Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500 bytes; however, an Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.

Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.

When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.

Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked. The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.

The next page explains authentication and association.

Authentication and association
3.3.4 This page describes WLAN authentication and association.


WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. This is a critical point to remember when considering WLAN security, troubleshooting and overall management.

Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process.

Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.

Authentication and Association types

• Unauthenticated and unassociated
• The node is disconnected from the network and not associated to an access point.
• Authenticated and unassociated
• The node has been authenticated on the network but has not yet associated with the access point.
• Authenticated and associated
• The node is connected to the network and able to transmit and receive data through the access point.

Methods of authentication


IEEE 802.11 lists two types of authentication processes.

The first authentication process is the open system. This is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment although the ability of low level network ‘sniffers’ to discover the SSID of the WLAN is high.

The second process is the shared key. This process requires the use of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof.

The problem of unauthorized entry into WLANs is being addressed by a number of new security solution technologies.

The next page explains radio waves and modulation.

The radio wave and microwave spectrums
3.3.5 This page describes radio waves and modulation.


Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna. However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air.

These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey.

Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals.

In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal.

The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter.

In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal.

A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal.

The first Interactive Media Activity explains electromagnetic fields and polarization.

The second Interactive Media Activity shows the names, devices, frequencies, and wavelengths of the EM spectrum.

The next page describes problems caused by signals and noise.

Wireless LAN organizations and standards / Wireless devices and topologies

Wireless LAN organizations and standards
3.3.1 This page will introduce the regulations and standards that apply to wireless technology. These standards ensure that deployed networks are interoperable and in compliance.


Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC).

A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS). DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps. The next standard approved was 802.11b, which increased transmission capabilities to 11 Mbps. Even though DSSS WLANs were able to interoperate with the Frequency Hopping Spread Spectrum (FHSS) WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task was simply to create a standard that matched the manufacturer’s solution.

802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps. All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates for DSSS only. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points.

802.11b devices achieve the higher data throughput rate by using a different coding technique from 802.11, allowing for a greater amount of data to be transferred in the same time frame. The majority of 802.11b devices still fail to match the 11 Mbps bandwidth and generally function in the 2 to 4 Mbps range.

802.11a covers WLAN devices operating in the 5 GHZ transmission band. Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ. 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps. In production networks, a more standard rating is 20-26 Mbps.

802.11g provides the same bandwidth as 802.11a but with backwards compatibility for 802.11b devices using Orthogonal Frequency Division Multiplexing (OFDM) modulation technology. Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate.

The next page explains the devices and topologies used in wireless networks.

Wireless devices and topologies
3.3.2 This page describes the devices and related topologies for a wireless network.


A wireless network may consist of as few as two devices. - The nodes could simply be desktop workstations or notebook computers. Equipped with wireless NICs, an ‘ad hoc’ network could be established which compares to a peer-to-peer wired network. Both devices act as servers and clients in this environment. Although it does provide connectivity, security is at a minimum along with throughput. Another problem with this type of network is compatibility. Many times NICs from different manufacturers are not compatible.

To solve the problem of compatibility, an access point (AP) is commonly installed to act as a central hub for the WLAN infrastructure mode. The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell. Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell could greatly vary. Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet). To service larger areas, multiple access points may be installed with a degree of overlap. The overlap permits "roaming" between cells. This is very similar to the services provided by cellular phone companies. Overlap, on multiple AP networks, is critical to allow for movement of devices within the WLAN. Although not addressed in the IEEE standards, a 20-30% overlap is desirable. This rate of overlap will permit roaming between cells, allowing for the disconnect and reconnect activity to occur seamlessly without service interruption.

When a client is activated within the WLAN, it will start "listening" for a compatible device with which to "associate". This is referred to as "scanning" and may be active or passive.

Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response. The authentication and association steps are completed.

Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes.

The first Interactive Media Activity shows the levels of the OSI reference model and the related networking devices.

The second Interactive Media Activity shows the addition of a wireless hub to a wired network.

The next page explains how wireless LANs communicate.

Signals and noise in optical fibers / Installation, care, and testing of optical fiber

Signals and noise in optical fibers
3.2.9 This page explains some factors that reduce signal strength in optical media.


Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media 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 interference that disturbs transmission on any other fiber. This 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 ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and wide-area networks (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 the signal 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 scattering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy.

Absorption is 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 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 because of the less than perfect total internal reflection in that rough area of the fiber. Any microscopic imperfections in the thickness or symmetry of the fiber will cut down on total internal reflection and the cladding will absorb some light energy.

Dispersion of a light flash also limits transmission distances on a fiber. Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber.

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. However, chromatic dispersion is a characteristic of both multimode and single-mode fiber. When wavelengths of light travel at slightly different speeds through glass than do other wavelengths, chromatic dispersion is caused. That is why a prism separates the wavelengths of light. Ideally, an LED or Laser light source would emit light of just one frequency. Then chromatic dispersion would not be a problem.

Unfortunately, lasers, and especially LEDs generate a range of wavelengths so chromatic dispersion limits the distance that can be transmitted on a fiber. If a signal is transmitted too far, what started as a bright pulse of light energy will be spread out, separated, and dim when it reaches the receiver. The receiver will not be able to distinguish a one from a zero.

The next page will discuss the installation, care, and testing of optical fiber.

Installation, care, and testing of optical fiber
3.2.10 This page will teach students how to troubleshoot 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 cracks in the core that will 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 incident angle of the ray will become less than the critical angle for total internal reflection. Instead of reflecting around the bend, some light rays will refract into the cladding and be lost.

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 cannot 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 properly polished to ensure that the ends are smooth. 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 carefully attached to the fiber end. Improperly installed connectors, improper splices, or the splicing of two cables with different core sizes will dramatically reduce the strength of a light signal.

Once the fiber-optic cable and connectors have been 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 a 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 will cause a big drop in the amount of light that reaches the receiver.

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 zeros and ones 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 is referred to as the optical link loss budget. Imagine a monthly financial budget. After all of the expenses are subtracted from initial income, enough money must be left to get through the month.

The decibel (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).

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 much additional detailed diagnostic information about a fiber link. They can be used to trouble shoot a link when problems occur.

This page concludes this lesson. The next lesson will discuss wireless media. The first page will discuss Wireless LAN organizations and standards.

Multimode fiber / Single-mode fiber / Other optical components

Multimode fiber
3.2.6 This page will introduce multimode fiber.


The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. Likewise, once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called "multimode" fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber.

Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B. The second fiber carries data from device B to device A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. Copper twisted-pair uses a wire pair to transmit and a wire pair to receive. Fiber-optic circuits use one fiber strand to transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are attached.

Until the connectors are attached, there is no need for shielding, because no light escapes when it is inside a fiber. This means there are no crosstalk issues with fiber. It is very common to see multiple fiber pairs encased in the same cable. This allows a single cable to be run between data closets, floors, or buildings. One cable can contain 2 to 48 or more separate fibers. With copper, one UTP cable would have to be pulled for each circuit. Fiber can carry many more bits per second and carry them farther than copper can.

Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.

The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass for its core. This glass has a lower index of refraction towards the outer edge of the core. Therefore, the outer area of the core is less optically dense than the center and light can go faster in the outer part of the core. This design is used because a light ray following a mode that goes straight down the center of the core does not have as far to travel 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.

Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. Standard multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1µ).

Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. There are two basic cable designs. They are the loose-tube and the tight-buffered cable designs. Most of the fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. The most 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 buildings.

The strength 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 is the outer jacket. The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color.

Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs) are two types of light source usually used with multimode fiber. Use one or the other. 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 (6,560 ft).

The next page describes single-mode fiber.

Single-mode fiber
3.2.7 This page will introduce single-mode fiber.


Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode fiber is usually yellow. The major difference between multimode and single-mode fiber is that single-mode allows only one mode of light to propagate through the smaller, fiber-optic core. The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of the single-mode fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter.

An 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. This greatly increases both the speed and the distance that data can be transmitted.

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. Although this distance is considered a standard, newer technologies have increased this distance and will be discussed in a later module. Multimode is only capable of carrying 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 inter-building connectivity.

Warming: The laser light used with single-mode has a longer wavelength than can be seen. The laser is so strong that it can seriously damage eyes. Never look at the near end of a fiber that is connected to a device at the far end. Never look into the transmit port on a NIC, switch, or router. Remember to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Be very careful.

Figure compares the relative sizes of the core and cladding for both types of fiber optic in different sectional views. The much smaller and more refined fiber core in single-mode fiber is the reason single-mode has a higher bandwidth and cable run distance than multimode fiber. However, it entails more manufacturing costs.

The next page introduces some components that are used with optical fiber.

Other optical components
3.2.8 This page explains how optical devices are used to transmit data.


Most of the data sent over a LAN is in the form of electrical signals. However, optical fiber links use light to send data. Something is needed to convert the electricity to light and at the other end of the fiber convert the light back to electricity. This means that a transmitter and a receiver are required.

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. There are two types of light sources used to encode and transmit the data through the cable:

• A light emitting diode (LED) producing infrared light with wavelengths of either 850 nm 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.

• Light amplification by stimulated emission radiation (LASER) a light source producing a thin beam of intense infrared light usually with wavelengths of 1310nm or 1550 nm. Lasers are used with single-mode fiber over the longer distances involved in WANs or campus backbones. Extra care should be exercised to prevent eye injury.

Each of these light sources can be lighted and darkened very quickly to send data (1s and 0s) at a high number of bits per second.

At the other end of the optical fiber from the transmitter is the receiver. The receiver 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 original 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 850, 1310, or 1550 nm of light that are 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 generates the voltage changes that represent the data 1s and 0s on a copper cable.

Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC). On single-mode fiber, the Straight Tip (ST) connector is frequently used.

In addition to the transmitters, receivers, connectors, and fibers that are always required on an optical network, repeaters and fiber patch panels are often seen.

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

Fiber patch panels similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers with various available fiber runs, or cable links.

The next page will discuss data loss in optical fiber.