Flashcards in Chapter 4. Deck (60):
How many Fresnel zones are there? How many before zones effect on communication?
Infinite, and only the first 2 have an effect.
How much signal is lost every 10 dB?
How would you connect multiple buildings together using an omni antenna? What if a building is a different height?
High gain omni using a dipole antenna on center building. Other buildings use highly directional pointing at omni. To fix a high problem you use a sector antenna with a down tilt or up tilt.
Indoor installations typically use low-gain omni's with a gain of what? and why?
2.14 dBi. Because when you increase you reduce vertical polarization so your horizontal or multi floor coverage reduces. So, when you increase horizontal gain your limiting the RF coverage to that floor and that floor only.
Information parameters from both RSSI and SQ metrics can be passed along from the PHY layer to what?
The MAC sublayer.
Is polarization very important indoors, and why?
Polarization is not as important for indoor communication because the polarization of the RF signal often changes when it is reflected, a common occurrence indoors.
What are highly directional antennas used for, what are they 2 types, and what do they look like?
They are used to provide highly focuses, high gain, RF signal to another location. Used for point to point linking of buildings over potentially long range. There are 2 types, parabolic dishes that look like satellite TV antennas or Grid antenna, which look like circular barbecue grills.
What are semi directional antennas ? How far can they travel outside ? What are some indoor purposes for them ? What are the 3 types of semi directional antennas ?
Designed to direct a signal in a specific direction. Used for short or medium communication. They are used for up to 1 mile communication outside. Inside they are used for libraries, warehouses, and places with long isles. Patch, Panel, Yagi.
What are the 3 things that MUST be known and calculated before a long distance point to point link will function? Is there a formula that can be used to calculate all of this?
* The 60 percent radius of the first Fresnel zone
* The height of the earth bulge
* the distance between the antenna in miles.
What are Yagi antennas and what are they used for?
They are semi directional and can be used for communication upwards of 2 miles.
What beamwidth and how is it measured?
The measurement of how broad or narrow the focus of an antenna is and his measured both horizontally and vertically.
It is the measurement from the center, or strongest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power or 3 dB.
What causes impedance mismatches? How do you prevent them?
Due to abrupt impedance changes between between the radio transmitter and cable and between the cable and the antenna. You prevent them from having all the components along the line have the same impedance.
What causes standing radio wave ?
When the AC radio signal travels along the cable to the antenna, some of the energy is reflected back toward the transmitter because of impedance mismatches.
What do most vendors include when they talk about RSSI metrics?
SQ or signal quality and RSSI value.
What does ISM stand for?
Industrial, scientific, and medical.
What happens when you increase the focus or the gain of an omni antenna in terms of polarization ?
The vertical polarization is reduced and the horizontal is increased.
What is a back lobe?
The RF signal behind the antenna.
What is a collinear antenna ?
A higher-gain omni antenna typically constructed by stacking multiple antennas on top of each other.
What is a good reason to use Grid antenna ?
In high wind load environments. They are less susceptible to wind.
What is a good rule of thumb when calculator fade margins indoors and the RSSI value of devices?
When deploying a WLAN indoors where high multipath or high noise floor conditions exist, the best practice is to plan for a fade margin of about 5 dB above vendors RSSI value.
What is a highly directional antenna?
Provides RF coverage to a very specific area.
What is a patch antenna ? What is another name for it?
Refers to a particular way of designing the radiating elements inside the antenna. Also known as a planar.
What is a sector antenna ?
Are special high-gain semi directional antenna that provides a pie-shaped coverage pattern. These antenna are typically installed in the middle of the coverage area.
What is a sectorized array?
Combining multiple sector antenna to provide 360 degrees of horizontal coverage.
What is a semi directional antenna ?
Provides RF coverage to a general area.
What is a standing radio?
The combination of incident and reflected waves traveling back and forth along the cable creates a resulting standing wave pattern along the length of the line.
What is an adaptive antenna?
Array that maneuvers the beam in the direction of a targeted receiver.
What is an antenna array ?
Is a group of two or more antennas that are integrated together to provide coverage
What is an azimuth chart and elevation chart?
Commonly known as radiation patterns. These radiation patterns are taken inside of a controlled environment where outside noise ca not alter the readings.
What is an omnidirectional antenna and why is its purpose ? What is a good antenna example of these?
Radiates RF energy in all directions. Used to provide general coverage in all directions. Dipole antenna or rubber ducks.
What is antenna diversity? What PHY used this ?
Pre 802.11 N and AC used this.
When the access point uses more than 1 antenna to receive a signal. The AP compares the signal that it is receiving on both antennas and uses whichever antenna has the higher signal strength to receive the frame of data. This sampling is performed on a frame to frame basis.
What is Beamforming, and what are three types ?
A method of concentrating RF energy. Concentrating a signal means that the power of the signal will be greater and the SNR at the receiver should therefore also be greater, providing a better transmission. The three types are, static beamforming, dynamic beamforming, and transmit beamforming.
What is dynamic beamforming?
Focuses the RF energy in a specific direction and in a particular shape. The radiation pattern of the signal can change on a frame by frame basic. This provides optimal power and signal to each station.
What is fade margin?
The level of desired signal above what is required. If the receiver has a receive sensitivity of -80 dBm a transmission will be successful as long as the signal received is greater than -80 dBm. The signal also needs to be adjusted for variables, such as weather, and other outside influences.
What is half-power points ?
The distance between the two half-power points on the horizontal and vertical axis is measured in degrees, giving the horizontal beam and the vertical width measurement.
What is impedance and what is the values name?
Impedance is a value of ohms of electrical resistance to an AC signal.
What is link budget and what is the purpose of the budget calculations?
Link budget is the sum of all the planned and expected gains and losses from the transmitting radio, through the RF medium, to the receiver radio. The purpose of the budgeting is to guarantee that the final received signal amplitude is above the receiver sensitivity threshold of the receiver radio.
What is MIMO ? What PHY used it?
A sophisticated form of antenna diversity. Wireless radio architecture that can receive or transmit using multiple antennas concurrently. Complex signal-processing techniques enable significant enhancements to reliability, range, and throughout in MIMO. 802.11 N and AC use this.
What is return loss?
Essentially the dB difference between the power sent to the antenna and the power reflected back, thus a higher value is better than a lower value ?
What is standing radio wave ratio?
A measurement of the change in impedances to an AC signal. Voltage standing waves exist because of impedance mismatches or variations between devices in an RF communication system.
What is static beamforming?
This is performed by using directional antennas to provide a fixed radiation pattern. Static beamforming uses multiple directional antennas, all clustered together by aimed away from the center point or location. For example having an antenna with 3 2.4 Ghz antennas in a sector providing non overlapping channel coverage plus 5 5.0 Ghz antennas.
What is switched diversity? What is it used with? What PHY used it?
Listens with multiple antennas. Multiple copies of the same signal arrive at the receivers antennas with different amplitudes. The signal with the best amplitude is chosen and the other signals are ignored. It is used with transmitting, as the antenna is used to transmit on the antenna that last received the best signal. Pre 802.11 N and AC used this.
What is the 6 dB rule? What is the NAME of this rule?
That for every 6 dB that is lost or gain the signal DISTANCE is cut in half or doubled.
Remember the 6 dB rule: +6 dB doubles the distance of the usable signal;
–6 dB halves the distance of the usable signal.
Inverse square law.
What is the first fresnel zone?
Technically the area around the point source, where the waves are in phase with the point source signal.
What is the Fresnel zone?
An imaginary, elongated, football-shaped area, that surrounds the path of the visual LOS between two point-to-point antennas.
What is the mathematical formula to calculate the radius of the first Fresnel zone at the mid=point between the two antennas ? What does this formula tell you about the antenna, and why is it so important?
radius = 72.2 * 3 /[D / (4*F)]
D = Distance of the link in miles
F = Transmitting frequency in Ghz.
What is the max distance of a point-to-point antenna before you need to take into account the Earth Bulge?
What is the maximum allowed encroachment of the Fresnel zone in percentages?
What is the second fresnel zone?
The area beyond the first fresnel zone, where the waves are out of phase with the point signal source.
What is the system operating margin (SOM)
The SOM is the difference between the actual received signal and the signal necessary for reliable communications. Part of your post assessment. Validation.
What is transmit beamforming? What is the downside to this?
Performed by transmitting multiple phase-shifting signals with the hope and intention that they arrive in phase at the location where the transmitter believes the receiver is located. It is a digital signal processing technology on the transmitting device that duplicates the transmitted signal on more than one antenna to optimize a combined signal at the client. The downside is that the client has to support this, and non do.
What is transmit diversity? What PHY used this? Pre 802.11 N and AC used this.
Transmitting out of an antenna where the last best received signal was heard.
What point of view is an elevation chart or e-plane?
What point of view is an H plane or Azimuth chart?
What should you do with the antenna positioning with a multiple antenna access point and why ?
You should align then slightly off parallel with each other to create multi-path, allowing better reception.
What type of antenna should used in high density areas such as meeting halls, gyms ... and why ?
Sector antennas. They mount on the ceiling and point downward, providing tight areas of coverage only. Omni spreads out way to far, and provides way to much coverage. Omni would utterly be overwhelmed with traffic.
Where can a planar antenna be used in a hospital ?
For the very long hallways. Depending on the beam width, gain, and attenuation of the building, this antenna might be able to cover the room on each side of the hallway as well.
Why can a 802.11 network adapter only estimate noise floor and SNR?
Because the network adapter can only understand binary data and does not have a Spectrum analyzer aboard.
Cant not actually sense interference.
Used math and complex algorithms to estimate SNR.
Newer chips are actually using the antenna on the chip to calculate SNR, but the radio can only work as a rudimentary spectrum analyzer and not an RF antenna at the same time.
Why do you not want to place high gain omni's to far off the ground?
Because the beam flattens outward and takes away from the vertical coverage so you cant have it to high off the ground.