Chapter 10 - MIMO Technology: HT and VHT

How many modulation and coding schemes are defined in 802.11ac? A. 8 B. 10 C. 64 D. 77 E. 256

B. 802.11ac defined only 10 MCSs, unlike 802.11n, which defined 77. 802.11n defined MCSs based on modulation, coding method, the number of spatial streams, channel size, and guard interval. 802.11ac defined 10 MCSs based on modulation and code rate.

Which PHY layer mechanism might be used to increase throughput for an HT/VHT radio in a clean RF environment with minimal reflections and low multipath? A. Maximal ratio combining B. 400-nanosecond guard interval C. Switched diversity D. Spatial multiplexing E. Spatial diversity

B. 802.11n/ac radios can use an 800-nanosecond guard interval; however, a shorter 400-nanosecond guard interval is also available. 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 should increase. However, if intersymbol interference occurs because of multipath, 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 only good RF environments. If throughput goes down because of a shorter GI setting, the default guard interval setting of 800 nanoseconds should be used instead.

Which of the following channel widths are supported in 802.11ac? (Choose all that apply.) A. 20 MHz B. 40 MHz C. 80 MHz D. 80+80 MHz E. 160 MHz

A, B, C, D, E. All of these are supported channel widths. The 160 MHz channel is actually made up of two 80 MHz channels, which can be side by side or separated.

Which HT protection modes would allow only for the association of 802.11a/g clients to an 802.11ac access point? (Choose all that apply.) A. Mode 0—Greenfield mode B. Mode 1—HT nonmember protection mode C. Mode 2—HT 20 MHz protection mode D. Mode 3—HT Mixed mode

A, B, C. Modes 0, 1, and 2 all define protection to be used in various situations where only 802.11n/ac stations are allowed to associate to an 802.11n/ac access point. Mode 3—HT Mixed mode—defines the use of protection when both HT/VHT and non-HT radios are associated to an 802.11ac access point.

802.11n (HT) radios are backward compatible with which of the following types of 802.11 radios? (Choose all that apply.) A. 802.11b radios (HR-DSSS) B. 802.11a radios (OFDM) C. 802.11 legacy radios (FHSS) D. 802.11g radios (ERP)

A, B, D. 802.11n (HT) radios are backward compatible with older 802.11b radios (HR-DSSS), 802.11a radios (OFDM), and 802.11g radios (ERP). HT radios are not backward compatible with legacy frequency-hopping radios.

What could an 802.11n (HT) radio use to increase throughput at the MAC sublayer of the Data-Link layer? (Choose all that apply.) A. A-MSDU B. A-MPDU C. Guard interval D. Block ACKs

A, B, D. The 802.11n amendment introduced two new methods of frame aggregation to help reduce overhead and increase throughput. Frame aggregation is a method of combining multiple frames into a single frame transmission. The two types of frame aggregation are AMSDU and A-MPDU. Block ACK frames are used to acknowledge AMPDUs. The Guard interval is used at the Physical layer.

How can a MIMO system increase throughput at the Physical layer? (Choose all that apply.) A. Spatial multiplexing B. A-MPDU C. Transmit beamforming D. 40 MHz channels

A, C, D. Spatial multiplexing transmits multiple streams of unique data at the same time. If a MIMO access point sends two unique data streams to a MIMO client that receives both streams, the throughput is effectively doubled. If a MIMO access point sends three unique data streams to a MIMO client that receives all three streams, the throughput is effectively tripled. Because transmit beamforming results in constructive multipath communication, the result is a higher signal-to-noise ratio and greater received amplitude. Transmit beamforming will result in higher throughput because of the higher SNR that allows for the use of more complex modulation methods that can encode more data bits. 40 MHz HT channels effectively double the frequency bandwidth, which results in greater throughput.

Which of the following modulation methods are supported with 802.11ac? (Choose all that apply.) A. BPSK B. BASK C. 32-QAM D. 64-QAM E. 256-QAM

A, D, E. The 802.11ac amendment supports BPSK, QPSK, 16-QAM, 64-QAM, and 256-QAM. BASK and 32-QAM do not exist.

A 3×3:2 MIMO radio can transmit and receive how many unique streams of data? A. Two B. Three C. Four D. Three equal and four unequal streams E. None—the streams are not unique data

A. MIMO radios transmit multiple radio signals at the same time and take advantage of multipath. Each individual radio signal is transmitted by a unique radio and antenna of the MIMO system. Each independent signal is known as a spatial stream, and each stream can contain different data from the other streams transmitted by one or more of the other radios. A 3×3:2 MIMO system can transmit two unique data streams. A 3×3:2 MIMO system would use three transmitters and three receivers; however, only two unique data streams are utilized.

Name a capability not defined for A-MPDU. A. Multiple QoS access categories B. Independent data payload encryption C. Individual MPDUs having the same receiver address D. MPDU aggregation

A. Multiple MPDUs can be aggregated into one frame. The individual MPDUs within an A-MPDU must all have the same receiver address. However, individual MPDUs must all be of the same 802.11e qualityof-service access category. Each aggregated MPDU is also encrypted and decrypted independently.

Which of these capabilities are considered mandatory for an 802.11n access point, as defined by the Wi-Fi Alliance's vendor certification program called Wi-Fi CERTIFIED n? (Choose all that apply.) A. Three spatial streams in receive mode B. WPA/WPA2 C. WMM D. Two spatial streams in transmit mode E. 2.4 GHz-40 MHz channels

B, C, D. Some of the mandatory baseline requirements of Wi-Fi CERTIFIED n include WPA/WPA2 certification, WMM certification, and support for 40 MHz channels in the 5 GHz U-NII bands. 40 MHz channels in 2.4 GHz are not required. 802.11n access points must support at least two spatial streams in both transmit and receive mode. Client stations must support one spatial stream or more.

Which frequencies are defined for 802.11n (HT) radio transmissions? (Choose all that apply.) A. 902-928 MHz B. 2.4-2.4835 GHz C. 5.15-5.25 GHz D. 5.25-5.35 MHz

B, C. Other 802.11 technologies are frequency dependent on a single RF band. For example, 802.11b/g radios can transmit in only the 2.4 GHz ISM band. 802.11a radios are restricted to the 5 GHz U-NII bands. 802.11ac radios can also transmit only in the 5 GHz U-NII bands. 802.11n radios are not locked to a single frequency band and can transmit on both the 2.4 GHz ISM band and the 5 GHz U-NII bands.

Which of the following technologies is part of explicit beamforming? (Choose all that apply.) A. Channel sounding B. Feedback matrix C. Sounding matrix D. Steering matrix E. Null data packet F. Channel matrix

B, D, E. The beamformer transmits an NDP announcement frame followed by an NDP frame. The beamformee processes this information and creates and transmits a feedback matrix. The AP uses the feedback matrices to calculate a steering matrix, which is used to direct the transmission.

802.11ac (VHT) radios are backward compatible with which of the following types of 802.11 technology? (Choose all that apply.) A. 802.11b (HR-DSSS) B. 802.11a (OFDM) C. 802.11g (ERP) D. 802.11n (HT)

B, D. 802.11ac (VHT) radios are backward compatible with all previous 5 GHz-compliant radios. This includes 802.11a (OFDM) radios and 5 GHz 802.11n (HT) radios.

MIMO radios use which mechanisms for transmit diversity? (Choose all that apply.) A. Maximal ratio combining (MRC) B. Direct-sequence spread spectrum (DSSS) C. Space-time block coding (STBC) D. Cyclic shift diversity (CSD)

C, D. Cyclic shift diversity (CSD) is a transmit diversity technique. Unlike STBC, a signal from a transmitter that uses CSD can be received by legacy 802.11g and 802.11a devices. Maximal ratio combining (MRC) is a receive diversity technique. DSSS is a spread spectrum technology used by legacy 802.11 SISO radios.

A WLAN consultant has recommend that a new 802.11n/ac network use only 40 MHz channels in the 5 GHz U-NII bands. Why would he recommend 40 MHz channels be used only in 5 GHz and not in 2.4 GHz? A. HT/VHT radios do not require DFS and TPC in the 5 GHz bands. B. HT/VHT radios get better range using TxBF in the 5 GHz bands. C. 40 MHz channels do not scale in the 2.4 GHz ISM band. D. 5 GHz VHT radios are less expensive than 2.4 GHz HT radios.

C. Deploying 40 MHz channels in the 2.4 GHz band does not scale properly because there is not enough available frequency space. Although 14 channels are available at 2.4 GHz, there are only three non-overlapping 20 MHz channels available in the 2.4 GHz ISM band. When the smaller channels are bonded together to form 40 MHz channels in the 2.4 GHz ISM band, any two 40 MHz channels will overlap. Channel reuse patterns are not possible with 40 MHz channels in the 2.4 GHz ISM band. The 5 GHz U-NII bands have much more frequency space; therefore, a 40 MHz channel reuse pattern is possible with careful planning.

The 802.11ac amendment defines a maximum of how many spatial streams for a client? A. One B. Two C. Four D. Eight

C. The 802.11ac amendment defines a maximum of four spatial streams for a client and eight spatial streams for an AP. However, most enterprise 802.11ac APs are 4×4:4, and most 802.11ac clients are 2×2:2.

Which 802.11ac MCS range defines all of the MCSs that are mandatory? A. MCS 0-2 B. MCS 0-4 C. MCS 0-6 D. MCS 0-7 E. MCS 0-8 F. MCS 0-9

D. MCS 0-7 are mandatory. MCS 8 and MCS 9 use 256-QAM, which is optional but will most likely be supported by most vendors.

Which new power-management method defined by the 802.11n amendment conserves power by powering down all but one radio? A. A-MPDU B. Power Save protection C. PSMP D. SM power save E. PS mode

D. Spatial multiplexing power save (SM power save) allows 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.

The guard interval is used as a buffer to compensate for which type of interference? A. Co-channel interference B. Adjacent cell interference C. RF interference D. HT interference E. Intersymbol interference

E. The guard interval acts as a buffer for the delay spread, and the normal guard interval is an 800-nanosecond buffer between symbol transmissions. The guard interval will compensate for the delay spread and help prevent intersymbol interference. If the guard interval is too short, intersymbol interference will still occur. HT/VHT radios also have the capability of using a shorter 400-nanosecond GI.