Wireless LAN IEEE 802.

11
Section 9

Wireless Communication Networks, N. Moayedian 53

IEEE 802.11 vs. Wi-Fi

• IEEE 802.11 is a standard

• Wi-Fi = “Wireless Fidelity” is a trademark
• Fidelity = Compatibility between wireless equipment from different
manufacturers
• Wi-Fi Alliance is a non-profit organization that does the compatibility
testing (WiFi.org)
• 802.11 has many options, and two pieces of equipment based on
802.11 can be incompatible.
• All equipment with the “Wi-Fi” logo has selected options such that
they will interoperate
Wireless Communication Networks, N. Moayedian 54
IEEE Standards Numbering System
• IEEE 802.* and IEEE 802.1* standards (e.g., IEEE 802.1Q-2011) apply
to all IEEE 802 technologies:
• IEEE 802.3 Ethernet
• IEEE 802.11 Wi-Fi
• IEEE 802.16 WiMAX

Wireless Communication Networks, N. Moayedian 55

IEEE Standards Numbering (Cont)
• IEEE 802.11* (e.g., 802.11i) standards apply to all Wi-Fi devices but
may not apply to ZigBee devices which are based on 802.15,
• Standards with all uppercase letters are base standards, e.g., IEEE
802.1AB-2009
• Standards with the lowercase are additions/extensions/revisions. It is
merged with the base standard in its next revision. e.g., IEEE 802.1w-
2001 was merged with IEEE 802.1D-2004
• Standards used to be numbered sequentially, e.g., IEEE 802.1a, …,
802.1z, 802.1aa, 802.1ab, …
• Recently, they started showing base standards in the additions, e.g.,
IEEE 802.1Qau-2010

Wireless Communication Networks, N. Moayedian 56

IEEE 802.11 Features
• Original IEEE 802.11-1997 was at 1 and 2 Mbps.
• Newer versions at 11 Mbps, 54 Mbps, 108 Mbps, 200 Mbps,…
• All versions use the “License-exempt” spectrum
• Need ways to share spectrum among multiple users and multiple LANs
Spread Spectrum (CDMA)
• Three Phys:
• Direct Sequence (DS) spread spectrum using ISM band
• Frequency Hopping (FH) spread spectrum using ISM band
• Diffused Infrared (850-900 nm) bands
• Supports multiple priorities
• Supports time-critical and data traffic
• Power management allows a node to doze off

Wireless Communication Networks, N. Moayedian 57

IEEE 802.11 Wireless LAN
IEEE 802.11 Year Max data rate Range Frequency
standard
802.11b 1999 11 Mbps 30 m 2.4 Ghz
802.11a 1999 5 Ghz
802.11g 2003 54 Mbps 30m 2.4 Ghz
802.11n (WiFi 4) 2009 600 70m 2.4, 5 Ghz
802.11ac (WiFi 5) 2013 3.47Gpbs 70m 5 Ghz
802.11ax (WiFi 6) 2021 14 Gbps 70m 2.4, 5 Ghz
802.11af 2014 35 – 560 Mbps 1 Km unused TV bands
(54-790 MHz)
802.11ah 2017 347Mbps 1 Km 900 Mhz
 all use CSMA/CA for multiple access, and have base-station and ad-hoc
network versions
Wireless Communication Networks, N. Moayedian
Wireless and Mobile Networks: 7- 58
802.11 LAN architecture

Internet
 wireless host communicates with
base station
• base station = access point (AP)
switch  Basic Service Set (BSS) (aka “cell”)
or router in infrastructure mode contains:
• wireless hosts
BSS 1
• access point (AP): base station
• ad hoc mode: hosts only

BSS 2
Wireless Communication Networks, N. Moayedian
Wireless and Mobile Networks: 7- 59
Comparison: infrastructure vs. ad-hoc vs. mesh networks
infrastructure
network
mesh network
AP: Access Point
AP

AP wired network
AP

ad-hoc network

Wireless Communication Networks, N.

60
Moayedian
802.11 – Classical architecture of an
infrastructure network 802.11 LAN
802.x LAN
• Station (STA)
• terminal with access mechanisms to the wireless STA1
medium and radio contact to the access point
• Basic Service Set (BSS) BSS1
• group of stations using the same radio frequency Portal
Access
• Access Point Point
• station integrated into the wireless LAN and the
distribution system Distribution System
• Portal Access
• bridge to other (wired) networks ESS Point
• Distribution System
• interconnection network to form one logical BSS2
network (EES: Extended Service Set) based
on several BSS

STA2 802.11 LAN STA3

Wireless Communication Networks, N.

61
Moayedian
802.11 - Architecture of an ad-hoc network
802.11 LAN

• Direct communication within a limited range

• Station (STA): STA1
IBSS1 STA3
terminal with access mechanisms
to the wireless medium
• Independent Basic Service Set STA2
(IBSS):
group of stations using the same
radio frequency
IBSS2

STA5

STA4 802.11 LAN

Wireless Communication Networks, N.

62
Moayedian
 802.11 - Architecture of a mesh network 802.11 LAN
802.x LAN
• Mesh BSS forming a meshed STA1
network with possibly
redundant paths using the BSS
Portal
Hybrid Wireless Mesh Protocol Access
Point Distribution
(HWMP) System
Mesh
802.11 LAN Gate
Mesh BSS
• Mesh Gate, AP andSTA 2

DS can be BSS
co-located in Access Distribution Mesh
System Mesh STA2
Gate
one device Point
Mesh STA3
Mesh STA1

Wireless Communication Networks, N.

Moayedian
Mesh STA5 Mesh STA4 63
IEEE standard 802.11 and wired LAN
fixed
terminal
mobile terminal

infrastructure
network

access point
application application
TCP TCP

IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

Wireless Communication Networks, N.

64
Moayedian
 802.11 - Layers and functions
• MAC • PHY
• access mechanisms, • clear channel assessment (carrier
fragmentation, encryption sense)
• MAC Management • modulation, coding
• synchronization, roaming, MIB, • PHY Management
power management • channel selection, MIB
LLC Station Management • Station Management
DLC

MAC MAC Management • coordination of all management

functions
PHY

PHY PHY Management

Wireless Communication Networks, N.

65
Moayedian
Physical layer
Wireless LAN IEEE 802.11

Wireless Communication Networks, N. Moayedian 66

ISM Bands
• Industrial, Scientific, and Medical bands. License exempt

• Ref: http://en.wikipedia.org/wiki/ISM_band
Wireless Communication Networks, N. Moayedian 67
802.11 - Physical layer (historical – not in
standard any longer)
• First version in 1997: IEEE 802.11
• 3 versions: 2 radio (typ. 2.4 GHz), 1 IR
• data rates 1 or 2 Mbit/s
• FHSS (Frequency Hopping Spread Spectrum) - obsolete
• spreading, despreading, signal strength, typ. 1 Mbit/s
• min. 2.5 frequency hops/s (USA), two-level GFSK modulation
• DSSS (Direct Sequence Spread Spectrum) – many products
• DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential
Quadrature PSK)
• preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s
• chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
• max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
• Infrared - obsolete
• 850-950 nm, diffuse light, typ. 10 m range
• carrier detection, energy detection, synchronization
• Two additional amendments in 1999:
• IEEE 802.11a-1999: 5-GHz band, 54 Mbps/20 MHz, OFDM
• IEEE 802.11b-1999: 2.4 GHz band, 11 Mbps/22 MHz
• Fourth amendment:
• IEEE 802.11g-2003: 2.4 GHz band, 54 Mbps/20 MHz, OFDM
Wireless Communication Networks, N. Moayedian 68
DSSS PHY packet format (legacy)
• Synchronization
• synch., gain setting, energy detection, frequency offset compensation
• SFD (Start Frame Delimiter)
• 1111001110100000
• Signal
• data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
• Service
• future use, 00: 802.11 compliant
• Length
• length of the payload
• HEC (Header Error Check)
• protection of signal, service and length, x16+x12+x5+1
128 16 8 8 16 16 variable bits
synchronization SFD signal service length HEC payload

PLCP preamble PLCP header

Wireless Communication Networks, N. Moayedian 69
 IEEE 802.11 HR/DSSS – PHY frame formats
(was 802.11b)
Long PLCP PPDU format
• High Rate Direct 128 16 8 8 16 16 variable bits
Sequence Spread synchronization SFD signal service length HEC payload
Spectrum @ 2.4GHz
PLCP preamble PLCP header

• Data rate 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

• 1, 2, 5.5, 11 Mbit/s, Short PLCP PPDU format (optional)

depending on SNR 56 16 8 8 16 16 variable bits

• User data rate max. short synch. SFD signal service length HEC payload
approx. 6 Mbit/s
PLCP preamble PLCP header
(1 Mbit/s, DBPSK) (2 Mbit/s, DQPSK)

96 µs 2, 5.5 or 11 Mbit/s
Wireless Communication Networks, N. Moayedian 70
 Channel selection (non-overlapping)
Europe (ETSI)

channel 1 channel 7 channel 13

2400 2412 2442 2472 2483.5

22 MHz [MHz]
US (FCC)/Canada (IC)

channel 1 channel 6 channel 11

2400 2412 2437 2462 2483.5

22 MHz [MHz]

Wireless Communication Networks, N. Moayedian 71

 IEEE 802.11 OFDM – PHY frame format (802.11a)
• OFDM @ 5GHz 4 1 12 1 6 16 variable 6 variable bits
• Data rates
rate reserved length parity tail service payload tail pad
• E.g. 6, 9, 12, 18, 24, 36, 48,
54 Mbit/s, depending on
SNR and channel width
PLCP header
• User throughput (1500 byte
packets): 5.3 (6), 18 (24), 24
(36), 32 (54)
• 6, 12, 24 Mbit/s mandatory
• Transmission range PLCP preamble signal data
• 100m outdoor, 10m indoor
• E.g., 54 Mbit/s up to 5 m, 12 1 variable symbols
48 up to 12 m, 36 up to 25
m, 24 up to 30m, 18 up to 6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
40 m, 12 up to 60 m
• Frequency
• Free 5.15-5.25, 5.25-5.35,
5.725-5.825 GHz ISM-band Wireless Communication Networks, N. Moayedian 72
Operating channels of 802.11a in Europe
(examples)
36 40 44 48 52 56 60 64 channel

5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz]
16.6 MHz

100 104 108 112 116 120 124 128 132 136 140 channel

5470 5500 5520 5540 5560 5580 5600 5620 5640 5660 5680 5700 5725
16.6 MHz [MHz]
center frequency = 5000 + 5*channel number [MHz]

Wireless Communication Networks, N. Moayedian 73

Operating channels for 802.11a / US U-NII
(examples)
36 40 44 48 52 56 60 64 channel

5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz]
16.6 MHz

center frequency = 5000 + 5*channel number [MHz]

149 153 157 161 channel

5725 5745 5765 5785 5805 5825 [MHz]

16.6 MHz

Wireless Communication Networks, N. Moayedian 74

OFDM in IEEE 802.11
• OFDM with 52 used subcarriers (64 in total)
• 48 data + 4 pilot
• (plus 12 virtual subcarriers)
pilot 312.5 kHz
• 312.5 kHz spacing
• 52*312.5 = 16.25 MHz
• 64*312.5 = 20 MHz

-26 -21 -7 -1 1 7 21 26 subcarrier

channel center frequency number

Wireless Communication Networks, N. Moayedian 75

IEEE 802.11 ERP – PHY frame formats (802.11g –
Wi-Fi 3)
• Extended Rate PHY @ 2.4GHz

• Data rates
• Builds on classical 1, 2 Mbit/s (DSSS) and 1, 2, 5.5, 11 Mbit/s (HR DSSS)
• Uses additionally OFDM for 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s (thus check 802.11
OFDM for frame formats)

•Many more options and modulation modes standardized but obsolete or

deprecated.

•Basically, it applies the old 802.11a @ 2.4 GHz.

Wireless Communication Networks, N. Moayedian 76

 IEEE 802.11 HT – PHY frame formats (802.11n) –
marketed as WiFi 4
• High Throughput (HT) Orthogonal Frequency Division Multiplexing (OFDM) system @ 2.4 and 5 GHz
• Based on the OFDM system, but now using up to 4 spatial stream operating in 20 MHz bandwidth
(additionally, 40 MHz bandwidth specified offering up to 600 Mbit/s)

Wireless Communication Networks, N.

77
Moayedian
IEEE 802.11 HT – PHY frame formats (802.11n)

Wireless Communication Networks, N. Moayedian 78

Very High Throughput (VHT) PHY – uses OFDM (802.11ac)

Source: IEEE Std 802.11-2016

Wireless Communication Networks, N. Moayedian 79
IEEE 802.11 VHT – High-speed for WLANs at 5 GHz –
marketed as WiFi 5
• Single link throughput > 500Mbit/s, multi-station > 1 Gbit/s
• Bandwidth up to 160 MHz (80 MHz mandatory), up to 8x MIMO, up to 256
QAM, beamforming, SDMA via MIMO
• Example home configuration:
• 8-antenna access point, 160 MHz
bandwidth, 6.77 Gbit/s
• 4-antenna digital TV, 3.39 Gbit/s
• 2-antenna tablet, 1.69 Gbit/s
• Two 1-antenna smartphones,
867 Mbit/s each

• Redefinition of many protocol fields and procedures!

Wireless Communication Networks, N. Moayedian 80
IEEE 802.11ax – High Efficiency WLAN – marketed
as WiFi 6(E)
• Increased number of non-overlapping channels at 6 GHz

Wireless Communication Networks, N. Moayedian 81

Improvements of 802.11ax over 802.11ac
• Centrally (AP) controlled MAC with dynamic bandwidth assignment using
OFDMA via Resource Units (RU, time-frequency resources, see LTE!)
• Multi-user MIMO in up- and downlink, AP sends trigger with scheduling
information (modulation, coding, RUs)
• Mix of assigned and random access RUs for uplinks
• Spatial frequency reuse via “coloring” of signals (distinguishes
own/neighboring network) plus adaptive power/sensitivity thresholds
• Two NAVs: own network and overlapping network to avoid misbehavior
• Dynamic fragmentation helps reducing overhead (fill available RUs)
• Longer guard intervals for better protection against signal delay spread
(outdoor conditions)

Wireless Communication Networks, N. Moayedian 82

Data rates for 802.11ax
• Values are for a single spatial stream
• Depending on number of streams devices with > 10 Gbit/s available
MCS Modulation Coding Data rate in Mbit/s per spatial stream
rate 20 MHz channels 40 MHz channels 80 MHz channels 160 MHz channels

1600 ns GI 800 ns GI 1600 ns 800 ns GI 1600 ns 800 ns GI 1600 ns 800 ns GI

GI GI GI
0 BPSK 1/2 8 8.6 16 17.2 34 36.0 68 72
1 QPSK 1/2 16 17.2 33 34.4 68 72.1 136 144
2 QPSK 3/4 24 25.8 49 51.6 102 108.1 204 216
3 16-QAM 1/2 33 34.4 65 68.8 136 144.1 272 282
4 16-QAM 3/4 49 51.6 98 103.2 204 216.2 408 432
5 64-QAM 2/3 65 68.8 130 137.6 272 288.2 544 576
6 64-QAM 3/4 73 77.4 146 154.9 306 324.4 613 649
7 64-QAM 5/6 81 86.0 163 172.1 340 360.3 681 721
8 256-QAM 3/4 98 103.2 195 206.5 408 432.4 817 865
9 256-QAM 5/6 108 114.7 217 229.4 453 480.4 907 961
10 1024-QAM 3/4 122 129.0 244 258.1 510 540.4 1021 1081
11 1024-QAM 5/6 135 143.4 271 286.8 567 600.5 1134 1201
Wireless Communication Networks, N.
83
Moayedian
MAC layer
Wireless LAN IEEE 802.11

Wireless Communication Networks, N. Moayedian 84

802.11 - MAC layer architecture

Source: IEEE Std 802.11-2016

Wireless Communication Networks, N. Moayedian 85
 How to access the medium in 802.11
• Distributed Coordination Function (DCF)
• Fundamental access method in 802.11, mandatory
• Also known as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
• Random backoff, certain fairness, refinement with RTS/CTS possible
• Point Coordination Function (PCF) (not really used, will be kicked out in the future)
• Contention free access, reservation of the medium
• Hybrid Coordination Function (HCF)
• QoS support by combining DCF and PCF
• Contention-based channel access (Enhanced Distributed Channel Access, EDCA) and controlled
channel access (HCF Controlled Channel Access, HCCA)
• Support of different priorities for, e.g., background, best effort, video, voice traffic (WiFi WMM
Designations)
• Mesh Coordination Function (MCF)
• Only in a MBSS, EDCA for contention-based access, MCCA (MCS Controlled Channel Access) for contention-free
access Wireless Communication Networks, N. Moayedian 86
 802.11 - MAC Inter Frame Space
• Priorities of packets defined through different inter frame spaces (not always
guaranteed)
• RIFS (Reduced IFS)
• shortest IFS, reduced overhead, only if no SIFS expected, for higher throughput
• SIFS (Short IFS)
• for ACK, CTS, polling response
• PIFS (PCF IFS)
EIFS
• used to gain priority access (PCF, TIM, …) AIFSi
• DIFS (DCF IFS)
…
• for “normal” asynchronous data service AIFSi
• AIFS (Arbitration IFS) RIFS
• variable depending on QoS
• EIFS (Extended IFS) DIFS DIFS
• IFS e.g. after an incorrect FCS PIFS
SIFS
• Additional “beamforming” IFSs medium busy contention next frame
t
direct access if
Wireless Communication Networks, N. Moayedian 87
medium is free  DIFS
802.11 - CSMA/CA access method I
• station ready to send starts sensing the medium (Carrier Sense based
on CCA, Clear Channel Assessment)
• if the medium is free for the duration of an Inter-Frame Space (IFS),
the station can start sending (IFS depends on service type)
• if the medium is busy, the station has to wait for a free IFS, then the
station must additionally wait a random back-off time (collision
avoidance, multiple of slot-time)
• if another station occupies the medium during the back-off time of
the station, the back-off timer stops (fairness)
contention window
DIFS DIFS
(randomized back-off
mechanism)
medium busy next frame

direct access if t
Wireless Communication Networks, N. Moayedian 88
medium is free  DIFS slot time (20µs)
802.11 - Competing stations - simple version
DIFS DIFS DIFS DIFS
boe bor boe bor boebusy
station1

boe busy
station2

busy
station3

boe busy boebor

station4

boe bor boe busy boebor

station5
t

busy medium not idle (frame, ack etc.) boe elapsed backoff time

packet arrival at MAC bor residual backoff time

Wireless Communication Networks, N. Moayedian 89

 802.11 - CSMA/CA access method II
• Sending unicast packets
• station has to wait for DIFS before sending data
• receivers acknowledge at once (after waiting for SIFS) if the packet was
received correctly (FCS)
• automatic retransmission of data packets in case of transmission errors, but
exponential increase of contention window
DIFS
data
sender
SIFS
ACK
receiver
DIFS
other data
stations t
waiting time contention

Wireless Communication Networks, N. Moayedian 90

802.11 – DCF with RTS/CTS
• Sending unicast packets
• station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the medium)
• acknowledgement via CTS after SIFS by receiver (if ready to receive)
• sender can now send data at once, acknowledgement via ACK
• other stations store medium reservations distributed via RTS and CTS
DIFS
RTS data
sender
SIFS SIFS
CTS SIFS ACK
receiver

NAV (RTS) DIFS

other NAV (CTS) data
stations t
defer access contention
Wireless Communication Networks, N. Moayedian 91
Fragmentation
DIFS
RTS frag1 frag2
sender
SIFS SIFS SIFS
CTSSIFS ACK1SIFS ACK2
receiver

NAV (RTS)
NAV (CTS)
NAV (frag1) DIFS
other NAV (ACK1) data
stations t
contention

Wireless Communication Networks, N. Moayedian 92

802.11 frame: addressing • Only the first three and
the last field are present
in all frames!

duration of reserved frame sequence # (for reliable data

transmission time (RTS/CTS) transfer)

2 2 6 6 6 2 6 0 - 2312 4
frame address address address seq address
control duration 1 2 3 control 4 payload CRC

2 2 4 1 1 1 1 1 1 1 1
protocol to from more power more
type subtype retry WEP rsvd
version AP AP frag mgt data

frame type (RTS, CTS, ACK, data)

Wireless Communication Networks, N. Moayedian
Wireless and Mobile Networks: 7- 93
 
Or
NAV information
Frame Types
 Short Id for PS-Poll Upper layer data
 2048 byte max
 256 upper layer
header

Frame Duration Address Address Address Sequence Address

DATA FCS
Control /ID 1 2 3 Control 4
2 2 6 6 6 2 6 0-2312 4 bytes

• Protocol Version
 IEEE 48 bit address  MSDU
• Frame Type & subtype
 Individual/Group  Sequence Number  CCIT CRC-32
• To DS and From DS  Universal/Local  Fragment Number Polynomial
• More Fragments  46 bit address
• Retry
• Power Management  BSSID –BSS
Identifier
• 0 : stay active
 TA - Transmitter
• 1: power save mode
 RA - Receiver
• More Data  SA - Source
• 1 : more data  no save mode  DA - Destination
• Or more Poll need
• WEP
Wireless Communication Networks, N. Moayedian 94
• Order
 Frame Subtypes

CONTROL : 01 DATA : 10 MANAGEMENT : 00

• RTS  Data  Beacon
• CTS  Data+CF-ACK  Probe Request & Response
• ACK  Data+CF-Poll  Authentication
 Data+CF-ACK+CF-Poll  Deauthentication
• PS-Poll
 Null Function  Association Request & Response
• CF-End & CF-End ACK  CF-ACK (nodata)  Reassociation Request &
 CF-Poll (nodata) Response
 CF-ACK+CF+Poll  Disassociation
 Announcement Traffic Indication
Message (ATIM)

Wireless Communication Networks, N. Moayedian 95

MAC address format (examples)
Example scenario to DS from address 1 address 2 address 3 address 4
DS
ad-hoc network 0 0 RA=DA TA=SA BSSID -
infrastructure 0 1 RA=DA TA=BSSID SA -
network, from AP
infrastructure 1 0 RA=BSSID TA=SA DA -
network, to AP
within mesh BSS 1 1 RA TA DA SA

AP: Access Point

DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address

Wireless Communication Networks, N. Moayedian 96

Special Frames: ACK, RTS, CTS
bytes 2 2 6 4
• Acknowledgement Frame Receiver
ACK Duration FCS
Control Address

bytes 2 2 6 6 4
Frame Receiver Transmitter
RTS Duration FCS
• Request To Send Control Address Address

bytes 2 2 6 4
Frame Receiver
CTS Duration FCS
• Clear To Send Control Address

Wireless Communication Networks, N. Moayedian 97

802.11 frame: addressing

Internet
H1 R1

802.3 Ethernet frame

R1 MAC addr H2 MAC addr

MAC dest addr MAC source addr

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 WiFi frame

Wireless Communication Networks, N. Moayedian
Wireless and Mobile Networks: 7- 98
802.11 - MAC management
• Synchronization
• try to find a LAN, try to stay within a LAN
• timer etc.
• Power management
• sleep-mode without missing a message
• periodic sleep, frame buffering, traffic measurements
• Association/Reassociation
• integration into a LAN
• roaming, i.e. change networks by changing access points
• scanning, i.e. active search for a network
• MIB - Management Information Base
• managing, read, write

Wireless Communication Networks, N. Moayedian 99

Synchronization using a Beacon (infrastructure)
beacon interval
(20ms – 1s)

B B B B
access
point
busy busy busy busy
medium
t
value of the timestamp B beacon frame

Wireless Communication Networks, N. Moayedian 100

Synchronization using a Beacon (ad-hoc)

beacon interval

B1 B1
station1

B2 B2
station2

busy busy busy busy

medium
t
value of the timestamp B beacon frame random delay

Wireless Communication Networks, N. Moayedian 101

Power management
• Idea: switch the transceiver off if not needed
• States of a station: sleep and awake
• Timing Synchronization Function (TSF)
• stations wake up at the same time
• Infrastructure
• Traffic Indication Map (TIM)
• list of unicast receivers transmitted by AP
• Delivery Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers transmitted by AP
• Ad-hoc
• Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)
• APSD (Automatic Power Save Delivery)
• more efficient method in 802.11e replacing above schemes offering scheduled (S-APSD) and
unscheduled service periods (U-APSD)
Wireless Communication Networks, N. Moayedian 102
Power saving with wake-up patterns (infrastructure)
TIM interval DTIM interval

D B T T d D B
access
point
busy busy busy busy
medium

p d
station
t
T TIM D DTIM awake

B broadcast/multicast p PS poll d data transmission

to/from the station

• Traffic Indication Map (TIM) : list of unicast receivers transmitted by AP

• Delivery Traffic Indication Map (DTIM) : list of broadcast/multicast receivers transmitted by AP
Wireless Communication Networks, N. Moayedian 103
Power management in adhoc
Ad-hoc Traffic Indication Map (ATIM)

Wireless Communication Networks, N. Moayedian 104

U-APSD(Automatic Power Save Delivery) – WMM Power Save
• Procedure for unicast data delivered to a STA in PS mode
• STA triggers release of buffered data from AP
• WMM Power Save based on legacy procedures plus optional U-APSD
• Advantages:
• No more polling needed
• Downlink data frames sent together in a fast sequence
• Trigger frame may already contain data – ideal e.g. for VoIP
• Applications specify PS behavior, i.e. sleep period
no more than max. service period length

ACK data / null data / null

AP

data / null ACK ACK

STA
Wireless Communication Networks, N. Moayedian
awake105
802.11 - Roaming
• No or bad connection? Then perform:
• Scanning
• scan the environment, i.e., listen into the medium for beacon signals or send probes into the
medium and wait for an answer
• Reassociation Request
• station sends a request to one or several AP(s)
• Reassociation Response
• success: AP has answered, station can now participate
• failure: continue scanning
• AP accepts Reassociation Request
• signal the new station to the distribution system
• the distribution system updates its data base (i.e., location information)
• typically, the distribution system now informs the old AP so it can release resources

• May take a long time …

Wireless Communication Networks, N. Moayedian 106
802.11: Association

 arriving host: must associate

with an AP
• scans channels, listening for
beacon frames containing AP’s
name (SSID) and MAC address
• selects AP to associate with
• then may perform
authentication BSS

• then typically run DHCP to get IP

address in AP’s subnet

Wireless Communication Networks, N. Moayedian

Wireless and Mobile Networks: 7- 107
802.11: passive/active scanning
BBS 1 BBS 2 BBS 1 BBS 2

1
1 1 AP 2 2 AP 2
AP 1 AP 1
2
2 3
3 4

H1 H1

passive scanning: active scanning:

(1) beacon frames sent from APs (1) Probe Request frame broadcast from H1
(2) association Request frame sent: H1 (2) Probe Response frames sent from APs
to selected AP (3) Association Request frame sent: H1 to
(3) association Response frame sent selected AP
from selected AP to H1 (4) Association Response frame sent from
selected AP to H1
Wireless Communication Networks, N. Moayedian
Wireless and Mobile Networks: 7- 108
Faster roaming using 802.11k, .11r and .11v
• Classical roaming is too slow, e.g., for VoIP • 802.11k: Optimized channel list
over WLAN  service interruption • Collect potential roaming networks prior to
1. 802.11 authentication message exchange roaming
2. Reassociation messages exchange
3. EAP-request/response identity exchange
4. Access request and challenge packet • 802.11r: Fast BSS Transition - only 4 steps
exchange left
5. EAP request/response 1. Client and AP exchange 802.11
authentication messages and nonce-values
6. RADIUS access request/accept exchange
2. Client and AP exchange reassociation
7. Success message to Client messages and temporal
8. Nonce-value exchange key/acknowledgment
9. Temporal key, acknowledgement exchange
• 802.11v: BSS Transition Management
• In this example 17 steps (all but 7. are • Manage information about alternative
exchanges)! access points
• See 802.1X for more details about • Disassociation Imminent can force client to
authentication roam

Wireless Communication Networks, N. Moayedian 109

 802.11 network standards
Frequency Stream Max. Approx. range
Release Frequency Band width
range, PHY Protocol data rate[22] MIMO Modulation
date[21] Indoor Outdoor
or type (GHz) (MHz) (Mbit/s) streams
802.11-1997
DSSS[23], FHSS[A] June 1997 2.4 22 1, 2 — DSSS, FHSS[A] 20 m 100 m
Wi-Fi 0
HR/DSSS[23] 802.11b Wi-Fi 1 Sep 1999 2.4 22 1, 2, 5.5, 11 — CCK, DSSS 35 m 140 m
802.11a Wi-Fi 2 Sep 1999 5 6, 9, 12, 18, 24, 36, 48, 35 m 120 m
802.11j Nov 2004 4.9, 5.0[B][24] 54 ? ?
OFDM 802.11y Nov 2008 3.7[C] (for 20 MHz ? 5,000 m C]
5, 10, 20 — OFDM
802.11p Jul 2010 5.9 bandwidth, 200 m 1000 m 25]
802.11bd Dec 2022 5.9, 60 divide by 2 and 4 for 500 m 1,000 m
ERP-OFDM[26] 802.11g Wi-Fi 3 Jun 2003 2.4 10 and 5 MHz) 38 m 140 m
802.11n 20 Up to 288.8[D] MIMO-OFDM
HT-OFDM[27] Oct 2009 2.4, 5 4 70 m 250 m 28]
(Wi-Fi 4) 40 Up to 600[D] (64-QAM)
20 Up to 693[D]
DL
802.11ac 40 Up to 1600[D]
VHT-OFDM[27] Dec 2013 5 8 MU-MIMO OFDM 35 m [29] ?
1–7 GHz (Wi-Fi 5) 80 Up to 3467[D]
(256-QAM)
160 Up to 6933[D]
20 Up to 1147[E]
802.11ax UL/DL
40 Up to 2294[E] 30 m 120 m
HE-OFDMA (Wi-Fi 6, May 2021 2.4, 5, 6 8 MU-MIMO OFDMA
80 Up to 5.5 Gbit/s[E] (98 ft) (390 ft)[F]
Wi-Fi 6E) (1024-QAM)
80+80 Up to 11.0 Gbit/s[E]
80 Up to 11.5 Gbit/s[E]
UL/DL
802.11be Dec 2024 160(80+80) Up to 23 Gbit/s[E] 30 m 120 m
EHT-OFDMA 2.4, 5, 6 16 MU-MIMO OFDMA
(Wi-Fi 7) (est.) 240(160+80) Up to 35 Gbit/s[E] (98 ft) (390 ft)[F]
(4096-QAM)
320(160+160) Up to 46.1 Gbit/s[E]
802.11bn May 2028 2.4, 5, 6, Multi-link (8192-QAM)
UHR 320 Up to 100 Gbit/s 16 ? ?
(Wi-Fi 8) (est.) 42, 60, 71 MU-MIMO OFDM
0.0625, 0.25
WUR[G] 802.11ba Oct 2021 2.4,Wireless
5 Communication Networks,
4, 20 N. Moayedian
— OOK (multi-carrier OOK) ? 110 ?
(62.5 kbit/s, 50 kbit/s)