Although the initial transmission amplitude will almost always be above 0 dBm (1 mW), the fi nal received signal amplitude will always be well below 0 dBm (1 mW) because of FSPL.
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Because RF communications can be affected by many outside infl uences, it is common to have a fade margin to provide a level of link reliability. By increasing the fade margin, you are essentially increasing the reliability of the link. Think of the fade margin as the buf- fer or margin of error for received signals that is used when designing and planning an RF system. After the RF link has been installed, it is important to measure the link to see how much buffer or padding there actually is. This functional measurement is known as the sys- tem operating margin (SOM). The SOM is the difference between the actual received signal and the signal necessary for reliable communications.
When Are Fade Margin Calculations Needed?
Whenever an outdoor WLAN bridge link is designed, link budget and fade margin calcu- lations will be an absolute requirement. For example, an RF engineer may perform link budget calculations for a 2-mile point-to-point bridge link and determine that the fi nal received signal is 5 dB above the receive sensitivity threshold of a radio at one end of a bridge link. It would seem that RF communications will be just fi ne; however, because of downfade caused by multipath and weather conditions, a fade margin buffer is needed.
A torrential downpour can attenuate a signal as much as 0.08 dB per mile (0.05 dB per kilometer) in both the 2.4 GHz and 5 GHz frequency ranges. Over long-distance bridge links, a fade margin of 25 dB is usually recommended to compensate for attenuation due to changes in RF behaviors such as multipath and due to changes in weather conditions such as rain, fog, or snow.
When deploying a WLAN indoors where high multipath or high noise fl oor conditions exist, the best practice is to plan for a fade margin of about 5 dB above the vendor’s rec- ommended receive sensitivity amplitude. For example, a –70 dBm or stronger signal falls above the RSSI threshold for the higher data rates for most WLAN vendor radios. During the indoor site survey, RF measurements of –70 dBm will often be used to determine cov- erage areas for higher data rates. In a noisy environment, RF measurements of –65 dBm utilizing a 5 dB fade margin is a recommended best practice.
E X E R C I S E 3 . 6
Link Budget and Fade Margin
In this exercise, you will use a Microsoft Excel fi le to calculate a link budget and fade mar- gin. You will need Excel installed on your computer.
1. From the book’s online resource area that can be accessed at www.sybex.com/go/
cwna4e, copy the fi le LinkBudget.xls to your desktop. Open the Excel fi le from your desktop.
E X E R C I S E 3 . 6 ( c o n t i n u e d )
2. In row 10, enter a link distance of 25 kilometers.
Note that the path loss due to a 25 kilometer link is now 128 dB in the 2.4 GHz frequency.
3. In row 20, enter 128 for path loss in dB.
4. In row 23, change the radio receiver sensitivity to –80 dBm.
Notice that the fi nal received signal is now –69 dBm, and the fade margin is only 11 dB.
5. Try increasing the “radio transmitter output power” to see how the connection would fare and to determine how much power would be needed to ensure a fade margin of 20 dB. You can also change the other components such as antenna gain and cable loss to ensure a fade margin of 20 dB.
Summary
This chapter covered six key areas of RF communications:
■ RF components
■ RF measurements
■ RF mathematics
■ RSSI thresholds
■ Link budgets
■ Fade margins
It is important to understand how each of the RF components affects the output of the transceiver. Whenever a component is added, removed, or modifi ed, the output of the RF communications is changed. You need to understand these changes and make sure that the system conforms to regulatory standards. The following RF components were covered in this chapter:
■ Transmitter
■ Receiver
■ Antenna
■ Isotropic radiator
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In addition to understanding the components and their effects on the transmitted signal, you must know the different units of power and comparison that are used to measure the output and the changes to the RF communications:
■ Units of power
■ Watt
■ Milliwatt
■ dBm
■ Units of comparison
■ dB
■ dBi
■ dBd
After you become familiar with the RF components and their effects on RF communica- tions, and you know the different units of power and comparison, you need to understand how to perform the actual calculations and determine whether your RF communication will be successful. It is important to know how to perform the calculations and some of the terms and concepts involved with making sure that the RF link will work properly. These concepts and terms are as follows:
■ Rule of 10s and 3s
■ Noise floor
■ Signal-to-noise ratio (SNR)
■ Receive sensitivity
■ Received signal strength indicator (RSSI)
■ Link budget
■ System operating margin (SOM)
■ Fade margin
Exam Essentials
Understand the RF components. Know the function of each of the components and which components add gain and which components add loss.
Understand the units of power and comparison. Make sure you are comfortable with the difference between units of power (absolute) and units of comparison (relative). Know all of the units of power and comparison, what they measure, and how they are used.
Be able to perform RF mathematics. There will be no logarithms on the test; however, you must know how to use the rule of 10s and 3s. You will need to be able to calculate a result based on a scenario, power value, or comparative change.
Understand the practical uses of RF mathematics. When all is said and done, the ultimate question is, Will the RF communication work? This is where an understanding of RSSI, SOM, fade margin, and link budget is important.
Be able to explain the importance of measuring the SNR and the noise floor. Understand that the ambient background level of radio energy on a specifi c channel can corrupt 802.11 data transmissions. Understand that the only device that can truly measure unmodulated RF energy is a spectrum analyzer.
Define RSSI. Understand that RSSI metrics are used by radios to interpret signal strength and quality. 802.11 radios use RSSI metrics for decisions such as roaming and dynamic rate switching.
Understand the necessity of a link budget and fade margin. A link budget is the sum of all gains and losses from the transmitting radio, through the RF medium, to the receiver radio.
The purpose of link budget calculations is to guarantee that the fi nal received signal ampli- tude is above the receiver sensitivity threshold of the receiver radio. Fade margin is a level of desired signal above what is required.
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Review Questions
1. What RF component is responsible for generating the AC signal?
A. Antenna B. Receiver C. Transmitter D. Transponder
2. A point source that radiates RF signal equally in all directions is known as what?
A. Omnidirectional signal generator B. Omnidirectional antenna C. Intentional radiator D. Nondirectional transmitter E. Isotropic radiator
3. When calculating the link budget and system operating margin of a point-to-point outdoor WLAN bridge link, what factors should be taken into account? (Choose all that apply.) A. Distance
B. Receive sensitivity C. Transmit amplitude D. Antenna height E. Cable loss F. Frequency
4. The sum of all the components from the transmitter to the antenna, not including the antenna, is known as what? (Choose two.)
A. IR
B. Isotropic radiator C. EIRP
D. Intentional radiator
5. The highest RF signal strength that is transmitted from an antenna is known as what?
A. Equivalent isotropically radiated power B. Transmit sensitivity
C. Total emitted power D. Antenna radiated power
6. Select the absolute units of power. (Choose all that apply.) A. Watt
B. Milliwatt C. Decibel D. dBm E. Bel
7. Select the units of comparison (relative). (Choose all that apply.) A. dBm
B. dBi C. Decibel D. dBd E. Bel
8. 2 dBd is equal to how many dBi?
A. 5 dBi B. 4.41 dBi C. 4.14 dBi
D. The value cannot be calculated.
9. 23 dBm is equal to how many mW?
A. 200 mW B. 14 mW C. 20 mW D. 23 mW E. 400 mW
10. A wireless bridge is configured to transmit at 100 mW. The antenna cable and connectors produce a 3 dB loss and are connected to a 16 dBi antenna. What is the EIRP?
A. 20 mW B. 30 dBm C. 2,000 mW D. 36 dBm E. 8 W
11. A WLAN transmitter that emits a 400 mW signal is connected to a cable with a 9 dB loss.
If the cable is connected to an antenna with 19 dBi of gain, what is the EIRP?
A. 4 W
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12. WLAN vendors use RSSI thresholds to trigger which radio card behaviors? (Choose all that apply.)
A. Receive sensitivity B. Roaming
C. Retransmissions D. Dynamic rate switching
13. Received signal strength indicator (RSSI) metrics are used by 802.11 radios to define which RF characteristics?
A. Signal strength B. Phase
C. Frequency D. Modulation
14. dBi is a measure of what?
A. The output of the transmitter
B. The signal increase caused by the antenna C. The signal increase of the intentional transmitter
D. The comparison between an isotropic radiator and the transceiver E. The strength of the intentional radiator
15. Which of the following are valid calculations when using the rule of 10s and 3s? (Choose all that apply.)
A. For every 3 dB of gain (relative), double the absolute power (mW).
B. For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 2.
C. For every 10 dB of loss (absolute), divide the relative power (mW) by a factor of 3.
D. For every 10 mW of loss (relative), multiply the absolute power (dB) by a factor of 10.
E. For every 10 dB of loss (relative), halve the absolute power (mW).
F. For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10.
16. A WLAN transmitter that emits a 100 mW signal is connected to a cable with a 3 dB loss.
If the cable is connected to an antenna with 7 dBi of gain, what is the EIRP at the antenna element?
A. 200 mW B. 250 mW C. 300 mW D. 400 mW
17. In a normal wireless bridged network, the greatest loss of signal is caused by what component?
A. Receive sensitivity B. Antenna cable loss C. Lightning arrestor D. Free space path loss
18. To double the distance of a signal, the EIRP must be increased by how many dBs?
A. 3 dB B. 6 dB C. 10 dB D. 20 dB
19. During a site survey of a point-to-point link between buildings at a manufacturing plant, the WLAN engineer determines that the noise floor is extremely high because of all the machinery that is operating in the buildings. The engineer is worried about a low SNR and poor performance due to the high noise floor. What is a suggested best practice to deal with this scenario?
A. Increase the access points’ transmission amplitude.
B. Mount the access points higher.
C. Double the distance of the AP signal with 6 dBi of antenna gain.
D. Plan for coverage cells with a 5 dB fade margin.
E. Increase the transmission amplitude of the client radios.
20. Which value should not be used to compare wireless network cards manufactured by differ- ent WLAN vendors?
A. Receive sensitivity B. Transmit power range C. Antenna dBi
D. RSSI