IEEE 802.11 Wireless LANs IEEE 802.11 Wireless LANs Sunghyun Choi Ph D Associate Professor Sunghyun Choi, Ph.D., Associate Professor Multimedia & Wireless Networking Lab. (MWNL) School of Electrical Engineering School of Electrical Engineering Seoul National University Email: [email protected]http://www.mwnl.snu.ac.kr
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School of Electrical EngineeringSchool of Electrical EngineeringSeoul National UniversityEmail: [email protected]
http://www.mwnl.snu.ac.kr
“Byeong Gi Lee and Sunghyun Choi, Broadband Wireless Access & Local Networks: Mobile WiMAX“Byeong Gi Lee and Sunghyun Choi, Broadband Wireless Access & Local Networks: Mobile WiMAXWireless Access & Local Networks: Mobile WiMAX and WiFi, Artech House, Norwood, USA, May 2008”Wireless Access & Local Networks: Mobile WiMAX and WiFi, Artech House, Norwood, USA, May 2008”
Talk OutlineTalk Outline
Introduction to IEEE 802.11 WLAN
Evolution of IEEE 802 11 WLAN Evolution of IEEE 802.11 WLAN
Baseline MAC of IEEE 802.11
IEEE 802.11e for QoS
IEEE 802.11n for high throughput
Conclusion
3
WLAN vs. Other SolutionsWLAN vs. Other Solutions
WLANV hi l
Mobility WAN
tdoo
r
Walk
Vehicle
808
Out
Fixed
aUMTSWideband Cellular
8
Wired LAN02.11a/g
802.11b
door Walk
Cellular
802.11ng
Mbps (Tx Rate)1 10 1000 1
Ind Fixed/
Desktop Bluetooth
Mbps (Tx Rate)1 10 1000.1
4
IEEE 802.11 Standard Overview IEEE 802.11 Standard Overview
Layers 1 and 2 One MAC and multiple PHYs
MAC
multiple PHYs
A li ti7
Layer
2.4 GHz 2.4 GHz
MAC
DS IRFH
Application
TCP4
7
.11a OFDM5 GHz
DS IRFH1 & 2 Mbps1 & 2 Mbps
IP3
.11a OFDM
.11b CCK
6~54 MbpsTook Off
Available since 2002 LLC
2802.2 5.5 & 11 Mbps
.11g OFDMT k ffMAC
PHY1802.11
6~54 Mbps Took off
O2.4 & 5 GHz
PHY1
5
.11n OFDM6.5~600 Mbps
Taking off
802.11 Standards802.11 Standards
6
Task Groups Finalized in 2008Task Groups Finalized in 2008
11k for Radio Resource11k for Radio Resource Measurement (RRM) enhancements provide mechanisms to higher layers for provide mechanisms to higher layers for
radio and network measurements.
11 f F t R i d f t BSS 11r for Fast Roaming and fast BSS transition Fast Roaming With QoS and security in mindQ y E.g.) fast handoff for VoIP hansets
7
Task Groups Finalized in 2008Task Groups Finalized in 2008
11y for 3650-3700 MHz Operation11y for 3650 3700 MHz Operation in the USA Support operation in licensed bands Support operation in licensed bands Cognitive radio functions (spectrum
sharing incumbent detection) forsharing, incumbent detection) for co-existence enhancements in non-exclusively licensed bandsnon exclusively licensed bands
8
Task Groups Finalized in 2009Task Groups Finalized in 2009
11n for Higher Throughput11n for Higher Throughput Provide much higher throughputs Maximum throughput of at least 100 Mb/s as Maximum throughput of at least 100 Mb/s, as
measured at MAC SAP Modifications to both PHY and MAC
11w for Protected Management FramesFrames Provide Advanced Security mechanisms
f d h Protect management frames to reduce the susceptibility of systems to attack
9
On-going StandardizationOn-going Standardization
802.11p / TGpWireless Access for the Vehicle Environment
Prioritization of Management Frames 802.11ae / TGae
10
Wireless LAN in the TV White Space 802.11af / TGaf
11p for Vehicular Environments11p for Vehicular Environments
Wi l f hi l Wireless access for vehicular environments (WAVE)( ) Inter-car and car-to-road communications Extension of 11a for 5.9 GHz ITS bandExtension of 11a for 5.9 GHz ITS band 5.850-5.925GHz Dedicated Short-Range
Communication (DSRC) bandCommunication (DSRC) band Over line-of-sight distances within 1 km
11s for ESS Mesh Networks 11s for ESS Mesh Networks
M h ki (M l i h i l ) Mesh networking (Multi-hop wireless) Define an 802.11 mesh using the MAC/PHY
layers Support auto-configuring paths between APs
lf fi i lti h t l iover self-configuring multi-hop topologies Layer-2 mesh path selection and forwarding
(routing at the link layer)(routing at the link layer) Advantageous properties of mesh networks
Robustness range extension and density Robustness, range extension and density Potential challenges such as power consumption and
security
13
Multi-Hop WirelessMulti-Hop Wireless
14
11ac & 11ad for VHT11ac & 11ad for VHT
V Hi h Th h Very High Throughput Wireless LAN Gigabit MAC and PHY
specifications Enable a maximum BSS throughput of at g p
least 1 Gbps, at MAC SAP The discussion of 802.11 VHT is divided
into two directions 11ac (Freq. < 6 GHz) 11ad (Freq. ≈ 60 GHz)
15
802.11ac (1)802.11ac (1)
E h f V Hi h Enhancements for Very High Throughput for operation in bands g p pbelow 6 GHz Below 6 GHz carrier frequency operationBelow 6 GHz carrier frequency operation
coexistence with legacy IEEE802.11a/n devices in the 5 GHz unlicensed band
16
802.11ac (2)802.11ac (2)
Ch l b di 80/100 MH Channel bonding – 80/100 MHz Advanced codingAdvanced coding FEC/ LDPC Network coding Network coding Interference cancellation coding
Advanced parallel communications Multi-user MIMO Cooperative wireless networking Single-hop relaySingle hop relay
17
802.11ad (1)802.11ad (1)
E h f V Hi h Enhancements for Very High Throughput in the 60 GHz Band g p(57 – 66GHz) Fast session transfer between 60 GHz andFast session transfer between 60 GHz and
2.4/5 GHz bands Maintain the 802 11 user experienceMaintain the 802.11 user experience Address coexistence with other systems in
the band (e g high-speed WPAN systemsthe band (e.g., high speed WPAN systems such as IEEE 802.15.3c, ECMA 387)
18
802.11ad (2)802.11ad (2)
U d l Usage model Desktop storage and display Video streaming High speed cable replacement (HDMI,High speed cable replacement (HDMI,
monitor) Wireless LAN and BackhaulWireless LAN and Backhaul To differentiate from 802.15.3c, VHT is
focusing its purpose on the core of 802 11focusing its purpose on the core of 802.11 which is data networking.
19
Other Task Groups (1)Other Task Groups (1)
11u for Wireless Interworking with11u for Wireless Interworking with External Network Interworking with 3G cellular Interworking with 3G cellular
11z for Extensions to Direct Link Setup(DLS) Does not require non-DLS capable access q p
point upgrades Supports power save modepp p Continues to allow operation of DLS in the
presence of existing DLS capable access p g ppoints
20
Other Task Groups (2)Other Task Groups (2)
11v for Network ManagementM t f AP STA t i Management of non-AP STAs to improve the overall performance.
11aa for Robust AV Transport Enhancing .11e for AV streamingg g
11ae for Prioritization of Management FramesManagement Frames Priority for latency and transmission
reliabilityreliability
11af for channel access and coexistence in the TV White Space
21
Baseline ProtocolBaseline Protocol
802.11 Reference Model802.11 Reference Model
23
Baseline Protocol Part I - PHYs
Baseline Protocol Part I - PHYsPart I - PHYsPart I - PHYs
Various PHYs of IEEE 802.11Various PHYs of IEEE 802.11
Wireless terminals Basic Service Area (BSA) Coverage area of one accessSTA1
PortalBSS1
AccessPoint
Coverage area of one access point (AP)
Basic Service Set (BSS) group of stations controlled by
1
Distribution System
AccessPoint
group of stations controlled by the same AP
Distribution System (DS)ESS Point
BSS2
Fixed infrastructure used to connect several BSSs to create an Extended Service Set (EES)
l
802.11 LAN
Portal bridge to other (wired) networks Every tx is via APSTA2 STA3 y
30
802.11 – Ad Hoc mode802.11 – Ad Hoc mode
Terminals communicate in a peer to peer basis802.11 LAN in a peer-to-peer basis
Independent BSS (IBSS) A STA can be a routerSTA1 A STA can be a router
to connect to the wireline network
BSS1
1STA3
STA2
BSS2
STA5
802.11 LANSTA4
5
31
Important ConceptsImportant Concepts Rate Sets BSS Basic Rate Set – shall by supported by allBSS Basic Rate Set shall by supported by all
stations Operational Rate Set – can be used by stations
b { } d { } l E.g., in 11b, {1,2} and {1,2,5.5,11}, respectively Control (ACK, RTS, CTS) and broadcast/multicast
frames (e.g., beacon) shall be transmitted withframes (e.g., beacon) shall be transmitted with one of the rates in BSS Basic Rate Set
Unicast vs. Broadcast In case of infrastructure BSS, uplink transmission
is always unicastS i S t ID (SSID) Service Set ID (SSID) A character set identifying each ESS Conveyed within beacon frames Conveyed within beacon frames Often called ESSID
32
Two Coordination FunctionsTwo Coordination Functions
Mandatory Distributed Coordination yFunction (DCF) For distributed contention-based channelFor distributed contention based channel
access
Optional Point Coordination Function Optional Point Coordination Function (PCF) For centralized contention-free channel
access
DCF only for most commercial 802.11 devices802.11 devices
33
802.11 MAC Architecture802.11 MAC Architecture
PCF sits on top of DCF PCF operation relies on DCF
Time-division-based packet-by-p ypacket transmission No transmission slots no control channels No transmission slots, no control channels,
no separate pilot channels, …
34
Distributed Coordination Function (DCF)Distributed Coordination Function (DCF)
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) similar to IEEE 802 3 Ethernet CSMA/CD similar to IEEE 802.3 Ethernet CSMA/CD
DIFSI di t h
Busy SIFSPIFS
DIFS
B k ffDIFS
Contention WindowImmediate access whenmedium is idle >= DIFS
BusyMedium
SIFS BackoffWindow
Slot Time
Next Frame
Defer Access Select Slot and decrement backoffas long as medium stays idle
Physical carrier-sense Physical carrier-sense Provided by PHY, and depends on PHY Clear Channel Assessment (CCA) by PHY Clear Channel Assessment (CCA) by PHY
Virtual carrier-senseP id d b MAC i N t k All ti V t Provided by MAC via Network Allocation Vector (NAV) counter
Each frame carries Duration value in the header Each frame carries Duration value in the header Any correctly received frame updates NAV if the
new NAV is largernew NAV is larger Assumes busy channel if non-zero NAV
irrespective of CCA!p
39
Stop-and-Wait ARQStop-and-Wait ARQ
Receiver of a directed frame returns an ACK
If ACK not received, sender retransmits ,after another backoff
For each unsuccessful frame transmission, CW doubles (from
CWmax=255 255
250
300
Example
CW doubles (from CWmin to CWmax)
127150
200
250
CW 2 (CW+1)-1
Reduces the collision CWmin=15
3163
0
50
100
probability0
1 2 3 4 5 6
41
Hidden TerminalHidden Terminal
STA 1 and STA 2 can see STA 3 but STA 1 and STA 2 can see STA 3, but they do not see each other May result in more collisions due to
the failure of carrier-sensing!the failure of carrier sensing!
42
RTS/CTS ExchangeRTS/CTS Exchange
A way to handle hidden terminals! Request-To-Send / Clear-To-SendRequest To Send / Clear To Send
(RTS/CTS) to reserve medium Works with virtual carrier sense Works with virtual carrier-sense
43
FragmentationFragmentation
One MSDU can be fragmented into multiple MPDUsmultiple MPDUs All the fragments have virtually the same
MAC header (except for the fragmentMAC header (except for the fragment number) Theoretically up to 11 fragments from oneTheoretically up to 11 fragments from one
MSDU since Max MSDU size = 2304 octets Min Fragment Threshold = 256 octets
44
Fragmentation Burst Fragmentation Burst
Fragments are transmitted with SIFS gintervals Backoff if a fragment transmission Backoff if a fragment transmission
fails
45
RTS & Fragment ThresholdsRTS & Fragment Thresholds
RTS Threshold Use RTS/CTS if MPDU_size > threshold Depending on the size of MPDU relative to RTS
threshold, the max retransmission limit is determined differently!
L R t Li it ( h t) 4 (7) b d f lt LongRetryLimit (short) = 4 (7) by default
Fragment Threshold Use fragmentation if MPDU_size > threshold
Default values of both are large enough g gsuch that none of them is used! Max MSDU size = 2304 bytes in 802.11Max MSDU size 2304 bytes in 802.11
46
Power Management (1)Power Management (1)
Without power management, a STA always senses medium Lots of power consumption for channel p p
sensing/receiving Power management allows STAs to g
go to doze state as much as possible without losing incoming p g gdata Active mode (AM) – always awake stateActive mode (AM) always awake state Power Save (PS) mode – switch between
awake and doze states
47
Power Management (2)Power Management (2)
Switch between AM and PS mode is i f d i f l finformed via a successful frame transmission with Power Mgmt bit (re)set
In BSS, AP buffers downlink frames, and announce it via beacon frames (in TIM field)
In IBSS, each STA buffers frames, and , ,announce it via ATIM frames
Power-saving STAs wake up periodically!Power saving STAs wake up periodically!
48
TIM & Dedicated TIM (DTIM) BeaconsTIM & Dedicated TIM (DTIM) Beacons
Time-axis
Beacon Interval DTIM interval
TIM (in Beacon) TIM DTIM TIM TIM DTIM
AP activity
Downlinkbuffered frame
Buffered framefor other station
Downlinkbuffered frame Broadcast
PS StationPS-Poll
PS Station(extreme low power) PS-Poll( p )
Beacon Transmissions Busy MediumIn Active State
49
IEEE 802.11e for QoS Provisioning
IEEE 802.11e for QoS Provisioningfor QoS Provisioningfor QoS Provisioning
Backward Compatible with Legacy MAC(Based on IEEE 802.11e-2005)
Limitations of Baseline MAC Limitations of Baseline MAC No notion of QoS and related signaling
R t i t d lli h d li Restricted polling scheduling PCF mandates round-robin scheduling
Superframe with alternating CFP and CP need to be short for short delay bound
AP assuming the full control over the medium during CFPg overlapping WLANs?
Uncontrollable/unpredictable frame Uncontrollable/unpredictable frame transmission times Just one frame per being polled
51
Just one frame per being polled
Prioritized vs. Parameterized QoSPrioritized vs. Parameterized QoS
Prioritized QoS (like DiffServ)Q ( ) Differentiated channel access for frames with
different user prioritiesdifferent user priorities 8 different user priorities (UPs)
802 1d b id t i il t 802.1d bridge supports similar concept
Parameterized QoS (like IntServ)Q ( ) QoS is characterized by a set of parameters
A traffic stream (TS) is set up between A traffic stream (TS) is set up between transmitter and receiver (and QoS AP or QAP)
52
Hybrid Coordination Function (HCF)Hybrid Coordination Function (HCF) Two access mechanisms Contention-based channel access Contention-based channel access Enhanced Distributed Channel Access (EDCA) Variation of legacy DCF Variation of legacy DCF
Traffic ID & Traffic StreamTraffic ID & Traffic Stream
Each MSDU from LLC carries one of 16Each MSDU from LLC carries one of 16 Traffic ID (TID) values 0~7 identify user priorities (UPs) 0~7 identify user priorities (UPs) 8~15 identify (parameterized) traffic streams IDs
(TSIDs)(TSIDs)
Traffic Stream (TS) is set up after admission control by QAPadmission control by QAP Up to 8 TSs per STA per direction TS can be set up for prioritized QoS if QAP
mandates admission control for specific priority traffictraffic
Begins when QAP receives a “trigger” g Q ggframe Trigger frame = uplink QoS data or NullTrigger frame = uplink QoS data or Null
frame associated with an admitted uplink or bidirectional TSPEC (with APSD=1 &or bidirectional TSPEC (with APSD 1 & Schedule=0)
Ends when QAP has attempted to Ends when QAP has attempted to transmit at least one buffered MPDU t AP QSTAto non-AP QSTA
70
Scheduled SPScheduled SP Non-AP QSTA wakes up to receive
frames during scheduled SPsframes during scheduled SPs Schedule element in ADDTS responseFi t h d l d SP t t First scheduled SP starts Lower 4 bytes of TSF timer = Service Start
TimeTime Subsequent scheduled SP starts Every Service Interval
71
IEEE 802.11e Part IV: Block ACK
IEEE 802.11e Part IV: Block ACKPart IV: Block ACK Part IV: Block ACK
Other Features of 802.11eOther Features of 802.11e
Block ACK Group of frames are ACKed with a single BlockACK frame A key mechanism to improve MAC efficiency (further
evolution expected as part of 11n) No ACK policy is also supported
Direct Link Protocol (DLP) Direct communication between non-AP STAs in
infrastructure mode
Both will enhance the efficiency of 802.11 Both requires a priori agreement between
communicating partiescommunicating parties
73
Two Types of Block AckTwo Types of Block Ack
Immediate Block Ack Suitable for high-bandwidth, low latency traffic
Delayed Block Ack Suitable for applications tolerant of moderate
latencyWith i i l HW h With minimal HW changes
Depends on whether BlockAck is t itt d i di t l fttransmitted immediately after BlockAckRequest frame Both of them are optional in 11e Usage of one type is agreed between
Aggregate MSDU (A-MSDU)Aggregate MSDU (A-MSDU) A-MSDU operation Mechanism to provide enhanced efficiency atMechanism to provide enhanced efficiency at
the top of the MAC layer Support for A-MSDU is mandatory at the
receiver where the A MSDU is carried in areceiver, where the A-MSDU is carried in a single (i.e., non A-MPDU) QoS Data MPDU under Normal Ack policy
86
A-MSDUA-MSDU
An A-MSDU is composed of MSDUs with the same TID valueTID value.
All the MSDUs are intended to be received by a single receiver, and necessarily they are allsingle receiver, and necessarily they are all transmitted by the same transmitter.
Maximum A-MSDU length Indicates maximum A-MSDU length.
Set to 0 for 3839 octets Set to 1 for 7935 octets
87
Aggregate MPDU (A-MPDU)Aggregate MPDU (A-MPDU) Robust Structure A-MPDU Aggregation is a purely-MAC function gg g p y
Architecturally at the “Bottom of MAC” PHY has no knowledge of MPDU boundaries
Control and data MPDUs can be aggregated The A-MPDU maximum length is 65535 octets
All h MPDU i hi A MPDU dd d h All the MPDUs within an A-MPDU are addressed to the same receiver address.
88
Basic A-MPDU ExchangeBasic A-MPDU Exchange
U UU U UUU UU U UUU U
Dat
a M
PDU
eD
ata
MPD
U
Dat
a M
PDU
r Tx
Act
ivity
xM
AC
Tx
RTS
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
Dat
a M
PDU
e
Agg
rega
teH
T P
PDU
Initi
ator
PHY
Tx
Lega
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PP
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Agg
rega
teH
T P
PDU
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Implicit Block Ack Protocol
RTS/CTSProtocol
nder
Tx
Act
ivi
TxM
AC
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Blo
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c
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TS
Blo
ck A
cy U y U
Res
pon
PH
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Lega
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Lega
cP
PD
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PD
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89
Value of A-MPDUValue of A-MPDU
RTS/CTS/A MPDU/BA sequence is typically RTS/CTS/A-MPDU/BA sequence is typically 2.5x more efficient than Data/Ack
RTS/CTS/A-MPDU/BA is 30% more efficient RTS/CTS/A MPDU/BA is 30% more efficient than A-MSDU/Ack
Enables BA and Data to be aggregatedEnables BA and Data to be aggregated (few % can be gained)
A-MPDU Support selective retransmission using MPDU delimiter
Higher overhead
Good for high error environment
91
Enhanced BA MechanismEnhanced BA Mechanism Implicit BAR: The originator may omit the inclusion of a BAR fr
ame in an aggregated frame and set QoS ack policy to “Normal Ack”.
Compressed BA: Defines a compressed variant of the 802.11e BA MPDUBA MPDU.
Partial State for Immediate BA reduces complexity of recipient
Aggregation frame
D1 D2 D3 D4InitiatorSIFS
Initiator
ResponderCompressed
BA
8 octetsFrame Control
Duration/ID RA TA BA Con
trolBA Starting Seq. Control BlockAckBitmap FCS
92
PSMP/MTBAPSMP/MTBA
Power-save Multi-Poll (PSMP)Power save Multi Poll (PSMP) PSMP sequence allows the AP to create effective service
periods Benefits from statistical multiplexing of retries, activity
cycles and rate variations In the VoIP application, benefit is up to 2x resulting from
sharing an allocation for retries within the current aggregate SP
Multi-TID Block Ack (MTBA)Multi TID Block Ack (MTBA) Allows for single frame to respond to (implicit) BAR for
multiple TID Shall be used within PSMP sequences instead of BA
93
PSMP Frame FormatPSMP Frame Format
A PSMP sequence with a duration of up to 8.184 ms
94
MTBA Frame FormatMTBA Frame Format
95
PSMP with MTBAPSMP with MTBA
Frames of different TID may be transmitted within a PSMP Frames of different TID may be transmitted within a PSMP-DTT or PSMP-UTT allocation of a (Scheduled or Unscheduled) PSMP sequence without regard to Access Category. MTBA efficiently carries BA for multiple TIDse c e t y ca es o u t p e s
PSMP schedules when a STA receives and when it may transmit.
DL Acknowledgement scheduled in the uplink & vice versag p UL data acknowledged by following PSMP sequence
96
PSMP BurstPSMP Burst Retransmission and resource allocation
97
Reverse Direction (RD)Reverse Direction (RD)
Allows a STA to share its TXOP with another STAanother STA The most significant benefit is obtained if this
reduces the number of channel access attempts
Some benefit from aggregating BA and Data t thtogether
Signalled by:D ti /ID fi ld hi h i th i i Duration/ID field, which carries the remaining duration of the TXOP
HT control Field which carries: RDG / More HT control Field, which carries: RDG / More PPDU, AC constraint
QoS Control field, which carries TID of traffic Q ,allowed in this RD