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2/12/2002 1
IEEE 802.11 Wireless Local Area
Networks
The future is wireless
Presented by Tamer Khattab and George Wong
Prepared for EECE571N - Advanced Networking
2/12/2002 2
WLAN Technology Overview
Physical layer technologies Architecture Transmission
Medium access control technologies
2/12/2002 4
ArchitectureArchitecture (Cont.) (Cont.)
• Network connectivity life time
BSS 2
BSS 1
Distribution System
DS
AP
APSTASTA
STASTA
direct connection
Wired 802.3 LAN
Portal
•Infrastructure
2/12/2002 7
Transmission MediumTransmission Medium
• Radio frequency transmission• Narrow-band transmission
• Spread spectrum transmission• direct sequence
• frequency hopping
• Infrared transmission• Laser diode sources
• Light emitting diode sources
2/12/2002 8
Wireless LAN Standards
•IEEE 802.11 wireless LAN standards
•ETSI HIPERLAN wireless LAN standards
2/12/2002 11
IEEE 802.11 Physical Layer
Medium type 2.4 GHz FHSS (2400 - 2483.2 GHz) 2.4 GHz DSSS (2400 - 2483.2 GHz) Diffused infrared DFIR (850 - 950 nm)
Rates: basic=1 Mbps, enhanced=2 Mbps
2/12/2002 12
FHSS
Band 2400-2483.5 MHz GFSK (Gaussian Frequency Shift
Keying) Sub-channels of 1 MHz Only 79 channels of the 83 are used Slow hopping 3 main sets each with 26 different
hopping sequences
2/12/2002 13
FHSS (Cont.)
Sequences within same set collide at max. on 5 channels
Min. hopping distance of 6 channels. No CDMA within same BSS Coexisting BSS in the same coverage
area use different sequences from the same hopping set.
2/12/2002 14
FHSS (Cont.)
Frequency
Time
Hopping distance >= 6 sub-channels(The distance in frequency between two consecutive hops)Sub-channel
1 MHz
400 ms
2/12/2002 15
FHSS (Cont.)
MAC data could be at 1 Mb/s or 2 Mb/s
Sync pattern 80 bit
SFD16 bit
PLW12 bit
HEC16 bit
4 bit Payload data(variable length)
PSF
PLCP preamble PLCP header PLCP_PDU
Physical layer header and preamble always at 1 Mb/s
2/12/2002 16
DSSS
Band 2400-2483.5 MHz DBPSK (Differential Binary Phase Shift
Keying) Band divided into 11 overlapping channels
each with bandwidth 11 MHz Coexisting BSS in the same coverage area
use channels separated by at least 30 MHz. 11 bit Barker sequence is used for
spreading No CDMA used within one BSS
2/12/2002 17
DSSS (Cont.)
Frequency (MHz)
Channel number
11 MHz
11
10
9
8
7
6
5
4
3
2
1
2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462
2/12/2002 18
DSSS (Cont.)
Sync pattern 128 bit
SFD16 bit
SG8 bit
HEC16 bit
SR8 bit
Payload data(variable length)
PLCP preamble PLCP header PLCP_PDU
MAC data could be at 1 Mb/s or 2 Mb/s
LN16 bit
Physical layer header and preamble always at 1 Mb/s
2/12/2002 19
Infra Red
Wave length near visible light 850-950 nm
PPM (Pulse Position Modulation) Diffused transmission technique
used Only used for indoor transmission
2/12/2002 20
IEEE 802.11a
5 GHz (5.15-5.25, 5.25-5.35, 5.725-5.825GHz)
OFDM (Orthogonal Freq. Div. Multiplexing)
52 Subcarriers BPSK/QPSK/QAM Forward Error Correction
(Convolutional) Rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps
2/12/2002 21
IEEE 802.11b
2.4 GHZ band DSSS (11-chip) Rates 5.5 and 11 Mbps M-arry modulation. Convolutional Codes Shorter Preamble
2/12/2002 24
Overview of the Protocol Layers IEEE 802.11 specifies a MAC layer that is designed to operate over wireless
channel IEEE 802.11 is in the same protocol layer as the IEEE 802.3
IEEE 802.2 Logical Link Control (LLC)
MAC Layer
Data Link Layer
Network Layer
IEEE 802.3 Ethernet
IEEE 802.11 Wireless EthernetPhysical
Layer
IEEE 802.5 Token Ring
IEEE 802.4Token Bus
2/12/2002 25
IEEE 802.11 – CSMA/CA CS – Carrier Sense
Each transmitter listens to the physical link before transmitting
MA – Multiple Access Many nodes are connected to the same
physical link.
CA – Collision Avoidance Methods used to avoid collision
2/12/2002 26
CSMA/CA Why not CSMA/CD?
Difficult to detect collision in a radio environment
Radio environment is not as well controlled as a wired broadcast medium, and transmissions from users in other LANs can interfere with the operation of CSMA/CD
Radio LANs are subject to the hidden-station problem
2/12/2002 27
Hidden-Station Problem A knows the existence of B C knows the existence of B B knows the existence of A and C However, A does not know the existence of C
BA C
2/12/2002 28
Hidden-Station Problem Since A and C are sufficiently distant from each other that they cannot
hear each other’s transmission (Carrier Sense doesn’t work!) This condition will result in the transmissions from the two stations, A and
C, proceeding and colliding at the intermediate station B (However, A and C cannot hear the collision!)
2/12/2002 29
Hidden-Station Problem CSMA/CA medium access control was developed
to prevent this type of collision
Data Frame Data Frame
A transmits data frame
B
A B C
C transmits data frame and collides
with A at B
(a)
(b)
Data FrameCA
2/12/2002 30
Exposed Node Problem Suppose B is sending to A. C is aware of this
communication because it hears B’s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’ transmission
This is not a problem since C’s transmisstion to D will not interfere with A’s ability to receive from B
B CA D
2/12/2002 31
Collision Avoidance IEEE 802.11 address these two problems,
hidden-station and exposed node problems, with an algorithms called Multiple Access with Collision Avoidance
Sender and receiver exchange control frames with each other before the sender actually transmit any data
The sender transmits a Request to Send (RTS) frame to the receiver and the receiver then replies with a Clear to Send (CTS) frame
2/12/2002 32
Collision Avoidance RTS includes a field that indicates how
long the sender wants to hold the medium CTS reserves channel for sender, notifying
(possible hidden) station For Neighbors
See CTS: keep quite See RTS but no CTS: ok to transmit
Receiver sends ACK when it receives the frame Neighbors keep silent until seeing ACK
2/12/2002 34
UBC Wireless Network UBC is currently deploying
the wireless network http://www.UNP.ubc.ca/
“It’s changed students’ lives” Christopher Macdonald, School of Architecture
“It was like winning the lottery” Alan Steeves, Research Engineer