The power emitted from wireless devices, especially in unlicensed bands, such as 2.4 GHz and 5 GHz, are regulated by the Federal Communications Commission (FCC). Wireless network professionals must calculate power levels (or RF signal strength) that are being transmitted by wireless devices to make sure their designs are complaint with FCC regulations. They also need to calculate power levels that are being received by wireless devices to make sure the signal is properly received at the destination.
RF power can be measured in two ways: on the linear scale, by the number of watts that are being transmitted; and on a relative scale, by the number of decibels (dBs) instead of watts.
Decibel milliwatt (dBm) is thelogarithmicpower ratio (in dB) of the measured power in milliwatts referenced to one milliwatt (mW). Notice that the reference point is specified as 1 mW = 0 dBm.
3’s and 10’s rules are shortcuts for estimating the increase or decrease of these power levels.
In this lab, students will practice basic RF calculations, including
1. converting from mW to dBm;
2. converting from dBm to mW; and
3. estimating power levels using the 3’s and 10’s rules.
Task 1: Converting between dBm and mW
Applying the 3’s and 10’s rules, the relationship between dBm and mW is estimated as shown in the following (partial) table.
3’s rule
10’s rule
……
……
0.125 mW = -9 dBm
0.001 mW = -30 dBm
0.25 mW = -6 dBm
0.01 mW = -20 dBm
0.5 mW = -3 dBm
0.1 mW = -10 dBm
1 mW = 0 dBm
1 mW = 0 dBm
2 mW = 3 dBm
10 mW = 10 dBm
4 mW = 6 dBm
100 mW = 20 dBm
8 mW = 9 dBm
1,000 mW = 30 dBm
……
……
Notice that as the mW value increases or decreased by the factor of 10, the dBm value increases and decrease by adding or subtracting 10. As the mW value doubles or halves, the corresponding dBm value increases and decrease by adding or subtracting 3.
1. Apply 3’s rule to estimate what 16 mWs is in dBm. _____
2. Apply 3’s rule to estimate what 0.0625 mWs is in dBm. _____
3. Apply 10’s rule to estimate what 10,000 mWs is in dBm. _____
4. Apply 10’s rule to estimate what 0.0001 mWs is in dBm. _____
5. Apply 3’s rule to estimate what 15 dBm is in mW. _____
6. Apply 3’s rule to estimate what -15 dBm is in mW. _____
7. Apply 10’s rule to estimate what 50 dBm is in mW. _____
8. Apply 10’s rule to estimate what -50 dBm is in mW. _____
Task 2: Estimating power levels using 3’s and 10’s rules
In cascaded systems with power gain and/or loss along a chain of subsystems, power levels given in dB forms can be directly added or subtracted to calculate the total power.
For instance, a transmitter with a power of 9 dBm, after a cable and connector loss of 3 dB, ends up with a power of 9 dBm ? 3 dB = 6 dBm or 4 mW. Notice that the simple addition and subtraction here doesn’t apply to powers given in mW.
If one were to subtract 3 dB from 8 mW (i.e., mixed measurement units), 8 mW should be converted to 9 dBm first. The result is in dBm which can be converted to mW when needed.
9. Applying the 3’s and 10’s rules, what’s 0.125 mW ? 30 dBm in dBm? _____
10. Applying the 3’s and 10’s rules, what’s 0.125 mW ? 30 dBm in mW? _____
11. Applying the 3’s and 10’s rules, what’s 0.0625 mW ? 40 dBm in dBm? _____
12. Applying the 3’s and 10’s rules, what’s 0.0625 mW ? 40 dBm in mW? _____
13. Applying the 3’s and 10’s rules, what’s 8 mW + 30 dBm in dBm? _____
14. Applying the 3’s and 10’s rules, what’s 8 mW + 30 dBm in mW? _____
15. Applying the 3’s and 10’s rules, what’s 16 mW + 40 dBm in dBm? _____
16. Applying the 3’s and 10’s rules, what’s 16 mW + 40 dBm in mW? _____
The same idea can be applied to conversions from dBm to mW when a dBm value is not readily available in the 3’s and 10’s rules table.
For instance, what is 13 dBm in mW?
First, 13 dBm = 0 dBm + 10 dBm + 3 dBm, where 0 dBm, 10 dBm, and 3 dBm are in the table.
Next, let’s apply the following 3’s and 10’s rules:
a) 0 dBm = 1 mW
b) When the dBm value increases by adding 10, the mW value increases by the factor of 10.
c) When the dBm value increases by adding 3, the mW value increases by the factor of 2.
17. Applying the 3’s and 10’s rules, what’s 7 dBm in mW (must show steps)?
Answer:
18. Applying the 3’s and 10’s rules, what’s 4 dBm in mW (must show steps)?
Answer:
19. Applying the 3’s and 10’s rules, what’s 9 dBm in mW (must show steps)?
Answer:
20. Applying the 3’s and 10’s rules, what’s -7 dBm in mW (must show steps)?
Answer:
NETW360 Week 1 iLab: Basic RF Calculations
Date:
Student’s Name:
Professor’s Name:
Task 1: Converting between dBm and mW
1. Apply 3’s rule to estimate what 16 mWs is in dBm. _____
2. Apply 3’s rule to estimate what 0.0625 mWs is in dBm. _____
3. Apply 10’s rule to estimate what 10,000 mWs is in dBm. _____
4. Apply 10’s rule to estimate what 0.0001 mWs is in dBm. _____
5. Apply 3’s rule to estimate what 15 dBm is in mW. _____
6. Apply 3’s rule to estimate what -15 dBm is in mW. _____
7. Apply 10’s rule to estimate what 50 dBm is in mW. _____
8. Apply 10’s rule to estimate what -50 dBm is in mW. _____
Task 2: Estimating power levels using 3’s and 10’s rules
9. Applying the 3’s and 10’s rules, what’s 0.125 mW ? 30 dBm in dBm? _____
10. Applying the 3’s and 10’s rules, what’s 0.125 mW ? 30 dBm in mW? _____
11. Applying the 3’s and 10’s rules, what’s 0.0625 mW ? 40 dBm in dBm? _____
12. Applying the 3’s and 10’s rules, what’s 0.0625 mW ? 40 dBm in mW? _____
13. Applying the 3’s and 10’s rules, what’s 8 mW + 30 dBm in dBm? _____
14. Applying the 3’s and 10’s rules, what’s 8 mW + 30 dBm in mW? _____
15. Applying the 3’s and 10’s rules, what’s 16 mW + 40 dBm in dBm? _____
16. Applying the 3’s and 10’s rules, what’s 16 mW + 40 dBm in mW? _____
17. Applying the 3’s and 10’s rules, what’s 7 dBm in mW (must show steps)?
Answer:
18. Applying the 3’s and 10’s rules, what’s 4 dBm in mW (must show steps)?
Answer:
19. Applying the 3’s and 10’s rules, what’s 9 dBm in mW (must show steps)?
Answer:
20. Applying the 3’s and 10’s rules, what’s -7 dBm in mW (must show steps)?
Answer:
NETW360 Week 2 iLab: RF Behavior Calculations
In this Lab, students use an online calculator to compute power, cable loss, antenna gain, free space path loss, link budget, and Fresnel zone clearance.
Go towww.swisswireless.org/wlan_calc_en.html and locate the online calculator. If the web page has been moved, try searching for “swiss wireless”, without the double quotation marks, on the Internet to locate the main page.
a) The impedance of the cable needs to match that of the antenna and wireless transceiver to avoid signal loss caused by the voltage standing wave ration (VSWR) effect.
b) Not all coaxial cables support the transmission of 2.4 GHz and 5 GHz signals.
c) Given an RF cable with a certain length, signal loss/attenuation in the cable increases with frequency.
d) Given a RF cable and a signal frequency, signal loss/attenuation in the cable increases with distance.
Use the calculator in the Loss in a Coaxial Cable at 2.45 GHz section to complete the following steps:
1. Next to Choose type of cable, select LMR 400. This is a TMS cable that supports both 2.4 GHz and 5 GHz RF signals. 100 feet of such cable used on the 2.5 GHz range decreases the signal strength by about 6.76 dB (that is, 6.76 dB signal loss per 100 feet).
2. Click in the Length (meter) box and type 30.48 (100 feet = 30.48 meters). Click m?dB. What is the loss at this length? _____
3. Click in the Length (meter) box and type 60.92 (200 feet = 30.48 meters). Click m?dB. What is the loss at this length? _____
4. When the cable length doubles, how does the loss change approximately? _____
Task 3: Antenna gain calculations
Antennas are often used to increase the power output of a transmitter. Antennas achieve this by focusing the existing power in a specific direction. Notice that the amount of power provided to the antenna from the transmitter does not change; the signal gain created by antennas is a passive gain.
Antenna gain in dBi or dBd is a parameter that describes the directionality characteristic of an antenna. Given a particular type of antenna, the higher the antenna gain, the more directional the antenna is, and the more focused the existing power is in a specific direction.
Parabolic or dish antennas are an example of highly directional antennas. Due to their relatively high antenna gain, dish antennas are typically used for point-to-point transmission links. The antenna gain of a parabolic antenna is directly related to the diameter of a dish antenna’s reflector and the signal frequency.
Use the calculator in the Antenna section to complete the following steps:
1. Next to frequency band, select 2.41–2.48 GHz.
2. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches). This is an optional antenna that could be added to an access point (AP).
3. Click D ? dB. What is the maximum theoretical antenna gain? _____
4. Next to frequency band, select 5.15–5.85 GHz.
5. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches)
6. Click D ? dB. What is the maximum theoretical antenna gain? _____
7. Given the same sized reflector, which signals, high-frequency or low-frequency, can be more efficiently focused by parabolic antennas (i.e., result in a higher antenna gain)?
Answer:
8. Next to frequency band, select 5.15–5.85 GHz.
9. Next to antenna diameter in meters, type 0.2 (0.1 meters = 7.8 inches)
10. Click D ? dB. What is the maximum theoretical antenna gain? _____
11. Given the same signal frequency, which dish antennas, large-sized or small-sized, are more efficient at focusing the signal (i.e., result in a higher antenna gain)?
Answer:
Task 4: Free space loss calculations
Free space path loss (FSPL) is the amount of signal loss/attenuation caused by signal dispersion over a distance. As does the light emitted from a flash light, RF signals spread out and weaken when propagating from an antenna. Notice that FSPL occurs regardless of the obstacles that cause reflection, diffraction, etc.; this is indicated by the “free space” phrase in its name.
Use the calculator in the Free space loss section to complete the following steps:
1. Next to frequency band, select 2.41–2.48 GHz.
2. Next to kilometers, type 0.1 (100 meters = 0.1 kilometers).
3. Click dB ? km. What is the free space path loss in dB? _____
4. Change the frequency band to 5.15–5.85 GHz.
5. Next to kilometers, type 0.1 (100 meters = 0.1 kilometers).
6. Click dB ? km. What is the free space path loss in dB? _____
7. How does the free space path loss for 802.11a (operating on the 5 GHz band) compare with 802.11g (operating on the 2.4 GHz band)?
Answer:
8. Next to frequency band, select 2.41–2.48 GHz.
9. Next to kilometers, type 0.02 (20 meters = 0.02 kilometers).
10. Click dB ? km. What is the free space path loss in dB? _____
11. Next to kilometers, type 0.04 (40 meters = 0.04 kilometers).
12. Click dB ? km. What is the free space path loss in dB? _____
13. Next to kilometers, type 0.08 (80 meters = 0.08 kilometers).
14. Click dB ? km. What is the free space path loss in dB? _____
15. When the distance doubles, how does free space path loss in dB change?
Answer:
16. Next to frequency band, select 17.1–17.3 GHz.
17. Next to kilometers, type 1.
18. Click dB ? km. What is the free space path loss in dB? _____
19. Next to kilometers, type 2.
20. Click dB ? km. What is the free space path loss in dB? _____
21. Next to kilometers, type 4.
22. Click dB ? km. What is the free space path loss in dB? _____
23. When the distance doubles, how does free space path loss in dB change?
Answer:
Without the calculator, we could use the 6 dB rule to estimate free space loss: doubling the distance results in a signal loss/attenuation of 6 dB.
Task 5: Link Budget Calculations
The ultimate goal of link budget calculations is to ensure that the received signal strength is above the receive sensitivity of the receiver. A link budget is computed by adding and subtracting gains and losses represented in dB forms from the original power level of the transmitter or IR.
Fade margin is the amount of desired signal (i.e., the received signal) above what is required (i.e., the receive sensitivity). If the receive sensitivity of a receiver is -75 dBm, and the received signal is measured as -75 dBm, a transmission link may or may not be successful. The fact is that the received signal strength cannot be maintained at -75 dBm, due to interference, obstacles, and weather conditions. A 10 dB to 25 dB margin is commonly planned to accommodate received signal strength fluctuations. This range may seem to be wide, but the longer a transmission link (especially outdoors), the higher the margin should be.
Answer:
Task 6: Link budget calculations
Between two point-to-point antennas, the area that surrounds the visual line of sight path (i.e., the straight line drawn between two antennas) is called the Fresnel zone. If trees, buildings, and other obstacles encroach on this football shaped area, RF signals could experience loss caused by reflection, scattering, and diffraction. This contributes to signal loss fluctuation, and could cause a transmission link to fail.
To determine if a tree or building is obstructing the Fresnel zone, the radius of the Fresnel zone at the location of the potential obstacle is calculated. Actions needed to maintain Fresnel zone clearance include removing the obstacle or raising the antenna. Often, this Fresnel zone clearance is relaxed by 40%, that is, only 60% of the Fresnel zone is clear of obstacles.
Use the calculator in the Fresnel ellipsoid section to complete the following steps:
1. Next to distance “D” between transmitter and receiver [meters], type 114. This is close to the maximum distance of an office WLAN.
2. Next to distance “d” between transmitter and obstacle [meters], type 65. This assumes an obstacle is at the midpoint between two antennas.
3. Click Compute radius. What’s the radius of the Fresnel zone at the middle point? _____
4. Next to distance “D” between transmitter and receiver [meters], type 16000. This refers to an approximately 10-mile outdoor point-to-point transmission link.
5. Next to distance “d” between transmitter and obstacle [meters], type 4800. The obstacle is about 3 miles from one antenna.
6. Click Compute radius. What’s the radius of the Fresnel zone at this specific location?
Answer:
NETW360 Week 2 iLab: RF Behavior Calculations
Date:
Student’s Name:
Professor’s Name:
Task 1: Power calculations
Use the calculator in the Power section to complete the following steps:
1. On the lower UNII-1 band (i.e., 5.150–5.250 GHz with 100 MHz channels), the maximum output power of the intentional radiator (IR) allowed by the FCC is 50 mW. The IR is also referred to as a wireless transmitter.
Click in the watts box and type 0.05 (50 mW = 0.05 watts). What is the dBm of 50 mW?
__________
2. On the middle UNII-1 band (i.e., 5.250–5.350 GHz with 100 MHz channels), the maximum output power of the intentional radiator (IR) allowed by the FCC is 250 mW.
Click in the watts box and type 0.25 (250 mW = 0.25 watts). What is the dBm of 250 mW?
__________
3. On the upper UNII-1 band (i.e., 5.725–5.825 GHz with 100 MHz channels), the maximum output power of the intentional radiator (IR) allowed by the FCC is 800 mW.
Click in the watts box and type the given power level in watts. What is the dBm of 800 mW?
__________
Scroll down to the Receive Sensitivity section. Review the information regarding receive sensitivity.
4. The receive sensitivity of a LinksysWUSB600N wireless network adaptor at 54 Mbps is -70 dBm. Click in the dBm box and type -70. What are the watts of -70 dBm?
__________
5. The receive sensitivity of a LinksysWUSB300N wireless network adaptor at 54 Mbps is -68 dBm. Click in the dBm box and type -68. What are the watts of -68 dBm?
__________
6. If you have to choose between these adaptors based on their receive sensitivity at the bit rate of 54 Mbps, which adaptor will potentially perform better in achieving the desired bit rate?
__________
Task 2: Cable loss calculations
Use the calculator in the Loss in a Coaxial Cable at 2.45 GHz section to complete the following steps:
1. Next to Choose type of cable, select LMR 400. This is a TMS cable that supports both 2.4 GHz and 5 GHz RF signals. 100 feet of such cable used on the 2.5 GHz range decreases the signal strength by about 6.76 dB (that is, 6.76 dB signal loss per 100 feet).
2. Click in the Length (meter) box and type 30.48 (100 feet = 30.48 meters). Click m?dB. What is the loss at this length? __________
3. Click in the Length (meter) box and type 60.92 (200 feet = 30.48 meters). Click m?dB. What is the loss at this length? __________
4. When the cable length doubles, how does the loss change, approximately? __________
Task 3: Antenna gain calculations
Use the calculator in the Antenna section to complete the following steps:
1. Next to frequency band, select 2.41–2.48 GHz.
2. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches). This is an optional antenna that could be added to an access point (AP).
3. Click D? dB. What is the maximum theoretical antenna gain? __________
4. Next to frequency band, select 5.15–5.85 GHz.
5. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches)
6. Click D? dB. What is the maximum theoretical antenna gain? __________
7. Given the same sized reflector, which signals, high-frequency or low-frequency, can be more efficiently focused by parabolic antennas (i.e., result in a higher antenna gain)?
__________
8. Next to frequency band, select 5.15–5.85 GHz.
9. Next to antenna diameter in meters, type 0.2 (0.1 meters = 7.8 inch
week 3
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NETWNETW360 Week 6 iLab
MHz).
5. Let’s assume some network parameters of the “linksys” access point have been changed. After several seconds, the information shown in the inSSIDer window is also changed as captured below.
6. Refer to the truncated list of detected access points from the middle section of the inSSIDer window shown below and answer the following questions.
a) Which CHANNEL(s) is (are) used by the “linksys” access point now?
b) What’s the MAX RATE of this access point or network?
c) What SECURITY standard is used by this access point or network?
d) What’s its NETWORK TYPE?
7. Refer to the detailed information about the “linksys” access point shown below and answer the following questions.
a) How many Co-Channels are listed here?
b) What’s the Overlappingvalue?
c) Explain what the Overlapping value means.
8. Refer to the graphical representation of the access points shown in the diagram below and answer the following questions:
a) What’s the transmission channel width (in MHz) of the “linksys” access point now?
b) If the “linksys” access point is meant to work in harmony with other access points in the same environment, what went wrong with the configuration changes?
Task 2: Monitoring Signal Strength
The amplitude of a wireless signal measured by WLAN and site survey tools is referred to as received signal strength indication (RSSI). Higher RSSI values are often associated with higher data rates and fewer retransmissions. RSSI is typically used by vendors for purposes such as roaming handoff and data rate switching. It could also be used, during a site survey, to mark the coverage boundary of an access point. For instance, Cisco recommends the minimum RSSI of -67 dBm for voice applications.
Notice that RSSI is an indicator of relative signal strength; it could be mapped to the actual signal strength based on each vendor’s specifications. Different vendors’ tools most likely provide different RSSI values in the same environment. Therefore, RSSI values from different vendors should not be directly compared to each other.
Another parameter typically used in site surveys to define the coverage boundary of an access point is the signal-to-noise (SNR) ratio. It is an indicator of how much stronger a measured signal is in relation to the noise level. Higher SNR values typically indicate higher data rates, fewer retransmissions, and better throughput. For instance, Cisco recommends the minimum SNR of 25 dB for voice applications.
The following inSSIDer screenshots demonstrate how the Signal and Link Score values change when the wireless client with inSSIDer installed moves away from the “linksys” access point. As explained in MetaGeek’s quick start guide for the inSSIDer Home software, Signal is “the amplitude level of the wireless network as seen by your computer’s wireless adapter, also known as RSSI.” Link Score is “a grade for each network calculated by its signal strength, channel power, and number of networks competing for airtime.”
1. Refer to the detailed information about the “linksys” access point shown below and answer the following questions.
a) What’s the Signal strength of this access point when the screenshot was taken?
b) What’s the Link Score?
2. The laptop with the insider software installed is moved away from the “linksys” access point for a certain distance in the same room, but hidden underneath a computer table.
Refer to the detailed information about the “linksys” access point shown below and answer the following questions.
a) What’s the Signal strength of this access point when the screenshot was taken?
b) What’s the Link Score?
3. At the same location, position the laptop with inSSIDer installed on top of the computer table.
Refer to the detailed information about the “linksys” access point shown below and answer the following questions.
a) What’s the Signal strength of this access point when the screenshot was taken?
b) What’s the Link Score?
c) What could be the reason that the signal strength is higher, although the distance between the access point and the wireless client (with inSSDIer installed) has not changed?
4. The laptop with the insider software installed is moved further away from the “linksys” access point into a different room across the hall.
Refer to the detailed information about the “linksys” access point shown below and answer the following questions.
a) What’s the Signal strength of this access point when the screenshot was taken?
Solution: devry netw360 all weeks ilabs latets 2015 spring [ all 7 ilabs ]