Chapter 4 - Radio Frequency Components, Measurements, and Mathematics

When calculating the link budget and system operating margin of a point-to-point outdoor WLAN bridge link, which 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

A, B, C, E, F. When radio communications are deployed, a link budget is the sum of all gains and losses from the transmitting radio, through the RF medium, to the receiver radio. Link budget calculations include original transmit gain and passive antenna gain. All losses must be accounted for, including free space path loss. Frequency and distance are needed to calculate free space path loss. The height of an antenna has no significance when calculating a link budget; however, the height could affect the Fresnel Zone and blockage to it.

Select the absolute units of power. (Choose all that apply.) a. Watt b. Milliwatt c. Decibel d. dBm e. Bel

A, B, D. Watts, milliwatts, and dBms are all absolute power measurements. One watt is equal to 1 ampere (amp) of current flowing at 1 volt. A milliwatt is 1/1,000 of 1 watt. dBm is decibels relative to 1 milliwatt.

The sum of all the components from the transmitter to the antenna, not including the antenna, is known as what? a. IR b. Isotropic Radiator c. EIRP d. Intentional radiator

A, D. IR is the abbreviation for intentional radiator. The components making up the IR include the transmitter, all cables and connectors, and any other equipment (grounding, lightning arrestors, amplifiers, attenuators, and so forth) between the transmitter and the antenna. The power of the IR is measured at the connector that provides the input to the antenna.

Which of the following are valid calculations when using the rule of 10s and 30s? (Choos 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.

A, F. The four rules of the 10s and 3s are as follows: For every 3 dB of gain (relative), double the absolute power (mW). For every 3 dB of loss (relative), halve the absolute power (mW). For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10. For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10.

The highest RF signal strength that is transmited from an antenna in known as what? a. Equivalent isotropically radiated power b. Transmit sensitivity c. Total emitted power d. Antenna radiated power

A. Equivalent isotropically radiated power, also known as EIRP, is a measure of the strongest signal that is radiated from an antenna.

A WLAN transmitter that emits 400 mW signal is connected to a cable with a 9 dB loss. If the cable is connected to an antenna with 19dBi of gain, what is the EIRP? a. 4 W b. 3000 mW c. 3500 mW d. 2 W

A. If the original transmit power is 400 mW and cabling induces a 9 dB loss, the power at the opposite end of the cable would be 50 mW. The first 3 dB of cable loss halved the absolute power to 200 mW. The second 3 dB of cable loss halved the absolute power to 100 mW. The final 3 dB of cable loss halved the power to 50 mW. The antenna with 19 dBi of gain passively amplified the 50 mW signal to 4,000 mW. The first 10 dBi of antenna boosted the signal to 500 mW. The next 9 dBi of antenna gain doubled the signal three times to a total of 4 watts. Since the cable loss is 9 dB and the antenna gain is 19 dBi, you could add the two together for a cumulative gain of 10 dB and then apply that gain to the 400 mW transmit signal to calculate an EIRP of 4,000 mW, or 4 W.

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

A. The received signal strength indicator (RSSI) is a metric used by 802.11 radio cards to measure signal strength (amplitude). Some vendors use a proprietary scale to also correlate to signal quality. Most vendors erroneously define signal quality as the signal-to-noise ratio (SNR). The signal-to-noise ratio is the difference in decibels between the received signal and the background noise (noise floor).

23 dBm is equal to how many mW? a. 200 mW b. 14 mW c. 20 mW d. 23mW e. 400mW

A. To convert dBm to mW, first calculate how many 10s and 3s are needed to add up to 23, which is 0 + 10 + 10 + 3. To calculate the mW, you must multiply 1 × 10 × 10 × 2, which calculates to 200 mW. The file ReviewQuestion9.ppt, available for download from www.wiley.com/go/cwnasg, shows the process in detail.

Select the units of comparison (relative). (Choose all that apply.) a. dBm b. dBi c. Decibel d. dBd e. Bel

B, C, D, E. The unit of measurement known as a bel is a relative expression and a measurement of change in power. A decibel (dB) is equal to one-tenth of a bel. Antenna gain measurements of dBi and dBd are relative measurements. dBi is defined as decibels relative to an isotropic radiator. dBd is defined as decibels relative to a dipole antenna.

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

B, D. Received signal strength indicator (RSSI) thresholds are a key factor for clients when they initiate the roaming handoff. RSSI thresholds are also used by vendors to implement dynamic rate switching, which is a process used by 802.11 radios to shift between data rates.

A WLAN transmitter that emits a 100 Mw signal is connected to a cable with a 3dB loss. If the cable were connected to an antenna with 7dBi of gain, what would be the EIRP at the antenna element? a. 200 mW b. 250 mW c. 300 mW d. 400 mW

B. If the original transmit power were 100 mW and cabling induced a 3 dB loss, the power at the opposite end of the cable would be 50 mW. The 3 dB of cable loss halves the absolute power to 50 mW. An antenna with 10 dBi of gain would boost the signal to 500 mW. We also know that 3 dB of loss halves the absolute power. Therefore, an antenna with 7 dBi of gain would amplify the signal to half that of a 10 dBi antenna. The antenna with 7 dBi of gain passively amplifies the 50 mW signal to 250 mW.

To double the effective distance of a signal at a specific power level, the EIRP must be increased by how many dBs? a. 3 dB b. 6 dB c. 10 dB d. 20 dB

B. The 6 dB rule states that increasing the amplitude by 6 decibels doubles the usable distance of an RF signal. The 6 dB rule is very useful for understanding antenna gain because every 6 dBi of extra antenna gain will double the usable distance of an RF signal.

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

B. dBi is defined as "decibel gain referenced to an isotropic radiator" or "change in power relative to an antenna." dBi is the most common measurement of antenna gain.

2 dBd is equal to how many dBi? a. 5 dBi b. 4.41dBi c. 4.14 dBi d. The value cannot be calculated

C. To convert any dBd value to dBi, simply add 2.14 to the dBd value.

A wireless bridge is configured to transmit at 100 mW. The antenna cable and connectors produce a 3dB loss and are connected to a 16 dBi antenna. What is the EIRP? a. 20 mW b. 30 dBm c. 2000 Mw d. 36 dBm e. 8 W

C. To reach 100 mW, you can use 10s and 2s and multiplication and division. Multiplying by two 10s will accomplish this. This means that on the dBm side, you must add two 10s, which equals 20 dBm. Then subtract the 3 dB of cable loss for a dBm of 17. Because you subtracted 3 from the dBm side, you must divide the 100 mW by 2, giving you a value of 50 mW. Now add in the 16 dBi by adding a 10 and two 3s to the dBm column, giving a total dBm of 33. Because you added a 10 and two 3s, you must multiply the mW column by 10 and two 2s, giving a total of 2,000 mW, or 2 W. Since the cable and connector loss is 3 dB and the antenna gain is 16 dBi, you can add the two together for a cumulative gain of 13 dB; then apply that gain to the 100 mW transmit signal to calculate an EIRP of 2,000 mW, or 2 W. The file ReviewQuestion10.ppt, available for download from www.wiley .com/go/cwnasg, shows the process in detail.

In a normal wireless bridged network, the greated loss of signal is cause by what component? a. Receive sensitivity b. Antenna cable loss c. Lightning arrestor d. Free space path loss

D. A distance of as little as 100 meters will cause free space path loss (FSPL) of 80 dB, far greater than any other component. RF components such as connectors, lightning arrestors, and cabling all introduce insertion loss. However, FSPL will always be the reason for the greatest amount of loss.

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 worrie about a low SNR and poor performance due to the high noise floor. What is a suggested best practive 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 6dBi of antenna gain d. Plan for coverage cells with a 5 dB fade margin e. Increase the transmission amplitude of the client radios

D. In a high multipath or noisy environment, a common best practice is to add a 5 dB fade margin when designing for coverage based on a vendor's recommended received signal strength or the noise floor, whichever is louder.

Which of the following is a better indicator of what outside influences are affecting an RF signal at a specific moment in time? a. RSSI b. SNR c. EIRP d. SINR

D. The signal-to-interference-plus-noise ratio (SINR) compares the primary signal to both interference and noise. Since interference fluctuates and can change quickly, this is a better indicator of what is happening to a signal at a specific time. The signal-to-noise (SNR) compares the signal to noise; however, noise is less likely to fluctuate drastically. The received signal strength indicator (RSSI) and equivalent isotropically radiated power (EIRP) are measurements of signal but do not relate the signal to other outside influences.

Which value should not be used to compare wireless network cards manufactured by different WLAN vendors? a. Receive sensitivity b. Transmit power range c. Antenna dBi d. RSSI

D. WLAN vendors execute received signal strength indicator (RSSI) metrics in a proprietary manner. The actual range of the RSSI value is from 0 to a maximum value (less than or equal to 255) that each vendor can choose on its own (known as RSSI_Max). Therefore, RSSI metrics should not be used to compare different WLAN vendor radios because there is no standard for the range of values or a consistent scale.

A point source that radiates RF signal in all directions is known as what? a. Omnidirectional signal generator b. Omnidirectional atenna c. Intentional radiator d. Nondirectional transmitter e. Isotropic radiator

E. An isotropic radiator is also known as a point source.