Linux+ Guide to Linux Certification

advertisement
CWNA Guide to Wireless
LANs, Second Edition
Chapter Four
IEEE 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
CWNA Guide to Wireless LANs, Second Edition
2
Introduction
Figure 4-2: OSI data flow
CWNA Guide to Wireless LANs, Second Edition
3
Introduction (continued)
Table 4-1: OSI layers and functions
CWNA Guide to Wireless LANs, Second Edition
4
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.
Amplitude
Center
Channel Frequency
Bandwidth
Channel
CWNA Guide to Wireless LANs, Second Edition
Frequency
5
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.
Amplitude
NB
WB
Frequency
Freq. Range
fL
CWNA Guide to Wireless LANs, Second Edition
fH
6
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.
Amplitude
Channel
Signal
Noise Floor
Thermal
Shot
Freq.
CWNA Guide to Wireless LANs, Second Edition
7
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.
Amplitude
Narrowband
Wideband
Frequency
CWNA Guide to Wireless LANs, Second Edition
8
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
CWNA Guide to Wireless LANs, Second Edition
9
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.
CWNA Guide to Wireless LANs, Second Edition
10
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
CWNA Guide to Wireless LANs, Second Edition
11
Narrowband Transmission (continued)
Figure 4-3: Narrowband transmission
CWNA Guide to Wireless LANs, Second Edition
12
Spread Spectrum Transmission
Figure 4-4: Spread spectrum transmission
CWNA Guide to Wireless LANs, Second Edition
13
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
CWNA Guide to Wireless LANs, Second Edition
14
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
CWNA Guide to Wireless LANs, Second Edition
15
Frequency Hopping Spread Spectrum
(continued)
Figure 4-6: FHSS error correction
CWNA Guide to Wireless LANs, Second Edition
16
FHSS
• FHSS - Acronym for frequency-hopping spread
spectrum. Bluetooth & HomeRF.
Amp.
1
Channel
3
2
4
Freq.
Wide Band
Frequency Hop Sequence: 1, 3, 2, 4
CWNA Guide to Wireless LANs, Second Edition
17
FHSS Timing
Amplitude
Hop
Time
Time
Data
Dwell
Time
Hop
Sequence
1
2
3
Channels
CWNA Guide to Wireless LANs, Second Edition
4
Frequency
18
FHSS System Block Diagram
FHSS
Data
Buffer
Antenna
1 3 2 4
Mixer
Carrier
Frequency
Sequence
Generator
1
3
2
4
Frequency Synthesizer
CWNA Guide to Wireless LANs, Second Edition
19
FHSS Channel Allocation
2.403 GHz
2.402 GHz
CH
2
Amplitude
CH
3
1 MHz
2.401.5 GHz
2.402.5 GHz
2.480 GHz
2.479 GHz
CH
79
CH
80
1 MHz
2.401.5 GHz
2.402.5 GHz
Freq.
2.400 GHz
CWNA Guide to Wireless LANs, Second Edition
2.4835 GHz
20
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
CWNA Guide to Wireless LANs, Second Edition
21
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
CWNA Guide to Wireless LANs, Second Edition
22
Direct Sequence Spread Spectrum
(continued)
Figure 4-7: Direct sequence spread spectrum (DSSS) transmission
CWNA Guide to Wireless LANs, Second Edition
23
DSSS
• DSSS - Acronym for direct-sequence spread
spectrum. WLAN, 802.11.
Amp.
Signal
1
1
Channel
3
2
4
Freq.
DSSS Band
CWNA Guide to Wireless LANs, Second Edition
24
DSSS Channel Allocation
Amplitude
Channels
1
2
3
4
5
6
7
8
9
10 11
Freq.
2.401 GHz
CWNA Guide to Wireless LANs, Second Edition
2.473 GHz
25
DSSS 3 Non-overlap Channels
Amplitude
Ch 1
(2.412 GHz)
Ch 6
(2.437GHz)
Freq.
22 MHz
2.401 GHz
2401 MHz
Ch 11
(2.462 GHz)
3MHz
2.473 GHz
2423 MHz
2426 MHz
CWNA Guide to Wireless LANs, Second Edition
26
DSSS System Block Diagram
Carrier
Frequency
Carrier
Generator
DSSS
Mixer
Pseudo – 11-bit Barker Code
Noise
Modulator
Chipping Code
Generator
CWNA Guide to Wireless LANs, Second Edition
Antenna
Data
Buffer
27
Comparing FHSS and DSSS
Frequency Hopping
Spread Spectrum, FHSS
Direct Sequence
Spread Spectrum, DSSS
Dwell Time
400 mS
Higher Cost
No
Dwell Time
Lower 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)
CWNA Guide to Wireless LANs, Second Edition
28
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
CWNA Guide to Wireless LANs, Second Edition
29
Orthogonal Frequency Division
Multiplexing (continued)
Figure 4-8: Multiple channels
CWNA Guide to Wireless LANs, Second Edition
30
Orthogonal Frequency Division
Multiplexing (continued)
Figure 4-9: Orthogonal frequency division multiplexing (OFDM)
vs. single-channel transmissions
CWNA Guide to Wireless LANs, Second Edition
31
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
CWNA Guide to Wireless LANs, Second Edition
32
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
CWNA Guide to Wireless LANs, Second Edition
33
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
CWNA Guide to Wireless LANs, Second Edition
34
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
CWNA Guide to Wireless LANs, Second Edition
35
IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-10: Data Link sublayers
CWNA Guide to Wireless LANs, Second Edition
36
IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-11: PHY sublayers
CWNA Guide to Wireless LANs, Second Edition
37
IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-12: PLCP sublayer reformats MAC data
CWNA Guide to Wireless LANs, Second Edition
38
IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-13: IEEE LANs share the same LLC
CWNA Guide to Wireless LANs, Second Edition
39
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
CWNA Guide to Wireless LANs, Second Edition
40
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
CWNA Guide to Wireless LANs, Second Edition
41
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
CWNA Guide to Wireless LANs, Second Edition
42
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
CWNA Guide to Wireless LANs, Second Edition
43
IEEE 802.11b Physical Layer
Standards (continued)
Table 4-2: 802.11b ISM channels
CWNA Guide to Wireless LANs, Second Edition
44
IEEE 802.11b Physical Layer
Standards (continued)
Table 4-3: IEEE 802.11b Physical layer standards
CWNA Guide to Wireless LANs, Second Edition
45
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
CWNA Guide to Wireless LANs, Second Edition
46
U-NII Frequency Band
Table 4-4: ISM and U-NII WLAN characteristics
Table 4-5: U-NII characteristics
CWNA Guide to Wireless LANs, Second Edition
47
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
CWNA Guide to Wireless LANs, Second Edition
48
Channel Allocation
Figure 4-16: 802.11a channels
CWNA Guide to Wireless LANs, Second Edition
49
Channel Allocation (continued)
Figure 4-17: 802.11b vs. 802.11a channel coverage
CWNA Guide to Wireless LANs, Second Edition
50
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 colocated for FHSS to achieve a throughput of 32
Mbps.
CWNA Guide to Wireless LANs, Second Edition
51
Co-location Comparison
40
3 Mbps FHSS
(sync)
11 Mbps DSSS
30
3 Mbps FHSS
(no sync)
20
10
1
5
10
15
20
Number of Co-located Systems
CWNA Guide to Wireless LANs, Second Edition
52
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
CWNA Guide to Wireless LANs, Second Edition
53
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
CWNA Guide to Wireless LANs, Second Edition
54
Physical Layer Standards (continued)
Table 4-6: 802.11a Rate field values
CWNA Guide to Wireless LANs, Second Edition
55
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
Table 4-7: 802.11a characteristics
CWNA Guide to Wireless LANs, Second Edition
56
Physical Layer Standards (continued)
Figure 4-19: Phase shift keying (PSK)
CWNA Guide to Wireless LANs, Second Edition
57
Physical Layer Standards (continued)
Figure 4-20: Quadrature phase shift keying (QPSK)
CWNA Guide to Wireless LANs, Second Edition
58
Physical Layer Standards (continued)
Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
CWNA Guide to Wireless LANs, Second Edition
59
Physical Layer Standards (continued)
Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
CWNA Guide to Wireless LANs, Second Edition
60
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
CWNA Guide to Wireless LANs, Second Edition
61
IEEE 802.11g Physical Layer
Standards (continued)
Table 4-8: IEEE 802.11g Physical layer standards
CWNA Guide to Wireless LANs, Second Edition
62
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
CWNA Guide to Wireless LANs, Second Edition
63
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
CWNA Guide to Wireless LANs, Second Edition
64
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
CWNA Guide to Wireless LANs, Second Edition
65
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
CWNA Guide to Wireless LANs, Second Edition
66
Download