CWNA Guide to Wireless LANs, Second Edition

CWNA Guide to Wireless LANs, Second Edition

Chapter FourIEEE 802.11 Physical Layer Standards

CWNA Guide to Wireless LANs, Second Edition 2

Objectives

• List and describe the wireless modulation schemes used in IEEE WLANs

• Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum

• Explain how orthogonal frequency division multiplexing is used to increase network throughput

• List the characteristics of the Physical layer standards in 802.11b, 802.11g, and 802.11a networks


Introduction

Figure 4-2: OSI data flow


Introduction (continued)

Table 4-1: OSI layers and functions


Telecommunication Channel

• Channel - a path along which information in the form of an electrical signal passes. Usually a range of contiguous frequencies involved in supporting information transmission.

Bandwidth

Amplitude

FrequencyChannel

CenterChannel Frequency


Narrow and Wide Band

• Narrow and Wide Band – a relative comparison of a group or range of frequencies used in a telecommunications system. Narrow Band would describe a small range of frequencies as compared to a larger Wide Band range.

Frequency

Amplitude

NB WB

Freq. RangefL fH


Noise Floor

• Noise –A disturbance, especially a random and persistent disturbance, that obscures or reduces the clarity of a signal. Anything you don’t want.

Channel

Noise FloorShot

Signal

Thermal

Amplitude

Freq.


Introduction to Spread Spectrum

• Spread Spectrum – a telecommunications technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information.

Frequency

Amplitude Narrowband

Wideband


Wireless Modulation Schemes

• Four primary wireless modulation schemes:– Narrowband transmission– Frequency hopping spread spectrum– Direct sequence spread spectrum– Orthogonal frequency division multiplexing

• Narrowband transmission used primarily by radio stations

• Other three used in IEEE 802.11 WLANs


Uses of Spread Spectrum

• Military - For low probability of interception of telecommunications.

• Civil/Military - Range and positioning measurements. GPS – satellites.

• Civil Cellular Telephony.

• Civil Wireless Networks – 802.11 and Bluetooth.


Narrowband Transmission

• Radio signals by nature transmit on only one radio frequency or a narrow portion of frequencies

• Require more power for the signal to be transmitted – Signal must exceed noise level

• Total amount of outside interference

• Vulnerable to interference from another radio signal at or near same frequency

• IEEE 802.11 standards do not use narrowband transmissions


Narrowband Transmission (continued)

Figure 4-3: Narrowband transmission


Spread Spectrum Transmission

Figure 4-4: Spread spectrum transmission


Spread Spectrum Transmission (continued)

• Advantages over narrowband:– Resistance to narrowband interference– Resistance to spread spectrum interference– Lower power requirements– Less interference on other systems– More information transmitted– Increased security– Resistance to multipath distortion


Frequency Hopping Spread Spectrum (FHSS)

• Uses range of frequencies – Change during transmission

• Hopping code: Sequence of changing frequencies– If interference encountered on particular frequency

then that part of signal will be retransmitted on next frequency of hopping code

• FCC has established restrictions on FHSS to reduce interference

• Due to speed limitations FHSS not widely implemented in today’s WLAN systems– Bluetooth does use FHSS


Frequency Hopping Spread Spectrum (continued)

Figure 4-6: FHSS error correction


FHSS

• FHSS - Acronym for frequency-hopping spread spectrum. Bluetooth & HomeRF.

Freq.

Amp.

1 2 3 4

Frequency Hop Sequence: 1, 3, 2, 4

Wide BandChannel


FHSS Timing

Frequency

Time

Amplitude

DwellTime

HopSequence

Channels

DataHopTime

12

34


FHSS System Block Diagram

Frequency Synthesizer

CarrierFrequency

DataBuffer

SequenceGenerator

Mixer

AntennaFHSS

1 23 4

1 23 4


FHSS Channel Allocation

2.400 GHz 2.4835 GHz

Amplitude

Freq.

1 MHz

2.401.5 GHz

2.402.5 GHz

2.402 GHz

2.403 GHz

CH2

CH3

1 MHz

2.401.5 GHz

2.402.5 GHz

2.479 GHz

2.480 GHz

CH79

CH80


FCC Rules for FHSS

• Prior to 8-31-00– Use 75 of the 79 channels

– Output Powermax = 1 Watt

– Bandwidthmax = 1 MHz

– Data Ratemax = 2 Mbps

• After 8-31-00– Only 15 of the 79 channels required

– Output Powermax = 125 mW

– Bandwidthmax = 5 MHz

– Data Ratemax = 10 Mbps


Direct Sequence Spread Spectrum (DSSS)

• Uses expanded redundant code to transmit data bits

• Chipping code: Bit pattern substituted for original transmission bits– Advantages of using DSSS with a chipping code:

• Error correction

• Less interference on other systems

• Shared frequency bandwidth

– Co-location: Each device assigned unique chipping code

• Security


Direct Sequence Spread Spectrum (continued)

Figure 4-7: Direct sequence spread spectrum (DSSS) transmission


DSSS

• DSSS - Acronym for direct-sequence spread spectrum. WLAN, 802.11.

Freq.

Amp.

1 2 3 4

DSSS BandChannel

1 Signal


DSSS Channel Allocation

2.401 GHz 2.473 GHz

11 22 33 44 55 66 77 88 99 1010 1111

Amplitude

Freq.

Channels


DSSS 3 Non-overlap Channels

2.401 GHz 2.473 GHz

Ch 1 Ch 6 Ch 11(2.412 GHz) (2.437GHz) (2.462 GHz)

Amplitude

Freq.

2401 MHz

22 MHz

3MHz

2426 MHz2423 MHz


DSSS System Block DiagramCarrier

Frequency

DataBuffer

Pseudo –Noise

Generator

Antenna

CarrierGenerator

11-bit Barker Code

Mixer

Modulator

DSSS

Chipping Code


Comparing FHSS and DSSS

Frequency Hopping

Spread Spectrum, FHSS

Direct Sequence

Spread Spectrum, DSSS

Dwell Time

400 mSHigher Cost

No

Dwell TimeLower Cost

Lower

Throughput (2 or 3 Mbps)

Lower

Interoperability

Higher

Throughput (11 Mbps)

Higher

Interoperability

Better NB Immunity to Interference

More User Density (79)

Poorer NB Immunity to Interference

Less User Density (3)


Orthogonal Frequency Division Multiplexing (OFDM)

• With multipath distortion, receiving device must wait until all reflections received before transmitting– Puts ceiling limit on overall speed of WLAN

• OFDM: Send multiple signals at same time– Split high-speed digital signal into several slower

signals running in parallel

• OFDM increases throughput by sending data more slowly

• Avoids problems caused by multipath distortion

• Used in 802.11a networks


Orthogonal Frequency Division Multiplexing (continued)

Figure 4-8: Multiple channels


Orthogonal Frequency Division Multiplexing (continued)

Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions


Comparison of Wireless Modulation Schemes

• FHSS transmissions less prone to interference from outside signals than DSSS

• WLAN systems that use FHSS have potential for higher number of co-location units than DSSS

• DSSS has potential for greater transmission speeds over FHSS

• Throughput much greater for DSSS than FHSS– Amount of data a channel can send and receive


Comparison of Wireless Modulation Schemes (continued)

• DSSS preferred over FHSS for 802.11b WLANs

• OFDM is currently most popular modulation scheme– High throughput– Supports speeds over 100 Mbps for 802.11a WLANs – Supports speeds over 54 Mbps for 802.11g WLANs


IEEE 802.11 Physical Layer Standards

• IEEE wireless standards follow OSI model, with some modifications

• Data Link layer divided into two sublayers:– Logical Link Control (LLC) sublayer: Provides

common interface, reliability, and flow control– Media Access Control (MAC) sublayer: Appends

physical addresses to frames


IEEE 802.11 Physical Layer Standards (continued)

• Physical layer divided into two sublayers:– Physical Medium Dependent (PMD) sublayer:

Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data

– Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions

• Reformats data received from MAC layer into frame that PMD sublayer can transmit

• “Listens” to determine when data can be sent



Figure 4-10: Data Link sublayers



Figure 4-11: PHY sublayers



Figure 4-12: PLCP sublayer reformats MAC data



Figure 4-13: IEEE LANs share the same LLC


Legacy WLANs

• Two “obsolete” WLAN standards: – Original IEEE 802.11: FHSS or DSSS could be used

for RF transmissions• But not both on same WLAN

– HomeRF: Based on Shared Wireless Access Protocol (SWAP)

• Defines set of specifications for wireless data and voice communications around the home

• Slow

• Never gained popularity


IEEE 802.11b Physical Layer Standards

• Physical Layer Convergence Procedure Standards: Based on DSSS– PLCP must reformat data received from MAC layer

into a frame that the PMD sublayer can transmit

Figure 4-14: 802.11b PLCP frame


IEEE 802.11b Physical Layer Standards (continued)

• PLCP frame made up of three parts:– Preamble: prepares receiving device for rest of

frame– Header: Provides information about frame– Data: Info being transmitted

• Synchronization field• Start frame delimiter field• Signal data rate field• Service field• Length field• Header error check field• Data field



• Physical Medium Dependent Standards: PMD translates binary 1’s and 0’s of frame into radio signals for transmission– Can transmit at 11, 5.5, 2, or 1 Mbps– 802.11b uses ISM band

• 14 frequencies can be used

– Two types of modulation can be used• Differential binary phase shift keying (DBPSK): For

transmissions at 1 Mbps

• Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps



Table 4-2: 802.11b ISM channels



Table 4-3: IEEE 802.11b Physical layer standards


IEEE 802.11a Physical Layer Standards

• IEEE 802.11a achieves increase in speed and flexibility over 802.11b primarily through OFDM– Use higher frequency– Accesses more transmission channels– More efficient error-correction scheme


U-NII Frequency Band

Table 4-5: U-NII characteristics

Table 4-4: ISM and U-NII WLAN characteristics


U-NII Frequency Band (continued)

• Total bandwidth available for IEEE 802.11a WLANs using U-NII is almost four times that available for 802.11b networks using ISM band

• Disadvantages:– In some countries outside U.S., 5 GHz bands

allocated to users and technologies other than WLANs

– Interference from other devices is growing• Interference from other devices one of primary

sources of problems for 802.11b and 802.11a WLANs


Channel Allocation

Figure 4-16: 802.11a channels


Channel Allocation (continued)

Figure 4-17: 802.11b vs. 802.11a channel coverage


Co-location

• FHSS has many more frequencies / channels then DSSS which only has 3 co-location channels.

• However 3 DSSS access points co-located at 11 Mbps each would result in a maximum throughput of 33 Mbps. It would require 16 access points co-located for FHSS to achieve a throughput of 32 Mbps.


Co-location Comparison

Number of Co-located Systems

1 10 15 205

10

20

30

4011 Mbps DSSS

3 Mbps FHSS (sync)

3 Mbps FHSS (no sync)


Error Correction

• 802.11a has fewer errors than 802.11b– Transmissions sent over parallel subchannels– Interference tends to only affect one subchannel

• Forward Error Correction (FEC): Transmits secondary copy along with primary information– 4 of 52 channels used for FEC– Secondary copy used to recover lost data

• Reduces need for retransmission


Physical Layer Standards

• PLCP for 802.11a based on OFDM• Three basic frame components: Preamble, header,

and data

Figure 4-18: 802.11a PLCP frame


Physical Layer Standards (continued)

Table 4-6: 802.11a Rate field values



• Modulation techniques used to encode 802.11a data vary depending upon speed

• Speeds higher than 54 Mbps may be achieved using 2X modes

Table 4-7: 802.11a characteristics



Figure 4-19: Phase shift keying (PSK)



Figure 4-20: Quadrature phase shift keying (QPSK)



Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)



Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)


IEEE 802.11g Physical Layer Standards

• 802.11g combines best features of 802.11a and 802.11b

• Operates entirely in 2.4 GHz ISM frequency

• Two mandatory modes and one optional mode– CCK mode used at 11 and 5.5 Mbps (mandatory)– OFDM used at 54 Mbps (mandatory)– PBCC-22 (Packet Binary Convolution Coding):

Optional mode• Can transmit between 6 and 54 Mbps


IEEE 802.11g Physical Layer Standards (continued)

Table 4-8: IEEE 802.11g Physical layer standards


IEEE 802.11g Physical Layer Standards (continued)

• Characteristics of 802.11g standard:– Greater throughput than 802.11b networks – Covers broader area than 802.11a networks– Backward compatible– Only three channels– If 802.11b and 802.11g devices transmitting in same

environment, 802.11g devices drop to 11 Mbps speeds

– Vendors can implement proprietary higher speed• Channel bonding and Dynamic turbo


Summary

• Three modulation schemes are used in IEEE 802.11 wireless LANs: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM)

• Spread spectrum is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band

• Spread spectrum transmission uses two different methods to spread the signal over a wider area: FHSS and DSSS


Summary (continued)

• OFDM splits a single high-speed digital signal into several slower signals running in parallel

• IEEE has divided the OSI model Data Link layer into two sublayers: the LLC and MAC sublayers

• The Physical layer is subdivided into the PMD sublayer and the PLCP sublayer

• The Physical Layer Convergence Procedure Standards (PLCP) for 802.11b are based on DSSS


Summary (continued)

• IEEE 802.11a networks operate at speeds up to 54 Mbps with an optional 108 Mbps

• The 802.11g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by 802.11a

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CWNA Guide to Wireless LANs, Second Edition

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