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CWNA > Chapter 18. > Flashcards

Chapter 18. Flashcards

1
Q

The benefits of using MIMO?

A

Increased throughput and range

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2
Q

What is mandatory for 802.11n and is used to reduce MAC layer overhead?

A

Support for A-MPDU and A-MSDU in receive mode.

Support for A-MPDU in transmit mode.

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3
Q

How does Support for Block ACK work on 802.11n?

A

Required for all devices. Sends a single Block ACK frame to acknowledge multiple received frames.

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4
Q

What is a Greenfield preamble?

A

Greenfield preamble cannot be interpreted

by legacy stations. The Greenfield preamble improves efficiency of the 802.11n networks with no legacy devices.

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5
Q

What is HT Duplicate mode?

A

Allows an AP to send the same data simultaneously on each 20 MHz channel within a bonded 40 MHz channel.

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6
Q

What is MIMO

A

MIMO requires the use of multiple radios and antennas, called radio chains. MIMO radios transmit multiple radio signals at the same time to take advantage
of multipath.

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7
Q

What is multipath?

A

Multipath is a propagation phenomenon that results in two or more paths of the same signal arriving at a receiving antenna at the same time or within nanoseconds of each other. Due to the natural broadening of the waves, the propagation behaviors of reflection, scattering, diffraction, and refraction will occur. A signal may reflect off an object or may scatter, refract, or diffract. These propagation behaviors can each result in multiple paths of the same signal.

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8
Q

Why are high multipath environments beneficial to MIMO?

A

Because the onboard digital processor will have an easier time differentiating between the unique data streams carried on the multiple RF signals

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9
Q

What is spatial multiplexing?

A

Transmitting multiple streams of data.

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10
Q

At a high level what are Space-time block coding (STBC) and cyclic shift diversity (CSD)?

A

They are transmit diversity techniques where the same transmit data is sent out of multiple antennas.

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11
Q

What is transmit beamforming?

A

Transmit beamforming (TxBF) is a technique where the same signal is transmitted over multiple antennas and the antennas act like a phased array.

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12
Q

What is a radio chain?

A

a single radio and all of its supporting architecture, including mixers, amplifiers, and analog/digital converters.

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13
Q

What do the numbers mean in 3x3

A

In a MIMO system, the first number always references the transmitters (TX), and the second number references the receivers (RX).

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14
Q

What is a spatial stream? Detailed.

A

A MIMO radio also has the ability to send independent unique data streams. Each independent data stream
is known as a spatial stream, and each unique stream can contain data that is different from the other streams transmitted by one or more of the other radio chains. Each stream will also travel a different path, because there is at least a half-wavelength of space between
the multiple transmitting antennas.

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15
Q

What is spatial diversity?

A

Occurs when multiple data streams follow different paths to the receiver because of the space between the transmitting antennas.

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16
Q

What is spatial multiplexing?

A

Sending multiple independent streams of unique data using spatial diversity.

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17
Q

What is the benefit of sending multiple unique data streams? Explain.

A

Throughput is drastically increased. If a MIMO access point sends two unique data streams to a MIMO client station that receives both streams, the throughput is effectively doubled. If a MIMO access point sends three unique data streams to a MIMO client station that receives all three streams, the throughput is effectively tripled.

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18
Q

What is the :3 in 3x3:3 mean?

A

How many independent special streams there are. a 3×3:2 MIMO system would use three transmitters and three receivers, but only two unique data streams are utilized. A 3×3:3 MIMO system would use three
transmitters and three receivers with three unique data streams.

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19
Q

What happens when a 2x2: client connects to a 3x3:3 access point?

A

3×3:3 access point and a 2×2:2 client device are communicating with each other, only two spatial streams will be used for unicast transmissions. When a
client radio joins a basic service set (BSS), the access point is advised about the client radio MIMO capabilities

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20
Q

What is the max amount of streams in 802.11n?

A

4x4:4

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21
Q

Why do mobile devices usually have 1x1:1 or at most 2x2:2 radio chains?`

A

Because having more would drain the battery to much.

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22
Q

Can each spatial stream use different modulations? Is this available on the market?

A

Yes, Multiple spatial streams can be sent with the same (equal) modulation or they can be sent using different (unequal) modulation. For example, a 3×3:3 MIMO radio can transmit three data streams using the same 64-quadrature amplitude modulation (QAM) technique. No vendor has ever implanted this. Ever.

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23
Q

MIMO takes advantage of multipath using what technology, INSTEAD of antenna diversity?

A

Spatial multiplexing to increase data capacity.

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24
Q

What is maximal ratio combining (MRC)?

A

When receive diversity is used, the signals may also be linearly combined by using a signal processing technique called maximal ratio combining (MRC). MRC algorithms are used to combine multiple received signals by looking at each unique signal and optimally combining the signals in a method that is additive as opposed to destructive.

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25
Q

What is Space-time block coding?

A

a method where the same information is transmitted
on two or more antennas. It is a type of transmit diversity. STBC can be used when the number of radio chains exceeds the number of spatial streams.

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26
Q

How does Space-Time block coding work?

A

By sending copies of the same information on multiple antennas, the actual rate of the data transmitted does not increase as transmit antennas are added. STBC does, however, increase the receiver’s ability to detect signals at a lower SNR than would be otherwise possible. The receive sensitivity of the radio system improves.

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27
Q

What is transmit diversity?

A

Transmit diversity techniques where the same transmit data is sent out of multiple antennas.

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28
Q

What is Cyclic shift diversity (CSD) and how does it differ from Space Time Block coding?

A

is another transmit diversity technique specified in the 802.11n standard. a signal from a transmitter that uses CSD can be received by legacy 802.11g and 802.11a devices.

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29
Q

What is transmit beamforming?

A

Can be used when there are more transmitting antennas than there are spatial data streams.

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30
Q

How does transmit beamforming work? Is it used today?

A

Transmit beamforming is a method that allows a MIMO transmitter using multiple antennas to adjust the phase and amplitude of the outgoing transmissions in a coordinated method. When multiple copies of the same signal are sent to a receiver, the signals will usually arrive out of phase with each other. If the transmitter (TX) knows about the RF characteristic of the receiver’s location, the phase of the multiple signals sent by a MIMO transmitter can be adjusted. When the multiple signals arrive at the receiver, they are in phase, resulting in constructive multipath instead of the destructive multipath caused by out-of-phase signals. Carefully controlling the phase of the signals transmitted from multiple antennas has the effect of emulating a directional antenna.

With some vendor-specific exceptions, 802.11n transmit beamforming has not been utilized due to the lack of client-side support for the technology

31
Q

Can Transmit beamforming be used together with spatial multiplexing? Do they actually work together?

A

Yes however, the number of spatial streams is constrained by the number of receiving antennas. In practice, transmit beamforming will probably be used
when spatial multiplexing is not the best option. As pictured in Figure 18.5, when utilizing transmit beamforming the transmitter will not be sending multiple unique spatial streams but will instead be sending multiple streams of the same data with the phase adjusted for each RF signal.

32
Q

What is sounding frames?

A

Transmitters that use beamforming will try to adjust the phase of the signals based on feedback from the receiver by using sounding frames. The transmitter is considered the beamformer, while the receiver is considered the beamformee. The beamformer and the
beamformee work together to educate each other about the characteristics of the MIMO channel.

33
Q

What is a steering matrix? What is this used with?

A

This exchange of sounding frames is used to measure the RF channel and create a commutative assessment on how to better steer RF energy to a receiver. Used with transmit beamforming,

34
Q

How does sounding frame work with explicit feedback?

A

When using implicit feedback, the beamformer sends a sounding frame and then receives long training symbols transmitted by the beamformee, which allows the MIMO channel between the beamformee and beamformer to be estimated by the beamformer. In other words, there is no direct feedback from the
beamformee and thus the beamformer creates the steering matrix.

35
Q

What is the difference between a 20MHz channel used in A or G using OFDM and 20 MHz channels used in N?

A

The 20 MHz channels used by HT radios have four extra subcarriers and can carry a little more data than a non-HT OFDM channel. As a result, the HT 20 MHz channel with a single spatial stream can provide
greater aggregate throughput for the same frequency space.

36
Q

How many subcarriers are used in a 802.11a/g channel and how many are used for data? Pilot? How many go unused?

A

64 subcarriers per 20 MHz channel (48 data subcarriers, 4 pilot subcarriers, 12 unused subcarriers)

37
Q

How many subcarriers are used in a 802.11 N channel and how many are used for data? Pilot? How many go unused?

A

64 subcarriers per 20 MHz channel (52 data subcarriers, 4 pilot subcarriers, 8 unused subcarriers)

38
Q

How many subcarriers are used in a 802.11 N 40 MHz wide channel and how many are used for data? Pilot? How many go unused?

A

128 subcarriers per 40 MHz channel (108 data subcarriers, 6 pilot subcarriers, 14 unused subcarriers)

39
Q

How are 40 MHz wide channels created and how can you see that channels are bonded in a spectrum analyzer?

A

he two 20 MHz channels used to form a 40 MHz channel are designated as primary and secondary and are indicated by two fields in the body of certain 802.11 management frames. The primary fields indicates the number of the primary channel. A positive or negative offset indicates whether the secondary channel is one channel above or one channel below the primary channel.

40
Q

What does it mean to be Forty MHz Intolerant ?

A

By default, 802.11n clients and APs will use 20 MHz channels when transmitting in the 2.4 GHz band. They can also advertise that they are Forty MHz Intolerant using various 802.11n management frames. Any 802.11n AP using a 40 MHz channel will be forced to switch back to using only 20 MHz channels if they receive the frames from nearby 802.11n 2.4 GHz stations that are intolerant.

41
Q

What is a guard interval?

A

802.11a/g radios use an 800-nanosecond guard interval (GI) between OFDM symbols. The guard interval is a period of time between symbols that accommodates the late arrival of symbols over long paths. In a multipath environment, symbols travel different paths, and therefore some symbols arrive later.

42
Q

What is intersymbol interference?

A

A “new” symbol may arrive at a receiver before
a “late” symbol has been completely received. This is known as intersymbol interference (ISI) and often results in data corruption.

43
Q

What is delay spread? What is its relation to guard interval?

A

The delay spread is the time differential between multiple paths of the same signal. Normal delay spread is 50 nanoseconds to 100 nanoseconds, and a maximum delay spread is about 200 nanoseconds.

The guard interval should be two to four times the length of the delay spread. Think of the guard interval as a buffer for the delay spread. The normal guard interval is an 800-nanosecond buffer between symbol transmissions. A guard interval will compensate for the delay spread and help prevent intersymbol interference. If the guard interval is too short, intersymbol interference can still occur.

44
Q

What is a short guard interval? What are come caveats to its usage?

A

a shorter 400-nanosecond guard interval is optional. A shorter guard interval results in a shorter symbol time, which has the effect of increasing data rates by about 10 percent. If the optional, shorter 400-nanosecond
guard interval is used with an 802.11n radio, throughput will increase; however, the odds of an intersymbol interference occurrence increases. If intersymbol interference does indeed occur because of the shorter GI, the result is data corruption. If data corruption occurs, layer 2 retransmissions will increase and the throughput will be adversely affected. Therefore, a
400-nanosecond guard interval should be used in RF environments without excessive multipath
which could cause late symbols to overlap with new symbols. If throughput goes down because
of a shorter GI setting, the default guard interval setting of 800 nanoseconds should be used
instead

45
Q

What is MCS or modulation and coding scheme?

A

How 802.11n data rates are defined.

46
Q

How are data rates (MCS) defined in 802.11n?

A

HT radios, however, define data rates based on numerous factors, including modulation, coding method, the number of spatial streams, channel size, and guard interval. Each modulation coding
scheme is a variation of these multiple factors.

47
Q

How many MCS values are there in 802.11n HT?

A
  1. modulation coding schemes exist for both 20 MHz HT channels and 40 MHz HT channels.
48
Q

What key factors determine data rate?

A

modulation type, the guard interval, and the number

of spatial streams all determine the eventual data rate.

49
Q

What is the layer 3-7 payload of an 802.11 data frame?

A

a MAC Service Data Unit (MSDU) is the layer 3–7 payload of an 802.11 data frame.

50
Q

What is a MPDU?

A

a MAC Protocol Data Unit (MPDU) is a technical name for an entire 802.11 frame. An MPDU consists of a layer 2 header, body and trailer

51
Q

What is a Physical Layer Convergence Procedure Protocol Data Unit (PPDU)?

A

When an MPDU (802.11 frame) is sent down from layer 2 to the Physical layer, a preamble and PHY header are added to the MPDU.

52
Q

What is a preamable and its main purpose?

A

The main purpose of the preamble is to use bits to synchronize transmissions at the Physical layer
between two 802.11 radios.

53
Q

What is the main purpose of the PHY header?

A

The main purpose of the PHY header is to use a signal feld to indicate how long it will take to transmit the 802.11 frame (MPDU) and to notify the receiver of the MCS (data rate) that is being used to transmit the MPDU.

54
Q

How many preambles does the 802.11n amendment define? What are they?

A

The 802.11n amendment defines the use of three PPDU structures that use three different preambles.
One of the preambles is a legacy format, and two are newly defined HT preamble. Non-HT legacy, HT Mixed., HT Greenfield.

55
Q

What is non-HT Legacy? What does it consist of? Or what are the header fields?

A

non-HT PPDU consists of a preamble that
uses legacy short and long training symbols, which are used for synchronization. An OFDM symbol consists of 12 bits. The header contains the signal field, which indicates the time needed to transmit the payload of the non-HT PPDU, which of course is the MPDU
(802.11 frame)

56
Q

What is the HT Mixed format? What does it contain?

A

The first part is the beginning of the preamble and contains the non-HT training symbols and legacy signal field that can be decoded by legacy 802.11a and 802.11g radios.

The second part is the HT Signal (HT-SIG) that contains information about the MCS, frame length, 20 MHz or 40 MHz channel size, frame aggregation, guard interval, and STBC. The HT Short Training Field (HT-STF) and HT Long Training Field (HT-LTF) are used for synchronization between MIMO radios

57
Q

Why is HT considered mandatory and all broadcast traffic is sent at 20 MHz?

A

When a 40 MHz channel is used, all broadcast traffic must be sent on a legacy 20 MHz channel so as to maintain interoperability with the 802.11a/g non-HT clients. Also, any transmissions to and from
the non-HT clients will have to use a legacy 20 MHz channel

58
Q

What is HT Greenfield?

A

An 802.11n radio in HT Greenfeld mode can receive frames from legacy devices; however, legacy devices cannot understand the HT Greenfeld preamble.
Therefore, any legacy device will interpret an HT Greenfeld transmission as noise. Greenfeld mode is almost never used because there are almost always legacy 802.11a/b/g client radios present in the environment.

59
Q

What is Frame aggregation ? What is the benefits?

A

a method of combining multiple frames into a
single frame transmission. The fixed MAC layer overhead is reduced, and overhead caused
by the random back off timer during medium contention is also minimized.

60
Q

What is Aggregate MAC Service Data Unit

(A-MSDU)? A frame aggregation tech? What happens at the receiving end?

A

The MSDU is the layer 3–7 payload of a data frame. Multiple MSDUs can be aggregated into a single frame transmission.

An 802.11n access point using A-MSDU aggregation would receive multiple 802.3 frames, remove the 802.3 headers and trailers, and then wrap the multiple MSDU payloads into a single 802.11 frame for transmission.

61
Q

What is the second method of frame aggregation known as aggregate MAC protocol data unit do? A-MPDU? Why is this not as good as Aggregate MAC Service Data Unit?

Why in fact then do most vendors use A-MSDU?

A

The second method of frame aggregation is known as Aggregate MAC Protocol Data Unit (A-MPDU). As you learned in earlier chapters, the MPDU is an entire 802.11 frame including the MAC header, body, and trailer. As pictured in Figure 18.16, multiple MPDUs can
be aggregated into a single frame transmission.
The individual MPDUs within an A-MPDU must all have the same receiver address. Also, the data payload of each MPDU is encrypted separately using the CCMP cipher.

MPDU aggregation has more overhead than
MSDU aggregation because each MPDU has an individual MAC header and trailer.

Most vendors use this because CRC errors can be detected in the individual MPDU frames and therefore
an entire A-MPDU does not need to be retransmitted, only the individual
MPDU that is corrupted. Therefore, A-MPDU is less susceptible to noise
than A-MSDU. The majority of WLAN vendors use A-MPDU.

62
Q

What is a multiple traffic ID block acknowledgment (MTBA) frame?

A

This is used when A-MPDU aggregate frames are used. MTBA is used to ACK all the frames it gets in a single ACK instead of sending an ACK for each frame it gets. Less overhead.

63
Q

What is a reduced interframe space (RIFS)?

A

802.11n introduced a new interframe space called RIFS that was even shorter than the old SIFS. The original SIFS was 16us, the new RIFS is 2us. However,, works on HT-Greenfield only.

64
Q

What is spatial multiplexing power save (SM

power save)?

A

The first new power save mode used by 802.11n. The purpose of SM power save is to allow a MIMO 802.11n device to power down all but one of its radios. For example, a 4×4 MIMO device with four radio chains
would power down three of the four radios, thus conserving power. SM power save defines
two methods of operation: static and dynamic.

65
Q

What happens when a static SM power save is utilized?

A

When static SM power save is utilized, a MIMO client station powers down all the client’s radios except for one single radio. Effectively, the MIMO client station is now the equivalent of a SISO radio that is capable of sending and receiving only one spatial stream.
The client uses an SM power save action frame to inform the access point that the MIMO
client is using only one radio and is capable of receiving only one spatial stream from the
AP. The SM power save action frame is also used to tell the AP that the client station has powered up all of its radios and now is capable of transmitting and receiving multiple spatial streams once again.

66
Q

How does dynamic SM power save work?

A

When dynamic SM power save is utilized, the MIMO client can also power down all but one of the client’s radios but can power up the radios again much more rapidly. The client station disables all but one of the radios after a frame exchange. An access point can trigger the client to wake up the sleeping radios by sending a request-to-send (RTS) frame. The client station receives the RTS frame, powers up the sleeping radios, and sends a clear-to-send (CTS) frame back to the access point. The client can now once again transmit and receive multiple spatial streams. The client uses an SM power save action frame to inform the AP
of the client’s dynamic power save state.

67
Q

What is the name of the field in 802.11n frame that defines the use of protection modes?

A

HT Protection field.

68
Q

How many HT protection modes are there? Name the,m.

A 
3. 
Mode 0, Greenfield (No protection)
Mode 1 - HT Nonmember protection mode
Mode 2. HT 20 MHz Protection Mode
Mode 3. Non-HT Mixed Mode.
69
Q

What is Mode 0—Greenfield (No Protection) Mode?

A

only HT radios are in use. All the HT client stations must also have the same operational capabilities.

70
Q

What is a mode 1 - HT Nonmember Protection Mode?

A

In this mode, all the stations in the BSS must
be HT stations. Protection mechanisms kick in when a non-HT client station or non-HT access point is heard that is not a member of the BSS. For example, an HT AP and stations may be transmitting on a 40 MHz HT channel. A non-HT 802.11a access point or client station is detected to be transmitting in a 20 MHz space that interferes with either the primary or secondary channel of the 40 MHz HT channel.

71
Q

What is mode 2 - HT 20 MHz Protection Mode?

A

In this mode, all the stations in the BSS must be
HT stations and are associated to a 20/40 MHz access point. If a 20 MHz–only HT station associates to the 20/40 MHz AP, protection must be used. In other words, the 20/40-capable HT stations must use protection when transmitting on a 40 MHz channel in order to prevent the 20 MHz–only HT stations from transmitting at the same time.

72
Q

What is Mode 3—Non-HT Mixed Mode?

A

This protection mode is used when one or more non-HT
stations are associated to the HT access point. The HT basic service set can be either 20 MHz or 20/40 MHz capable. If any 802.11a/b/g radios associate to the BSS, protection will be used. Mode 3 will probably be the most commonly used protection mode because most basic service sets will most likely have legacy 802.11a/b/g devices as members.

73
Q

How do are RTS/CTS and CTS-to-Self used in protection mode on 802.11n networks?

A

an HT STA will precede HT transmissions with either an RTS/CTS control frame exchange or a CTS-to-Self control frame using modulation and coding understandable to the STAs that are being protected against. The Duration ID within these control frames causes STAs to update their network allocation
vector (NAV).

CWNA (19 decks)

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