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Advanced Digital Signal Processing
Next Generation Wireless LANs
Advanced Channel Access Techniques
Section 10.1, 10.2
Student
Tashiro (M1)
Nishirura (M1)
2014/6/10
Adviser
Chosokabe (M2)
Prof. Ochi
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
1
Contents
 10 Advanced Channel Access Techniques
10.1 PCF
 10.1.1
 10.1.2
 10.1.3
 10.1.4
CFP Structure and Timing
PCF Access Procedure
PCF Transfer Procedure
PCF Limitations
10.2 HCCA
 10.2.1
 10.2.2
 10.2.3
 10.2.4
 10.2.5
 10.2.6
2014/6/10
Traffic Streams
Controlled Access Phases
Polled TXOP
TXOP requests
Use of RTS / CTS
HCCA Limitations
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
2
MAC Architecture
 Channel Access Techniques in the 802.11 Standard
In this section, we focus on contention-free channel access.
MAC Architecture
Contention-free
Point
Coordination
Function
(PCF)
option
Contention-based
HCF
Controlled
Access
(HCCA)
HCF
Contention
Access
(EDCA)
11e : mandatory
11e : mandatory
Distributed Coordination Function (DCF)
CSMA/CA
2014/6/10
Random Backoff Time
QoS
HCF :
Hybrid
Coordination
Function
mandatory
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
3
10.1 PCF
 Overview of PCF (point coordination function)
The PCF provides contention-free frame transfer.
All STAs inherently obey the medium access rules of the PCF.
 Because these rules are based on the DCF, and they set their NAVs.
When polled by the PC, a CF-Pollable STA may transmit only
one MPDU.
Contention-free Period (CFP)
The PC resides in the AP (option)
②
AP/PC
No right to transmit
①
③
NAV
STA 1
CF-Pollable (option)
2014/6/10
①
①
NAV
NAV
STA 2
CF-Pollable (option)
STA 3
non-CF-Pollable
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
4
10.1.1 CFP Structure and Timing
 Establishing the CFP
Each CFP begins with a Beacon frame that contains a DTIM
DTIM : delivery traffic indication message
element.
The PC generates CFPs at the CFPRate.
 CFPRate is defined as a number of DTIM intervals.
 The CFPRate value is communicated to other STAs in the CFPPeriod
field in the CF Parameter Set of the Beacon frames.
Element ID
Length
CFP
Count
CFP
Period
CFP MaxDuration
CFP DurRemaining
1
1
1
1
2
2
octets
 The CFPPeriod field indicates the number of DTIM intervals between
the start of CFPs.
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
5
10.1.1 CFP Structure and Timing
 Length of the CFP
The length is controlled by the PC, with maximum duration
specified in the value of the CFPMaxDuration field.
The CFPDurRemaining field specifies the maximum time
from the transmission of this beacon to the end of this CFP.
The PC terminates any CFP before or at the CFPMaxDuration.
transmission of a control frame of subtype which includes CF-End
CF Parameter Set element format
2014/6/10
Element ID
Length
CFP
Count
CFP
Period
CFP MaxDuration
CFP DurRemaining
1
1
1
1
2
2
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
octets
6
10.1.1 CFP Structure and Timing
 Example : Beacons and CFPs
The CFP is 2 DTIM intervals.
The DTIM interval is 3 beacon intervals.
The CFPMaxDuration value is about 2.5 beacon intervals.
CFPPeriod value is 2 DTIM intervals
CFPDurRemaining
value in beacon
DTIM
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
Beacon
Beacon
Beacon
Beacon
DTIM
Beacon
CFP
CP
Beacon
Beacon
Beacon
CFP
DTIM
7
10.1.1 CFP Structure and Timing
 CFP / CP Alternation
In the case of a busy medium due to DCF traffic, the beacon is
delayed for the time required to complete the current DCF
frame exchange.
A CFP is foreshortened by the amount of the delay.
CFP Repetition Interval
CFP Repetition Interval
Nominal beacon transmission time
Delay due to a busy medium
DTIM
2014/6/10
CFP
CP
DTIM
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
Beacon
Beacon
Beacon
Beacon
Beacon
CFP
Beacon
Foreshortened
CP
Beacon
Beacon
CFP
Beacon
Busy Medium
DTIM
8
10.1.2 PCF Access Procedure
 Control of the Medium
The PC gains control for the entire CFP by waiting for a PIFS
(PCF IFS) time, which is shorter than DIFS.
All STAs in the BSS (other than the PC) set their NAVs to the
CFPMaxDuration value at the start time of each CFP.
 This prevents most contention by preventing non-polled transmissions.
CFP Repetition Interval
PIFS = SIFS + Slot Time
PIFS
AP/PC
Beacon
DIFS = PIFS + Slot Time
CFP
frame
SIFS < PIFS < DIFS
・・・
CP
CF-End
・・・
SIFS
DIFS
PIFS
SIFS
Other STAs
set
reset
NAV
CFPMaxDuration
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
9
10.1.3 PCF Transfer Procedure
 Frame Types Available during the CFP
AP/PC
AP/PC
STA
Frame Types between the PC and the CF-Pollable STAs
Data + CF-Poll
Data + CF-ACK + CF-Poll
SIFS
Data + CF-ACK
SIFS
Data *
SIFS
ACK Control *
CF_Poll (no data)
CF-ACK + CF_Poll (no data)
Data *
Management *
Data + CF-ACK
CF-ACK (no data)
Data (Frame body is Null) *
Frame Types between the PC and the Non-CF-Pollable STAs
Data *
2014/6/10
Management *
SIFS
ACK Control *
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
* : Also used in CP
10
10.1.3 PCF Transfer Procedure
 PCF Frame Transfer
Dx : Frames sent from the PC to the STA x.
Ux : Frames sent from the STA x to the PC.
CFP
SIFS
Beacon
AP/PC
PIFS
U1
+ ACK
D1
+ Poll
STA 1 (CF-Pollable)
SIFS
STA 2 (CF-Pollable)
STA 3 (CF-Pollable)
U1
+ ACK
SIFS
PIFS
SIFS
D3
+ Poll
D2 + ACK
+ Poll
D1
+ Poll
SIFS
CP
SIFS
CF-End
ACK
SIFS
No response to CF-Poll
D2 + ACK
+ Poll
U2
+ ACK
SIFS
D3
+ Poll
set
ACK
SIFS
reset
NAV
CFPMaxDuration
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
11
10.1.3 PCF Transfer Procedure
 No Response within the SIFS Period
Unacknowledged Frame from the PC to the STA
CASE 1
SIFS
PIFS
D1
+ Poll
AP/PC
CASE 2
D2
+ Poll
ACK
U1
+ ACK
STA 1 (CF-Pollable)
Retry
D2
+ Poll
STA 2 (CF-Pollable)
D1
+ Poll
SIFS
D1
+ Poll
STA 1 (CF-Pollable)
CFP
SIFS
U1D1
+ Poll
+ ACK
Retry
D2
STA 2
(non-CF-Pollable)
D2
D2
ACK
If the STA is CF-Pollable, the PC may
retry the unacknowledged frame the next
time the STA is polled.
2014/6/10
D1
+ Poll
AP/PC
D1
+ Poll
PIFS
PIFS
D1
+ Poll
SIFS
D1
+ Poll
SIFS
CFP
D2
ACK
SIFS Retry
Regardless of CF-Pollability of the STA, the
PC may retransmit the unacknowledged frame
after a PIFS time.
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
12
10.1.4 PCF Limitations
 Problems with a PCF
Non-CF-Pollable STAs cannot transmit any Data frames
during the CFP.
The PCF operates without the CSMA/CA contention window
randomization and backoff of the DCF,
There is a risk of repeated collisions if multiple, overlapping,
point-coordinated BSSs are operating on the same PHY
channel.
AP 2/PC
AP 1/PC
In general,
the PCF is not implemented.
2014/6/10
Risk of Collisions
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
13
Contents
 10 Advanced Channel Access Techniques
10.1 PCF
 10.1.1
 10.1.2
 10.1.3
 10.1.4
CFP Structure and Timing
PCF Access Procedure
PCF Transfer Procedure
PCF Limitations
10.2 HCCA
 10.2.1
 10.2.2
 10.2.3
 10.2.4
 10.2.5
 10.2.6
2014/6/10
Traffic Streams
Controlled Access Phases
Polled TXOP
TXOP requests
Use of RTS / CTS
HCCA Limitations
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
14
10.2 HCCA
 Extension of PCF
in comparison with PCF…
Polling more frequently, introduce TXOP for more efficiency
 Polling in both CP and CFP
STA is more frequently polled than PCF
Less delay time than PCF
 HC(resides in AP) grants TXOP to STA
Efficiency Improved
TXOP depends on QoS Requirements from STA
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
15
10.2.1 Traffic Streams
 HCCA is supported parameterized QoS
1. STA registers QoS requirements for traffic stream(TS)
written in TSPEC elements:
e.g. Max Service Interval : maximum time between polls
2. HC establishes polling schedules
 Purpose
to reserve HC resources
to modify HC’s scheduling behavior
STA
2014/6/10
QoS requirements
for TS
Establish
Polling Schedules
HC(AP)
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
16
10.2.1 Traffic Streams
 TS : set of MSDUs travelling one direction
downlink : Max 8 TSs, uplink : Max 8 TSs
Identified by direction, TSID,
station address
Identified by direction, TSID
MSDUs
Uplink TS
HC(AP)
MSDUs
Downlink TS
STA1
2014/6/10
STA2
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
17
10.2.1 Traffic Streams
 TCLAS element
parameters that identify kind of packets belonging to TS
(e.g. packet which belongs to UDP Port xx)
HC assigns MSDUs to appropriate TS based on TCLAS
 Traffic admitted in the context of the TSPEC…
sent using EDCA, HCCA, hybrid(HEMM)
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
18
10.2.1.1 TS Setup and maintenance
 From TS Setup to TS deletion
 1. ADDTS Request
STA transmit QoS Requirement for TS to HC
 2. ADDTS Response
HC transmit whether the requirement is admitted or not
 3. Data transfer
 4. DELTS
if TS is not needed
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
19
10.2.1.1 TS Setup and maintenance
STA
ADDTS Request(include TSPEC, TCLAS(optional))
HC
ADDTS Response
Success, Invalid Parameters,
Rejected, or Rejected with suggest changes
If not success, renegotiate with AP
*TSPEC Parameters can be renegotiated
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
20
10.2.1.2 Data Transfer
 MSDUs are classified above MAC, and transmitted
 STA : transmit when polled
Poll based on HC’s schedule
STA
2014/6/10
Transmit when polled
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
AP
21
10.2.1.3 TS Deletion
 by DELTS frame(STA or HC transmit)
if correctly received, TS will be deleted
 decided by HC or STA
no longer required, no activity, etc
DELTS(transmitted by STA or HC)
STA
TS Deletion
HC
 also deleted on when disassociation or reassociation
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
22
10.2.2 Controlled access phases
 CAP
duration HC control medium
response frames follow HC initiated transmissions
 HC gains CAP by PIFS
After PIFS, HC polls to STA
PIFS
Polling
HC
STA
Data Frame
DIFS
CFP or CP
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
23
10.2.3 Polled TXOP
CFPoll
Ack
Ack
HC
SIFS
SIFS
Frame
STA
2014/6/10
Frame
TXOP written in TXOPLimit
(QoS Control Field : in CF-Poll)
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
24
10.2.4 TXOP requests
 STA : make request more TXOP
TXOP Request / Queue Size(both in QoS Data Frame)
Set either (not both)
 TXOP Request
additional TXOP duration request
 Queue Size
frames wait for transmit
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
25
10.2.5 Use of RTS/CTS
10.2.6 HCCA limitations
 Stations may send RTS
for more robust
 HCCA has not been widely implemented
EDCA is remarkably effective(Fixed data rate)
Little demand for HCCA
Collision to Neighbor QoS BSS
CSMA/CA is effective
2014/6/10
Copyrights 2014 Ochi - Kurosaki Laboratory . All Rights Reserved.
26
Today’s Homework
 Overlapping point-coordinated BSSs are operating on the
same PHY channel.
Two PCs send their Beacon frames after the carrier-sense for a
PIFS period so that they can start CFPs, but the timing is quite
the same by chance.
The PCs don’t work properly.
Please try to address this problem ! How do you solve it ?
Attention : This HW is outside the scope of the 802.11 standard.
AP 1/PC
AP 2/PC
Beacon
AP 1/PC
PIFS
AP 2/PC
STA 1 STA 2
Beacon
PIFS Collision
STA 1
(Both of the STAs are CF-Pollable)
2014/6/10
Busy
Medium
Collision
STA 2
27
appendix
10.1.3 PCF Transfer Procedure
 PCF Frame Transfer
Dx : Frames sent from the PC to the STA x.
Ux : Frames sent from the STA x to the PC.
CFP
SIFS
AP/PC
Beacon
PIFS
Other STAs
CP
SIFS
PIFS
D2 + ACK
+ Poll
D1
+ Poll
SIFS
set
SIFS
SIFS
D3
+ Poll
U1
+ ACK
CF-End
ACK
No response to CF-Poll
reset
NAV
CFPMaxDuration
No Response within the SIFS Period
 A CF-Pollable STA does not respond to a CF-Poll.
 A non-CF-Pollable STA does not return the ACK frame.
The PC resumes control and transmits its next frame after a PIFS period.
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
28
appendix
10.1.3 PCF Transfer Procedure
 No Response within the SIFS Period
Unacknowledged Frame from the STA to the PC
Successful Case
SIFS
Retransmission Case
SIFS ACK to U1
D1
+ Poll
PIFS
D2 + ACK
+ Poll
D1
+ Poll
U1
+ ACK
AP/PC
D1
+ Poll
AP/PC
U1
+ ACK
CFP
Retry (STA)
D1
+ Poll
U1
+ ACK
ACK
D1
+ Poll
U1
+ ACK
STA 1 (CF-Pollable)
U1
+ ACK
Retry (AP)
D2
+ Poll
D2 + ACK
+ Poll
STA 2 (CF-Pollable)
D2
+ Poll
SIFS
D1
+ Poll
STA 1 (CF-Pollable)
No ACK to U1
ACK
STA 2 (CF-Pollable)
SIFS
CFP
The STA does not retransmit the unacknowledged frame unless it is polled again by the PC.
or
The STA decides to retransmit the unacknowledged frame during the CP.
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
29
appendix
10.1.3 PCF Transfer Procedure
 Various Cases
The STA has no frame to send when polled by the PC
The STA responds to CF-Poll by using a Null frame.
Data frame whose frame body is null
 The null response is required to permit a “no-traffic” situation to be
distinguished from a collision.
Termination of the PCF before CFPMaxDuration
 The end of the PCF is signaled by transmission of a Control frame of
subtype which includes CF-End by the PC.
CF-End + CF-ACK
CF-End
 A STA that receives either of these frames resets its NAV.
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
30
appendix
10.1.3 PCF Transfer Procedure
 CFPMaxDuration Limit
The Minimum Value for CFPMaxDuration
(2×MaxMPDUTime) +
(the time required to send the initial Beacon frame) +
(the time required to send CF-End frame of the CFP)
The Maximum Value for CFPMaxDuration
(BeaconPeriod×DTIMPeriod×CFPRate) -
[MaxMPDUTime+(2×aSIFSTime)+(2×aSlotTime)+
(8×ACKSize)]
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
31
appendix
10.1.3 PCF Transfer Procedure
 Polling List Processing
The PC sends a CF-Poll to at least one STA during each CFP
when there are entries in the polling list.
The PC issues polls to a subset of the STAs on the polling list
in order by ascending AID value.
Example : Polling List
AID Fixed Field
Order
AID
1
AID 3
2
AID 4
3
AID 6
4
AID 9
Association ID
2
octets
Range : 1 to 2007
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
32
appendix
10.1.3 PCF Transfer Procedure
 Polling List Update Procedure
A STA indicates its CF-Pollability in the Association Request
and Reassociation Request frames.
Capability Information Fixed Field
B0
B1
B2
B3
B4
ESS
IBSS
CF
Pollable
CF-Poll
Pequest
Privacy
B5
B15
Reserved
2
octets
CFPollable
CF-Poll
Request
0
0
STA is not CF-Pollable.
0
1
STA is CF-Pollable, not requesting to be placed on the CF-Polling list
1
0
STA is CF-Pollable, requesting to be placed on the CF-Polling list.
1
1
STA is CF-Pollable, requesting never to be polled.
2014/6/10
Meaning
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
33
appendix
10.1.3 PCF Transfer Procedure
 Polling List Update Procedure
APs set the CF-Pollable and CF-Poll Request subfields in the
following management frames :
 Beacon
 Probe Response
 Association Response
 Reassociation Response
CFPollable
CF-Poll
Request
0
0
No point coordinator at AP.
0
1
Point coordinator at AP for delivery only (no polling).
1
0
STA is CF-Pollable, requesting to be placed on the CF-Polling list.
1
1
Point coordinator at AP for delivery and polling.
2014/6/10
Meaning
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
34
appendix
TS Negotiation Frame Format
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
35
appendix
TSPEC Elements
2014/6/10
Copyrights 2014 Ochi and Kurosaki Laboratory. All Rights Reserved.
36
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