Transcript 802.11 WLAN technologies   IEEE 802.11 standards and rates  IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band )  IEEE 802.11b (1999)

802.11 WLAN technologies
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IEEE 802.11 standards and rates
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IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band )
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IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi
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IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz
band)
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IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz)
backward compatible to 802.11b
IEEE 802.11 networks work on license free industrial, science,
medicine (ISM) bands:
26 MHz
902
928
83.5 MHz
2400
2484
100 mW
200 MHz
5150
5350
255 MHz
5470
200 mW
indoors only
5725 f/MHz
1W
1
802.11 Logical architecture
LLC
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provides addressing and data link control
provides common interface, reliability, and flow control
MAC provides
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frames and headers
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access to wireless medium
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joining the network
authentication & privacy
Network
LLC
MAC
FHSS DSSS IR
PHY
802.11
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CSMA/CA
Three physical layers (PHY)
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Frames
Modulation
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FHSS: Frequency hopping spread spectrum
DSSS: Direct Sequence SS
OFDM: Orthogonal frequency division multiplexing
2
802.11 - Layers and functions
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MAC
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access mechanisms,
fragmentation, encryption
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MAC Management
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PLCP Physical Layer Convergence Protocol
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synchronization, roaming,
MIB, power management
PMD Physical Medium Dependent
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MAC Management
PLCP
PHY Management
PMD
Station Management
DLC
PHY
MAC
channel selection, MIB
Station Management
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LLC
modulation, coding
PHY Management
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clear channel assessment
signal (carrier sense)
coordination of all
management functions
3
DSSS – 802.11b
DS-transmitter

Chipping code: Bit pattern substituted for original
transmission bits
 Advantages of using DSSS with a chipping code:
 Error correction
 Each device assigned unique chipping code
 Security
4
802.11 - Layers and functions
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MAC
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access mechanisms,
fragmentation, encryption
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MAC Management
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PLCP Physical Layer Convergence Protocol
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synchronization, roaming,
MIB, power management
PMD Physical Medium Dependent
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MAC Management
PLCP
PHY Management
PMD
Station Management
DLC
PHY
MAC
channel selection, MIB
Station Management

LLC
modulation, coding
PHY Management


clear channel assessment
signal (carrier sense)
coordination of all
management functions
5
PLCP (802.11b)
Note:
To send even one bit payload
reliably, you will have to form
a packet with the PLCP preamble
and the PLCP header.
* PLCP - Physical Layer Convergence Procedure
** PMD - Physical Medium Dependent
6
802.11 - Carrier Sensing
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In IEEE 802.11, carrier sensing is performed
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at the air interface (physical carrier sensing)
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detects presence of other users by analyzing all
detected packets
Detects activity in the channel via relative signal
strength from other sources
at the MAC layer (virtual carrier sensing)
done by sending MPDU duration information in the
header of RTS/CTS and data frames
Channel is busy if either mechanisms indicate it to be
Duration field indicates the amount of time (in
microseconds) required to complete frame transmission
Stations in the BSS use the information in the duration
field to adjust their network allocation vector (NAV)
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7
802.11 DCF – basic access
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If medium is free for DIFS time, station sends 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 –RTS/CTS
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If medium is free for DIFS, station can send RTS with reservation
parameter
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
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
data
t
contention
9
Bit rates and modulation in 802.11b
Modulation
Bit rate
DBPSK
DQPSK
CCK
CCK
1 Mbit/s
2 Mbit/s
5.5 Mbit/s
11 Mbit/s
DB/QPSK = Differential
Binary/Quaternary PSK
CCK = Complementary
Code Keying
Defined in
802.11
Defined in
802.11b
Automatic fall-back to a
lower bit rate if channel
becomes bad
10
OFDM – 802.11g
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With multipath distortion, receiving device must wait until
all reflections received before transmitting
 Puts ceiling limit on overall speed of WLAN
OFDM: Send multiple signals at same time
 Split high-speed digital signal into several slower
signals running in parallel
OFDM increases throughput by sending data more slowly
Avoids problems caused by multipath distortion
Used in 802.11g networks
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OFDM – 802.11g (continued)
Orthogonal frequency division multiplexing (OFDM) vs. singlechannel transmissions
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Bit rates and modulation in 802.11g
Modulation
Bit rate
Coding
rate
Coded bits
/ symbol
Data bits
/ symbol
BPSK
BPSK
QPSK
QPSK
16-QAM
16-QAM
64-QAM
64-QAM
6 Mbit/s
9 Mbit/s
12 Mbit/s
18 Mbit/s
24 Mbit/s
36 Mbit/s
48 Mbit/s
54 Mbit/s
1/2
3/4
1/2
3/4
1/2
3/4
2/3
3/4
48
48
96
96
192
192
288
288
24
36
48
72
96
144
192
216
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IEEE 802.11g and 802.11b
interworking (1)
802.11g and 802.11b interworking is based on two
alternatives regarding the 802.11g signal structure:
Preamble/Header
Payload
802.11b
DSSS
DSSS
802.11g, opt.1
DSSS
OFDM
802.11g, opt.2
OFDM
OFDM
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IEEE 802.11g and 802.11b
interworking (2)
Option 1 (*): The preamble & PLCP header part of 802.11g
packets is based on DSSS (using BPSK at 1 Mbit/s or QPSK
at 2 Mbit/s), like 802.11b packets.
802.11g and 802.11b stations compete on equal terms for
access to the channel (CSMA/CA). However, the 802.11g
preamble & header is rather large (compared to option 2).
802.11g, opt.1
802.11g, opt.2
DSSS
OFDM
OFDM
OFDM
(*) called DSSS-OFDM in the 802.11g standard
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IEEE 802.11g and 802.11b
interworking (3)
Option 2 (*): The preamble & header of 802.11g packets
is based on OFDM (using BPSK at 6 Mbit/s).
Now, 802.11b stations cannot decode the information in
the 802.11g packet header and the CSMA/CA scheme will
not work properly. Solution: Stations should use the
RTS/CTS mechanism before transmitting a packet.
802.11g, opt.1
802.11g, opt.2
DSSS
OFDM
OFDM
OFDM
(*) called ERP-OFDM (ERP = Extended Rate PHY) in the 802.11g standard
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802.11 - Layers and functions

MAC



access mechanisms,
fragmentation, encryption
MAC Management

synchronization, roaming,
MIB, power management
PLCP Physical Layer Convergence Protocol

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PMD Physical Medium Dependent


MAC Management
PLCP
PHY Management
PMD
Station Management
DLC
PHY
MAC
channel selection, MIB
Station Management
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LLC
modulation, coding
PHY Management


clear channel assessment
signal (carrier sense)
coordination of all
management functions
17
802.11 - MAC management
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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
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802.11 - Synchronization
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All STAs within a BSS are synchronized to a common clock
 Infrastructure mode: AP is the timing master
 periodically transmits Beacon frames containing Timing
Synchronization function (TSF)
 Receiving stations accepts the timestamp value in TSF
 Ad hoc mode: TSF implements a distributed algorithm
 Each station adopts the timing received from any beacon
that has TSF value later than its own TSF timer
This mechanism keeps the synchronization of the TSF timers in a
BSS to within 4 s plus the maximum propagation delay of the
PHY layer
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802.11 - Power management
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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?)
20
802.11 - Roaming
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Bad connection in Infrastructure mode? Perform:
scanning of environment
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listen into the medium for beacon signals or send probes into the
medium and wait for an answer
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send Reassociation Request
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receive Reassociation Response
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station sends a request to a new AP(s)
success: AP has answered, station can now participate
failure: continue scanning
AP accepts Reassociation Request and
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signals 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 Mobility
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Standard defines the following mobility types:
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No-transition: no movement or moving within a local BSS
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BSS-transition: station movies from one BSS in one ESS to another
BSS within the same ESS
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ESS-transition: station moves from a BSS in one ESS to a BSS in a
different ESS (continuos roaming not supported)
Especially: 802.11 don’t support roaming with GSM!
- Address to destination
mapping
- seamless integration
of multiple BSS
ESS 1
ESS 2
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802.11 MAC frame format
Data 0 - 2312 FCS
PHY IEEE 802.11
bytes
2
Frame
control
2
6
6
6
2
6
Duration/ Address Address Address Seq. Address
ID
1
2
3
control
4
0-2312
4
Frame body
FCS
MAC header
Protocol Type
version
bits
2
2
Sub-type
bits
4
Sub-type Info
12
To
DS
1
From
DS
1
More Retry Pwr
Frag
Mgmt
1
1
1
More WEP Order
Data
1
1
1
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Field explanations
To DS
From DS
Message
0
0
station-to-station frames in an IBSS;
all mgmt/control frames
0
1
From AP to station
1
0
From station to AP
1
1
From one AP to another on DS
To DS
From DS
Address 1
Address 2
Address 3
Address 4
0
0
DA
SA
BSSID
N/A
0
1
DA
BSSID
SA
N/A
1
0
BSSID
SA
DA
N/A
1
1
RA
TA
DA
SA
RA: Receiver Address TA: Transmitter Address
DA: Destination Address SA: Source Address
BSSID: MAC address of AP in an infrastructure BSS
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AP1
MAC
A
MAC
B
MAC
C
25
AP1
MAC
A
MAC
B
MAC
C
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