Chapter 19. Flashcards
When was 802.11ac developed and ratified?
December 2013.
What is the theoretical max speed of 802.11ac?
6.933 Gbps
What is the max speed of phase 802.11ac?
1.3 Gbps.
What are the primary 3 tech enhancements to 802.11ac that gives the first wave speeds of 1.3 Gbps?
Increased channel bandwidth, more radio chains and
spatial streams, and an enhanced modulation and coding scheme are the three technologies that help the first deployments of 802.11ac provide data rates of up to 1.3 Gbps.
What is the max QAM rating of N and AC?
64-QAM for N and 256-QAM for AC.
How are 80 MHz channels created?
80 MHz channel combines two 40 MHz channels. The two 40 MHz channels that make up the 80 MHz channel must be adjacent.
How are 160 MHz channels created? Do they have to be adjacent?
As you might deduce, the 160 MHz channel is made up of two 80 MHz channels; however, the two 80 MHz channels do not have to be adjacent. If the channels are adjacent, then it is referred to as a 160 MHz channel. If they are not adjacent, then it is referred to as an 80+80 MHz channel.
What is a major draw back to using 40 MHz channel widths in a larger environment, and contention?
With 802.11n, an access point (AP) could be configured to use either 20 MHz channels or 40 MHz channels. When an AP was configured for a 40 MHz channel, it could not transmit until both the primary and secondary 20 MHz channels were available. A neighboring 20 MHz AP could be transmitting on either of the 20 MHz channels and force the 40 MHz AP to wait before it could transmit, reducing the performance capabilities of the 802.11n AP.`
What is dynamic bandwidth operation? Give an example.
allows the AP to choose the channel width on a per-frame basis. As an example, when an 802.11ac AP, operating at 80 MHz on channels 36, 40, 44, and
48, wants to transmit, it must first check to see if all four channels are available If one of the channels, such as 36, is currently being using by another AP, instead of
waiting for all four channels to be available to perform an 80 MHz transmission, the AP can transmit using a 40 MHz transmission on channels 44 and 48.
What does an 802.11ac AP have to do before it can transmit on a 160 MHz wide channel?
in order to transmit using a 160 MHz channel, the access point must do a clear channel assessment across the entire 160 MHz frequency range, and all eight 20 MHz channels must be available before the transmission can begin.
How many phase and amplitude levels are possible with 256 QAM and what is the binary data size that each QAM represents?
256-QAM identifies 256 unique values, using 16 different levels of phase shift and 16 different levels of
amplitude shift. Because there are 256 distinct values, each value is able to represent 8 bits.
Describe in basic detail what happens when a 64-QAM radio transmits data?
When a 64-QAM radio transmits data, it modifies the amplitude and phase of the wave and then transmits it. The receiving radio must then take the signal and identify the amplitude and phase modifications that were made to identify which of the 64 symbols was
transmitted. This is not always easy because noise and interference can make it difficult to identify the values of the transmitted signal.
Why is it important for clients to be close to an AP for the client to gain the usage of 256-QAM?
256-QAM is used for the highest modulation coding sets. To achieve these higher data rates, higher signal-to-noise ratios are needed. This also means that the clients need to be close to the AP in order to achieve these data rates.
How much additional antenna gain will you need to get 256-QAM vs 64-QAM?
5 db. 256-QAM is used for the highest modulation coding sets. To achieve these higher data
rates, higher signal-to-noise ratios are needed.
What is unequal modulation?
the use of different modulation and coding
schemes at the same time on different spatial streams.
How many MCS options are there for 802.11ac?
10 MCS options.
What is does the error-correcting code used by each MCS in 802.11ac mean?
Error-correcting codes add redundant information to assist with error recovery. The code rate is represented by a fraction. The first number (numerator) represents the quantity of user data bits, relative to the number of
bits on the channel (denominator)—the higher the code rate, the more data transmitted and the less redundancy provided.
802.11ac radios in tablets and smartphones will most likely remain 1×1:1 do to what reason?
Battery life is critical.
In 802.11ac how many MCS values are mandatory
The first 8 modulation and coding schemes are mandatory.
Does 802.11ac have to worry about interference on b/g/n? Does it have to worry about interference from other radios using a different modulation scheme?
No because it ONLY operates in the 5ghz band.
It also doesn’t have to worry about different modulation schemes because all other 5ghz radios use OFDM.
All 802.11ac frames are transmitted using the _____ frame format?
A-MPDU.
Why does 802.11ac make Reduced Interframe Space (RIFS) obsolete?
The higher transmission speeds of 802.11ac.
Why did 802.11ac get rid of RIFS?
By transmitting A-MPDU frames, 802.11ac removes the need for multiple transmissions of individual frames with individual headers and individual ACKs. An A-MPDU frame reduces the per-frame overhead and only requires a single block ACK. Because of this, the 802.11ac amendment states that the use of RIFS is not supported and is obsolete.
Why would 802.11ac need to send out RTS/CTS when using large channel widths? What are they steps? What happens when the channels are busy that are bonded?
Because sometimes clients are at the far edge of each AP’s signal coverage to where each station can hear each other very well. They can hear each other because with large channel widths sometimes they bonded channels overlap with other AP’s wide channels.
When a station wants to send out a lot of data using its wide band, it needs to make sure that the entire channel is available and it needs to reserve the channel, otherwise the transmission will be corrupted.
- So the AP sends out an RTS/CTS frame. Using an 802.11a frame format, AP1 attempts to send an RTS frame across each of the four channels that make up AP1’s 80 MHz channel. If all four channels are available, and if Station1 receives the RTS frames, it will transmit four CTS frames.
- As neighboring nodes hear the RTS or CTS, they set their network allocation vector (NAV) to the RTS duration value or the CTS duration value and defer any transmissions until their NAV timer counts down to zero.
- When AP1 receives all four CTS frames, it knows that all four channels are available and
reserved and it can perform an 80 MHz data transmission. After the data is transmitted and
the data frame is received correctly, Station1 will send four block ACK frames.
If some of the channels are busy that are bonded, the AP will then only send data using the free channels.
