802 11 QoS Overview
802.11
Philippe Klein
IEEE Plenary Meeting – Nov 08 Dallas, TX
avb-phkl-802-11-qos-overview-0811-1
Disclaimer
This presentation is not a highly detailed technical
presentation but a crash course aiming to provide
- prior to the joint meeting with the 802.11 WG- a
basic understanding
g of the QoS mechanisms
currently defined in the IEEE 802.11-2007
specifications
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Abbreviations & Acronyms
y
ADDTS
AIFS
AIFSN
AP
APSD
BSS
CSMA/CA
CW
DELTS
DIFS
DLS
EDCA
EDCAF
EIFS
HCCA
HCF
MAC
MPDU
NAV
PIFS
PS
QoS
S-APSD
SIFS
SP
STA
TXOP
U-APSD
WMM
add traffic stream
arbitration interframe space
arbitration interframe space number
access point
automatic power-save delivery
basic service set
carrier sense multiple access with collision avoidance
contention window
delete traffic stream
distributed (coordination function) interframe space
direct-link setup
enhanced distributed channel access
enhanced distributed channel access function
extended interframe space
HCF controlled channel access
hybrid coordination function
medium access control
MAC protocol data unit
network allocation vector
point (coordination function) interframe space
power save (mode)
quality of service
scheduled automatic power-save delivery
short interframe space
service period
station
transmission opportunity
unscheduled automatic power-save delivery
WiFi Multimedia
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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802 11 MAC Legacy
802.11
L
Access
A
Protocol
P t
l
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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802.11 MAC Access Protocol
1. Distributed Coordinated Function (DCF)
Contention access
Based on Carrier Sense Media Access /Collision Avoidance
(PHY Carrier Sense & Network Allocation Vector)
2. Point Coordinated Function (PCF)
Contention Free access:
Access P
A
Point
i t (AP) coordinates
di t access ffor AP and
d STA
transmissions
Optional !!!
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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DCF Contention Mechanism
DIFS
Backoff Time
PIFS
Busy Medium
SIFS
Contention Window
Next Frame
Slot time
Deferred Access
Decrement Backoff as
long as medium is idle
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Interframe Spacing
p
g in 802.11
SIFS
Control Frame or next frame fragment may be sent here
Beacon or New PCF Frame may be sent here
PIFS
New DCF Frame may be sent here
DIFS
Frame Error Recovery done here
(allow other frame exchanges to
complete correctly before another
transmission is allowed)
EIFS
ACK
Slot time
Slot time
time
802 11b
802.11b
802 11
802.11a
802 11
802.11g
SIFS
10 µs
16 µs
10 µs
Slot time
20 µs
9 µs
9 µs
PIFS (= SIFS + 1 x Slot time)
30 µs
28 µs
19 µs
DIFS (= SIFS + 2 x Slot time)
50 µs
34 µs
28 µs
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Random Backoff Time
Backoff Time = Random() * SlotTime
Random() = Pseudo
Pseudo-random
random [0
[0,CW]
CW]
CWmin =< CW <= Cwmax
CW:
● Incremented by 2n-1 on each retransmission
● Reset after successful Txm
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Distributed Coordinated Function
DIFS
DIFS
DIFS
DIFS
FRAME
Station A
Defer
FRAME
Station B
Defer
FRAME
Station C
Defer
FRAME
Station D
Defer
FRAME
Station E
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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PCF: Point Coordinator Function
● Poll and response protocol to eliminate the
possibility of contention for the medium
medium.
● A Point Coordinator (PC) - always located in an
Access Point - controls the PCF
● To operate in PCF, stations register to the PC
● PC regularly
g
y polls
p
the registered
g
stations for traffic
while delivering traffic to theses stations.
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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PCF Operation
p
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Co-existence of PCF and DCF
CFP Repetition Interval
Contention Free
Period (CFP)
B
PCF
CFP Repetition Interval
Contention Period
(CP)
DCF
Contention Free
Period (CFP)
B
NAV
DCF
Time
Beacon
Beacon
NAV
PCF
Contention Period
(CP)
● CFP/C ratio is programmable in the AP
● Beacon marks the beginning of the Contention Free Period
(CFP)
● CF-End ends the CFP
● NAV prevent outstanding DCF traffic to interfere with CFP
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Qualityy of Service Limitations
Q
● Both PHY’s DCF and PCF methods do not
differentiate between traffic types or sources and
do not provide QoS
● PCF provides only iso-synchronous
iso synchronous services
● worse latency access only is deterministic
● access latencyy is function of the number of registered
g
stations, the CF traffic and the CFP/CP ratio
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Medium Access Control
Q lit off S
Quality
Service
i
Enhancements
(previously 802.11e amendment ,
now partt off 802.11-2007)
802 11 2007)
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Major
j Enhancements in 802.11e
● Basic Elements for QoS
Traffic Differentiation
Concept of Transmission Opportunity (TXOP)
● New Contention Channel Access
Enhanced Distributed Channel Access (EDCA)
● New Contention Free Channel Access
HCF Controlled Channel Access (HCCA)
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Major
j Enhancements in 802.11e
● Automatic Power-Save Delivery (APSD)
● New
N
mechanisms
h i
ffor higher
hi h th
throughput
h t
Block Acknowledgement (Block Ack)
Direct Link Setup (DLS)
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Enhanced Distributed Channel Access (EDCA)
(
)
● Same Contention Based that EDC but…
1 8 priority
1.
i i levels
l
l per station
i (id
(identical
i l to IEEE
802.1D priority tags) mapped to 4 access
categories (AC)
2. Different Arbitration InterFrame Space
(AIFS) per access categories
3. TxOP Burst Limits
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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EDCA Frame Priorities
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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EDCA Implementation
p
Model
802.1D tagged MSDU
Mapping to Access Category
Txm
queues
Resolve
internal
collision
Voice
Data
Video
Data
Best
Efforts
Background
AC VO
AC_VO
AC VI
AC_VI
AC BE
AC_BE
AC BK
AC_BK
AIFSN,
CWmin,
CWmax
AIFSN,
CWmin,
CWmax
AIFSN,
CWmin,
CWmax
AIFSN,
CWmin,
CWmax
Txm attempt
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Traffic Prioritization & Collision Resolution
● Depends
epe ds o
on 2 ttiming
g pa
parameters
a ete s tthat
at vary
a y for
o eac
each AC:
C
1.
2.
AIFSN (Arbitrary Inter-Frame Space Number)
CW (Contention Window)
(and TxOP Limit for AC_VI and AC_VO)
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Arbitration InterFrame Space
p
AIFS (AC i-1)
CW
AIFS (AC i)
DIFS
CW
PIFS
Busy Medium
SIFS
Contention Window
Next Frame
Deferred Access
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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EDCA + Admission Control
Admitted
flows
AP
Rejected
Video stream
AC2 (video)
AC3 (Voice)
AC2 (video)
AC2 (video)
AC1
STA1
STA3
STA2
●
Capability Negotiation of Admission Control per Access Category
(AC) occurs during association:
● STA sends an Add Traffic Stream (ADDTS) request with Txm
Specifications (TSPEC) prior to sending packet on a particular
AC.
● TSPEC can be for upstream, downstream, or bi
bi-directional
directional traffic.
● AP has proprietary algorithm to accept/deny ADDTS request.
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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802.11 TSPEC Element
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802.11 TSPEC Main Parameters
User Priority
0 (lowest) to 7 (highest)
Maximum MSDU size
maximum size of the packets (octets)
Maximum Burst Size
maximum size of the data burst that can be transmitted at the
peak data rate (octets)
Minimum PHY rate
physical bit rate assumed by the scheduler for transmit time
and admission control calculations (b/s)
Peak data Rate
maximum bit rate allowed for transfer of the packets
M
Mean
d
data
t rate
t
average bit rate
t for
f transfer
t
f off the
th packets
k t (b/
(b/s))
Delay bound
maximum delay allowed to transport a packet across the
wireless interface (including queuing delay, in ms)
Nominal MSDU size
nominal size of the packets (octets)
Maximum Service
Interval
interval required by TS in this TSPEC between the start of two
successive TXOPs
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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EDCA Limitations
● Still no guaranty of service provided but..
● Probabilistic
P b bili ti priority
i it mechanism
h i
only
l tto allocate
ll
t
bandwidth based on traffic categories
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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HCF Controlled Channel Access (HCCA)
(
)
● HCF = Hybrid Coordination Function
● Hybrid
H b id C
Coordinator
di t (HC) operates
t att Q
QoS-AP
S AP
● Controls the iteration of Contention Free Period
(CFP) and Contention Period (CP)
● Schedule Transmit Opportunities for contention free
accesses
IEEE 802.1 Plenary Meeting , Nov 08 Dallas
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HCF Controlled Channel Access (HCCA)
(
)
● Similar to PCF
● Same polling mechanism to assign Transmission
Opportunity (TXOP) to QoS-STA
● but
● Polling can be issued in both CFP and CP (in
Controlled Access Periods)
● HC Polling is scheduled according to Transmit
specifications (TSPECs)
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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PCF vs HCF Comparison
p
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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802.11e Media Access Summaryy
CAP
CFP
CF-Poll
CF
Poll
CF-End
Controlled Access Period
Contention Free Period
Contention Free – Poll
Contention Free – End
CP
EDCF
QAP
QSTA
Contention Period
Enhanced Distributed Coord. Function
QoS Access Point
Qos Station
TBTT
TXO
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
Target Beacon Transmission Time
Transmission Opportunity
29
802.11e
802
11e
Automatic Power-Save Delivery
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Automatic Power-Save Deliveryy ((APSD))
● Enhancing Power Saving mode in QoS Basic
Service Set
● QoS-AP automatically delivers downlink frames,
which belong to some specified Access Category
Category,
to Power-Saving stations
● Two types of delivery mechanism
● Unscheduled APSD (U-APSD)
● Scheduled APSD (S-APSD)
(S APSD)
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Unscheduled APSD (U-APSD)
(
)
1 P
1.
Power-saving
i Q
QoS-STA
S STA wakes
k up and
d send
d a “t
“trigger”data
i
”d t
frame belonging to “trigger-enabled” AC to QoS-AP
2. After receiving “trigger”
trigger frame, a Service Period (SP) is
started
3. QoS-AP send frames belonging to “delivery-enabled” AC
to QoS-STA
Q SS
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Scheduled APSD (S-APSD)
(
)
Voice
Packet
B
Voice
Packet
Ack
Ack
Voice
Packet
Ack
Voice
Packet
Poll
Ack
Poll
Q-STA
B
Q-AP
Voice Frame
Voice Frame
20 mS
Service
Start Time
Service
Start Time
1. QoS-STA negotiates a APSD Schedule with QoS-AP
2. QoS-AP start transmitting the frames of the specified
T ffi Stream
Traffic
St
att Service
S i St
Startt Ti
Time and
d the
th ffollowing
ll i
periods
3 QoS
3.
QoS-STA
STA must wake up at Service Start Time and the
following periods to receive frames
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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802.11e
802
11e
Throughput Enhancements
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Block ACK
● Improves channel efficiency by aggregating several
acknowledgments into one frame.
● 2 types of Block Ack mechanisms:
● immediate and delayed
Immediate ACK
Delayed ACK
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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Direct Link Protocol
● Ability to exchange data directly between two
stations in the network
network, without traversing the AP
AP.
QoS AP
QoS STA2
QoS STA1
5: DLS Data
IEEE 802.1 Plenary Meeting , Nov 08 Dallas TX
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WMM ((Wi-Fi Multimedia™))
● Certification program initiated by the Wi-Fi
Alliance (www.wi-fi.org)
(www wi fi org)
● Specification document:
Wi-Fi Alliance Technical Committee Quality of Service
(QoS) Task Group, WMM (including WMM™ Power
Save) Specification Version 1.1
● Subset of IEEE 802.11e : EDCA & U-APSD only
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Thank you
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