Chapter 8.

Question 1

Why does a WLAN use CSMA/CA instead of CSMA/CD?

Answer 1

802.11 wireless radios are not capable of transmitting and receiving at the same time, so they are not capable of detecting a collision during their transmission. For this reason, 802.11 wireless networking uses CSMA/CA instead of CSMA/CD to try to avoid collisions

Question 2

What does a wireless radio do when it has determined that no other stations are transmitting?

Answer 2

the 802.11 radio will choose a random backoff value. The station will then wait an additional period of time, based on that backoff value, before transmitting. During this time, the station continues to monitor to make sure that no other stations begin transmitting. Because of the half-duplex nature of the RF medium, it is necessary to ensure that at any given time only one 802.11 radio has control of the medium.

Question 3

What is CSMA/CA?

Answer 3

CSMA/CA is a process used to ensure that only one 802.11 radio is transmitting at a time.

Question 4

What is distributed coordination function?

Answer 4

A medium access method that utilizes multiple checks and balances to try to minimize collisions.

Question 5

Explain what each letter is in CSMA?

Answer 5

Carrier Sense determines whether the medium is busy.

Multiple Access ensures that every radio gets a fair shot at the medium (but only one at a time).

Collision Avoidance means only one radio gets access to the medium at any given time, hopefully avoiding collisions.

Question 6

How do AP’s know if a collision occurred? Describe what an ACK is.

Answer 6

Every time an 802.11 radio transmits a unicast frame, if the frame is received properly, the 802.11 radio that received the frame will reply with an acknowledgment (ACK) frame.

The ACK frame is a method of delivery verification of unicast frames.

Question 7

What does 802.11n and 802.11ac radios do with frames and acknowledging them?

Answer 7

They make use of frame aggregation, which groups multiple unicast frames together. The delivery of aggregated frames is verified using a block ACK.

Question 8

How is a frame checked for errors?

Answer 8

Used a CRC in the trailer or Cyclic Redundancy Check. If any portion of the unicast frame is corrupted, the CRC will fail and the receiving 802.11 radio will not send an ACK frame to the transmitting 802.11 radio. If an ACK frame is not received by the original radio, the unicast frame is not acknowledged and will have to be retransmitted.

Question 9

What is the fundamental access method of 802.11 communications?

Answer 9

Distributed coordination function.

Interframe space
Duration/ID field
Carrier sense
Random backoff timer

Question 10

What are the 4 main components of DCF (Distributed Coordination Function)? What are there purposes? Do they function at the same time?

Answer 10

Interframe space
Duration/ID field
Carrier sense
Random backoff timer

These 4 components are the checks and balances that work together at the same time to ensure that only one 802.11 radio is transmitting on the half-duplex medium,.

They do function at the same time.

Question 11

What is interframe space (IFS)

Answer 11

A period of time that exists between transmissions of wireless frames.

Question 12

What are the 6 times of interframe spaces, shortest to longest.

Answer 12

Reduced interframe space (RIFS), highest priority
Short interframe space (SIFS), second highest priority
PCF interframe space (PIFS), middle priority.
DCF interframe space (DIFS), lowest priority
Arbitration interframe space (AIFS), used by QoS stations.
Extended interframe space (EIFS), used after receipt of corrupted frames.

Question 13

What are the two most common interframe spaces used?

Answer 13

The two most common interframe spaces used

are the SIFS (Short Interframe Space) and the DIFS (DCF Interframe Space).

Question 14

What are two reasons for the existence of interframe spaces?

Answer 14

Interframe spaces are one line of defense used by CSMA/CA to ensure that only certain types of 802.11 frames are transmitted following certain interframe spaces.

It insures that higher priority frames such as ACK’s are categorized as SIFS which allows them to transmit first ahead of other frame types.

Question 15

What is a duration/ID field?

Answer 15

Duration/ID field. When a client transmits a unicast frame, the Duration/ID field contains a value from 0 to 32,767. The Duration/ID value represents the time, in microseconds, that is required to transmit an active frame exchange process so that other radios do not interrupt the process.

Question 16

What does a client do with the duration/ID field?

Answer 16

The client that is transmitting the data frame calculates how long it will take to receive an ACK frame and includes that length of time in the Duration/ID field in the MAC header of the transmitted unicast data frame..

Question 17

Draw an example of SIFS + ACK to demonstrate duration value.

Answer 17

Make drawing

Question 18

What does the duration/ID field contain the majority of the time?

Answer 18

A value that is used to reset other station’s NAV (Network Allocation Vector) timers.

Question 19

What is the first step that an 802.11 CSMA/CA device needs to do to begin transmitting? What does it do?

Answer 19

Perform carrier sense. This is a check to see whether the medium is busy.

Question 20

What are the two ways that a carrier sense is performed?

Answer 20

Virtual Carrier and Physical Sense.

Question 21

What is a virtual carrier sense?

Answer 21

A timer mechanism known as the network allocation vector. The NAV timer maintains a prediction of future traffic on the medium based on duration value information seen in a previous frame transmission.

Question 22

How does an AP use the NAV timer to know when to send data?

Answer 22

When an AP is not transmitting it is listening. When an AP is listening it will hear a transmission from another station. It will look at the header of the frame and determine whether the duration field contains a duration value or an ID value. If the field contains a duration, the listening station will set its NAV timer to this value. The AP knows that when the timer reaches 0 the medium will no longer be busy. All other listening stations to do the same.

Question 23

What is physical carrier sense?

Answer 23

A second line of defense used by CSMA/CA. This is is used because sometimes an AP or station for whatever reason can not hear the duration timer\ID of another transmitting station and did not set its NAV timer.

Question 24

What does a station do when it is performing physical carrier sense?

Answer 24

listening to the channel to see whether any other transmitters are taking up the channel

Question 25

What are the 2 purposes to a physical carrier sense?

Answer 25

The first purpose is to determine whether a frame transmission is inbound for a station to
receive. If the medium is busy, the radio will attempt to synchronize with the transmission.

The second purpose is to determine whether the medium is busy before transmitting. This
is known as the clear channel assessment (CCA). The CCA involves listening for RF transmissions at the Physical layer. The medium must be clear before a station can transmit.

Question 26

Are virtual sense and physical carrier sense happening at the same time?

Answer 26

Yes, always.

Question 27

At what layer of the OSI do virtual and physical carrier sense operate in?

Answer 27

Virtual is a layer 2.

Physical is a physical layer protection.

Question 28

Once carrier sense has reached the point of discovering that the medium is physically clear, and that all timers have reached zero. What is the next process?

Answer 28

The client now will begin contending for the medium by creating a random backoff timer.

Question 29

How does a station create a random backoff timer?

How is this created?

Answer 29

By choosing a random number from a range called a contention window value. After the random number is chosen, the number is multiplied by the slot time value.

It is created by…..Slot time sizes are dependent on the physical layer specification (PHY) in use (DSSS, OFDM,
etc.). This starts a random backoff timer. The random backoff timer is the final timer used
by a station before it transmits. The station’s backoff timer begins to countdown ticks of a
clock known as slots. When the backoff time is equal to 0, the client can reassess the channel
and, if it is clear, begin transmitting

Question 30

Break down the process of creating a backoff timer value.

Answer 30

1. An OFDM station selects a random number from a contention window of 0–15. For this example, the number chosen is 4.
2. The station multiplies the random number of 4 by a slot time of 9μs.
3. The random backoff timer has a value of 36μs (4 slots)
4. For every slot time during which there is no medium activity, the backoff time is decremented by a slot time
5. The station decrements the backoff timer until the timer is zero.
6. The station transmits if the medium is clear.

Question 31

What coordination function/medium contention mechanism did 802.11e bring to the table?

Answer 31

The 802.11e quality of service amendment added a new coordination function to 802.11 medium contention, known as Hybrid Coordination Function (HCF).

Question 32

What is HCF (Hybrid Coordination Function) controlled Channel Access?

Answer 32

HCF defines the ability for an 802.11 radio to send multiple frames when transmitting on the RF medium.

Question 33

What is the primary difference between DCF (Distributed Control Function) / PCF (Point Coordination Function) and HCF (Hybrid Coordination Function)?

Answer 33

HCF defines the ability for an 802.11 radio to send multiple frames when transmitting on the RF medium. When an HCF-compliant radio contends for the medium, it receives an allotted amount of time to send frames.

Question 34

What is a transmit opportunity?

Answer 34

HCF period of time to send frames.

Question 35

What is it called when HCF sends multiple frames at once? And what type of interframe spacing is used?

Answer 35

Frame burst. SIFS interframe spacing is used.

Question 36

What is enhanced distributed channel access? What is it an extension of? What is the purpose?

Answer 36

A wireless media access method that provides differentiated access that directs traffic to 4 access category QoS priority queues.

This is an extension to DCF (Distributed Coordinated Function).

The EDCA medium access method prioritizes traffic using priority tags that are identical to 802.1D priority tags. Priority tags provide a mechanism for implementing QoS at the MAC level.

Question 37

What is block acknowledgment?

Answer 37

Improves channel efficiency by aggregating several acknowledgments into one single acknowledgement frame.

Question 38

What is the immediate Block ACK? How does it work? What is the purpose?

Answer 38

Designed for use with low-latency traffic. an originator station sends a block of QoS data frames to a recipient station. The originator requests acknowledgment of all the QoS data frames by sending a BlockAckReq frame. Instead of acknowledging each unicast frame independently, the block of QoS data frames are all acknowledged by a single Block ACK.

A bitmap in the Block ACK frame is used to indicate the status of all the received data frames. If only one of the frames is corrupted, only that frame will need to be retransmitted. The use of a Block ACK instead of
a traditional ACK is a more efficient method that cuts down on medium contention overhead.

Question 39

What purpose was the 802.11e amendment?

Answer 39

The 802.11e amendment defned the layer 2 MAC methods needed to meet the QoS requirements for time-sensitive applications over IEEE 802.11 wireless LANs.

Question 40

How many access categories does WMM provide? What are they?

Answer 40

4.

WMM Voice
WMM Video
WMM Best Effort
WMM Background

Question 41

WMM is based on what mechanism

Answer 41

Enhanced distributed channel access.

Question 42

What is WMM Voice Priority?

Answer 42

This is the highest priority. It allows multiple and concurrent VoIP calls with low latency and
toll voice quality

Question 43

WMM Video priority?

Answer 43

This supports prioritized video traffic before
other data traffic. A single 802.11g or 802.11a
channel can support three to four SDTV video
streams or one HDTV video stream.

Question 44

What is WMM Best Effort Priority?

Answer 44

This is traffic from applications or devices, such
as Internet browsing, that don’t need QoS
priority, such as legacy devices. This traffic
is not as sensitive to latency but is affected by
long delays

Question 45

What is WMM Background Priority?

Answer 45

This is low-priority traffic that does not have strict throughput or latency requirements. This traffic includes file transfers and print jobs.

Question 46

What device power changes came out of 802.11e?

Answer 46

Power-saving mechanisms to increase the battery life via advanced power-saving mechanisms.

Question 47

What is WMM-Admission Control? What was its purpose and how did it work?

Answer 47

Which defines the use of management frames for the signaling between an AP and a client station. Client stations can request to send a traffic stream (TS) of frames of a particular WMM access category.
A traffic stream can be unidirectional or bidirectional.

How it works : An AP will evaluate a request frame from a client station against the network load and channel conditions. If the AP can accommodate the request, it accepts the request and grants the client station the medium time for a traffic stream. If the request is rejected, the client device is not allowed to initiate the requested traffic stream and may decide to delay the traffic stream, associate with a different AP, or establish a best-effort traffic stream outside the operation of WMM-Admission Control. WMM-Admission Control improves the performance for time-sensitive data such as video and voice. WMM-Admission Control improves the reliability of applications in progress by preventing oversubscription of bandwidth.

Question 48

What is the purpose of airtime fairness? What happens in a network without airtime fairness on? How does airtime fairness work?

Answer 48

Airtime fairness came about because of the development of high speed modern devices that can send and receive data very quickly. Since wifi is contention based speed of transmission is paramount to the overall health of the network. Faster devices can use up less airtime because they transmit very quickly. Slowly devices transmitting can take a lot of airtime away from the rest of the network.

Without airtime both stations will statistically get an equal number of times to access the RF medium even though one of the stations is capable of transmitting at a higher rate and requires much less airtime to transmit the same amount of data.

Purpose : the station with the higher data rate transmission is given priority before the station with the lower data rates. Effectively this is a much better use of transmission time because the higher data rate station does not have to remain idle waiting during the lower data rate transmission. A faster station can transmit a set amount of frames in a much shorter time period, and the slower rate station still send all
its frames in about the same period as before. Airtime fairness effectively achieves better
time management of the medium by cutting down on wait times. The net result is better performance, higher capacity, and more throughput over the Wi-Fi network.