Chap 17 - Wireless Signals and Modulation Flashcards
What is a Hertz?
A unit of measurement of frequencies
What range of frequencies are considered radio waves?
From 3 kHz to 300 GHz
What are 2 main frequency ranges used for wireless LAN communication?
- 2.4 GHz
- 5 GHz
What are the common names for 2.4 GHz and 5 GHz?
- 2.4 GHz Band
- 5 GHz Band
What frequency range does the 2.4 GHz Band encompass?
2.400 GHz to 2.4835 GHz
What is done in order to keep everything orderly when referring to frequencies?
Bands are divided up into channels each of which refers to a specific frequency.
How many channels does the 2.4 GHz band contain?
14 Channels
In the 2.4 GHz band except for channel 14 what is the channel separation or channel width?
0.005 GHz (aka 5 MHz)
What does the center for a frequency define?
Where the actual channel is located.
What 3 facts define bandwidth?
- The actual frequency range needed for the transmitted signal
- That part of the signal above and below the center frequency.
- Ex: 22 MHz bandwidth is 11 MHz above and 11 MHz below center of frequency
What does a Spectral Mask do?
Allows a wireless device to ignore that part of a signal that exceeds the bandwidth boundaries.
How is phase measured, what is in phase and what is out of phase?
- Measured in degrees - 0 to 360
- If 2 signals start and end at the exact same time are ‘in phase’
- Signals that start or end at different times are out of phase
How does phase affect a signal?
- In phase - the signals tend to add to each other
- Signals that are 180 degrees out of phase tend to cancel each other out
What is wavelength?
- A measure of the physical distance that a wave travels over one complete cycle
- As frequency increases wavelength decreases
What are the wavelengths of a 2.4 GHz signal and a 5 GHz signal?
- 2.4 GHz signal is 4.92 inches
- 5 GHz signal is 2.36 inches
What is amplitude?
- The height from the top peak to the bottom peak of the signal’s waveform
- Sometimes used in reference to RF power
What is the unit of measure for the power of RF signal, what range do wireless networks operate in, and what type of measurement is this considered to be?
- Watts
- Wireless networks range from 1 mw to 100 mw
- Considered to be an absolute measurement
In logarithmic functions what is the reference number?
The number that the mW of interest is being compared to.
What is gain?
The increase in dB resulting from the addition of an antenna to the transmitter.
What is an isotropic antenna?
- The ideal antenna
- Does not exist
- Tiny point
- Radiates RF equally in every direction
What is the Effective Isotropic Radiated Power (EIRP)?
The actual power radiated from the antenna calculated by adding transmitter power, antenna gain, and subtracting dB loss from the cable
What causes a signal to weaken the further away you get?
The wave has a three-dimensional curved shape that expands as it travels. It is this expansion or spreading that causes the signal strength to weaken.
How quickly does signal strength fall off?
Signal strength falls off quickly near the transmitter but more slowly farther away.
What 2 things is signal loss a function of?
- Frequency
- Distance
What is the free space loss when you are 1 meter away from the transmitter?
46 dBm
Which has a greater free space path loss, 2.4GHz or 5 GHz?
- 5GHz
- The higher the frequency the greater the loss.
On wireless LAN devices what is the EIRP level range when leaving the transmitter?
- In mW - from 100mW down to 1mW
- In dB - from +20dB to 0dB
What is the signal strength range when arriving at the receiver?
- In mW - 1mW down to 0mW
- In dB - from 0dB down to -100dB
As defined by the 802.11 standard, what is the unit of measure for a received signal strength and what is the range of values?
- Received Signal Strength Indicator (RSSI) scale
- Values range from 0 (weakest) up to 255 (strongest)
What is sensitivity level?
A receiver’s threshold that divides intelligible, useful signals from unintelligible ones.
What has to happen for a receiver to understand the data contained in a signal?
The signal has to have a power level stronger than the sensitivity level
What is noise?
Signals that are received on the same frequency as the one you are trying to receive
What is the noise floor?
The noise level, or the average signal strength of the noise,
What is the Signal to Noise Ratio (SNR)?
- The difference between the signal and the noise
- The higher the better.
What is the Carrier Signal?
This basic RF signal is called a carrier signal because it is used to carry other useful information.
What are 3 goals of modulating schemes?
- Carry data at a pre-defined rate
- Reasonably immune to interference and noise
- Be practical to transmit and receive
What are the only 3 attributes a modulation scheme can alter?
- Frequency, but only varying slightly above or below the carrier stream
- Phase
- Amplitude
What are Narrow Band transmissions and what are 2 examples?
- Transmissions that require little extra bandwidth other than that centered on the frequency
- AM and FM radio
What is Spread Spectrum?
When data is carried over a wide range of frequencies.
What are the 2 common spread-spectrum categories?
- Direct Sequence Spread Spectrum (DSSS)
- Orthogonal Frequency Division Multiplexing (OFDM)
What is Direct Sequence Spread Spectrum?
What bands?
Channels?
What 2 things do the channels support?
How wide are the channels?
- Used in the 2.4 GHz band
- A small number of fixed, wide channels
- They support complex phase modulation schemes and somewhat scalable data rates
- Wide enough to augment the data by spreading it out and making it more resilient to disruption
What is Orthogonal Frequency Division Multiplexing?
What band is it used in?
How is the data sent on the 20MHz channel?
How is each channel divided?
What are modulated?
What are they modulated with?
- Used in both 2.4 and 5 GHz bands
- A single 20 MHz channel contains data that is sent in parallel over multiple frequencies
- Each channel is divided into many subcarriers (also called subchannels or tones)
- Both phase and amplitude are modulated with quadrature amplitude modulation (QAM) to move the most data efficiently
What 5 things does the 802.11 standard define?
- RF signals and measurements
- Modulation and coding
- Bands and channels
- Data rates and increasing bandwidth
- Wireless management, QoS, and security
What is a BSS and what does it include?
- Basic Service Set
- Typically includes an AP and any clients associated to it
What is 802.11b?
Bands?
Data rates?
Spatial Streams?
Channel width?
- 2.4 GHz
- Data rates - 1, 2, 5.5, and 11 Mbps
- 0 spatial streams
- Channel width - 22 MHz
What is 802.11g?
Bands?
Data rates?
Spatial Streams?
Channel width?
- 2.4 GHz
- Data rates - 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
- 0 spatial streams
- Channel width - 22 MHz
What is 802.11a?
Bands?
Data rates?
Spatial Streams?
Channel width?
- 5 GHz
- Data rates - 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
- 0 spatial streams
- Channel width - 20 MHz
What is 802.11n?
Bands:
Data rates:
Number of spatial streams:
Channel width:
Nickname:
- 2.4 and 5 GHz
- Data rates - up to 150 Mbps per spatial stream
- 4 spatial streams
- Channel width - 20 or 40 MHz
- High Throughput (HT) techniques
What is 802.11ac?
Bands?
Data rates?
Number of data rates?
Spatial Streams?
Channel width?
Nickname?
- 5 GHz band
- Data rates - up to 866 Mbps per spatial stream
- 320 different data rates
- 4 spatial streams
- Channel width - 20, 40, 80, 160 MHz
- Very High Throughput (VHT)
What is 802.11ax?
Bands?
Data rates?
Number of data rates?
Spatial Streams?
Channel width?
Nickname?
Special capability?
- 2.4 and 5 GHz
- Data rates - Up to 1.2 Gbps per spaitial stream
- 8 spatial streams
- Channel width - 20, 40 80, and 160 MHz
- High Efficiency Wireless, aka Wifi 6
- Multiple devices can transmit during the same window of air time
How does 802.11ax get such high data rates?
How is interference avoided?
Which category of spread spectrum is used?
What does it use this category to do?
How is channel air time allocated?
- Through better transmit power control and BSS marking or “coloring” methods
- Uses OFDM Access (OFDMA)
- Uses OFDM to schedule and control access to the wireless medium
- Channel air time allocated as resource units that can be used for transmission by multiple devices simultaneously
What is a Wireless Chain?
- A wireless device that used a single transmitter and a single receiver
- aka SISO (Single In, Single out)
What is SISO
- Single In - Single Out
- A single wireless chain
- One transmitter and one receiver
What is MIMO and which 802.11 standards use this?
- Multiple In - Multiple Out
- One device with multiple antennas, multiple transmitters, and multiple receivers
- Used by 802.11n, 802.11ac and 802.11ax
What is T X R?
- A term used to describe an AP capability where T is the number of transmitters, and R is the number of receivers.
- A 2x3 MIMO device has 2 transmitters and 3 receivers
What are 3 ways extra radios in MIMO can be leveraged?
Extra radios can be used to :
- Improve received signal quality
- Improve transmission to specific client locations
- Carry data to and from multiple clients simultaneously.
What is Spatial Multiplexing?
A way to increase data throughput, data can be multiplexed or distributed across two or more radio chains—all operating on the same channel, but separated through spatial diversity.
How is Spatial Multiplexing accomplished?
- Data is distributed across multiple radio chains all transmitting on the same channel but separated by spatial diversity.
- The receiver can tell the signals apart because the signals arrive either:
- out of phase or
- at a different amplitude
In the MIMO specification how is the number of Spatial Streams specified?
- With a colon after the MIMO specification
- Ex: 3 x 3 : 2 means 3 transmitters, 3 receivers with 2 spatial streams
Would 3 spatial streams require 3 transmitters?
No, the number of spatial streams depends on the processing capacity and feature set of the device
What happens when two devices have a mismatched number of spatial streams?
- They negotiate the wireless connection by informing each other of their capabilities
- They then use the lowest number they have in common
What is Transmit Beamforming (TxBF)?
When the phase of the signal is altered as it is fed into each transmitting antenna so that the resulting signals will all arrive in phase at a specific receiver.
How does a radio know how to transmit data using Transmit Beamforming?
- Transmitter uses explicit feedback from the device at the far end, enabling the transmitter to make the appropriate adjustments to the transmitted signal phase.
- As T×BF information is collected about each far end device, a transmitter can keep a table of the devices and phase adjustments so that it can send focused transmissions to each one dynamically.
What is Maximal-Ratio Combining (MRC) ?
What does it improve and what does it provide?
- A signal is transmitted from multiple antennas. The end station uses mutiple antennas to receive the various copies of the signal. It then merges the best parts of each copy into a reconstructed signal.
- This improves sensitivity and provides a better SNR.
What is Dynamic Rate Shifting (DRS)?
- A way for transmitter and receiver to adjust their modulation and coding schemes based on the current RSSI and SNR conditions.
- The worse the condition the simpler the scheme and the slower the data rate
What is QAM?
What does QAM stand for?
What uses QAM?
What does QAM do?
- Quadrature Amplitude Modulation
- Used by OFDM
- Modulates both phase and amplitude of subcarriers (aka subchannels or tones)
Can the Effective Isotropic Radiated Power (EIRP) be measured at the destination?
No, EIRP is a measurement taken at the source of the RF.
The following 802.11 standards have nicknames for the technology that they use. What are these nicknames for:
- 11n
- 11ac
- 11ax
- 802.11n - High Throughput (HT)
- 802.11ac - Very High Throughput (VHT)
- 802.11ax - High Efficiency Wireless, also Wifi-6
