Chapter 8- 802.11 Medium Access
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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.
• 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 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.
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
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
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
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
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
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
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
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Interframe Space
Virtual Carrier Sense
Physical Carrier Sense
Random back-off timer
Pg 254
Interframe Space (IFS)
• The amount of time between transmissions
– Actual length of time depends on network speed
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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
Interframe Space (IFS)
• Only certain types of frames are sent after
certain interframe spaces
– Only ACK and CTS after SIFS
Pg 254
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
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
Duration/ID field
Pg 256
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
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
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
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
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
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
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
Enhanced Distributed Channel Access (EDCA)
Pg 259
Enhanced Distributed Channel Access (EDCA)
• Defines four access categories base on User
Priority level
• From lowest to highest priority
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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
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
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
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
Block Acknowledgement (BA)-Immediate
Pg 261
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 (WMM)
Pg 262
W i-Fi Multimedia-PS
• Power saving mechanisms for increased
battery life
Pg 262
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 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.