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IEEE802.11

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Intended for home or office use (primarily indoor) ... include AMD, Apple, Cisco, Compaq, Dell, Epson, Ericsson, Fujistu, Gateway, ... – PowerPoint PPT presentation

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Title: IEEE802.11


1
IEEE802.11

2
What is IEEE 802.11?
  • Standard for wireless local area networks
    (wireless LANs) developed in 1990 by IEEE
  • Intended for home or office use (primarily
    indoor)
  • 802.11 standard describes the MAC layer, while
    other substandards (802.11a, 802.11b) describe
    the physical layer
  • Wireless version of the Ethernet (802.3) standard

3
Wi-Fi Standard (802.11x)
  • Mission promote 802.11 interoperability as the
    global wireless LAN standard
  • Wi-Fi Board members include AMD, Apple, Cisco,
    Compaq, Dell, Epson, Ericsson, Fujistu, Gateway,
    HP, IBM, Intel, Microsoft, NEC, Nokia, Nortel,
    Philips, Samsung, Sharp, Sony, TDK, Toshiba,

4
802.11b LAN Configuration
Access Point Device
5
Example 802.11b Wireless LAN Equipment
6
Intended Use
  • Wireless Internet access inside hotel lobbies,
    conference rooms, etc.
  • Wireless with your
  • Latte?
  • Wireless home networking
  • Wireless at the Airport

7
Wi-Fi Market in the News
  • Wireless LAN equipment market
  • 969 Million in 2000 to estimated 4.5 Billion in
    2006
  • In 2001
  • Microsoft adds 802.11 in Windows XP
  • Major hotel chains install Wi-Fi Internet access
  • 462 Starbucks stores offer wireless Internet
  • Microsoft joins WECA board (the 802.11 alliance)
  • Intel Joins WECA board
  • Most PC/Laptop manufacturers offer Wi-Fi

8
802.11b Interoperability
  • The Wireless Ethernet Compatibility Alliance
    (WECA) certification provides a standard for
    wireless technology, ensuring interoperability of
    products from different vendors.

WECA Certification Mark
9
Competing Short-Range Wireless Technologies
  • Short-range wireless solutions
  • Bluetooth
  • 802.11 (Wi-Fi)
  • HomeRF (not as popular)
  • Who will prevail?
  • 802.11 more suitable for wireless LANs (office,
    hotel, airport,)
  • Bluetooth is designed for personal area networks
    smart appliances, printers, scanners, etc.

10
Commercial Wireless Data Communications
802.11a Technology
Near Future
Digital Cellular
Bluetooth Technology
Home RF
Proprietary Systems
802.11b Wireless LANs
Present
Cellular
11M
128K
721K
54M
19K
384K
Data Rate Bits per second
11
IEEE 802.11b Technology
  • Standard adopted by Institute of Electrical and
    Electronics Engineers (IEEE) in September 1999.
  • The standard is believed to be very good, will be
    widely accepted and will allow hardware prices to
    decrease.
  • Developed by consortium of major companies with
    focus on interoperability.
  • Optimized for wireless LANs.
  • Uses radio frequency signals in unlicensed 2.4GHz
    band to send and receive data.
  • Uses Direct Sequence Spread Spectrum (DSSS) RF
    method.
  • Equipment dynamically selects lower data rates as
    RF signal quality decreases 11, 5.5, 2, 1
    Mbits.
  • Allows roaming among radio access points.

12
IEEE 802.11b Technology More Specifically
  • 802.11b defines how the RF channel is used,
    allowing multiple devices to communicate on the
    channel as if it were a wire.
  • This in itself can form a standalone network as
    if wired devices were connected to a hub.
  • The Access Point sits on both the wireless
    network segment (space) and the wired segment,
    acting as a bridge from the wireless to the wired
    segments.
  • A bridge forwards data packets from one side to
    the other at the MAC layer.

13
Flavors of 802.11x
  • 802.11 (1 Mbps)
  • Older standard
  • 802.11b (11 Mbps)
  • Current technology
  • 802.11g (22 Mbps)
  • Expected future standard

14
802.11 Under the Hood (Wireless Ethernet)
  • Why cant we use regular Ethernet for wireless?
  • Ethernet A sees B, B sees C, ? A sees C
  • Wireless Hidden node problem
  • A sees B, B sees C, yet A does not see C

C
A
B
15
802.11 Under the Hood (Wireless Ethernet)
  • Why cant we use regular Ethernet for wireless?
  • Ethernet B sees C, C sees D ? B C cant send
    together
  • Wireless B can send to A while C sends to D

B
C
D
A
16
802.11 Under the Hood The Protocol
  • Sender A sends Request-to-Send (RTS)
  • Receiver B sends Clear-to-Send (CTS)
  • Nodes who hear CTS cannot transmit concurrently
    with A (red region)
  • Nodes who hear RTS but not CTS can transmit
    (green region)
  • Sender A sends data frame
  • Receiver B sends ACK
  • Nodes who hear the ACK can
  • now transmit

CTS
RTS
B
A
17
802.11 Collision Resolution
  • Two senders might send RTS at the same time
  • Collision will occur corrupting the data
  • No CTS will follow
  • Senders will time-out waiting for CTS and retry
    with exponential backoff

18
More Wireless Challenges
  • Problem mobility
  • Nodes can move and get out of touch
  • How to deal with intermittent connectivity?
  • Solution hierarchical structure
  • A set of access points hooks into a wired
    distribution system
  • Access points cover the target area
  • Roaming nodes connect to nearest access point at
    any given time

19
Hierarchical Structure
  • Communication between roaming nodes is via the
    distribution system

Distribution System
A
E
F
AP1
AP3
AP2
C
D
B
20
Choosing the Access Point Active Scanning
  • Roaming node periodically sends probe frame
  • All APs within range reply with a probe response
    frame
  • Roaming node chooses best AP and replies to it
    with association request
  • AP acknowledges request with association
    response. AP notifies previous AP (if any) of the
    established association (handover).

21
Choosing the Access Point Passive Scanning
  • APs advertise themselves periodically by sending
    beacon frames
  • Roaming nodes can associate themselves with an AP
    by sending the association request
  • AP acknowledges request as before with an
    association response

22
Frame Format
Immediate Sender (AP3)
Intermediate Destination (AP1)
Source (A)
Ultimate Destination (E)
Control
Duration
Addr1
Addr2
Addr3
Addr4
Control
Data
CRC
Distribution System
Frame Type (RTS,CTS,) ToDS FromDS
A
E
F
AP1
AP3
AP2
C
D
B
23
Physical Properties
  • 802.11
  • Frequency hopping
  • Direct sequence
  • Diffused infrared

24
Range from Access Point to Computer
100 Feet
Access Point
5.5Mb
11 Mb
Computers
11 Mbits
5.5Mb
2 Mb
1 Mb
0 Mb
Range 0 to 700 feet
25
Electrical Engineering Building
3rd
2nd
1st
Basement
26
Network Setup
  • Basic Network Setup is Cellular
  • Mobile Terminals (MT) connect with Access Points
    (AP)
  • Standard also supports ad-hoc networking where
    MTs talk directly to MTs

27
IEEE 802.11 Physical Layers
802.11b 802.11a
Modulation Scheme DSSS OFDM
Spectrum (GHz) 2.4 2.485 5.15-5.35, 5.725-5.825
Data Rate (Mbps) 1 11 6 - 54
Subchannels 11 overlapping 8 independent
Interference Microwave, Cordless Phones,Bluetooth, HomeRF, Light Bulbs! HyperLAN II
Availability Today Late August?
Cost 250 AP, 100 PC Card ??? (same)
28
Media Access Control- Ethernet
  • CSMA/CD (Carrier Sense Multiple Access with
    Collision Detection)
  • If media is sensed idle, transmit
  • If media is sensed busy, wait until idle and then
    transmit immediately
  • Collisions can occur if more than one user
    transmits at the same time
  • If a collision is detected, stop transmitting.
  • Reschedule transmission according to exponential
    backoff

29
Media Access Control (802.11)
  • Would like to use CSMA
  • Nice for bursty traffic
  • Make for seamless replacement of wired LANs with
    wireless LANS
  • Use CSMA, but cant use CD
  • PT/PR ratio is too high
  • Dont want to waste energy on mobiles
  • Use Collision Avoidance instead
  • Dont always start transmitting immediately after
    someone else

30
CSMA/CA Details
  • SIFS (Short Interframe Space)
  • DIFS (Distributed Interframe Space)
  • Scenario
  • B and C want to transmit, but A currently has
    control of medium
  • B randomly selects 7 slots of backoff, C selects
    4 slots
  • C transmits first, then B

31
What is HIPERLAN/2?
  • European standard developed by ETSI/BRAN
    (European Telecommunications Standards
    Institute/Broadband Radio Access Networks)
  • Physical Layer is very similar to 802.11a (OFDM
    operating in the 5 GHz spectrum)
  • Standard based on wireless ATM (Asynchronous
    Transfer Mode)

32
HIPERLAN/2 MAC
BCH Miscellaneous header FCH Details how the
DL and UL phases will be allocated ACH Feedback
on which resource requests were received RCH
Random access resource request
33
Performance Comparison
34
Conclusions
  • IEEE 802.11 is a widely accepted standard in the
    United States for wireless LANs
  • Primarily a cellular random access scheme with
    provisions for ad hoc networking and contention
    free access
  • 802.11b products are available now, but better to
    wait for 802.11a products later this year
  • HIPERLAN/2 is being pushed in Europe
  • Wireless ATM solution for real-time traffic
  • Standard reflects the network topology
  • There is an effort to agree on one world-wide
    standard, keep your fingers crossed
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