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Flashcards in Chapter 8. Deck (49):

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

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


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

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.


What is CSMA/CA?

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


What is distributed coordination function?

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


Explain what each letter is in CSMA?

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.


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

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.


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

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


How is a frame checked for errors?

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.


What is the fundamental access method of 802.11 communications?

Distributed coordination function.


What are the 4 main components of DCF? What are there purposes? Do they function at the same time?

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.


What is interframe space (IFS)

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


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

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.


What are the two most common interframe spaces used?

The two most common interframe spaces used
are the SIFS and the DIFS.


What are two reasons for the existence of interframe spaces?

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.


What is a duration/ID field?

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.


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

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..


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

Make drawing


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

A value that is used to reset other station's network allocation vector ) NAV) timers.


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

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


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

Virtual Carrier and Physical Sense.


What is a virtual carrier sense?

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.


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

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. This allows all other listening stations to do the same thing.


What is physical carrier sense?

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.


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

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


What are the 2 purposes to a physical carrier sense?

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.


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

Yes, always.


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

Virtual is a layer 2.
Physical is a physical layer protection.


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?

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


How does a station create a random backoff timer?
How is this created?

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


Break down the process of creating a backoff timer value.

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.


What is DCF?

Distributed coordinated function.


What did 802.11e bring to the table?

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


What is HCF controlled Channel Access?

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


What is the primary difference between DCF (Distributed control function and PCF point coordination function and HCF?

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.


What is a transmit opportunity?

HCF period of time to send frames.


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

Frame burst. SIFS interframe spacing is used.


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

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.


What is block acknowledgment?

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


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

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.


What purpose was the 802.11e amendment?

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.


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


WMM Voice
WMM Video
WMM Best Effort
WMM Background


WMM is based on what mechanism

Enhanced distributed channel access.


What is WMM Voice Priority?

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


WMM Video priority?

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.


What is WMM Best Effort Priority?

This is traffic from applications or devices, such
as Internet browsing, that cannot provide QoS
capabilities, such as legacy devices. This traffic
is not as sensitive to latency but is affected by
long delays


What is WMM Background Priority?

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


What came out of 802.11e?

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


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

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.


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

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 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. Notice that the faster station transmitted all eight frames in a much shorter time period, and the slower rate station still sent all
eight 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.