IEEE 802.11 Wireless Local Area Networks - PowerPoint PPT Presentation

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IEEE 802.11 Wireless Local Area Networks

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IEEE 802.11 Wireless Local Area Networks The future is wireless Presented by Tamer Khattab and George Wong Prepared for EECE571N - Advanced Networking – PowerPoint PPT presentation

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Title: IEEE 802.11 Wireless Local Area Networks


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
WLAN Technology Overview
  • Physical layer technologies
  • Architecture
  • Transmission
  • Medium access control technologies

3
Architecture
  • Network connectivity life time
  • Ad-hoc

4
Architecture (Cont.)
  • Network connectivity life time
  • Infrastructure

5
Architecture (Cont.)
  • Connection type
  • Point-to-point

6
Architecture (Cont.)
  • Connection type
  • Broadcast

7
Transmission Medium
  • Radio frequency transmission
  • Narrow-band transmission
  • Spread spectrum transmission
  • direct sequence
  • frequency hopping
  • Infrared transmission
  • Laser diode sources
  • Light emitting diode sources

8
Wireless LAN Standards
  • IEEE 802.11 wireless LAN standards
  • ETSI HIPERLAN wireless LAN standards

9
Wireless LAN Standards
  • IEEE 802.11 wireless LAN standards

10
Physical Layer
  • IEEE 802.11 Wireless Ethernet

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 basic1 Mbps, enhanced2 Mbps

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

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.

14
FHSS (Cont.)
Time
400 ms
Frequency
1 MHz
15
FHSS (Cont.)
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

17
DSSS (Cont.)
11 MHz
Channel number
11 10 9 8 7 6 5 4 3 2 1
Frequency (MHz)
2412 2417 2422
2427 2432 2437 2442
2447 2452 2457
2462
18
DSSS (Cont.)
19
Infra Red
  • Wave length near visible light 850-950 nm
  • PPM (Pulse Position Modulation)
  • Diffused transmission technique used
  • Only used for indoor transmission

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

21
IEEE 802.11b
  • 2.4 GHZ band
  • DSSS (11-chip)
  • Rates 5.5 and 11 Mbps
  • M-arry modulation.
  • Convolutional Codes
  • Shorter Preamble

22
Product Samples
2.4 GHz FHSS ISA
2.4 GHz FHSS PCMCIA
5 GHz DSSS PCMCIA
23
MAC Layer
  • IEEE 802.11 Wireless Ethernet

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

Network Layer
Data Link Layer
IEEE 802.2 Logical Link Control (LLC)
IEEE 802.3 Ethernet
IEEE 802.11 Wireless Ethernet
IEEE 802.5 Token Ring
IEEE 802.4 Token Bus
MAC Layer
Physical Layer
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

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

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

B
A
C
28
Hidden-Station Problem
  • Since A and C are sufficiently distant from each
    other that they cannot hear each others
    transmission (Carrier Sense doesnt 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!)

29
Hidden-Station Problem
  • CSMA/CA medium access control was developed to
    prevent this type of collision

30
Exposed Node Problem
  • Suppose B is sending to A. C is aware of this
    communication because it hears Bs 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 Cs transmisstion to
    D will not interfere with As ability to receive
    from B

B
C
A
D
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

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

33
Collision Avoidance
B
C
A
D
34
UBC Wireless Network
  • UBC is currently deploying the wireless network
  • http//www.UNP.ubc.ca/
  • Its changed students lives Christopher
    Macdonald, School of Architecture
  • It was like winning the lottery Alan Steeves,
    Research Engineer

35
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