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CSCD 439/539Wireless Networks and Security

Lecture 6Physical Layer Continued …

Fall 2007

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Introduction

• 802.11 g, a and n

• OFDM

• MIMO and other enhancements, n

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Intro to 802.11a

• 802.11a was approved in September 1999, two years after 802.11 standard approved– Operates in 5 GHz unlicensed national information

infrastructure (UNII) band– Spectrum is divided into three “domains,” each having

restrictions imposed on the maximum allowed output power

• First 100 MHz in the lower frequency portion is restricted to a maximum power output of 50 mW

• Second 100 MHz has a higher 250 mW maximum• Third 100 MHz, which is mainly intended for

outdoor applications, has a maximum of 1.0 W power output

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Intro to 802.11a

• 802.11a– Offered an alternative to the overcrowded

band 2.4 GHz, 5GHz– The 5GHz ISM bandwidth is not continuous– There are two areas 5.15GHz - 5.35GHz and

5.725GHz - 5.825Ghz– More details about 802.11a later …

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Intro to OFDM

• 802.11a and 802.11g based on OFDM– Orthogonal Frequency Division Multiplexing

• Revolutionized Wi-Fi and other cellular products by allowing faster throughput and more robustness

• OFDM makes highly efficient use of the available spectrum

– This characteristic will be important in coming years as wireless networks dominate especially in enterprise environments

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OFDM Based on FDM

• Recall …– Frequency division multiplexing (FDM) is a

technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system

– Each signal travels within its own unique frequency range (carrier)

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FDM• Comment:

– FDM Access transmissions are the least efficient networks since each analog channel can only be used one user at a time

Each User has their own channel

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OFDM based on FDM• In OFDM, data divided among large number of

closely spaced carriers– The "frequency division multiplex" part of the name– The entire bandwidth is filled from a single source of

data– Instead of transmitting data serially, data is

transferred in a parallel– Divided among multiple subcarriers– Only a small amount of the data is carried on each

carrier, which besides the obvious benefit of being parallel

– Provides benefits related to the radio nature of wireless

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OFDM

• An OFDM signal consists of – Several closely spaced modulated carriers– When modulation of any form - voice, data,

etc. is applied to a carrier• Sidebands spread out on either side• A receiver must be able to receive the whole signal

to be able to demodulate the data• So, when signals are transmitted close to one

another they must be spaced with a guard band between them

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Traditional View with Guards

Traditional view of signals carrying modulation

Receiver filter passband: one signal selected

Guards

Guard bands waste the spectrum

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OFDM

• Making the subcarriers mathematically orthogonal – Breakthrough for OFDM– Enables OFDM receivers to separate

subcarriers via an Fast Fourier Transform• Eliminate the guard bands• OFDM subcarriers can overlap to make full use of

the spectrum• Peak of each subcarrier spectrum, power in all the

other subcarriers is zero

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OFDM• OFDM offers higher data capacity in a given spectrum

while allowing a simpler system design

Others are have zero power

Power

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OFDM

• Shows parallel nature of subcarriers

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Benefits of OFDM

• Radio signals are imperfect – General challenges of RF signals include

• Signal-to-noise ratio• Self-interference (intersymbol interference or ISI)• Fading owing to multipath effects

– Same signal arrives at a receiver via different paths

– Briefly look at multipath fading …

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Multipath Fading

• The mobile or indoor radio channel is characterized by multipath reception– Sent signal contains not only a direct line-of-sight radio wave,

but also a large number of reflected radio waves– Even worse in urban areas, the line-of-sight is often blocked by

obstacles, and collection of differently delayed waves is received by a mobile antenna

– These reflected waves interfere with direct wave, causes significant degradation link performance

– Reason is that waves arrive at slightly different times, so they are out of phase with original wave

• Will randomly boost or cancel out parts of the signal

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Multipath Fading

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Benefits of OFDM

• Main way to prevent Intersymbol Interference errors– Transmit a short block of data (a symbol)– Wait until all the multipath echoes fade before

sending another symbol– Waiting time often referred to as guard interval

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Benefits of OFDM

• Longer the guard intervals - more robust system to multipath effects– But during guard interval, system gets no use

from the available spectrum– Longer the wait, the lower the effective

channel capacity

• Some guard interval is necessary for any wireless system– Goal is to minimize that interval and maximize

symbol transmission time

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Benefits of OFDM

• OFDM meets this challenge by dividing transmissions among multiple subcarriers.– Same guard interval can then be applied to

each subcarrier, while the symbol transmission time is multiplied by the number of subcarriers

– With 802.11a, there are 52 channels, so the system has 52 times the transmission capacity compared to single channel

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Benefits of OFDM

• Using multiple subcarriers also makes OFDM systems more robust to fading– Fading typically decreases received signal

strength at particular frequencies, so problem affects only a few of the subcarriers at any given time and …

– Error-correcting codes provide redundant information that enables OFDM receivers to restore information lost in these few erroneous subcarriers

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802.11a OFDM

• 802.11a specifies eight non-overlapping 20 MHz channels in the lower two bands– Each divided into 52 sub-carriers (four of which carry pilot data)

of 300-kHz bandwidth each

• Four non-overlapping 20 MHz channels are specified in the upper band

• The receiver processes the 52 individual bit streams, reconstructing the original high-rate data stream– Four complex modulation methods are employed, depending on

the data rate that can be supported by channel conditions between the transmitter and receiver.

– Include BPSK, QPSK, 16-QAM, and 64-QAM.

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Trying to Use 802.11a

• Advantage– Since 2.4 GHz band is heavily used, using 5 GHz

band gives 802.11a the advantage of less interference

• Disadvantage– However, high carrier frequency also brings

disadvantages– It restricts use of 802.11a to almost line of sight,

necessitating use of more access points– It also means that 802.11a cannot penetrate as far as

802.11b since it is absorbed more readily, other things (such as power) being equal.

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Trying to Use 802.11a• 802.11a products started shipping in 2001

– Lagged 802.11b products slow availability of the 5 GHz components needed to implement products

• 802.11a was not widely adopted because 802.11b was already widely adopted

• Because of 802.11a's disadvantages, poor initial product implementations, making its range even shorter, and because of regulations– Manufacturers of 802.11a equipment responded to lack of

market success by improving the implementations – Plus making technology that can use more than one 802.11

standard.– There are dual-band, or dual-mode or tri-mode cards that can

automatically handle 802.11a and b, or a, b and g, as available– Similarly, there are mobile adapters and access points which

can support all these standards simultaneously

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Comparing 802.11a and 802.11b

– The throughput of 802.11a is 2 to 4.5 times better than 802.11b up to a certain range …• Example: At 225 ft, 802.11a averages

yielded 5.2 Mbps compared to 1.6 Mbps for 802.11b

• Next slide shows this as a graph

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Throughput Range Performance

• Averaged throughput performance for 1500 byte packets: 802.11a thoughputs always better by 2 to 4.5 times up to 225 ft.

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802.11g

• June 2003, a third modulation standard ratified– 802.11g– Works in 2.4 GHz band (like 802.11b) but operates at

a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like 802.11a

– 802.11g hardware will work with 802.11b hardware– Older networks, 802.11b node significantly reduces

the speed of an 802.11g network

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802.11g

• The modulation scheme used in 802.11g– OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s,

and reverts to CCK, like 802.11b for 5.5 and 11 Mbit/s – DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s

• Even though 802.11g operates in same frequency band as 802.11b– Achieve higher data rates because of its similarities to 802.11a

• The maximum range of 802.11g devices is slightly greater than that of 802.11b devices– Range in which a client can achieve full (54 Mbit/s) data rate

speed is much shorter than that of 802.11b

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Beyond 802.11a and b, 802.11g

• Despite its major acceptance, 802.11g suffers from same interference as 802.11b in the already crowded 2.4 GHz range

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802.11n … A miracle or …

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802.11n Introduction

• 802.11n is the long anticipated update to Wi-Fi standards 802.11a/b/g– Expected to bring a 4x increase in throughput– Improvement in range– 802.11n expected to be ratified by the IEEE

within the next 18 months

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802.11n Timeline

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802.11n

• 802.11n utilizes a larger number of antennas

• The number of antennas relates to the number of simultaneous streams– Two receivers and two transmitters (2x2) or

four receivers and four transmitters (4x4)– The standards requirement is a 2x2 with a

maximum two streams, but allows 4x4

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802.11n

• Solutions based on the 802.11n standard will operate in the 2.4-GHz, the 5-GHz radio band, or both– Backward compatibility with preexisting

802.11a/b/g deployment– Majority of Wi-Fi devices and access points

deployed are dual-band• Operate in both 2.4-GHz and 5-GHz frequencies

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802.11n Features

• Wireless solutions based on 802.11n standard employ several techniques to improve throughput, reliability, and predictability of wireless

• Three primary innovations are– Multiple Input Multiple Output (MIMO) technology– Channel bonding (40MHz Channels)– Packet aggregation

• These techniques allow 802.11n solutions to achieve an approximate fivefold performance increase over current 802.11a/b/g networks

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MIMO

• 802.11n builds on previous standards by adding multiple-input multiple-output (MIMO)

– MIMO uses multiple transmitter and receiver antennas to improve the system performance

– MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end

• Similar to our ability to localize, using just our two ears, origin of specific sounds or to isolate and understand one conversation fragment from midst of assorted cocktail party chatter

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MIMO

• 802.11n access points and clients transmit two or more spatial streams, and employ multiple receive antennas and advanced signal processing to recover multiple transmitted data streams– MIMO-enabled access points use spatial multiplexing to transmit

different bits of a message over separate antennas– Provide greater data throughput– Allow for more robust, resilient wireless LANs

• Whereas previous wireless technologies had problems dealing with signal reflections, MIMO actually uses these reflections to increase the range and reduce "dead spots" in the wireless coverage area

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MIMO Technology• Multiple independent streams are transmitted

simultaneously to increase the data rate

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MIMO

• Performance gain is result of MIMO smart antenna technology– Allows wireless access points to receive signals more

reliably over greater distances than with standard diversity antennas

– Example, distance from access point at which an 802.11a/g client communicating with a conventional access point might drop from 54 Mbps to 48 Mbps or 36 Mbps, same client communicating with a MIMO access point may be able to continue operating at 54 Mbps

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Channel Bonding

• Most straightforward way to increase capacity of a network is to increase the operating bandwidth– However, conventional wireless technologies

limited to transmit over one of several 20-MHz channels

– 802.11n networks employ technique called channel bonding to combine two adjacent 20-MHz channels into a single 40-MHz channel

– Technique more than doubles the channel bandwidth

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Channel Bonding

– Channel bonding most effective in 5-GHz frequency given greater number of available channels

• 2.4-GHz frequency has only 3 non-overlapping 20-MHz channels

• Thus, bonding two 20-MHz channels uses two thirds of total frequency capacity

– So, IEEE has rules on when a device can operate in 40MHz channels in 2.4GHz space to ensure optimal performance

– Cisco expects greatest benefits of channel bonding will be realized in the 5-GHz frequency

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Packet Aggregation

• In conventional wireless transmission methods– Amount of channel access overhead required

to transmit each packet is fixed, regardless of the size of the packet itself

– As data rates increase, time required to transmit each packet shrinks

– Overhead cost remains same, potentially becoming much greater than packet itself at high speeds delivered with 802.11n

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Packet Aggregation

• 802.11n technologies increase efficiency by aggregating multiple packets of application data into a single transmission frame

• 802.11n networks can send multiple data packets with the fixed overhead cost of just a single frame

• Packet aggregation is more beneficial for certain types of applications such as file transfers because can aggregate packet content– Real-time applications (e.g. voice) don’t benefit from packet

aggregation because its packets would be interspersed at regular intervals

– And combining packets into larger payload would introduce unnecessary latency

– Voice and other multimedia applications still benefit from other effects of MIMO

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802.11n Advantages• Applications benefit most from 802.11n wireless LANs include:

• Environments and applications that require sharing of large files, including anything from advanced design and engineering applications to users in a conference room collaborating on a large Microsoft PowerPoint presentation

• Voice and video applications that demand high-quality transmissions, such as video conferencing and IPTV services that use multiple streams of high-definition video

• Facilities with challenging RF characteristics including warehouses, manufacturing floors and retail locations

• Disaster recovery, backup, and storage applications

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Summary

• Looked at 802.11 a,g and n• Big part of increase in speed was OFDM

– Very important currently for other media transmission, HD TV, Cellular phone and Wi-Max networks

• 802.11n holds tremendous promise– Not there yet … mixed results in testing with various

products– However, expected standard within the year– Should provide some improvement over existing Wi-Fi

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Finish

Next time • Bluetooth and Wi-Max• Briefly talk about what you found for tools