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Wireless Fundamentals > Chapter 6 > Flashcards

Chapter 6 Flashcards

1
Q

After data has been encoded using a chipping method, the transmitter needs to modulate the signal to create a carrier signal containing the chips for DSSS and HR-DSSS. What where these 2 modulations called?

A

Differential binary phase shift keying (DBPSK)

Differential quadrature phase shift keying (DQPSK)

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2
Q

Are FHSS and DSSS backward compatible?

A

No.

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3
Q

How does Barker Code work? How much damage can be done before the entire sequence fails?

A

The system converts the 1 bit of data into a series of bits that are referred to as chips.
The chip is created using a Boolean XOR on the data bit and a fixed length bit sequence. The is the barker code.
The sequence of chips is then spread across a wider frequency space.
11 chips are created and use 22 MHz of frequency carrier space.
The receiver then takes the 11 chips and converts them back into data. 9/11 chips can be corrupted and the receiving radio will still be able to interpret the sequence and convert them back into a single data bit.

Each data will have an additional 11 bits of redundancy code added to it.

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4
Q

How does convolutional coding work?

A

Convolutional coding uses a ratio between the bits transmitted vs. the bits encoded to provide these different levels. The lower the ratio, the less resistant the signal is to interference and the greater the data rate will be.

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5
Q

How does OFDM work at a high level? What does this mean for interference?

A

52 subcarriers are used per channel to transmit data at lower data rates, but because there are so many subcarriers, overall data rates are higher.

Also, because of the lower subcarrier data rates, delay spread is a smaller percentage of the symbol period, which means that ISI (Intersymbol Interference) is less likely to occur. OFDM tech is more resistant to the negative effects of multipath.

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6
Q

How large was U-NII-2 extended frequency wise? How many channels? What must it support?

A

255 MHz wide, and uses 12 channels. Must support DFS.

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7
Q

How many channels did U-NII-1 allow? How wide was this band?

A

Four 20 MHz wide channels, the band was 100 MHz wide.

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8
Q

How many channels does the 802.11 - 2012 allow in the ISM band?

A

14 channels. 11 allowed in the US.

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9
Q

How many of the 52 subcarriers are used to send data in OFDM and what are the remaining subcarriers used for?

A

48 are used to transmit data.
4 others are known as pilot carriers. They are used as a reference for phase and amplitude by the demodulator, allowing the receiver to sync itself as it demodulates the data in the other subcarriers.

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10
Q

How many subcarriers per channel using OFDM? And how wide are the subcarrier’s?

A

52 subcarriers per channel and each sub carrier is 312.5 KHz wide.

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11
Q

How wide and how many channels did U-NII-2 create? What must AP’s support to use this band?

A

100 MHz wide. 4 channels. DFS.

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12
Q

What amendment introduced the use of 256-QAM?

A

802.11ac

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13
Q

What are the 3 frequency ranges of 802.11 ISM band? How wide are they? What is the name of each band?

A

902 MHz - 928 MHz (26 MHz wide) (Industrial)

  1. 4 Ghz - 2.5 GHz (100 MHz wide) (Scientific)
  2. 725 - 5.875 GHz (150 MHz wide) (Medical)
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14
Q

What are the limiting factors of the 900 MHz ISM band?

A

Slow throughput
Most of the world actually uses this band for GPS.

Baby monitors and home phones can use this band.

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15
Q

What are the modulation techs that are used with OFDM for the higher data rates?

A

Quadrature amplitude modulation.

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16
Q

What are the modulation techs that are used with OFDM for the lower data rates?

A

Binary phase shift keying and quadrature phase shift keying.

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17
Q

What do radios do when signal attenuation/corruption starts to effect their performance?

A

DRS (Dynamic Rate Shifting) to force itself down to a lower modulation rate

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18
Q

What does delay spread mean?

A

The delay between the main signal and the reflected signal.

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19
Q

What does Dwell time mean with FHSS? What is the max dwell time in ms? How many sub frequencies are there?

A

The defined amount of time that the FHSS system transmits on a specific frequency before it switches to the next frequency in the hop set.

Max dwell time for FHSS is 400 ms per carrier frequency during any 30 second time period.

75 frequencies 1 MHz wide.

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20
Q

What does intersymbol interference mean and what causes it? What spread technology is not easily affected by this and why?

A

If the delay spread is too great, data from the reflected signal may interfere with the same data stream from the main signal

Spread spectrum systems are not as susceptible to ISI because they spread their signals across a range of frequencies. These various frequencies produce different delays in multipath, such that some wavelengths may be affected by ISI whereas others may not.

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21
Q

What does narrowband transmission mean?

A

Uses very little bandwidth to transmit the data that it is carrying.

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22
Q

What does processing gain mean?

A

The task of adding additional, redundant information to the data that is being transmitted.

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23
Q

What does Spread spectrum mean? How does it work?

A

Uses more bandwidth than is necessary to carry its data. Spread spectrum technology takes the data that is to be transmitted and spreads it across the frequency that it is using.

24
Q

What does U-NII stand for?

A

Unlicensed National Information Infrastructure

25
Q

What first happens to data before it gets transmitted over 802.11 for DSSS or HR-DSSS?

A

It gets encoded, either by Barker Code or CCK (Complementary Code Keying).

26
Q

What is DQPSK?

A

Differential quadrature phase shift keying (DQPSK) utilizes four phase shifts, allowing each of the four phase shifts to modulate 2
chips (00, 01, 10, 11) instead of just 1 chip, doubling the speed.

27
Q

What is a constellation diagram? What do the axis positions represent?

A

A constellation diagram, also known as a constellation map, is a two dimensional diagram often used to represent QAM modulation. A constellation diagram is divided into four quadrants, and different locations in each quadrant can be used to represent data bits.
Horizontal axis can be used to represent phase shifts.
Vertical can be used to represent amplitude.

28
Q

What is a downside to narrow band transmissions and what does spread spectrum do to mitigate this?

A

Narrowband signals take up a single or very narrow band of frequencies, intentional jamming or unintentional interference of this frequency range is likely to cause disruption in the signal.

Spread spectrum uses a wider range of frequency space, it is typically less susceptible to intentional jamming or unintentional interference from outside sources. Interference needs to be spread out across the entire band for Spread Spectrum to be affected.

29
Q

What is Differential binary phase shift keying (DBPSK) ?

A

Utilizing two phase shifts, one that represents a 0 chip and another that represents a 1 chip.

30
Q

What is DSSS or direct sequence spread spectrum? What was the speed, where did it come from? What was its successor? How fast?

A

Originally specified in the primary, or root 802.11 standard and provides 1-2 MBps.

Its success HR-DSSS from 802.11b was 5.5 and 11 MBps.

31
Q

What is frequency hopping spread spectrum and how does it work?

A

it transmits data by using a small frequency carrier space, then hops to another small frequency carrier space and transmits data,
then to another frequency, and so on. It uses a frequency for a set period of time then hops to the next. This is called dwell time. Each time the dwell time reaches 0 the system switches to another frequency.

32
Q

What is hop time?

A

The amount of time it takes for the transmitter to change from one frequency to another. This is usually done in microseconds. This is considered overhead, since this is wasted time.

33
Q

What is hopping sequence? How does it work?

A

This is a series of small carrier frequencies. Instead of transmitting on 1 set channel, an FHSS radio transmits on a sequence of subchannels called hops. Each time a hop sequence is used, it is repeated. The sender and the receiver needs to be in synch during these hops for successful data transmission to occur.

34
Q

What is quadrature amplitude modulation?

A

A hybrid of phase and amplitude modulation.

35
Q

What is the bandwidth of U-NII-3? How many channels did it support?

A

Five 20 MHz wide channels.

36
Q

What was the error correction method used with OFDM and how did it work?

A

It used forward error correction that allows the receiving station to detect and repair corrupted bits. Uses convolutional coding.

37
Q

How wide is an ISM band using DSSS and HR-DSSS?

A

22 MHz

38
Q

What is the distance in MHz between the center of ISM channels?

A

5 MHz

39
Q

How wide is the OFDM spread using A, G, N, AC amendments?

A

20 MHz wide.

40
Q

What is the critical criteria necessary for ISM bands to be considered non-overlapping?

A

Five channels apart or 25 MHz.

41
Q

What is the transmit spectrum mask?

A

The architecture of each channels side lobes.

42
Q

What is the significance between transmit spectrum mask, center frequency, and nine overlapping channels on the ISM band?

A

Side lobes are far weaker than the center frequency upwards of -47 dBm loss. However, this can still cause adjacent channel interference of you are extremely close to the AP.

Client and AP need to be between 5-10ft away to prevent transmit spectrum mask co-channel interference.

43
Q

What is the separation in MHz on 5 GHz UNII bands between center frequencies?

A

20 MHz

44
Q

How wide is a 5 GHz channel in total with transmit spectrum mask?

A

30 MHz

45
Q

How many channels are available right now for channel reuse plans on 5 GHz?

A

25 channels

46
Q

How many 40 MHz channels are available in the current 5 Ghz frequency spectrum?

A

12

47
Q

How many channels could be made available if the entire 5 Ghz spectrum became available?

A

37 channels, at 20 MHz.

48
Q

Do transmit spectral mask interference exist in the 5 GHz band?

A

Yes, same distance 5-10ft.

49
Q

What is the WLAN communities definition of adjacent channel interference?

A

An adjacent channel is considered to be the prior or or next numbered channel. For example 3 is adjacent to 4.

50
Q

What are 2 things that affect data rates?

A

Encoding / Modulation.

51
Q

What is the proper term used to describe changes in speed of encoding and modulation?

A

Data Rate.

52
Q

What is the average loss in percentage of data rate on the 2.4 GHz band and why?

A

50% due to carrier sense multiple access collision avoidance taking up medium throughout and RTS/CTS and medium contention.

53
Q

What is the average speed in percentage that you would get from advertising data rates on N/AC ?

A

60% -70% So you would get that percentage of the advertised speed.

54
Q

What percentage of medium contention is usually used by CSMA/CA?

A

35%

55
Q

Why do you use the term aggregate throughput when describing throughput in RF?

A

Because the bandwidth is dependent on how many clients are connected to the specific medium. For example; advertised speed for 6 is 54 Mbps. 20 Mbps effective…. with four clients connected, they each get 5 Mbps.

Wireless Fundamentals (19 decks)

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