CWNA Guide to Wireless LANs, Second Edition

Chapter FourIEEE 802.11 Physical Layer Standards

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

OSI data flow

Introduction (continued)

OSI layers and functions

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

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)

Narrowband transmission

Spread Spectrum Transmission

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)

FHSS error correction

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)

Direct sequence spread spectrum (DSSS) transmission

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)

Multiple channels

Orthogonal Frequency Division Multiplexing (continued)

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

IEEE 802.11 Physical Layer Standards (continued)

Data Link sublayers

IEEE 802.11 Physical Layer Standards (continued)

PLCP sublayer reformats MAC data

IEEE 802.11 Physical Layer Standards (continued)

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

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

IEEE 802.11b Physical Layer Standards (continued)

• 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

IEEE 802.11b Physical Layer Standards (continued)

802.11b ISM channels

IEEE 802.11b Physical Layer Standards (continued)

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

U-NII characteristics

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

802.11a channels

Channel Allocation (continued)

802.11b vs. 802.11a channel coverage

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Channel Setup

There are two critical steps for a good WLAN deployment: 1. Determine placement of access points or bridges – • This includes determining where they should be placed and deciding how many

are required for the desired coverage. • Very few gaps in the coverage should be left. • These gaps are essentially dead air and the client will lack connectivity in these

locations. • As discussed before, bandwidth requirements have an impact on the

coverage areas. 2. Map out the channel assignments – • There should be as little overlap as possible between channels that use the

same frequency.

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Multiple Overlapping Networks Coverage Current Thinking:

1. Only three of the 11 channels used by wireless hubs in the US can be allocated simultaneously.

2. Reason:

1. In North America, the 802.11b spectrum ranges form 2411 MHz to 2473 MHz, and is divided up into 11 channels. Channels are spaced 5 MHz apart from the center.

2. However, each channel is 22 MHz wide, so there is a great overlap

Ch Start Fqy Mid Fqy End Fqy1 2401 2412 24232 2406 2417 24283 2411 2422 24334 2416 2427 24385 2421 2432 24436 2426 2437 24487 2431 2442 24538 2436 2447 24589 2441 2452 2463

10 2446 2457 246811 2451 2462 2473

Channels 1, 6, and 11 are the safe channels to use.

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An entire 22MHz is not simply swallowed up in a rectangular pattern with power on the vertical axis and frequency on the horizontal – instead it's more of a parabola, centered around the midpoint of the frequency. Thus, as you get further away from the center, the power drops off substantially. According to Burton's analysis, when three channels separate 802.11b frequencies, there is only about 4% of interference. This is the case between frequencies 1 and 4, and 8 and 11. Between 4 and 8, the interference is substantially less than 1%.

Burton’s Analysis

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

802.11a PLCP frame

Physical Layer Standards (continued)

802.11a Rate field values

Physical Layer Standards (continued)

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

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

802.11a characteristics

Physical Layer Standards (continued)

Phase shift keying (PSK)

Physical Layer Standards (continued)

Quadrature phase shift keying (QPSK)

Physical Layer Standards (continued)

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

Physical Layer Standards (continued)

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