IEEE802.11
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
Wi-Fi Standard (802.11x)
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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,
802.11b LAN Configuration
Access Point
Device
Example 802.11b Wireless
LAN Equipment
Intended Use
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Wireless Internet access inside hotel
lobbies, conference rooms, etc.
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Wireless at the Airport
Wireless with your
Latte?
Wireless home networking
Wi-Fi Market in the News
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Wireless LAN equipment market
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In 2001:
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$969 Million in 2000 to estimated $4.5 Billion in 2006
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
802.11b Interoperability
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The Wireless Ethernet
Compatibility Alliance
(WECA) certification
provides a standard for
wireless technology,
ensuring interoperability of
products from different
vendors.
WECA Certification Mark
Competing Short-Range
Wireless Technologies
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Short-range wireless solutions:
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Bluetooth
802.11 (Wi-Fi)
HomeRF (not as popular)
Who will prevail?
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802.11 more suitable for wireless LANs
(office, hotel, airport,…)
Bluetooth is designed for personal area
networks – smart appliances, printers,
scanners, etc.
Commercial Wireless
Data Communications
802.11a
Technology
Near
Future
Digital Cellular
Bluetooth
Technology
Present
Cellular
19K
Home RF
Proprietary
Systems
802.11b
Wireless LANs
128K
Data Rate
384K
721K
Bits per second
11M
54M
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,
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.
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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.
Flavors of 802.11x
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802.11 (1 Mbps)
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802.11b (11 Mbps)
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Older standard
Current technology
802.11g (22+ Mbps)
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Expected future standard
802.11 Under the Hood
(Wireless Ethernet)
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Why can’t 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
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A
C
B
802.11 Under the Hood
(Wireless Ethernet)
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Why can’t we use regular Ethernet for
wireless?
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Ethernet: B sees C, C sees D  B & C can’t
send together
Wireless: B can send to A while C sends to D
B
A
C
D
802.11 Under the Hood
The Protocol
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Sender A sends Request-to-Send (RTS)
Receiver B sends Clear-to-Send (CTS)
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Nodes who hear CTS cannot transmit concurrently with A
(red region)
 Nodes who hear RTS but not CTS can transmit (green
region)
CTS
Sender A sends data frame
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Receiver B sends ACK
Nodes who hear the ACK can
now transmit
RTS
A
B
802.11 Collision Resolution
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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
More Wireless Challenges
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Problem: mobility
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Nodes can move and get “out of touch”
How to deal with intermittent connectivity?
Solution: hierarchical structure
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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
Hierarchical Structure:
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Communication between roaming nodes
is via the distribution system
Distribution System
A
E
AP1
B
C
AP2
F
AP3
D
Choosing the Access Point
Active Scanning
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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).
Choosing the Access Point
Passive Scanning
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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
Frame Format
Immediate
Sender (AP3)
Intermediate
Ultimate
Destination
Destination (E)
(AP1)
Control Duration Addr1 Addr2 Addr3 Control Addr4
A
E
B
C
AP2
F
Data
CRC
Frame Type (RTS,CTS,…)
ToDS
FromDS
Distribution System
AP1
Source
(A)
AP3
D
Physical Properties
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802.11
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Frequency hopping
Direct sequence
Diffused infrared
Range from Access Point to
Computer
100 Feet
5.5Mb
Access Point
11 Mb
Computers
0 Mb
11 Mbits
1 Mb
2 Mb
5.5Mb
Range = 0 to 700 feet
Electrical Engineering Building
3rd
2nd
1st
Basement
Network Setup
•Basic Network Setup is Cellular
•Mobile Terminals (MT) connect with Access Points (AP)
MT
AP
MT
AP
MT
AP
Ethernet
•Standard also supports ad-hoc networking where MT’s
talk directly to MT’s
IEEE 802.11 Physical Layers
802.11b
802.11a
Modulation Scheme DSSS
OFDM
Spectrum (GHz)
2.4 – 2.485
5.15-5.35, 5.7255.825
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)
Media Access Control- Ethernet
Desktop System
Desktop System
Desktop System
Ethernet
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CSMA/CD (Carrier Sense Multiple Access with
Collision Detection)
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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
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If a collision is detected, stop transmitting.
Reschedule transmission according to exponential backoff
Media Access Control (802.11)
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Would like to use CSMA
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Use CSMA, but can’t use CD
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Nice for bursty traffic
Make for seamless replacement of wired LANs with wireless
LANS
PT/PR ratio is too high
Don’t want to waste energy on mobiles
Use Collision Avoidance instead
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Don’t always start transmitting immediately after someone
else
CSMA/CA Details
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SIFS (Short Interframe Space)
DIFS (Distributed Interframe Space)
Packet A
B
C
ACK
SIFS
Packet C
DIFS
ACK
SIFS
Packet B
DIFS
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
What is HIPERLAN/2?
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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)
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
Performance Comparison
Conclusions
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IEEE 802.11 is a widely accepted standard in the
United States for wireless LANs
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HIPERLAN/2 is being pushed in Europe
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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
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