Decibels (Optional) / Time and frequency of signals (Optional) / Analog and digital signals (Core)

Decibels (Optional)
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:


dB = 10 log10 (Pfinal / Pref)

dB = 20 log10 (Vfinal / Vref)

In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.

The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.

Pfinal is the delivered power measured in watts.

Pref is the original power measured in watts.

Vfinal is the delivered voltage measured in volts.

Vref is the original voltage measured in volts.

The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.

In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:

• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?

• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?

• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?

The next page will explain how an oscilloscope is used to analyze and view signals.

Time and frequency of signals (Optional)
4.1.5 One of the most important facts of the information age is that characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice.


If the voltage is graphed over time, the patterns that represent the voice will be displayed. An oscilloscope is an important electronic device used to view electrical signals such as voltage waves and pulses. The x-axis on the display represents time and the y-axis represents voltage or current. There are usually two y-axis inputs, so two waves can be observed and measured at the same time.

The analysis of signals with an oscilloscope is called time-domain analysis. The x-axis or domain of the mathematical function represents time. Engineers also use frequency-domain analysis to study signals. In frequency-domain analysis, the x-axis represents frequency. An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis.

Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. Frequency modulation (FM) radio signals use frequencies that are different from television or satellite signals. When listeners change the station on a radio, they change the frequency that the radio receives.

The next page examines the variations of network signals.

Analog and digital signals (Core)
4.1.6 This page will explain how analog signals vary with time and with frequency.


First, consider a single-frequency electrical sine wave, whose frequency can be detected by the human ear. If this signal is transmitted to a speaker, a tone can be heard.

Next, imagine the combination of several sine waves. This will create a wave that is more complex than a pure sine wave. This wave will include several tones. A graph of the tones will show several lines that correspond to the frequency of each tone.

Finally, imagine a complex signal, like a voice or a musical instrument. If many different tones are present, the graph will show a continuous spectrum of individual tones.

The Interactive Media Activity draws sine waves and complex waves based on amplitude, frequency, and the phase.

The next page will discuss noise.

Decibels (Optional)

Decibels (Optional)
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:


dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vref)

In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.

The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.

Pfinal is the delivered power measured in watts.
Pref is the original power measured in watts.
Vfinal is the delivered voltage measured in volts.
Vref is the original voltage measured in volts.

The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.

In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:

• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?

• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?

• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?

The next page will explain how an oscilloscope is used to analyze and view signals.

Sine and Square waves (Core) / Exponents and logarithms (Optional)

Sine waves and square waves (Core)
4.1.2 Sine waves, or sinusoids, are graphs of mathematical functions. Sine waves are periodic, which means that they repeat the same pattern at regular intervals. Sine waves vary continuously, which means that no adjacent points on the graph have the same value.


Sine waves are graphical representations of many natural occurrences that change regularly over time. Some examples of these occurrences are the distance from the earth to the sun, the distance from the ground while riding a Ferris wheel, and the time of day that the sun rises. Since sine waves vary continuously, they are examples of analog waves.

Square waves, like sine waves, are periodic. However, square wave graphs do not continuously vary with time. The wave maintains one value and then suddenly changes to a different value. After a short amount of time it changes back to the original value. Square waves represent digital signals, or pulses. Like all waves, square waves can be described in terms of amplitude, period, and frequency.

The next page reviews exponents and logarithms.

Exponents and logarithms (Optional)
4.1.3 In networking, there are three important number systems:


• Base 2 – binary
• Base 10 – decimal
• Base 16 – hexadecimal

Recall that the base of a number system refers to the number of different symbols that can occupy one position. For example, binary numbers have only two placeholders, which are zero and one. Decimal numbers have ten different placeholders, the numbers 0 to 9. Hexadecimal numbers have 16 different placeholders, the numbers 0 to 9 and the letters A to F.

Remember that 10 x 10 can be written as 102. 102 means ten squared or ten raised to the second power. 10 is the base of the number and 2 is the exponent of the number. 10 x 10 x 10 can be written as 103. 103 means ten cubed or ten raised to the third power. The base is ten and the exponent is three. Use the Interactive Media Activity to calculate exponents. Enter a value for x to calculate y or a value for y to calculate x.

The base of a number system also refers to the value of each digit. The least significant digit has a value of base0, or one. The next digit has a value of base1. This is equal to 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers.

Numbers with exponents are used to easily represent very large or very small numbers. It is much easier and less error-prone to represent one billion numerically as 109 than as 1000000000. Many cable-testing calculations involve numbers that are very large and require exponents. Use the Interactive Media Activity to learn more about exponents.

One way to work with the very large and very small numbers is to transform the numbers based on the mathematical rule known as a logarithm. Logarithm is abbreviated as "log". Any number may be used as a base for a system of logarithms. However, base 10 has many advantages not obtainable in ordinary calculations with other bases. Base 10 is used almost exclusively for ordinary calculations. Logarithms with 10 as a base are called common logarithms. It is not possible to obtain the logarithm of a negative number.

To take the log of a number use a calculator or the Interactive Media Activity. For example, the log of (109) = 9. It is possible to take the logarithm of numbers that are not powers of ten. It is not possible to determine the logarithm of a negative number. The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signal intensity on copper, optical, and wireless media.

The next page will explain how to calculate decibels.

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.