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CWAP - Chapter 7 Exam Questions and correct Answers
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CWAP - Chapter 7 Exam Questions and correct Answers
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CWAP - Chapter 7What do 802.11 STA, including both AP STA and non-AP STA use to contend for the
wireless channel? - Question and answersCSMA/CA
CSMA/CA causes WLAN's to have a much_____throughput-to-data rate ratio than
wired LANS - Question and answerslower
What are two other protocols that use CSMA/CA but have never been implemented in
the real world - Question and answersPoint Coordination Function (PCF) and HCF-
controlled channel access (HCCA) Both protocols involve the AP taking over control of
the wireless channel and dictating which STAs are allowed to send through the use of
polling.
In regards to medium contention which protocol is used for non QoS STA's to contend
for the medium? - Question and answersDistributed Coordination Function (DCF). This
also means that these STA's can only send null and data frames rather than QoS data
and QoS null frames.
Also note that AP's must also follow the same contention rules as STA's
What two carrier sense protocols are used by STA's to indicate whether a channel is
busy or idle - Question and answersPhysical Carrier Sense - also known as clear
channel assessment
Virtual Carrier Sense - also known as Network Allocation Vector
Both QoS and non-QoS devices use the CCA and the NAV in the same way. So before
a STA attempts to transmit data, both the CCA and the NAV must be checked in order
to verify that the channel is idle.
How does Clear Channel Assessment work? - Question and answersCCA simply
involves listening to the channel. If the channel is considered occupied, then the CCA
indicates a state of busy. If the channel is considered clear, then the CCA indicates a
state of idle.
The CCA is set to busy if a high enough level of energy is detected coming from a valid,
modulated 802.11 bits.
Energy detection level varies by transmit power
, What is the process if CCA detects modulated energy levels? - Question and
answersThe CCA will go busy for 15 microseconds if DSSS modulation is being used or
for 4 microseconds if OFDM modulation is being used. If at the end of that period
modulated bits continue to be detected on the channel at the requisite energy levels, the
CCA will continue to stay busy and 802.11 devices, both AP's and STA's, will continue
to stay quiet.
What are two important factors to remember about CCA? - Question and answersCCA
only applies to 802.11 modulation. Interference from non-802.11 devices does not
cause the CCA to go into a busy state
CCA may not keep ALL devices within a BSS quiet. If an AP or STA is too far away to
detect data transmissions at the requisite energy level, the CCA may go into the idle
state even though the channel is still occupied
If an AP or STA sees the CCA as idle while the channel is still occupied, it could lead to
collisions. That's why 802.11 WLANS have the NAV, which acts as a virtual alternative
What are some of the reasons CCA may fail to keep other devices on the channel quiet
- Question and answersToo far from the transmitting device
obstructions blocking the RF path from the transmitting device
Interference in the RF path to the transmitting device
How do STA's set their NAV timer? - Question and answersAP's and STA's set their
NAV value according to the duration value inside the 802.11 header. The duration value
is contained within the Duration/ID field of all 802.11 frames except the power save poll.
The duration value will set the NAV only if the AP or STA viewing the header is not the
receiver frame.
To prevent all STA's from transmitting the moment the medium is free another protocol
is used for medium contention - Question and answersInterframe Space
What is Interframe Space (IFS) - Question and answersThe IFS is a quiet period that
APs and STA's must wait before any 802.11 frame transmission. There are many
different IFS's used depending on the situation
What are the 4 rules that define which IFS will be used before a frame transmissions -
Question and answersIf the arbitration has been completed, then a reduced IFS (RIFS)
or short IFS (SIFS) will be used. In most cases, the SIFS is used. The RIFS is only used
between consecutive frames transmitted by the same 802.11n device
If arbitration has not been determined, then an arbitration IFS (AIFS) or DCF IFS (DIFS)
will be used. The AIFS is used for WLAN's that support 802.11e QoS, and the DIFS is
used for WLANs that do not support 802.11e QoS
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