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Chapter 8- 802.11 Medium Access. CSMA/CA vs CSMA/CD DCF PCF HCF BA WMM. Exam Essentials. Understand the similarities and differences between CSMA/CA and CSMA/CD. Understand both access methods and know what makes them similar and what makes them different.

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Chapter 8 802 11 medium access l.jpg
Chapter 8- 802.11 Medium Access

  • CSMA/CA vs CSMA/CD

  • DCF

  • PCF

  • HCF

  • BA

  • WMM


Exam essentials l.jpg
Exam Essentials

  • Understand the similarities and differences between CSMA/CA and CSMA/CD.

    • Understand both access methods and know what makes them similar and what makes them different.

  • Define the four checks and balances of CSMA/CA and DCF.

    • Understand that virtual carrier-sense, physical carrier-sense, interframe spacing, and the random back-off timer all work together to ensure that only one 802.11 radio is transmitting on the half-duplex medium.

  • Define virtual and physical carrier-senses.

    • Understand the purpose and basic mechanisms of the two carrier-senses.

  • Explain DCF and PCF.

    • Defne the basic operations of both Distributed Coordination Function and Point Coordination Function.


Exam essentials3 l.jpg
Exam Essentials

  • Define HCF quality of service mechanisms.

    • Hybrid Coordination Function defnes the use of TXOPs and access categories in EDCA as well as the use of TXOPs and polling during HCCA.

  • Understand Wi-Fi Multimedia (WMM) certification and its importance now and in the future.

    • WMM is designed to provide quality of service capabilities to 802.11 wireless networks. WMM is a partial mirror of the 802.11e amendment. WMM currently provides for traffc priority via four access categories.


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CSMA/CA vs. CSMA/CD

  • Media Access Control (MAC) describes how the media (wired or wireless) is used

    • Polling, token passing, contention based

  • CSMA/CD is for ethernet wired networks

  • CSMA/CA for wireless 802.11

  • Stations using either access method must first listen to see whether any other device is transmitting. If another device is transmitting, the station must wait until the medium is available.

Pg 252


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CSMA/CA vs. CSMA/CD

  • The difference is when the coast is clear

  • CSMA/CD node can immediately begin transmitting.

    • If a collision occurs while a CSMA/CD node is transmitting, the collision will be detected and the node will temporarily stop transmitting.

  • 802.11 wireless stations are not capable of transmitting and receiving at the same time, so they are not capable of detecting a collision during their transmission.

    • 802.11 wireless networking uses CSMA/CA instead of CSMA/CD to try to avoid collisions.

Pg 252


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CSMA/CA

  • IF CSMA/CA station sees no transmissions it will wait a random interval

    • Keep watching the medium

    • If still clear after interval, transmit

    • If not, start over

  • Because only 1 station can use the frequency at a time

    • Half duplex

Pg 252


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CSMA/CA

  • Standard Distributed Coordination Function (DCF) is defined to allow access of multiple stations

    • Checks and balances to keep the line clear

    • Minimize chances of collision

Pg 252


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Collision Detection

  • Since 802.11 stations cannot detect collisions, how do they know when they happen?

  • Every unicast frame requires an acknowledgement

    • Broadcast and multicast don’t

    • ACK Frame

  • When ACK Frame is received, original station knows the frame is received

Pg 253


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Collision Detection

  • Receiving device will also check the CRC

    • If frame is corrupt, no ACK frame

    • IF no ACK frame, sender assumes delivery failure

Pg 253


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Distributed Coordination Function (DCF)

  • Fundamental method of 802.11 communication

  • Mandatory access method

    • Point Coordination Functions (PCF) is optional

    • 802.11 also has Hybrid Coordination Function (HCF)

  • Four Main parts

    • Interframe Space

    • Virtual Carrier Sense

    • Physical Carrier Sense

    • Random back-off timer

Pg 254


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Interframe Space (IFS)

  • The amount of time between transmissions

    • Actual length of time depends on network speed

  • Short interframe space (SIFS), 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 with retransmissions

Pg 254


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Interframe Space (IFS)

  • Only certain types of frames are sent after certain interframe spaces

    • Only ACK and CTS after SIFS

Pg 254


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Interframe Space (IFS)

  • Two most commonf are SIFS and DIFS

  • The IFS is a backup to the virtual carrier sense

  • SIFS < PIFS < DIFS < AIFS < EIFS

Pg 254


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Duration/ID field

  • Field in the Frame Header

  • Value between 0 and 32,767

  • Time, in microseconds, required to transmit the ACK plus the SIFS interval

    • ACK frame has 0 duration

  • Basically states how long the medium will be busy after the frame is transmitted

    • Used to reset the network allocation vector (NAV) timer on other stations on network

    • Special PS-Poll frame-duration is used as an ID

Pg 255



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Carrier Sense

  • Check to see if the medium is busy

    • Virtual Carrier Sense

    • Physical Carrier Sense

  • Both occur at the same time/in partnership

Pg 256


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Virtual Carrier Sense

  • Time called the Network Allocation Vector (NAV) watches the Duration field in transmitted frames

    • Set NAV timer to the duration value in last transmitted frame

    • Knows the network will be busy that long

  • Station cannot use network unless NAV timer is 0

    • Help prevent collisions

Pg 256


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Physical Carrier Sense

  • Actual listening to the RF to see if there is traffic

    • Just in case you didn’t see previous frame or couldn’t read the duration

  • Check to see if there is an incoming frame

  • Check to see if medium is busy before transmitting

    • Clear channel assesment (CCA)

Pg 256


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Random Back-off Timer

  • Contention window is the period of time that stations can compete for access to the network

  • Station selects a random back-off value

    • 0 to initial contention window

  • Back-off is multiplied by slot time

    • Slot time depends on technology in use

  • When back-off timer reaches 0, AND channel is clear, they can transmit

    • If channel is busy, start over

Pg 258


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Point Coordination Function (PCF)

  • Optional access control method

    • Polling

  • AP is point coordinator

    • Only work in a BSS

    • Not in ad-hoc/IBSS

  • Both AP and client station must support PCF

  • AP will switch between DCF and PCF

    • PCF time is contention free period (CFP)

    • DCF is contention period (CP)

  • Not implemented yet

Pg 259


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Hybrid Coordination Function (HCF)

  • Added in 802.11e

    • Enhanced Distributed Channel Access (EDCA)

    • HCF Controlled Channel Access (HCCA)

  • DCF and PCF require contention for each frame

  • HCF defines ability to send multiple frames

    • Transmit Opportunity (TXOP)

    • During TXOP period, client station can send a frame burst

      • Uses Short Interframe Space (SIFS)

Pg 259


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Enhanced Distributed Channel Access (EDCA)

  • Provides differentiated access using eight user priority levels

    • Extension of DCF

  • Uses frame tags similar to 802.1D standard

    • QoS standard at MAC (layer 2) level

    • Define priority values

  • With priority queuing

    • Data waiting in higher priority queues transmits before lower priority queues

Pg 259



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Enhanced Distributed Channel Access (EDCA)

  • Defines four access categories base on User Priority level

  • From lowest to highest priority

    • AC_BK (Background)

    • AC_BE (Best Effort)

    • AC_VI (Video)

    • AC_VO (Voice

  • For each category, Enhanced Distributed Cannel Access Function (EDCAF) is used

    • Frames with higher category have lower back off values and are more likely to get a TXOP

Pg 259


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H CF Controlled Channel Access (HCCA)

  • Uses a QoS aware centralize coordinator

    • Hybrid Coordinator (HC)

  • Built into access point and has a higher priority

    • Can allocate TXOP to itself and other stations to provide controlled access phase (CAP)

    • Contention free transfer for QoS data

Pg 261


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Block Acknowledgement (BA)

  • Part of 802.11e

  • Allows for a single acknowledgement for multiple frames

    • Reduces overhead

  • Two Types

    • Immediate

      • For low latency traffic

    • Delayed

      • For latency tolerant traffic

Pg 261


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Block Acknowledgement (BA)-Immediate

  • Originator sends a block of QoS data frames to station

  • Originator requests acknowledgement of all outstanding QoS data

    • Block AckReq frame

      • Recipient can send a single ack frame for all received frames

    • Can request a single frame from block be retransmitter

Pg 261



W i fi multimedia wmm l.jpg
W i-Fi Multimedia (WMM)

  • For latency sensitive data

    • Real time voice and video

  • Voice, video, audio, have less tolerance of latency (cumulative delay)

  • 802.11e had layer 2 MAC methods to meet QoS requirements

    • WiFi Alliance created WMM

  • Because WMM is based on EDCA mechanisms, 802.1D priority tags from the Ethernet side are used to direct traffic to four access-category priority queues. The WMM certification provides for traffic prioritization via four access categories

Pg 262



W i fi multimedia ps l.jpg
W i-Fi Multimedia-PS

  • Power saving mechanisms for increased battery life

Pg 262


Exam essentials32 l.jpg
Exam Essentials

  • Understand the similarities and differences between CSMA/CA and CSMA/CD.

    • Understand both access methods and know what makes them similar and what makes them different.

  • Define the four checks and balances of CSMA/CA and DCF.

    • Understand that virtual carrier-sense, physical carrier-sense, interframe spacing, and the random back-off timer all work together to ensure that only one 802.11 radio is transmitting on the half-duplex medium.

  • Define virtual and physical carrier-senses.

    • Understand the purpose and basic mechanisms of the two carrier-senses.

  • Explain DCF and PCF.

    • Defne the basic operations of both Distributed Coordination Function and Point Coordination Function.


Exam essentials33 l.jpg
Exam Essentials

  • Define HCF quality of service mechanisms.

    • Hybrid Coordination Function defnes the use of TXOPs and access categories in EDCA as well as the use of TXOPs and polling during HCCA.

  • Understand Wi-Fi Multimedia (WMM) certification and its importance now and in the future.

    • WMM is designed to provide quality of service capabilities to 802.11 wireless networks. WMM is a partial mirror of the 802.11e amendment. WMM currently provides for traffc priority via four access categories.


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