14 - Understand Wireless Principles Flashcards
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A ___ can be defined as an electromagnetic field that radiates from a sender. This __ propagates to a receiver, which receives its energy. Light is an example of this, as the eye can interpret light and send its energy to the brain, which in turn transforms this light into the impression of colours.
radio wave, wave
Different waves have different sizes, typically expressed in __
metres
Another unit of measurement is __ that expresses how often a wave occurs per second
hertz
Depending on the type of information sent, certain ____ are more efficient than others in the air because they have different properties
wave groups
___ determines how often a signal is seen. A wave is always sent at the speed of light because it is an electromagnetic field. Therefore, the wave takes shorter or longer time to travel once cycle depending on its length
frequency
Cycles per second, so 2 cycles per second is 2hz..
Frequency
A signal that occurs 1 million times per second
megahertz
A signal that occurs 1 billion times per second
gigahertz
___ is defined as the physical distance that the wave covers in one cycle
Wavelength
the size of the cycle pattern of an electromagnetic wave
Wavelength
How is a radio wavelength measured?
metres
___ can be defined as the strength of the signal. In a graphical representation, ___ is seen as the distance between the highest and lowest crests of the cycle.
amplitude
___ also affects the signal because it represents the level of energy that is injected in one cycle. The more energy that is injected in a cycle, the higher the ___
amplitude
Amplification can be __ or __
active, passive
What is active amplication?
When applied power is increased
What is passive amplication?
This is accomplished by focussing the energy is one direction by using an antenna
Amplitude can also be decreased – this decrease is called ___.
attenuation
The attenuation of the signal strength on its way between a sender and receiver
free path loss
As the signal or wave travels away from the AP, it is affected by any ___ that it encounters
obstacles
The word “__” refers to the fact that loss of energy is simply as a result of distance, not obstacle. Using the word __ is important when referring to __ path loss, because ‘path loss’ takes into consideration other sources of loss
free
What causes ____ is not distance itself as there is no physical reason why a signal is weaker farther away from the source
free path loss
___ is a measurement of how well a device can receive the signal. More commonly this is called signal value. __ is usually expressed in decibels to 1 milliwatt (dBm)
Received Signal Strength Indicator (RSSI)
___ is a grade value which can range from 0 (no signal or no reference) to a maximum of 255. Many vendors use a maximum that is lower than 255 (e.g 100 or 60)
RSSI
From this RSSI grade value, an equivalent __ is displayed. Again this depends on the vendor
dBm
__ can be caused by wireless devices such as cordless phones and microwaves. This is measured in decibels from 0 to -120. The ___ level is the amount of interference in your wifi signal, so the lower value, the better. A typical __ would be -95 dB
Noise (noise floor)
__ is the evaluation of signal strength after its been affected by noise
Signal to noise ratio (SNR)
SNR compromises of two values:
- RSSI
* Noise (any signal that interferes with your signal)
To calculate the SNR, subtract the ___ value from the ___. Because both values are usually expressed as negative numbers, the result is a positive number expressed in decibels.
noise, RSSI
___measure the amount of power relative to a reference, or to an isotropic antenna
Decibels dB
When converting between decibels and milliwatts, use the rules of “_ and __”:
3, 10
What is the rule of 3 and 10?
- 0 dB = same power
- +3 dB = 2 mW (2 times the power
- -3 dB = .5 mW (1/2 the power)
- +10 dB = 10 mW (10 times the power
- -10 dB = .1 mW (1 tenth of power)
Decibels are used extensively in wifi networks to compare __
power
Transmitters express power in ___
milliwatts
An ___ sends an electromagnetic field. Engineers need to compare the power of ___s without using the indirect value of the current that is sent, and they do so by measuring the power gain relative to the reference ___
antenna
The scale that is used to compare the powers that antennas radiate to an isotropic antenna is called ___
dBi (the “I” stands for isotropic)
The logarithm progression for the dBi scale obeys the same rules as for the other ___ scales (3 and 10).
decibels
Comparing antennas give a measure of their __.
gain
__ measures how much energy is radiated from an antenna toward the main beam
effective isotropic-radiated power (EIRP)
When an antenna concentrates the energy it receives from the cable in one direction, it is said to be more powerful (in this direction) than an antenna that radiates the energy in all directions because there is more __ in this one direction
signal
Wifi engineers need a way to determine how much energy is actually radiated from the antenna toward the main beam. This measure is called ___
EIRP
___ tries to express, in isotropic equivalents, how much energy is radiated in the beam. In mathematical terms, ___, expressed in dBm, is simply the amount of Tx power plus the gain (on dBi) of the antenna. The signal might go through a cable in which some power might be lost, so the cable loss must be deducted.
EIRP
EIRP can be expressed as..
EIRP = Tx power (dBm) + antenna gain (dBi) – cable loss (dB).
Two main families of antennas are ___ and ___.
omnidirectional, directional
Because a directional antenna is stronger in a specific direction, it is said to add more __. A directional antenna will be more powerful in that specific directional than an omnidirectional antenna
gain
Antennas radiate in 3D but providing a 3D view of the radiation pattern relies on specific software. Therefore, vendors usually provide two views:
horizontal plane, elevation plane
a _____ represents the radiation pattern as seen from the top. This shows how the signal spreads ahead, behind, to the right, left, but not how the signal spreads up or down.
horizontal plane (H-plane or azimuth chart)
The ___ represents the radiation pattern as seen from the side of the antenna. This chart shows how the signal spreads ahead, behind, to the top, bottom, but not how it spreads left or right. Side view.
elevation plane (E-plane)
AP and clients that only support 802.11a/b/g are considered legacy. The systems use a single transmitter, talking to a single receiver to provide wifi connection. A legacy device that used ___ has only one radio that switches between antenna. The radio determines which antenna provides the strongest signal and switches to the best. Only one is used at a time
SISO
802.11n/ac makes use of multiple antennas and radios, which are combined with advance signal processing methods to implement a technique known as ___. Several transmitter antennas send several frames over several paths. This improves reliability, and better SNR, therefore reduces the likelihood packets will be dropped or lost
MIMO
MIMO incorporates…
- Maximal Ratio Combining (MRC)
- Beamforming
- Spatial Multiplexing
A receiver with multiple antennas uses ___ to optimally combine energies from multiple receive chains. An algorithm eliminates out-of-phase signal degradation.
Maximal Ratio Combining (MRC)
__ is the counterpart of Tx beamforming and takes place on the receiver side, usually on the AP.
Maximal Ratio Combining (MRC)
Tx ___ is a technique that is used when there is more than one Tx antenna. The signal that is sent from each antenna can be coordinated so that the signal at the receiver is dramatically improved, even if the antenna is far from the sender. This is generally used when the receiver has only one antenna
beamforming
____requires both an 802.11n/ac-capable transmitter and receiver. Requiring a minimum of two receivers and a single transmitter per band while supporting as many as four transmitters and receivers per band.
spatial multiplexing
Under ___, a signal stream is broken into multiple individual streams, each of which is transmitted from a different antenna, using its own transmitter. Because there is space between each antenna, each signal follows a different path to the receiver. This phenomenon is known as spatial diversity.
spatial multiplexing
With 802.11n, a device ca Tx multiple spatial streams at once but only to a single address. For individually addressed frames, it means only a single device or user receives data at a time. This is called ___.
SU-MIMO
802.11ac provides a feature call ___ where an AP is able to use its antenna resources to Tx multiple frames up to four different clients all at the same time over the same frequency spectrum.
MU-MIMO
What are the 802.11 connection states between a wireless station and an AP?
- Not authenticated or associated
- Authenticated but not yet associated
- Authenticated and associated
A mobile station must be in an __ and ___ state before an AP bridging will occur.
authenticated, associated
The mobile station and AP will exchange a series of ______ to get to an authenticated and associated state.
802.11 management frames
A controller-based architecture allows the splitting of 802.11 functions between the AP, which processes real-time portions of the protocol, and the Cisco Wireless LAN controller, which manages items that are not time-sensitive. This model is called ___.
split MAC
The AP processes the portions that have real-time requirements:
- Frame-exchange handshaking between client and AP
- Transmitting of beacon frames
- Buffering and transmitting of frames for clients in a power-save operation
- Responding to probe request frames from clients, forwarding notifications of received probe requests to the WLC
- Providing real-time signal quality information to the Cisco WLC
- Monitoring all radio channels for noise, interference, and other WLANs, and monitoring the presence of other Aps
- Providing wireless encryption and decryption of 802.11 frames
The WLC provides the following functions:
- Authentication (PSK, EAP and so on)
- RF management: Control of RF space for APs and clients
- Client addressing
- Seamless roaming
- QoS
- AP configuration management
- AP image management
___ is an open protocol that enables a WLC to manage a collection of wireless APs. ___ control messages are exchanged between the WLC and AP across an encrypted tunnel. This protocol includes the WLC discovery and joins process, AP configuration and firmware push from the WLC, and statistics gathering and wireless security enforcement. ___-capable APs are authenticated before being able to download any configuration from the WLC.
CAPWAP (control and provisioning of wireless access points)
Client data is encapsulated with a ___ that contains valuable information about the client RSSI and SNR and then is sent to the WLC which forwards the data as needed.
CAPWAP header
There are two functions that run on a Cisco WLC, ___ and ____. These functions can run on the same WLC.
Mobility Agent (MA) and Mobility Controller (MC)
What are the functions of the MA?
- Terminates CAPWAP tunnels
- Maintains the client database
- Configures and enforces security and QoS policies for wireless clients
What are the functions of the MC?
- Provides mobility management tasks, including roaming, RRM, wireless intrusion prevention, and guest access
- MA reports local and roamed client states to MC. The MC builds a database of client stations across all the Mas
The ___ is where the wireless user is seen to be within the wired portion of the network. The __ anchors the client IP address. The network needs to keep the wireless user on the same subnet with the same address as it roams. If the IP address changed on a VOIP call, the call would drop.
PoP (Point of presence)
What is the function of the PoP?
- Anchors Client IP address
* Used for security policy application
Before a user roams, PoP and __ are in the same place. The ___ is the WLC where the client AP CAPWAP is terminated. If the user roams, the __ may move from WLC to WLC for example, and the PoP may stay fixed.
PoA (point of attachment)
What is the fucntion of the PoA?
- Moves with user AP connectivity
* Used for user mobility and QoS policy application
