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Wireless LAN
TLI348 Wireless systems Wireless LAN p.1/41
Outline
History & General information
General architecture
Link level
Physical level
Summary
Related resources
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Why WLAN ?
GSM/HSCSD/GPRS/EDGE/. . . complex infrastructure licensed radio resources may be too expensive
IrDA short distance point-to-point connection
BlueTooth short distance no proper roaming
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WLAN network
unlicensed radio resources
bandwidth up to 54 Mbps
distance up to 400 m
up to 50 nominal users
carrier sense multiple access with collision avoidance (CSMA/CA)
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IEEE standards
802.2 Logical link control
802.3 Ethernet (CSMA/CD)
802.4 Token Ring
802.10 Security
802.11 Wireless LAN (CSMA/CA)
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802.11 standards
802.11 (developed in 1997)
802.11b (ratified in 1999)
802.11a (ratified in 1999, production starts in 2001)
802.11g (ratified in 2003)
802.11e (draft, Quality-of-Service)
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Scope of the 802.11 standards
applicationpresentation
sessiontransport
networklink MAC
physical PHY
WLAN only specifies the link and the physical level, thus enabling high-levelprotocol to function in the same way.
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General structure
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Ad-hoc network
communication is established between multiple stations (STAs)
all stations have a fair access to the network (BSS)
BSS
STA
STA
STA
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Infrastructure network
the access point(AP) connects the WLAN network to the backbone(DS)
set of APs and DS forms the extended service network (ESS)
DS
BSS 2
AP
STA
STA
BSS 1
AP
STASTA
AP
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Mobility types
no transition: STA moves within a cell
BSS transition: STA moves from one cell to another within the sameESS
ESS transition: STA leaves the ESS
BSS 2
AP
STA
BSS 1
AP
STA
STAno
transiti
on
BSS
tran
sitio
n
ESS
tran
sitio
n
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Roaming procedure
roaming type nomadic roaming seamless roaming
roaming decision
signal/noise level packet loss . . .
roaming direction preemptive AP discovery roam-time discovery active/passive scanning
roaming
deassociation from the old AP association with a new AP
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Operation modes
Distributed coordination function (DCF) used in ad-hoc and infrastructure networks all stations have equal rights to access the medium
advantages: simplicity, cheap solutions disadvantages: collisions, degraded Quality-of-Service (QoS)
Point coordination function (PCF) used in infrastructure networks only behaviour of all stations is coordinated by PC
advantages: coordinated access to the medium, no collisions,better functioning
disadvantages: increased complexity, ineffective bandwidthallocation
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Link level
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Frame format
2 2 6 6 6 2 6 0-2312 4
Frame
Control
Duration/
IDAddress 1 Address 2 Address 3
Sequence
ControlAddress 4
Frame
BodyFCS
Frame Control
Duration/ID Time necessary to transmit data Association Identity (AID)
Address X BSSID - identifier of the cell DA - destination address SA - source address RA - receiving station address TA - transmitting station address
Sequence Control consists of the Sequence and Fragment NumberFrame Body
FCS - frame control sequence calculated over all fields
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Frame control field
2 2 4 1 1 1 1 1 1 1 1
Protocol
versionType Subtype
To
DS
From
DS
More
FragRetry
Pwr
Mgmt
More
DataWEP Order
protocol version determines the version
type and subtype fields identify the function of the frame
To DS is set if a frame is destined for the DS
From DS is set if a frame exits the DSMore Frag is set if there is another frame to follow
Retry is set if this frame is a retransmission
Pwr Mgmt indicates the power management mode
More Data indicates that there are more dataWEP indicates that data in the frame body has been processed by WEP
Order indicates the Strictly Ordered class
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Types of frames
Management
authentication/deauthentication association/reassociaton/deassociation probe beacon
Control power-save polls acknowledgments
Data user data contention-free polls/acknowledgements
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Distributed coordination functioning
Backoff time
as the medium becomes idle, all STAs wait for DIFS seconds
after, all STAs generate randomly the backoff time
STA with the elapsed backoff timer begins to transmit
STA1 busy medium DIFS -
backoff - data
STA2 busy medium DIFS -
backoff -
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Distributed coordination functioning
Acknowledgments
The SIFS interval, which is shorter than DIFS, is used to send the controldata.
busy medium DIFS -
backoff -
busy medium SIFS-
control data
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Distributed coordination functioning
Simple interaction
STA1 sends data to STA2
STA2 acknowledges the received data
DIFS -
STA1 data busy medium DIFS -
STA2 busy mediumSIFS
- ACKDIFS
-
Other busy medium busy medium DIFS -
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Point coordination functioning
PC sends data to STA1 and asks whether it has data to send
STA1 receives data, acknowledges the CF-Poll and sends data
PC acknowledges data from STA1, sends data to STA2 and asks
whether it has dataSTA2 receives data, acknowledges the CF-Poll and sends data
PC acknowledges data and end the CF period
PC PIFS-
B SIFS-
dataCF-Poll
SIFS-
dataCF-ACK+CF-Poll
SIFS-
SIFS-
CF-ACKCF-End
STA1 SIFS-
dataCF-ACK
SIFS-
STA2 SIFS-
dataCF-ACK
SIFS-
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Physical level
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Structure of the physical level
The physical layer consists of two sublayers:
Physical Layer Convergence Procedure (PLCP)
Physical Medium Dependent (PMD)
modulation frequency band
MAC
PHYPLCP
PMD
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PLCP frame (802.11)
PLCP preamble and PLCP header are always encoded at the rate of1Mbit/s so that these data can always be decoded. 1Mbit/s -
128 16 8 8 16 16
Sync SFD Signal Service Length CRC MPDU
PLCP Preamble - PLCP Header -
PPDU -
Sync: synchronization field
SFD: start frame delimiterSignal: PHY modulation
Service: reservedLength: number of microseconds required to transmit the MPDU
CRC: protects Signal, Service, and Length fieldsTLI348 Wireless systems Wireless LAN p.26/41
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Short PLCP frame (802.11b)
Sync field is shorter (56 bits)
PLCP header is encoded at the rate of 2Mbit/s
1Mbit/s - 2Mbit/s -
56 16 8 8 16 16
Sync SFD Signal Service Length CRC MPDU
PLCP Preamble - PLCP Header -
PPDU -
Short PLCP frame is optional in 802.11b and is mandatory in 802.11g.
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PLCP frame (802.11a)
6 Mbit/s -
16s 24 16 6
Sync Signal Service MPDU Tail
4 1 12 1 6
Rate X Length Parity Tail
PLCP header -
Sync: synchronization field
Signal: describes the way data are encoded Rate: rate of data Length: length of PSDU Parity: parity bit for bits 0 16 Tail: additional time for decoder to switch to the necessary mode
Service: descrambler initialization
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Long & short hybrid PLCP frame (802.11g)
DSSS is used to encode preamble and header (802.11b)
OFDM is used to encode data (802.11a)
often referred to as the DSSS-OFDM modulation
1Mbit/s -
128 16 8 8 16 16
Sync SFD Signal Service Length CRC Sync Signal MPDU Tail
OFDM PSDU -
1Mbit/s - 2Mbit/s -
56 16 8 8 16 16
Sync SFD Signal Service Length CRC Sync Signal MPDU Tail
OFDM PSDU -
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Modulations
Frequency Hopping Spread Spectrum (FHSS) defined in 802.11
Direct Sequence Spread Spectrum (DSSS) defined in 802.11
High-Rate Direct Sequence Spread Spectrum (HR/DSSS) defined in 802.11b extended in 802.11g
Orthogonal Frequency Division Multiplexing (OFDM) defined in 802.11a
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DSSS
data are sent in the form of chips (11Mchips/s)
11 consecutive chips constitute one symbol (1Msymbol/s)
the chipping sequence 10110111000 is used to encode one bit
Differential Binary Phase Shift Keying (DBPSK) (1 bit/symbol)
Input Phase change
0 0
1
Differential Quadrature Phase Shift Keying (DQPSK) (2 bits/symbol)
Input Phase change
00 001 /2
11
10 3/2 or (/2)
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HR/DSSS
The Complementary Code Keying (CCK) modulation is used to encode thesequence of chips.
data are sent in the form of chips (11Mchips/s)
8 consecutive chips form one symbol (11/8 Msymbol/s)
c = {ej(1+2+3+4), ej(1+3+4), ej(1+2+4),ej(1+4),
ej(1+2+3), ej(1+3),ej(1+2), ej1}
CCK modulation at 5.5 Mbit/s (4 bits/symbol) bits (0,1) define the phase change using DQPSK bits (2,3) encode the basic symbol. i.e. the sequence of chips
CCK modulation at 11 Mbit/s (8 bits/symbol) bits (0,1) define the phase change using DQPSK bits (2,3) (4,5) (6,7) encode the basic symbol using QPSK
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Extended Rate PHY (ERP)
A single-carrier modulation scheme that encodes the payload using thepacket binary convolutional code (PBCC).
ERP-PBCC-22 clock works at 11MHz symbol rate is 11Msymbol/s 2 bits are transferred per symbol (22 Mbit/s)
ERP-PBCC-33 clock works at 16.5MHz symbol rate is 16.5Msymbol/s 2 bits are transferred per symbol (33 Mbit/s)
These data rates are optional and may be not supported by an STA.
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OFDM
each OFDM symbol consists of 52 subcarriers (NST) 48 data subcarriers (NSD) 4 pilot subcarriers (NSP)
each subcarrier can encode 1 or more bits (NBPSC) BPSK 1 bit per subcarrier QPSK 2 bits per subcarrier 16-QAM 4 bits per subcarrier 64-QAM 6 bits per subcarrier
convolutional code produce data bits at a certain rate (R) 1/2 2/3 3/4
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Frequency band 5GHz (802.11a)
Frequency band 5.15 5.825 GHz is used lower subband (5.15 5.25) middle subband (5.25 5.35)
upper subband (5.725 5.825)Each subband accommodates 4 non-overlapping channels
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Summary
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WLAN specifications
802.11b 802.11g 802.11a
Year 1999 2003 1999
Number of channels 3 3 8(4)
Data rates (Mbps) 33, 22, 11, 5.5, 2,
1
54, 48, 36, 24,
18, 12, 11, 9, 6,
5.5, 2, 1
54, 48, 36, 24,
18, 12, 9, 6
Indoor range 30m (11Mbps)
91m (1Mbps)
30m (54Mbps)
91m (1Mbps)
12m (54Mbps)
91m (6 Mbps)
Outdoor range (LOS) 120m (11Mbps)
460m (1Mbps)
120m (54Mbps)
460m (1Mbps)
30m (54 Mbps)
305m (6Mbps)
Wireless band (GHz) 2.4 2.497 2.4 2.497 5.15 5.875
Modulation DSSS
HR/DSSS (CCK)
ERP-PBCC
DSSS
DSSS-OFDM
OFDM
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Hardware vendors
Abocom
Accton
Acer
Acrowave
Actiontec
Adaptec
Airvast
Alfa
Allied Telesyn
Allnet
Alloy
Askey
Asus
Buffalo
Cameo
CC&C
Belkin
Cisco/Aironet
Compaq
CompuShack
D-Link
Dell
Delta Networks
Eusso
IST Wave
Fujitsu Siemens
GemTek
Global Sun
HP
IBM
Intel
LG
LinkPro
LinkSys
Microsoft
Motorola
Netgear
Nokia
Nortel
Proxim
Samsung
Sitecom
SMC
Sparklan
3Com
Tellus
W-Link
Z-Com
Zyxel
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Useful links & resources
William Stallings. Data and Computer Communication
Pejman Roshan, Jonathan Leary. 802.11 Wireless LAN Fundamentals.Cisco Press
Bruce Alexander. 802.11 Wireless Network Site Surveying andInstallation. Cisco Press
IEEE Wireless Standardshttp://standards.ieee.org/getieee802/802.11.html
WLAN forumhttp://www.wlanforum.com
Linux WLAN pagehttp://www.linux-wlan.org
NS-2 simulatorhttp://www.isi.edu/nsnam
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