Lecture 11:

802.11 WLAN’s and Other Recent Technological Developments

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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 Networ

k 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 (activ

e)

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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 modes1. 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 (I

EEE 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 Frequency Hopped Spread Spectrum 79 channels Each 1 MHz wide Dwell time less than 400 msec

DSSS 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 p

er symbol Better range than 802.11a

About 7 times larger range Named “Wi-Fi” by the Wireless Ethernet Compatib

ility Alliance (www.wi-fi.com) Goal is to promote interoperability between vendors’ p

roducts.

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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. 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 messa

ges. 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 ra

ndom 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 connecti

ng with base stations. Association - to connect to base stations. Disassociation - to disassociate with base stations Reassociation - change a preferred base station, with

out losing data in the handover. Distribution - determine how to route frames sent to

the base station. Integration - handles translation from the 802.11 for

mat into another format required by a destination network.

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Station Services - For activity during communications Authentication - stations identify themselves as val

id 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 informatio

n sent over a wireless LAN confidential. Data delivery - ways to transmit and receive data a

s 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 en

d 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 Licensed bands below 3.5 GHz. Greater than 1 Mbps. Vehicle mobility up to 250 km/hr. Seeks “spectral efficiencies, sustained user data rate

s 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 M

BWA. 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 lo

wer bit rate. Multipath delay spread is not significant compared to t

hese 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 anywhe

re 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 neede

d. 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 frequenc

y 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 locatio

n adaptive → must follow mobile units

Sectoring → primitive non-adaptive form of SDMA Use narrow Rx (not Tx) antenna beam at base station t

o focus in on mobile users base station "hears" very well from one direction decreases ACI & CCI from all other directions by signif

icant amount (10-15 dB) acts as spatial “filter” (only receives signals well from c

ertain 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 existi

ng 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. 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 applicat

ions. 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.