Fundamentals of Wireless LANs 1.2

1. Fundamentals of Wireless LANs 1.2 Module 3: Wireless Radio Technology

2. Module Overview

3. Module Overview • In this module, the student will learn about wireless technology and radio waves. • This module will explore the technology and the mathematics of radio, so that the reader can understand how invisible radio waves work to make so many things possible, including WLANs.

4. Waves – Sine Waves • A waveform is a representation of how alternating current (AC) varies with time. • The most familiar AC waveform is the sine wave, which derives its name from the fact that the current or voltage varies with the mathematical sine function of the elapsed time • Frequency measured in cycles per second or Hertz (Hz). • A million cycles per second is represented by megahertz (MHz) • A billion cycles per second represented by gigahertz (GHz)

5. Sine Wave There is an inverse relationship between time and frequency: t = 1/f f = 1/t

6. Sine Wave Properties • Amplitude – The distance from zero to the maximum value of each alternation is called the amplitude. • Period – The time it takes for a sine wave to complete one cycle is defined as the period of the waveform. • The distance traveled by the sine wave during this period is referred to as its wavelength. • Wavelength – Indicated by the Greek lambda symbol λ. It is the distance between one value to the same value on the next cycle. • Frequency – The number of repetitions or cycles per unit time is the frequency, typically expressed in cycles per second, or Hz.

7. Watts • One definition of energy is the ability to do work. • There are many forms of energy, including: • electrical energy • chemical energy • thermal energy • gravitational potential energy • The metric unit for measuring energy is the Joule. • Energy can be thought of as an amount. • 1 Watt = I Joule of energy / one second • If one Joule of energy is transferred in one second, this is one watt (W) of power.

8. Watts • The U.S. Federal Communications Commission allows a maximum of 4 watts of power to be emitted in point-to-multipoint WLAN transmissions in the unlicensed 2.4-GHz band. • Typical WLAN NICS transmit at 100 mW. • Typical Access Points can transmit between 30 to 100 mW (plus the gain from the Antenna).

9. Watts • Power levels on a single WLAN segment are rarely higher than 100 mW, enough to communicate for up to three-fourths of a kilometer or one-half of a mile under optimum conditions. • Access points generally have the ability to radiate from 30 to100 mW, depending on the manufacturer. • Outdoor building-to-building applications (bridges) are the only ones that use power levels over 100 mW.

10. Decibels • The decibel (dB) is a unit that is used to measure electrical power. • The dB is measured on a base 10 logarithmic scale • The base increases ten-fold for every ten dB measured • The formula for calculating dB is: dB = 10 log10 (Pfinal/Pref)

11. Calculating dB • dB = The amount of decibels. • This usually represents a loss in power such as when the wave travels or interacts with matter, but it can also represent a gain as when traveling through an amplifier. • Pfinal = The final power. • This is the delivered power after some process has occurred. • Pref = The reference power. • This is the original power. • There are also some general rules for approximating the dB and power relationship: • An increase of 3 dB = Double the power • A decrease of 3 dB = Half the power • An increase of 10 dB = Ten times the power • A decrease of 10 dB = One-tenth the power

12. Decibel Reference The power gain or loss in a signal is determined by comparing it to this fixed reference point, the milliwatt.

13. dB milliWatt (dBm) • dB milliWatt (dBm) – This is the unit of measurement for signal strength or power level. • If a person receives a signal at one milliwatt, this is a loss of zero dBm. However, if a person receives a signal that is 0.001 milliwatt, then a loss of 30 dBm occurs. • This loss is represented as -30 dBm. • To reduce interference with others, the 802.11b WLAN power levels are limited to the following: • 36 dBm EIRP by the FCC • 20 dBm EIRP by ETSI EIRP = Effective Isotropic Radiated Power

14. dB dipole (dBd) • dB dipole (dBd) – This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency. • A dipole antenna is the smallest, least gain practical antenna that can be made.

15. dB isotropic (dBi) • dB isotropic (dBi) – This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna. • An isotropic antenna cannot exist in the real world, but it is useful for calculating theoretical coverage and fade areas. • A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. • For example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd.

16. Effective Isotropic Radiated Power • Effective Isotropic Radiated Power (EIRP) – is defined as the effective power found in the main lobe of a transmitter antenna. • EIRP is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna. http://en.wikipedia.org/wiki/EIRP

17. Gain • Gain – This refers to the amount of increase in energy that an antenna adds to an RF signal. • There are different methods for measuring gain, depending on the chosen reference point. • Cisco Aironet wireless is standardized on dBi to specify gain measurements. • Some antennas are rated in dBd. • To convert any number from dBd to dBi, simply add 2.14 to the dBd number.

18. Electromagnetic Waves – EM Waves • The EM spectrum is simply a name that scientists have given to the set of all types of radiation. • Radiation is energy that travels in waves and spreads out over distance. • All EM waves travel at the speed of light in a vacuum and have a characteristic wavelength (λ) and frequency (f) which can be determined by using the following equation: c = λ x f, where c = the speed of light (3 x 108 m/s) • EM waves exhibit the following properties: • reflection or bouncing • refraction or bending • diffraction or spreading around obstacles • scattering or being redirected by particles

19. Visible Light EM Radiation EM waves can be classified by their frequency in Hz or their wavelength in meters.

20. Eight EM Sections • Power waves – These are the slowest of all EM radiation and therefore also have the lowest energy and the longest wavelength. • Radio waves – This is the same kind of energy that radio stations emit into the air for a radio to capture and play. However, other things such as stars and gases in space also emit radio waves. Many communication functions use radio waves. • Microwaves– Microwaves will cook popcorn in just a few minutes. In space, astronomers use microwaves to learn about the structure of nearby galaxies. • Infrared (IR) light – Infrared is often thought of as being the same thing as heat, because it makes our skin feel warm. In space, IR light maps the dust between stars. • Visible light – This is the range that is visible to the human eye. Visible radiation is emitted by everything from fireflies to light bulbs to stars. It is also emitted by fast-moving particles hitting other particles. • Ultra-violet (UV) light – It is well known that the sun is a source of ultraviolet (UV) radiation. It is the UV rays that cause our skin to burn. Stars and other hot objects in space emit UV radiation. • X-rays – A doctor uses X-rays to look at bones and a dentist uses them to look at teeth. Hot gases in the universe also emit X-rays. • Gamma rays – Natural and man-made radioactive materials can emit gamma rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma rays. However, the biggest gamma-ray generator of all is the universe, which makes gamma radiation in many ways. Increasing frequency and energy / decreasing wavelength The EM spectrum has eight major sections, which are presented in order of increasing frequency and energy, and decreasing wavelength:

21. ISM Bands of Spectrum In the US, it is the FCC that regulates spectrum use. In Europe, the European Telecommunications Standards Institute (ETSI) regulates the spectrum usage.

22. Noise • A very important concept in communications systems, including WLANs, is noise. • In the context of telecommunications, noise can be defined as undesirable voltages from both natural and technological sources. • Since noise is just another signal that produces waves, the noise will be added to other signals – including wireless data! • Sources of noise in a WLAN include the electronics in the WLAN system, plus radio frequency interference (RFI), and electromagnetic interference (EMI) found in the WLAN environment. • Gaussian, or white noise affects all frequencies equally. • Narrowband interference would only interfere with some radio stations or channels of a WLAN.

23. Modulation Techniques • A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel. • For WLANs, the carrier frequency is 2.4 GHz or 5 GHz. • Using carrier frequencies in WLANs has added complexity because the carrier frequency is changed by frequency hopping or direct sequence chipping, to make the signal more immune to interference and noise.

24. Spread Spectrum (SS) • Spread-spectrum technology makes data transmission possible in the ISM bands • SS diffuses radio signals over a wide range of frequencies • The FCC requires that devices using the ISM bands use SS transmissions for data • By spreading data transmission over a wide range of frequencies, the transmission will look like noise to other non 802.11 devices • This also allows spread-spectrum devices to be more resilient to noise

25. Spread-Spectrum Technologies • 802.11 uses three types of spread-spectrum technologies: • Frequency Hopping (FHSS) systems jump from one frequency to another – legacy • Direct Sequence (DSSS) spread the signal over a wide range of frequencies – 802.11b/g • Orthogonal Frequency Division Multiplexing (OFDM) – 802.11a/g

26. Frequency Hopping • Frequency hopping (FH) systems are the least costly to produce but allow for the lowest data rates • FH rapidly changes from one frequency to another during data transmission using a predetermined pattern • This pattern is pseudorandom which means it is practically, never the same • The receiver radio is synchronized to the hopping sequence of the transmitting radio to enable the receiver to be on the right frequency at the right time. • The amount of time a sender stays at a particular frequency is known as the dwell time

27. FHSS • FHSS is a spread spectrum technique that uses frequency agility to spread data over more than 83 MHz of spectrum. • Frequency agility is the ability of a radio to change transmission frequency quickly, within the useable RF frequency band.

28. FHSS (cont.) • Frequency hopping avoids interference between two stations using the same band by using different hopping sequences • If any two stations do interfere with each other, the interference is for such a short time that it appears as transient noise

29. Direct Sequence Spread Spectrum • In the US, each channel operates from one of 11 defined center frequencies and extends 11 MHz in each direction • For example, Channel 1 operates from 2.401 GHz to 2.423 GHz, which is 2.412 GHz plus or minus 11 MHz. Channel 2 uses 2.417 plus or minus 11 MHz, and so on. • There is significant overlap between adjacent channels. Center frequencies are only 5 MHz apart, yet each channel uses 22 MHz of analog bandwidth. • In fact, channels should be co-located only if the channel numbers are at least five apart. Channels 1 and 6 do not overlap, Channels 2 and 7 do not overlap, and so on. • In Europe, ETSI has defined a total of 14 channels, which allows for four different sets of three non-overlapping channels.

30. Direct Sequence Spread-Spectrum (DSSS) • Whereas FHSS uses each frequency for a short period of time in a repeating pattern, DSSS uses a wide frequency range of 22 MHz all of the time. • Non-overlapping channels have 25 MHz of frequency between them which gives them a 3MHz buffer • Each data bit becomes a chipping sequence, or a string of chips that are transmitted in parallel, across the frequency range. • This is also referred to as the chipping code

31. Chipping Code Example 1 = 00110011011 0 = 11001100100 0 = 11001100100 1 = 00110011011

32. 802.11b Channels—FCC

33. 2.4 GHz Channel Sets Regulatory Domain Channel Identifier Center Frequency Americas Europe, Middle East and Asia Japan Israel X X X X X X X X X X X X X X X X X X X X X X X X X X X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2412 MHz 2417 MHz 2422 MHz 2427 MHz 2432 MHz 2437 MHz 2442 MHz 2447 MHz 2452 MHz 2457 MHz 2462 MHz 2467 MHz 2472 MHz 2484 MHz X X X X X X X X X X X X X X X X X X

34. Channels- 2.4 GHz DSSS • 11 “chips per bit” means each bit sent redundantly • 11 Mbps data rate • 3 access points can occupy same area 11 Channels – each channel 22 MHz wide 1 set of 3 non-overlapping channels 14 Channels – each channel 22 MHz wide 4 sets of 3 non-overlapping channels, only one set used at a time

35. Non-overlapping Channels - again

36. 802.11b Throughput • 802.11b uses three different types of modulation, depending upon the data rate used: • Binary phase shift keyed (BPSK) — BPSK uses one phase to represent a binary 1 and another to represent a binary 0, for a total of one bit of binary data. • BPSK is utilized to transmit data at 1 Mbps.  • Quadrature phase shift keying (QPSK) — With QPSK, the carrier undergoes four changes in phase and can thus represent two binary bits of data. • QPSK is utilized to transmit data at 2 Mbps. • Complementary Code Keying (CCK) — CCK uses a complex set of functions known as complementary codes to send more data by representing 4 or 8 binary bits. • CCK is can transmit data at 5.5 Mbps (4 bits) and 11 Mbps (8bits).

37. Complementary Code Keying (CCK) • CCK is an alternative encoding method to PSK which can encode 4 to 8 bits into a code word • The benefit of CCK is that it uses an 8-bit encoding scheme instead of an 11-bit encoding scheme to produce 1.375 times as much data transmission as PSK • When CCK encodes 4 binary bits at a time it produces 5.5Mbps of throughput and when CCK encodes 8 bits at a time it produces 11Mbps of throughput

38. DSSS Modulation and Data Rates The ‘D’ in the beginning stands for Differential http://en.wikipedia.org/wiki/Phase-shift_keying

39. Orthogonal Frequency Division Multiplexing • The 802.11a and 802.11g standards both use orthogonal frequency division multiplexing (OFDM), to achieve data rates of up to 54 Mbps. • OFDM works by breaking one high-speed data carrier into several lower-speed subcarriers, which are then transmitted in parallel. • Each high-speed carrier is 20 MHz wide and is broken up into 52 subchannels, each approximately 300 KHz wide • OFDM uses 48 of these subchannels for data, while the remaining four are used for error correction. http://www.wave-report.com/tutorials/OFDM.htm http://en.wikipedia.org/wiki/COFDM

40. OFDM (52 of 64 sub-carriers used) OFDM Subcarriers

41. 802.11a Modulation • The 802.11a standard specifies that all 802.11a-compliant products must support three basic data rates which include: Binary Phase Shift Keying (BPSK) – encodes 125 Kbps of data per channel, resulting in a 6,000-Kbps, or 6 Mbps Quadrature Phase Shift Keying (QPSK) – encodes to 250 Kbps per channel, yielding a 12 Mbps data rate. 16-level Quadrature Amplitude Modulation (16-QAM) – encodes 4 bits per hertz, achieving a data rate of 24 Mbps. • In addition, the standard also lets the vendor extend the modulation scheme beyond 24 Mbps. 64-level Quadrature Amplitude Modulation (64-QAM), which yields 8 bits per cycle or 10 bits per cycle, for a total of up to 1.125 Mbps per 300-KHz channel. With 48 channels, this results in a 54 Mbps data rate.

42. Refraction • A surface is considered smooth if the size of irregularities is small relative to the wavelength. Otherwise, it is considered to be rough. • Electromagnetic waves are diffracted around intervening objects. • If the object is small relative to the wavelength, it has very little effect and the wave will pass around the object undisturbed. • However, if the object is large a shadow will appear behind the object and a significant amount of energy is reflected back toward the source.

43. Refraction Sub-Refraction • Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere. • Amount of refractivity depends on the height above ground. • Refractivity is usually largest at low elevations. • The refractivity gradient (k-factor) usually causes microwave signals to curve slightly downward toward the earth, making the radio horizon father away than the visual horizon. • This can increase the microwave path by about 15%, Refraction (straight line) Normal Refraction Earth

44. Refraction • Radio waves also bend when entering different materials. • This can be very important when analyzing propagation in the atmosphere. • It is not very significant in WLANs, but it is included here, as part of a general background for the behavior of electromagnetic waves.

45. Reflection • Reflection is the light bouncing back in the general direction from which it came. • When waves travel from one medium to another, a certain percentage of the light is reflected. • This is called a Fresnel reflection.

46. Reflected Waves • When a wireless signal encounters an obstruction, normally two things happen: • Attenuation – The shorter the wavelength of the signal relative to the size of the obstruction, the more the signal is attenuated. • Reflection – The shorter the wavelength of the signal relative to the size of the obstruction, the more likely it is that some of the signal will be reflected off the obstruction.

47. Microwave Reflections • Microwave signals: • Frequencies between 1 GHz – 30 GHz (this can vary among experts). • Wavelength between 12 inches down to less than 1 inch. • Microwave signals reflect off objects that are larger than their wavelength, such as buildings, cars, flat stretches of ground, and bodes of water. • Each time the signal is reflected, the amplitude is reduced.

48. Reflection • Reflection is the light bouncing back in the general direction from which it came. • Consider a smooth metallic surface as an interface. • As waves hit this surface, much of their energy will be bounced or reflected. • Think of common experiences, such as looking at a mirror or watching sunlight reflect off a metallic surface or water. • When waves travel from one medium to another, a certain percentage of the light is reflected. • This is called a Fresnel reflection (Fresnel coming later).

49. Reflection • Radio waves can bounce off of different layers of the atmosphere. • The reflecting properties of the area where the WLAN is to be installed are extremely important and can determine whether a WLAN works or fails. • Furthermore, the connectors at both ends of the transmission line going to the antenna should be properly designed and installed, so that no reflection of radio waves takes place.

50. Reflections