Chapter 8

Question 1

What is CSMA?

Answer 1

Carrier Sense Multiple Access

Basic method with carrier detection

Question 2

ACK?

Answer 2

If Cyclic redundancy check CRC on received frame fails, receiving radio will not send an ACK
if ACK is not received,802.11 frame must be retransmitted after CSMA/CA process again.
802.11 Cannot detect if a collision occurs

Question 3

DCF?

Answer 3

Distributed Coordination Function
Fundamental/Mandatory access mode of 802.11 standard
Provides CSMA/CA

Main components Include:

• InterFrame Space
• Duration / ID Field
• Carrier Sense (Physical and Virtual)
• Random Back-Off Timer

Question 4

IFS?

Answer 4

Time period between Wireless Transmissions

Question 5

Types of IFS?

Answer 5

Reduced, Highest priority
Short, Second Highest
PCF, Middle Priority
DCF, lowest priority
Arbitration. used by QoS Stations
Extended, Used with retransmissions

Question 6

Time order of IFS?

Answer 6

RIFS < SIFS < PIFS < DIFS < AIFS < EIFS

Question 7

Why does IFS vary?

Answer 7

Due to transmission speed of the network

SIFS and DIFS are the most common

Question 8

Duration / ID Field

Answer 8

Field in the MAC header of an 802.11 unicast frame
- Value from 0 to 32767
- Represents the time in microseconds to transmit
- Includes the time for ACK plus one SIFS interval
Every Wireless Frame has a Duration Value
Used to set the NAV value in contention

Question 9

Carrier Sense?

Answer 9

Physical Carrier Sense

Virtual Carrier Sense

Question 10

Physical Carrier Sense?

Answer 10

Performed by stations not Transmitting or Receiving

Listening to any 802.11 traffic

Question 11

Virtual Carrier Sense?

Answer 11

Uses a logical timer called the Network Allocation Vector

Stations can not transmit unless the NAV value is 0

Question 12

Detailed Virtual Carrier Explanation?

Answer 12

Uses the Network Allocation Vector (NAV) timer mechanism

Maintains a predication of future traffic on the medium, based on Duration

Question 13

Physical Carrier Sense?

Answer 13

Performed constantly when a station is not transmitting or receiving
Station listens to see if channel is busy

Question 14

2 Purposes of Physical Carrier Sense?

Answer 14

Determine if a frame transmission is inbound for a station to receive
Determine if the medium is busy before transmitting - Clear Channel Assessment

Question 15

What happens at the same time?

Answer 15

Virtual and physical carrier senses

Question 16

Random Back-Off Timer?

Answer 16

Final Timer used prior to Transmission
Reduces the change the two or more wireless stations transmit simultaneously
Occurs after medium is inactive for DIFS interval and a station wants to transmit

Question 17

What is PCF?

Answer 17

Point Coordination Function
Optional 802.11 media access method
Uses a form of polling
AP acts as a Point Coordinator (PC)
Will only work with BSS or ESS
Both AP and station must support PCF
When functioning in PCF mode it is known as the contention free period
When functioning in DCF mode, it is known as contention period (CP)
PCF has yet to implemented by any vendors

Question 18

What is HCF?

Answer 18

Hybrid Coordination Function
Added by 802.11e QoS amendment
Combines capabilities from DCF and PCF
Adds enhancements to create to channel-access methods
Defines ability to send multiple frames sequentially, known as frame burst.

Question 19

What is TXOP?

Answer 19

When contending for the medium, station receives an allotted time to send frames, call Transmit Opportunity (TXOP)

Question 20

What is BA?

Answer 20

Block Acknowledgement
Introduced in 802.11e
Aggregates ACK Frames
Two types used:
- Immediate (for low latency)
- Delayed (for latency tolerant)
More efficient and cuts down on medium contention overhead

Question 21

What is WMM?

Answer 21

Wi-Fi Multimedia
Created by the Wifi Alliance
- Partially Mirrors 802.11e
- Further defines communications for time sensitive frames like voice
802.1D priority tags are used to direct traffic to four access category priority queues.

Question 22

What did Wi-Fi alliance also define?

Answer 22

WMM-PS (Power Save) to increase battery life.

WMM-Admission Control, which defines the use of management frames for the signaling between an AP and a client station.