192620010
Mobile & Wireless Networking
Lecture 7:
Wireless LAN
[Schiller, Section 7.3]
[Reader, Part 6]
[Optional: "IEEE 802.11n Development: History, Process, and Technology",
Perahia, IEEE Communications Magazine, July 2008]
Geert Heijenk
Mobile and Wireless Networking
2013 / 2014
Outline of Lecture 9
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Wireless LAN
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General Characteristics / IEEE 802.11 standard
IEEE 802.11 Physical Layers
Medium Access Control
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l
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CSMA/CA
CSMA with RTS/CTS
MAC Quality of Service Enhancements
MAC Frame Format
MAC Management
IEEE 802.11: Important Developments
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Design goals for wireless LANs
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global, seamless operation
low power for battery use
no special permissions or licenses needed to use the LAN
robust transmission technology
simplified spontaneous cooperation at meetings
easy to use for everyone, simple management
protection of investment in wired networks
security (no one should be able to read my data), privacy (no
one should be able to collect user profiles), safety (low radiation)
transparency concerning applications and higher layer
protocols, but also location awareness if necessary
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Comparison: infrastructure vs. ad-hoc networks
infrastructure
network
AP: Access Point
AP
AP
wired network
AP
ad-hoc network
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IEEE 802.11 - Architecture of an infrastructure network
Station (STA)
802.11 LAN
STA1
802.x LAN
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Basic Service Set (BSS)
BSS1
Portal
Access
Point
Access
Point
ESS
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group of stations using the same
radio frequency
Access Point
Distribution System
q
station integrated into the wireless
LAN and the distribution system
Portal
q
BSS2
bridge to other (wired) networks
Distribution System
q
STA2
terminal with access mechanisms
to the wireless medium and radio
contact to the access point
802.11 LAN
STA3
interconnection network to form
one logical network (ESS:
Extended Service Set) based
on several BSS
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IEEE 802.11 - Architecture of an ad-hoc network
Direct communication within a
limited range
802.11 LAN
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STA1
STA3
IBSS1
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STA2
IBSS2
STA5
STA4
Station (STA):
terminal with access
mechanisms to the wireless
medium
Independent Basic Service Set
(IBSS):
group of stations using the
same radio frequency
Exception:
q  IEEE 802.11p for Vehicular
Networks:
communication without setting
up a basic service set.
802.11 LAN
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IEEE standard 802.11
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
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IEEE 802.11 - Layers and functions
PLCP Physical Layer Convergence Protocol
MAC
q
access mechanisms,
fragmentation, encryption
MAC Management
q
synchronization, roaming,
authentication, MIB, power
management
q
PMD Physical Medium Dependent
PHY Management
MAC Management
PLCP
PHY Management
PMD
channel selection, MIB
q
Station Management
coordination of all management
functions
Station Management
DLC
PHY
MAC
modulation, coding
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LLC
clear channel assessment
signal (carrier sense)
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The Physical Layer
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IEEE 802.11
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Direct Sequence Spread Spectrum (DSSS) PHY
–  2.4 GHz : RF : 1 – 2 Mbps
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Frequency Hopping Spread Spectrum (FHSS) PHY
–  2.4 GHz : RF : 1- 2 Mbps
l
Infrared (IR) PHY
–  Indoor : IR : 1 and 2 Mbps
q
IEEE 802.11b
l
High Rate DSSS PHY
–  2.4 GHz : 5.5 Mbps – 11 Mbps
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IEEE 802.11a
l
OFDM PHY
–  5 GHz : 6-54 Mbps
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IEEE 802.11g
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OFDM PHY
–  2.4 GHz : 6-54 Mbps
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IEEE 802.11n
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OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz)
–  Extension of a/g: 6.5 – 600 Mbps
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IEEE 802.11ac
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OFDM PHY + MIMO (up to 8x8) + Multi-User MIMO (up to 4 simultaneous users)
+ 80 MHz + 256 QAM
–  extension of n in 5GHz: up to 4 x 1.69 Gbit/s
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IEEE 802.11ad
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WiGig in 60 GHz, upto 7 Gbps
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IEEE 802.11a
Data rate
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6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR
User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)
6, 12, 24 Mbit/s mandatory
Transmission range
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100m outdoor, 10m indoor
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E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40
m, 12 up to 60 m
Frequency
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Free 5 GHz ISM-band
Special Advantages/Disadvantages
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Advantage: fits into 802.x standards, free ISM-band, available, simple
system, uses less crowded 5 GHz band
Disadvantage: stronger shading due to higher frequency
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IEEE 802.11g
Frequency
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Free 2.4 GHz ISM-band
But shared with many legacy WLANs and other systems, e.g. Bluetooth
Backward compatible with IEEE 802.11b
Data rate (as 802.11a)
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6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR
User throughput significantly lower in presence of 802.11b stations
Transmission range
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Somewhat higher than 802.11a
Special Advantages/Disadvantages
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Advantage: backward compatibility, better propagation conditions
Disadvantage: crowded band, lower speed due to backward compatibility
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802.11 b/g Channel selection (non-overlapping)
Europe (ETSI)
channel 1
2400
2412
channel 7
channel 13
2442
2472
22 MHz
2483.5
[MHz]
US (FCC)/Canada (IC)
channel 1
2400
2412
channel 6
channel 11
2437
2462
22 MHz
2483.5
[MHz]
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IEEE 802.11a/g – PHY frame format
4
1
12
1
rate reserved length parity
6
16
tail service
variable
6
variable
payload
tail
pad
bits
PLCP header
PLCP preamble
12
signal
data
1
6 Mbit/s
variable
symbols
6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
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802.11 - MAC layer I
Access methods
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DCF CSMA/CA (mandatory)
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DCF w/ RTS/CTS (optional)
l
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access point polls terminals according to a list
HCF EDCA (optional)
l
q
reduces hidden terminal problem
PCF (optional)
l
q
collision avoidance via randomized „back-off“ mechanism
minimum distance between consecutive packets
ACK packet for acknowledgements (not for broadcast)
CSMA/CA with priority levels
HCF CCA (optional)
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Improved polling
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802.11 - MAC layer II
Priorities
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defined through different inter frame spaces
no guaranteed, hard priorities
SIFS (Short Inter Frame Spacing)
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PIFS (PCF IFS)
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highest priority, for ACK, CTS, polling response
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
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lowest priority, for asynchronous data service
DIFS
medium busy
DIFS
PIFS
SIFS
contention
direct access if
medium is free ≥ DIFS
next frame
t
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802.11 - CSMA/CA access method (DCF) I
DIFS
DIFS
medium busy
direct access if
medium is free ≥ DIFS
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contention window
(randomized back-off
mechanism)
next frame
t
slot time
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)
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802.11 - competing stations - simple version
DIFS
DIFS
station1
station2
DIFS
boe
bor
boe
busy
DIFS
boe bor
boe
busy
boe busy
boe bor
boe
boe
busy
station3
station4
boe bor
station5
busy
bor
t
busy
medium not idle (frame, ack etc.)
boe elapsed backoff time
packet arrival at MAC
bor residual backoff time
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802.11 - Binary Exponential Backoff
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Stations choose their backoff time randomly from contention
window
Ideal contention window size is trade-of between acceptable load
and experienced delay
Initial contention window size (CWmin) is 7 slots
(backoff time between 0 and 7)
After collision (no ack), contention window is “doubled” until CWmax
= 255 is reached:
7 -> 15 -> 31 -> 63 -> 127 -> 255
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802.11 - CSMA/CA access method (DCF) II
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Sending unicast packets
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station has to wait for DIFS before sending data
receivers acknowledge at once (after waiting for SIFS) if the packet
was received correctly (CRC)
automatic retransmission of data packets in case of transmission
errors
DIFS
sender
data
SIFS
receiver
ACK
DIFS
other
stations
waiting time
data
t
contention
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802.11 – CSMA/CA with RTS/CTS
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Sending unicast packets
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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
sender
RTS
data
SIFS
receiver
other
stations
CTS SIFS
SIFS
ACK
DIFS
NAV (RTS)
NAV (CTS)
defer access
data
t
contention
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Fragmentation
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
NAV (RTS)
NAV (CTS)
other
stations
NAV (frag1)
NAV (ACK1)
ACK2
DIFS
data
t
contention
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802.11e - MAC services (QoS support)
•
•
•
Distributed Coordination Function
Point Coordination Function (not implemented)
+ Hybrid Coordination Function (HCF)
•
backward compatible with DCF
HCF:
•  HCF Controlled Channel Access (HCCA)
•  Enhanced Distributed Channel Access (EDCA)
•
•
Differentiation priority levels
4 access categories with separate queues in each STA
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EDCA
Multiple Access Categories with their own transmission queue
Differentiation by means of:
•  Arbitray Inter Frame Spacing (AIFS)
•  Initial Contention Window (CWmin)
•  Maximum Contention Window (CWmax)
•  Transmission Opportunity Limit (TXOP Limit)
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DCF Summary
Immediate access when
medium is free >=DIFS
DIFS
Contention Window
DIFS
Im
M
PIFS
Busy Medium
SIFS
Backoff-Window
D
Next Frame
Slot Time
Defer Access
Select Slot and Decrement Backoff as long
as medium is idle
g. 1. Access methods in IEEE 802.11 DCF and IEEE 802.11e E
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tion algorithm with different parameters. These paramet
EDCA Operation (AIFS)
AIFS[j]
Immediate access when
Medium is free >= DIFS/AIFS[i]
AIFS[i]
DIFS
Contention Window
PIFS
DIFS/AIFS
Busy Medium
SIFS
Backoff Slots
Next Frame
Slot Time
Defer Access
Select Slot and Decrement Backoff as long
as medium is idle
2.11e EDCA: (a) basic access in DCF and (b) basic access in EDCA
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802.11 – MAC Frame format
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Types
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Duration /ID
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important against duplicated frames due to lost ACKs
Addresses
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for NAV during RTS/CTS and fragmentation, and CFP
Sequence control
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control frames, management frames, data frames
receiver, transmitter (physical), BSS identifier, sender (logical)
Miscellaneous
q
checksum, frame control, data
bytes
2
2
6
6
6
2
6
Frame Duration/ Address Address Address Sequence Address
Control
ID
1
2
3
Control
4
bits
2
2
4
1
1
1
1
1
1
1
0-2312
4
Data
CRC
1
Protocol
To From More
Power More
Type Subtype
Retry
WEP Order
version
DS DS Frag
Mgmt Data
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MAC address format
scenario
ad-hoc network
infrastructure
network, from AP
infrastructure
network, to AP
infrastructure
network, within DS
to DS from
DS
0
0
0
1
address 1 address 2 address 3 address 4
DA
DA
SA
BSSID
BSSID
SA
-
1
0
BSSID
SA
DA
-
1
1
RA
TA
DA
SA
DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address
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Special Frames: ACK, RTS, CTS
Acknowledgement
bytes
ACK
Request To Send
2
Frame
Control
bytes
RTS
2
Frame
Control
2
6
Receiver
Duration
Address
4
CRC
2
6
6
Receiver Transmitter
Duration
Address Address
4
CRC
Clear To Send
bytes
CTS
2
Frame
Control
2
6
Receiver
Duration
Address
4
CRC
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802.11 - MAC management
Synchronization
q
q
try to find a LAN, try to stay within a LAN
timer etc.
Power management
q
q
sleep-mode without missing a message
periodic sleep, frame buffering, traffic measurements
Association/Reassociation
q
q
q
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
q
managing, read, write
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Synchronization using a Beacon (infrastructure)
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
B
beacon frame
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Synchronization using a Beacon (ad-hoc)
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
value of the timestamp
B
busy
beacon frame
t
random delay
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Power management
Idea: switch the transceiver off if not needed
States of a station: sleep and awake
Timing Synchronization Function (TSF)
q
stations wake up at the same time
Infrastructure
q
Traffic Indication Map (TIM)
l
q
list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)
l
list of broadcast/multicast receivers transmitted by AP
Ad-hoc
q
Ad-hoc Traffic Indication Map (ATIM)
l
l
l
announcement of receivers by stations buffering frames
more complicated - no central AP
collision of ATIMs possible (scalability?)
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Power saving with wake-up patterns (infrastructure)
TIM interval
access
point
DTIM interval
D B
T
busy
medium
busy
T
d
D B
busy
busy
p
station
d
t
T
TIM
D
B
broadcast/multicast
DTIM
awake
p PS poll
d data transmission
to/from the station
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Power saving with wake-up patterns (ad-hoc)
ATIM
window
station1
B1
station2
B
beacon frame
awake
beacon interval
A
B2
random delay
a acknowledge ATIM
B2
D
a
B1
d
A transmit ATIM
t
D transmit data
d acknowledge data
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802.11 - Roaming
No or bad connection? Then perform:
Scanning
q
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
q
station sends a request to one or several AP(s)
Reassociation Response
q
q
success: AP has answered, station can now participate
failure: continue scanning
AP accepts Reassociation Request
q
q
q
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
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IEEE 802.11 – standards and amendments
802.11e: MAC Enhancements – QoS
802.11h: Spectrum Managed 802.11a (DCS, TPC) –
802.11i: Enhanced Security Mechanisms
IEEE 802.11-2007: new release of the standard that includes a, b, d, e, g, h, i & j.
802.11n: > 100 Mbps (MIMO)
802.11p: inter-vehicle communications
802.11s: Mesh Networking
IEEE 802.11-2012: new release of the standard that includes k – z.
802.11ac: 1 Gbps in 5 GHz band (> bw, > mimo-channels, 256 QAM)
802.11ad: 7 Gbps in 60 GHz band (a.o. beamforming)
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