Transcript Lecture 12: 802.11 WLAN’s and Other Recent Technological

Lecture 11:
802.11 WLAN’s and Other Recent
Technological Developments
 This lecture provides discussion of the latest
technological advances in the following areas.
 Wireless Local Area Networks based on the IEEE 802.11
family of standards
 IEEE 802.16 and 802.20
 Fourth generation cellular systems
 Emerging technologies considered to be most promising in
the further development of wireless technologies
 OFDM
 Ultra Wideband
 Space-time processing
 The goal is to give insight into areas of potential
research and economic development.
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I. The IEEE 802 Family of Standards
 The Institute of Electrical and Electronics
Engineers
 A technical, professional, and student society.
 Publishes many journals and magazines.
 Also has developed a few technical standards.
 Most notably Local Area Network standards.
 Ethernet (802.3) and others.
 802.11 is the working group for Wireless LAN’s
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 Created by the IEEE LAN /MAN Standards Committee
(LMSC)
 Started in 1980
 Working Groups
 802.1 High Level Interface (HILI) Working Group (active)
 802.2 Logical Link Control (LLC) Working Group
(hibernating)
 802.3 CSMA/CD Working Group (active) – Ethernet,
standard for wired LAN’s
 802.4 Token Bus Working Group (hibernating)
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 802.5 Token Ring Working Group (hibernating)
 802.6 Metropolitan Area Network (MAN)
Working Group (hibernating)
 802.7 Broadband Technical Adv. Group (BBTAG)
(hibernating)
 802.9 Integrated Services LAN (ISLAN) Working
Group (hibernating)
 802.10 Standard for Interoperable LAN Security
(SILS) Working Group (hibernating)
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 ** 802.11 Wireless LAN (WLAN) Working Group (active)
 802.12 Demand Priority Working Group (hibernating)
 802.14 Cable-TV Based Broadband Communication
Network Working Group (disbanded, no publications)
 802.15 Wireless Personal Area Network (WPAN) Working
Group (active)
 ** 802.16 Broadband Wireless Access (BBWA) Working
Group (active)
 802.17 Resilient Packet Ring (RPR) (active)
 802.18 Radio Regulatory Technical Advisory Group (active)
 802.19 Coexistence Technical Advisory Group (active)
 ** 802.20 Mobile Wireless Access Working Group (active)
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 IEEE 802.11 Wireless LAN’s
 Source: Andrew S. Tanenbaum, Computer
Networks, Fourth Edition, Prentice Hall, 2003.
Pages 68-71, 267-270, and 292-302.
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II. Background
 As stated before, 802.11 WLAN’s are prime
competitors for providing high speed data access
within buildings, including public places like
airports and restaurants.
 802.11 was first standardized in 1997.
 802.11 – 1 Megabit per second (Mbps) and 2 Mbps
capabilities.
 In unlicensed 2.4 GHz band (ISM).
 802.11b (1999) – 11 Mbps at 2.4 GHz
 802.11a (1999) – 54 Mbps at 5.8 GHz
 802.11g (2001) – 54 Mbps at 2.4 GHz
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III. 802.11 Operation
 Two operating modes
1. With a base station
 Base station is called an access point.
2. Without a base station
 Computers talk to each other directly
 Ad hoc networking approach.
 Defines how devices cooperate without a central
controller.
 Especially concerned with how to cope with
packet collisions.
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 Compatibility with Ethernet
 Since Ethernet was a very popular LAN standard
(IEEE 802.3) for wired environments, 802.11 was
made compatible with it.
 802.11 Physical Layers
 802.11 – 3 modes – 1 to 2 Mbps in 2.4 GHz band
 Infrared
 FHSS
 DSSS
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 Infrared
 0.85 to 0.95 microns, 1 Mbps or 2 Mbps
 FHSS
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Frequency Hopped Spread Spectrum
79 channels
Each 1 MHz wide
Dwell time less than 400 msec
 DSSS
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Direct Sequence Spread Spectrum
1 Mega symbols per second (Megabaud)
1 Mbps is one bit per symbol using Differential BPSK
2 Mbps is two bits per symbol using Differential QPSK
11 chips per symbol (11 Mega chips per second)
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 Uses a bandwidth of 22 MHz per channel
 No security from DSSS, since all stations use the
same chip sequence
 Allows 11 frequency channels to be used in the 2.4
GHz ISM band.
 Channels are spaced 5 MHz apart and overlap
 Overlapping coverage areas should use different
channels.
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 802.11a – 54 Mbps in the 5.8 GHz band
 Uses OFDM (Orthogonal Frequency Division
Multiplexing)
 More on OFDM later in the lecture
 48 frequencies each at 250,000 symbols per second
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 802.11b – up to 11 Mbps in the 2.4 GHz band
 Not a follow-up to 802.11a
 The 802.11b standard was approved first and got to
market first.
 1, 2, 5.5, and 11 Mbps
 Rate may be adapted to achieve best performance
under current noise and load.
 In practice, 11 Mbps is nearly always used.
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 Uses HR-DSSS (High rate DSSS)
 1 and 2 Mbps rates use the same technique as 802.11
 5.5 and 11 Mbps run at 1.375 Mbaud with 4 or 8 bits
per symbol
 Better range than 802.11a
 About 7 times larger range
 Named “Wi-Fi” by the Wireless Ethernet
Compatibility Alliance (www.wi-fi.com)
 Goal is to promote interoperability between vendors’
products.
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 802.11g – 54 Mbps in the 2.4 GHz band
 Two upgrade options from 802.11b.
 First upgrade: Add a 256-state convolutional code
to 802.11b CDMA.
 Creates rates of 22 and 33 Mbps.
 Higher rates are possible because of the coding gain
 Second upgrade. Uses OFDM like 802.11a.
 Uses direct sequence SSM for the header, then OFDM
for the payload.
 Payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54
Mbps.
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 Problems Unique to Wireless LAN’s
 Traditional Ethernet LAN’s
 Listen until the channel is not busy
 Send a message
 If it collides with another message, wait a random
time then retry.
 Called CSMA/CD (Carrier Sense Multiple Access with
Collision Detection)
 Assumes all stations can hear all the other
transmissions
 Assumes that a collision can be detected.
 But not all collisions can be detected when using
wireless. Additional challenges in Wireless
LAN’s
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 Problem: All users may not be able to hear each
other.
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 Problem (a): Hidden node problem
 C is sending to B.
 A cannot hear C and thinks it could also transmit to B.
 A’s and C’s packets will collide at B.
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 Problem (b): Exposed node problem.
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A is transmitting to station X.
If B listens, it will think the radio channel is busy.
So it will falsely conclude it cannot send to C.
But C would hear no interference if B sent a packet
to it.
 C would not also hear the one from A, since C is
out of range from A.
 So, B could have transmitted but will not.
 Note: B can send to C, but cannot receive from C.
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 Solution: RTS and CTS
 Potential senders send a Request to Send (RTS).
 Tells how long of a message it wishes to send.
 Potential receiver sends a Clear to Send (CTS) in
response.
 Also tells how long of a message will be sent.
 Assumption: If I hear something from Y, I am in Y’s
range and Y is in mine.
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 How does this RTS/CTS approach solve the hidden
node problem?
 How does this solve the exposed node problem?
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The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.
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 C is within range of A but not within range of B.
 It hears the RTS from A but not the CTS from B.
 As long as it does not interfere with the CTS, it is
free to transmit while the data frame is being sent.
 solve the exposed node problem
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 D is within range of B but not A.
 It does not hear the RTS but does hear the CTS.
 Hearing the CTS tips it off that it is close to a
station that is about to receive a frame, so it defers
sending anything until that frame is expected to be
finished.
 solve the hidden node problem
 E hears both control messages and, like D, must
be silent until the data frame is complete.
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 Notes:
 This assumes all stations have the same range.
 Collisions still might occur between RTS messages.
 For example, B and C could both send RTS frames to
A at the same time, These will collide and be lost.
 The RTS/CTS procedure slows down
communications somewhat.
 Summary:
 Hear a CTS, don’t send.
 Only hear an RTS, assume okay
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 This approach is called CSMA/CA
 Carrier Sense Multiple Access (CSMA)
 Stations listen to the channel
 Collision Avoidance (CA)
 RTS/CTS are used to prevent collisions of data packets
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 802.11 Distributed Coordination Function (DCF)
 Used when there are no access points (ad hoc mode)
 Uses CSMA/CA
 The figure below shows the timing for all stations.
 When A is sending a packet to B.
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 A sends RTS, waits for CTS, sends data, then waits for
Acknowledgement (ACK).
 B sends CTS and then ACK when done.
 C hears the RTS
 Learns the intended length of the transmission
 Creates an internal Network Allocation Vector (NAV) from
this information.
 NAV tells C how long to wait until it should try to send its
own RTS.
 Note: If C does not also hear the CTS, it can abandon its
NAV.
 D hears the CTS (not the RTS)
 D also uses a NAV from info in the CTS.
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 A fragment burst
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 802.11 Point Coordination Function (PCF)
 The access point polls other stations to give those
stations a chance to send something.
 No collisions occur, so CSMA/CA is not needed
here.
 DCF and PCF are used simultaneously.
 This is done by coordinating the amount of time
between successive messages.
 Different amounts of dead time are required
between messages.
 Messages in 802.11 are called frames.
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 Interframe spacing in 802.11
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 Message waiting times
 Short InterFrame Spacing (SIFS)
 To next control message (ACK, CTS, etc.)
 Or next fragment in a block of fragments all being sent
in succession.
 Only one station is expected to send one of these
messages.
 PCF InterFrame Spacing (PIFS)
 Now the base station (access point) is allowed to try to
send a polling message.
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 DCF InterFrame Spacing (DIFS)
 Now any station can send an RTS to attempt
to grab the channel.
 If a collision of RTS occurs, stations wait a
random amount of time and try again.
 Extended InterFrame Spacing (EIFS)
 Used for a station to tell that it has received a
bad or unknown frame.
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 PCF will not always transmit when it has a
chance.
 This would starve DCF.
 A time interval is defined.
 In the first part of the superframe, the AP polls in a
round-robin fashion all stations configured for
polling.
 The AP idles for the remainder of the superframe.
 Which allows DCF.
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The 802.11 data frame.
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 802.11 Services
 Several services are provided by 802.11 to perform
necessary functions.
 Distribution Services – Related to stations
connecting with base stations.
 Association - to connect to base stations.
 Disassociation - to disassociate with base stations
 Reassociation - change a preferred base station,
without losing data in the handover.
 Distribution - determine how to route frames sent to
the base station.
 Integration - handles translation from the 802.11
format into another format required by a destination
network.
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 Station Services - For activity during
communications
 Authentication - stations identify themselves as
valid before being permitted to send data.
 Deauthentication - to make sure a user that leaves
can no longer use the network.
 Privacy - encryption capabilities to keep
information sent over a wireless LAN confidential.
 Data delivery - ways to transmit and receive data
as has been discussed already.
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 802.11 is working on several security issues.
 To get people to use the security features.
 To make them easier to use.
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V. IEEE 802.16
 IEEE 802.16 - Broadband Wireless Access
(BBWA) Working Group
 Called the IEEE 802.16 WirelessMAN Standard
 Published April 2002.
 Designed as an alternative to fiber, cable modems,
or DSL.
 Much quicker to deploy and potentially less costly.
 Consists of point-to-multipoint connections between
end locations and base stations located on buildings or
poles.
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 Operates in various frequencies in the range of
10 to 66 GHz.
 Uses line-of-sight connections. What are the
benefits and drawbacks of using line-of-sight?
 Antennas would need to be installed on the outside
of a building.
 The higher the frequency, the more difficult to
penetrate through walls, vegetation, etc.
 Some non line-of-sight is being considered in an
amendment for 2-11 GHz (802.16a).
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 Range and Data Rate
 Range: Up to 31 miles.
 Data Rate: 70 Mbps.
 Quality of Service
 The standard defines different handling of packets,
depending on whether they are voice/video or data.
 Modulation is Adaptive
 Adjusted almost instantaneously for optimal data transfer.
 Uses Reed-Solomon block coded FEC.
 In combination with QPSK, 16-QAM, or 64-QAM.
 Also uses a convolutional code to protect critical data, such
as frame control and initial accesses.
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VI. IEEE 802.20
 IEEE 802.20 - Mobile Broadband Wireless
Access (MBWA) Working Group
 Goals
 Packet based air interface
 Optimized for the transport of Internet Protocol
based services.
 Affordable, ubiquitous, always-on and
interoperable multi-vendor mobile broadband
wireless access networks.
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 Scope
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Licensed bands below 3.5 GHz.
Greater than 1 Mbps.
Vehicle mobility up to 250 km/hr.
Seeks “spectral efficiencies, sustained user data
rates and numbers of active users that are all
significantly higher than achieved by existing
mobile systems.”
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 Flash OFDM
 A very interesting technology for possible use in
MBWA.
 By Flarion
(http://www.flarion.com/products/flash_ofdm.asp).
 Claims:
 3 times greater physical layer capacity by using
OFDM instead of CDMA.
 Dedicated bandwidth to each flow for QoS
 Adaptive error control coding.
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 Main ideas with MBWA
 Design networks for data first.
 Support voice as a data service.
 Protect voice quality using special packet
prioritization mechanisms.
 Can achieve substantial increases in spectral
efficiency.
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VII. OFDM
 Orthogonal Frequency Division Multiplexing
 Enabled by new capabilities for hardware-based
Digital Signal Processing.
 Instead of transmitting one signal in a frequency
band, transmit many signals at different carriers.
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 Each one is narrower bandwidth
 With lower bit rate.
 Small frequency spacing between carriers.
 And overlap is allowed because the carriers are
chosen carefully (to be orthogonal).
 Conceptual picture:
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Specification from the 802.11a standard:
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 Benefits
 Each signal has longer symbol time because of a
lower bit rate.
 Multipath delay spread is not significant compared to
these long symbol times
 Makes spectrum usage more efficient.
 Can individually adjust modulation and power for each
signal as needed.
 Better immunity to narrowband interference, since
narrowband interference only affects a small
fraction of the subcarriers.
 Can use bands of frequency that are not contiguous.
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 Power is spread out across many frequencies.
 An alternative form of spread spectrum.
 Review: How does this compare with the other two
approaches for SSM have we seen?
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Signals are
“narrowband”
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VIII. Ultra Wideband
 Ultra Wideband (UWB) modulation uses
baseband pulse shapes that have extremely fast
rise and fall times, in the sub-nanosecond range.
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 Also known as impulse radio.
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 Produces a very wide spectrum
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 From near DC to several GHz
 But power spectral density is not very large
anywhere within the range.
 The device is transmitting nothing for much more time
than it is transmitting something.
 Extremely low cost devices can be used.
 Since no frequency up or down conversion is
needed.
 Can support extremely high data rates.
 Since the bandwidth range is so high.
 Possibly for next generation WLAN’s.
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 Timing can be randomized
 Not at periodic intervals.
 So only receivers know when the pulses will occur.
 So once again we have the idea of transmissions
looking like background noise.
 Also can be encrypted to provide further security.
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 A radically different approach
 No spectrum allocations!
 FCC has approved UWB
 Because it has such low power in any one
frequency range.
 Does not interfere with existing users.
 But that is debated by those users!
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IX. Space-Time Processing
 Several antennas in base stations and mobiles.
 Provides transmit and receive diversity.
 New research has created ways of allowing
antennas to be closer together.
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 Directional Antennas
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 Uses adaptive multi-beam antenna arrays
 multi-beam → serve different groups of users by
location
 adaptive → must follow mobile units
 Sectoring → primitive non-adaptive form of SDMA
 Use narrow Rx (not Tx) antenna beam at base station
to focus in on mobile users
 base station "hears" very well from one direction
 decreases ACI & CCI from all other directions by
significant amount (10-15 dB)
 acts as spatial “filter” (only receives signals well from
certain points in space)
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 How does it work?
 Antenna + Digital Signal Processing (DSP)
Technology
 Antenna array (many individual antenna elements)
required to have multiple beams can create a
focused capability by combining the signals from
the multiple antennas in a weighted fashion, high
directivity can be accomplished
 adaptive → change pattern width & direction vs.
time
 requires significant DSP solutions
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 SDMA technology is currently being deployed
 To in some cases greatly increase capacity of
existing systems.
 Common terminology
 Smart antennas
 Adaptive antenna arrays
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X. Where is the Future?
 There is no clear picture about what would be
involved in 4th Generation Wireless Systems.
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Cellular or Ad hoc?
WLAN based or Cellular?
CDMA, OFDM, UWB, or something else?
Many opinions.
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 Some goals are clear, however.
 More and more like the Internet
 So wireless truly looks like a “wire” to data
applications. Better managed.
 Much more adaptive to channel conditions
 Adaptive channels, modulation, coding, diversity,
multiple access schemes, etc.
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 More tightly integrated with applications and
data networking protocols.
 For example, wireless services specially adapted for
web browsing.
 Cross-layer approaches can achieve large efficiency
benefits.
 Less dependent on spectrum allocations.
 Easier roaming between countries and providers.
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 But the real issue is economic viability.
 3G is in deployment, so it obviously must precede
4G to some degree.
 And high quality engineers are needed to make
it all become a reality!
 That’s you.
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