Chapter 6 "understanding 802.11 Frame Types"

Front 1

What is the difference between 802.3 Ethernet switches and 802.11 Wireless Access Points when it comes to forwarding frames between two hosts?

Back 1

Switches do not intervene or actively participate in the exchange. Wireless Access Points do because they are the central "hub/manager" of a BSS

Front 2

If a client wants to do client-to-client communication, it must coordinate with the AP to use what IEEE standard?

Back 2

802.11z - Direct Link Setup (DLS)

Front 3

IEEE 802.11 networks are based on what traditional addresses?

Back 3

MAC Addresses

Front 4

Wireless clients know the APs MAC address by what name?

Back 4

Basic Service Set Identifier (BSSID)

Front 5

Name the nine fields in a 802.11 frame.

Back 5

1. Frame Control Field

2. Duration/ID Field

3. Address1 Field

4. Address2 Field

5. Address3 Field

6. Sequence Control Field

7 Address4 Field

8. Data Field

9. Frame Check Sequence (FCS) Field

Front 6

An 802.11 frame motion is indicated by two bits. What are they?

Back 6

To DS and From DS

Front 7

How many possible destinations are there for a wireless frame?

What are they?

Back 7

Four

1. Management/Control/DLS

2. DS to Client

3. Client To DS

4. Wireless Bridge or Mesh

Front 8

What three examples would each frame motion bit be set to 0?

Back 8

1. When an AP sends a management/control frame (broadcast)

2. When a client sends a management frame to an AP (AP is destination)

3. When a client sends a frame directly to another client via DLS (Direct Link Setup)

Front 9

What is another special case that is neither in the BSS nor in the DS so both directions are set to 1?

Back 9

Mesh AP Networks

Front 10

What field in the 802.11 frame always contains the receiver address (RA)?

Back 10

The Address1 field

Front 11

What field in the 802.11 frame always contains the transmitter address (TA)?

Back 11

The Address2 field

Front 12

What field contains the final destination address when the RA is not the final recipient?

Back 12

The Address3 Field

Front 13

This field can also contain the original source address when the TA is not the originator.

Back 13

The Address3 Field

Front 14

What field does not come into play unless a frame is being transported from one AP to another AP across a wireless link?

Back 14

The Address4 Field

Front 15

What are the two frame motion bits used for management, control or DLS?

Back 15

To DS - 0

From DS - 0

Front 16

What are the two frame motion bits used for DS to client?

Back 16

To DS - 0

From DS - 1

Front 17

What are the two frame motion bits used for client to DS?

Back 17

To DS - 1

From Client - 0

Front 18

What are the two frame motion bits used for Wireless bridge or mesh?

Back 18

To DS - 1

From DS - 1

Front 19

If a frame motion bit set is 0 and 0, where is the frame going?

Back 19

Management, Control, or DLS

Front 20

If a frame motion bit set is 0 and 1, where is the frame going?

Back 20

DS to Client

Front 21

If a frame motion bit set is 1 and 0, where is the frame going?

Back 21

Client to DS

Front 22

If a frame motion bit set is 1 and 1, where is the frame going?

Back 22

Wireless bridge or Mesh

Front 23

What are the three addresses for a management. control, or DLS frame?

Back 23

Address1 (RA): Destination Address

Address2 (TA): Source Address

Address3: BSSID

(Address3 can contain the final destination or the original source)

Front 24

What are the three addresses for a DS to Client Frame?

Back 24

Address1 (RA): Destination Address

Address2 (TA): BSSID

Address3: Source Address

Front 25

What are the three addresses for a Client to DS Frame?

Back 25

Address1 (RA): BSSID

Address2 (TA): Source Address

Address3: Destination Address

Front 26

What are the four addresses for a frame destined for a wireless bridge or mesh network?

Back 26

Address1 (RA): Destination Radio

Address2 (TA): Source Radio

Address3: Destination Address

Address4: Source Address

Front 27

If an AP receives a fame that is destined for the DS, what does it to to the frame?

Back 27

Converts it to an 802.3 wired frame

Front 28

What is a function each device uses to coordinate the use of a wireless channel?

Back 28

Distributed Coordination Function (DCF)

Front 29

Devices based on the 802.3 and 802.11 standards must use what technique to determine if the media is available before transmitting?

Back 29

Carrier Sense Multiple Access (CSMA)

Front 30

What two-fold process can devices use to detect a channel in use?

Back 30

Physical Carrier Sense and Virtual Carrier Sense

Front 31

When trying to detect a channel in use, what process is used to listen to the channel to overhear any other transmission that might be occurring?

Back 31

Physical Carrier Sense

Front 32

When a wireless client hears a frame that is destined for its own MAC address it receives it for processing, otherwise if the MAC is different, it determines the channel is busy. What is this process called?

Back 32

Clear Channel Assessment (CCA)

Front 33

When trying to detect a channel in use, what process has to include a duration field in the Duration/ID frame header field?

Back 33

Virtual Carrier Sense

Front 34

What indicates how many microseconds are required for the whole frame to be sent over the channel and effectively reserves the channel for that length of time?

Back 34

The duration field in the Duration/ID frame header

Front 35

What must each wireless client use to predict when the channel will become free?

Back 35

A Network Allocation Vector (NAV)

Front 36

Each time a frame is overheard on the channel, what happens to the NAV?

Back 36

The frame duration is added to the NAV

Front 37

Wired devices can detect collisions in real time. This is known as what?

Back 37

CSMA/CD (collision detection)

Front 38

Wireless clients CANNOT detect when a collision occurs at all. Instead, 802.11 devices must try to _______ collisions. This is known as what?

Back 38

CSMA/CA (collision avoidance)

Front 39

How do wireless clients avoid collisions?

Back 39

They back off and wait a random time before transmitting

Front 40

When a client has a frame to transmit, it chooses a random number (0 to 31) of timeslots to use as what?

Back 40

a backoff timer

Front 41

The total time a client spends waiting to transmit is called what?

Back 41

The Contention Window (backoff timer + NAV)

Front 42

What are periods that provide a safety cushion between frames?

Back 42

Interframe Space Periods

Front 43

What interframe space period has the shortest period of time, and is used before each data frame during a burst of 802.11n frames?

Back 43

Reduced Interframe Space (RIFS)

Front 44

What interframe space period is used between data frames and frame acknowledgements, or CTS 802.11g (DSSS & OFDM) protection mode control frames?

Back 44

Short Interframe Space (SIFS

Front 45

What interframe space period is the default period used after mfost standard priority frame types?

Back 45

Distributed Interframe Space (DIFS)

Front 46

What interframe space period is the longest period of time and used after collisions and before retransmitted frames?

Back 46

Extended Interframe Space (EIFS)

Front 47

What are the four steps a device must follow before it can transmit on a channel?

Back 47

1. Wait for DIFS

2. Wait for its own backoff timer

3. Listen

4. Wait for contention window (backoff timer + NAV) - if clear, the device may transmit.

Front 48

Each time a wireless client receives a unicast frame, it must send what to the sender?

Back 48

A unicast acknowledgement

(except for 802.11n & 802.11e standards, which send bursts at a time)

Front 49

If a transmitted frame fails and is not acknowledged, what does the sending client do?

Back 49

The sending client will choose a new backoff timer value that is double the previous choice.

Front 50

Can a wireless client transmit before another client that has already been waiting?

Back 50

Yes, due to the random backoff timer, the second client can have a shorter window.