1. CWNA Guide to Wireless LANs, Second Edition
Chapter Four
IEEE Physical Layer Standards

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 b, g, and a networks
CWNA Guide to Wireless LANs, Second Edition

3. Introduction Figure 4-2: OSI data flow
CWNA Guide to Wireless LANs, Second Edition

4. Introduction (continued)
Table 4-1: OSI layers and functions
CWNA Guide to Wireless LANs, Second Edition

5. 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.
Center
Channel Frequency
Amplitude
Bandwidth
Frequency
Channel
CWNA Guide to Wireless LANs, Second Edition

6. 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 fH
CWNA Guide to Wireless LANs, Second Edition

7. 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
The noise floor is the level caused by the environment surrounding a wireless LAN.
Thermal and shot noise usually set the noise floor in a quite RF environment.
Thermal Noise is the fluctuating voltage across a resistance due to the random motion of free charge caused by thermal agitation.
Shot Noise is caused by the quantized and random nature of current flow. Think of it as noisy current.
Signal
Noise Floor
Thermal
Shot
Freq.
CWNA Guide to Wireless LANs, Second Edition

8. 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.
Narrowband
Amplitude
Wideband
Frequency
CWNA Guide to Wireless LANs, Second Edition

9. 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 WLANs
CWNA Guide to Wireless LANs, Second Edition

10. 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 – and Bluetooth.
CWNA Guide to Wireless LANs, Second Edition

11. 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 standards do not use narrowband transmissions
CWNA Guide to Wireless LANs, Second Edition

12. Narrowband Transmission (continued)
Figure 4-3: Narrowband transmission
CWNA Guide to Wireless LANs, Second Edition

13. Spread Spectrum Transmission
Figure 4-4: Spread spectrum transmission
CWNA Guide to Wireless LANs, Second Edition

14. 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

15. 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

16. Frequency Hopping Spread Spectrum (continued)
Figure 4-6: FHSS error correction
CWNA Guide to Wireless LANs, Second Edition

17. FHSS
FHSS - Acronym for frequency-hopping spread spectrum. Bluetooth & HomeRF.
Amp.
Acronym for frequency-hopping spread spectrum. FHSS is one of two types of spread spectrum radio, the other being direct-sequence spread spectrum. FHSS is a transmission technology used in WLAN transmissions where the data signal is modulated with a narrowband carrier signal that "hops" in a random but predictable sequence from frequency to frequency as a function of time over a wide band of frequencies. The signal energy is spread in time domain rather than chopping each bit into small pieces in the frequency domain. This technique reduces interference because a signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency at the same time. If synchronized properly, a single logical channel is maintained.
The transmission frequencies are determined by a spreading, or hopping, code. The receiver must be set to the same hopping code and must listen to the incoming signal at the right time and correct frequency in order to properly receive the signal. Current FCC regulations require manufacturers to use 75 or more frequencies per transmission channel with a maximum dwell time (the time spent at a particular frequency during any single hop) of 400 ms. 1 2 3 4
Freq.
Channel
Wide Band
Frequency Hop Sequence: 1, 3, 2, 4
CWNA Guide to Wireless LANs, Second Edition

18. FHSS Timing Time Amplitude Data Hop Time Dwell Time Hop Sequence 1 2 34
Channels
Frequency
CWNA Guide to Wireless LANs, Second Edition

19. FHSS System Block Diagram
Antenna
FHSS
Data
Buffer 1 3 2 4
Mixer
Carrier
Frequency
Sequence
Generator 1 3 2 4
Frequency Synthesizer
CWNA Guide to Wireless LANs, Second Edition

20. FHSS Channel Allocation
2.403 GHz
2.480 GHz
2.402 GHz
2.479 GHz CH 2 CH 3 CH 79 CH 80
Amplitude
1 MHz
1 MHz
GHz
GHz
GHz
GHz
Freq.
GHz
2.400 GHz
CWNA Guide to Wireless LANs, Second Edition

21. FCC Rules for FHSS Prior to 8-31-00 After 8-31-00
Use 75 of the 79 channels
Output Powermax = 1 Watt
Bandwidthmax = 1 MHz
Data Ratemax = 2 Mbps
After
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

22. 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

23. Direct Sequence Spread Spectrum (continued)
Figure 4-7: Direct sequence spread spectrum (DSSS) transmission
CWNA Guide to Wireless LANs, Second Edition

24. DSSS DSSS - Acronym for direct-sequence spread spectrum. WLAN, 802.11.
Amp.
Signal 1
Acronym for direct-sequence spread spectrum. DSSS is one of two types of spread spectrum radio, the other being frequency-hopping spread spectrum. DSSS is a transmission technology used in WLAN transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission. 1 2 3 4
Freq.
Channel
DSSS Band
CWNA Guide to Wireless LANs, Second Edition

25. DSSS Channel Allocation
Amplitude
Channels 1 2 3 4 5 6 7 8 9 10 11
There are only three non-overlapping channels: 1, 6, and 11. However there are several two non-overlapping channels: 1, 5; 2, 7; 7, 11; etc.
Freq.
2.401 GHz
2.473 GHz
CWNA Guide to Wireless LANs, Second Edition

26. DSSS 3 Non-overlap Channels
Amplitude
Ch 1
Ch 6
Ch 11
(2.412 GHz)
(2.437GHz)
(2.462 GHz)
Freq.
22 MHz
3MHz
2.473 GHz
These are the only three that do not overlap as a group. The 3-MHz gaps are sometimes called guard bands because the isolate the channels from one another. There are other pairs that do not overlap for example channel 2 and 7.
2.401 GHz
2401 MHz
2423 MHz
2426 MHz
CWNA Guide to Wireless LANs, Second Edition

27. DSSS System Block Diagram
Carrier
Frequency
Antenna
DSSS
Mixer
Carrier
Generator
11-bit Barker Code
Pseudo –
Noise
Generator
Data
Buffer
Barker Code =
Modulator
Chipping Code
CWNA Guide to Wireless LANs, Second Edition

28. Comparing FHSS and DSSS
Frequency Hopping
Spread Spectrum, FHSS
Direct Sequence
Spread Spectrum, DSSS
Dwell Time
400 mS
Higher Cost No
Lower Cost
Lower
Throughput (2 or 3 Mbps)
Interoperability
Higher
Throughput (11 Mbps)
Better NB Immunity to Interference
More User Density (79)
Poorer NB Immunity to Interference
Less User Density (3)
Cost of FHSS is greater than DSSS. Sample of throughput of , b. FHHS has slow radio sync up while DSSS is very fast. FHHS long latency time.
CWNA Guide to Wireless LANs, Second Edition

29. 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 a networks
CWNA Guide to Wireless LANs, Second Edition

30. Orthogonal Frequency Division Multiplexing (continued)
Figure 4-8: Multiple channels
CWNA Guide to Wireless LANs, Second Edition

31. Orthogonal Frequency Division Multiplexing (continued)
Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions
CWNA Guide to Wireless LANs, Second Edition

32. 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

33. Comparison of Wireless Modulation Schemes (continued)
DSSS preferred over FHSS for b WLANs
OFDM is currently most popular modulation scheme
High throughput
Supports speeds over 100 Mbps for a WLANs
Supports speeds over 54 Mbps for g WLANs
CWNA Guide to Wireless LANs, Second Edition

34. 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

35. 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

36. IEEE 802.11 Physical Layer Standards (continued)
Figure 4-10: Data Link sublayers
CWNA Guide to Wireless LANs, Second Edition

37. IEEE 802.11 Physical Layer Standards (continued)
Figure 4-11: PHY sublayers
CWNA Guide to Wireless LANs, Second Edition

38. IEEE 802.11 Physical Layer Standards (continued)
Figure 4-12: PLCP sublayer reformats MAC data
CWNA Guide to Wireless LANs, Second Edition

39. IEEE 802.11 Physical Layer Standards (continued)
Figure 4-13: IEEE LANs share the same LLC
CWNA Guide to Wireless LANs, Second Edition

40. Legacy WLANs Two “obsolete” WLAN standards:
Original IEEE : 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

41. 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: b PLCP frame
CWNA Guide to Wireless LANs, Second Edition

42. 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

43. 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

44. IEEE 802.11b Physical Layer Standards (continued)
Table 4-2: b ISM channels
CWNA Guide to Wireless LANs, Second Edition

45. IEEE 802.11b Physical Layer Standards (continued)
Table 4-3: IEEE b Physical layer standards
CWNA Guide to Wireless LANs, Second Edition

46. IEEE 802.11a Physical Layer Standards
IEEE a achieves increase in speed and flexibility over b primarily through OFDM
Use higher frequency
Accesses more transmission channels
More efficient error-correction scheme
CWNA Guide to Wireless LANs, Second Edition

47. 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

48. U-NII Frequency Band (continued)
Total bandwidth available for IEEE a WLANs using U-NII is almost four times that available for b 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 b and a WLANs
CWNA Guide to Wireless LANs, Second Edition

49. Channel Allocation Figure 4-16: 802.11a channels
CWNA Guide to Wireless LANs, Second Edition

50. Channel Allocation (continued)
Figure 4-17: b vs a channel coverage
CWNA Guide to Wireless LANs, Second Edition

51. 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.
DSSS 3 AP X 11 Mbps = 33 Mbps.
FHSS 16 AP X 2 Mbps = 32 Mbps
CWNA Guide to Wireless LANs, Second Edition

52. Co-location Comparison40
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

53. 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

54. Physical Layer Standards
PLCP for a based on OFDM
Three basic frame components: Preamble, header, and data
Figure 4-18: a PLCP frame
CWNA Guide to Wireless LANs, Second Edition

55. Physical Layer Standards (continued)
Table 4-6: a Rate field values
CWNA Guide to Wireless LANs, Second Edition

56. Physical Layer Standards (continued)
Modulation techniques used to encode a data vary depending upon speed
Speeds higher than 54 Mbps may be achieved using 2X modes
Table 4-7: a characteristics
CWNA Guide to Wireless LANs, Second Edition

57. Physical Layer Standards (continued)
Figure 4-19: Phase shift keying (PSK)
CWNA Guide to Wireless LANs, Second Edition

58. Physical Layer Standards (continued)
Figure 4-20: Quadrature phase shift keying (QPSK)
CWNA Guide to Wireless LANs, Second Edition

59. Physical Layer Standards (continued)
Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
CWNA Guide to Wireless LANs, Second Edition

60. Physical Layer Standards (continued)
Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
CWNA Guide to Wireless LANs, Second Edition

61. IEEE 802.11g Physical Layer Standards
802.11g combines best features of a and b
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

62. IEEE 802.11g Physical Layer Standards (continued)
Table 4-8: IEEE g Physical layer standards
CWNA Guide to Wireless LANs, Second Edition

63. IEEE 802.11g Physical Layer Standards (continued)
Characteristics of g standard:
Greater throughput than b networks
Covers broader area than a networks
Backward compatible
Only three channels
If b and g devices transmitting in same environment, g devices drop to 11 Mbps speeds
Vendors can implement proprietary higher speed
Channel bonding and Dynamic turbo
CWNA Guide to Wireless LANs, Second Edition

64. Summary
Three modulation schemes are used in IEEE 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

65. 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 b are based on DSSS
CWNA Guide to Wireless LANs, Second Edition

66. Summary (continued)
IEEE a networks operate at speeds up to 54 Mbps with an optional 108 Mbps
The g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by a
CWNA Guide to Wireless LANs, Second Edition