Chapter 3

1. Chapter 3 • Radio Frequency Components, Measurement and mathematics

2. Exam Essentials • Understand the RF components. • Know the function of each of the components and which components add gain and which components add loss. • Understand the units of power and comparison. • Make sure you are very comfortable with the difference between units of power (absolute) and units of comparison (relative). Know all of the units of power and comparison, what they measure, and how they are used. • Be able to perform RF mathematics. • There will be no logarithms on the test; however, you must know how to use the rule of 10s and 3s. You will need to be able to calculate a result based on a scenario, power value, or comparative change.

3. Exam Essentials • Understand the practical uses of RF mathematics. • When all is said and done, the ultimate question is, Will the RF communication work? This is where an understanding of RSSI, SOM, fade margin, and link budget is important. • Define RSSI. • Understand that RSSI metrics are used by radios to interpret signal strength and quality. 802.11 radios use RSSI metrics for decisions such as roaming and dynamic rate switching. Understand the necessity of a link budget and fade margin. A link budget is the sum of all gains and losses from the transmitting radio, through the RF medium, to the receiver radio. The purpose of link budget calculations is to guarantee that the final received signal amplitude is above the receiver sensitivity threshold of the receiver radio. Fade margin is a level of desired signal above what is required.

4. Communications • For successful communications you need: • Two or more devices that want to communicate • Medium of communication • Set of rules to use • This chapter covers the medium • Components of RF • Transmission of RF signal • Role of each device Pg 64

5. RF Components • Transmitter • Antenna • Receiver • Intentional Radiator • Equivalent Isotropically Radiated Power (EIRP) Pg 65

6. Transmitter • Transmitter begins RF communication • Generates the AC signal • Modifies the signal based on modulation technique • Carrier Signal goes through cable or direct to antenna • Transmitter also sets the power level, or transmission amplitude • Dictated by regulation and can be adjusted • Often integrated with receiver-Transciever Pg 65

7. Antenna • Collects AC signal from transmitter and radiates RF waves • With receiver, takes RF Waves and directs to receiver • Receiver converts back to bits and bytes • Antenna signal is compared to a theoretical isotropic radiator • Point source that radiates equally in all directions Pg 66

8. Antenna • Antenna Power can be modified by • Adding power-Active Gain • Focusing Energy-Passive Gain • Like a lens Pg 66

9. Receiver • Takes carrier signal from antenna and convert back into 1 and 0 • Signal is much decreased from original amplitude • Free Space Path Loss • Also altered due to interference and multipath Pg 67

10. Intentional Radiator • Device that intentionally generates and emits radio frequency energy by radiation or induction • On purpose as opposed to by-product • All components up to the antenna • Transmitter, cable, connectors, equipment • The antenna can alter by focusing • FCC dictates max power, usually in MilliWatts (mW) or decibels relative to 1 milliwatt (dBm) Pg 67

11. Equivalent Isotropically Radiated Power (EIRP) • Highest signal strength transmitted from a particular antenna • Measure of the actual output of the antenna • Affected by shape of antenna • FCC will also regulate this power level Pg 67

12. Units of Power and Comparison • Key ideas for wireless are coverage and performance • To measure power, we can measure absolute or relative power • Absolute is compared to a known scale • Relative is to another signal Pg 69

13. Units of Power and Comparison • Comparative units can help: • compare coverage areas for different signals • Measure gain or loss • Measure the change in power Pg 69

14. Units of Power and Comparison Pg 69

15. Watt • Basic unit of power • 1 ampere of current at 1 volt • Volts x Amps • Ability to move/push/etc Pg 70

16. MilliWatt (mW) • 1/1000 th of a watt • Most 802.11 equipment is measured in milliwatts • Usually 1 to 100 mw • FCC may allow up to 1 W in some cases, but it isn’t usually needed except in point to point. Pg 70

17. Decibel (dB) • Base unit of comparison, not of power • Represents the difference between two values • Compare the power of two transmitters • Compare the output of a transmitter and received at the receiver • From the term bel • Bell Labs • 10 to 1 ratio Pg 70

19. Decibel (dB) • Bels are logarithmic • Use the log10 to calculate Pg 70

20. Decibel (dB) • Decibels are 10 x a bel • bel=log10(P1/P2) • decibel= 10Xlog10(P1/P2) • No log math on the test! • We use decibels instead of watts as it is easier to write in many cases. Pg 70

21. Decibel (dB) Pg 72

22. dBi • Antennas are compared to isotropic radiators • The difference between the theoretical isotropic radiator and the actual antenna can be measures in decibels isotropic (dBi) • Relative measurement • Change in power relative to an antenna • Measure of antenna gain • Measured at focus point • Always a gain, not a loss • No-gain or unity gain (0 dBi) • Think antenna Gain Pg 73

23. dBd • A second relative measure of strength • Decibel dipole • Decibel gain relative to a dipole antenna • Can also compare to dBi • Standard dipole is 2.14 dBi • If an antenna is 3 dBd the total is additive • 2.14+3=5.14 dBi Pg 73

24. dBm • Absolute measurement • Decibels relative to 1 mw of power • So 100 mW= +20dBm • Can also calc from a dBm value • PmW=log-1(PdBm/10) • 1 mW is reference and 0 dBm is 1 mW Pg 74

25. dBm • Why use dBm? • Easier to grasp -100dBm than .0000000001 mW • Also, the 6dB rule • If you double the distance between a received and transmitter, the received signal will decrease by 6 dB. • Also, every 6dBi of gain will double the usable distance of the RF signal • Also helps when adding units • If transmitter is +20dBm and the antenna is 5 dBi, the EIRP is 25 dBm Pg 74

26. Inverse Square Law • The 6 dB rule is based on Isaac Newton’s inverse square law. • Change in power is equal to the square of the change in distance • If you double the distance, the power will change by (2xD)2 • FSPL = 36.6 + (20log10(f)) + (20log10(D)) • FSPL = path loss in dB • f = frequency in MHz • D = distance in miles between antennas • FSPL = 32.4 + (20log10(f)) + (20log10(D)) • FSPL = path loss in dB • f = frequency in MHz • D = distance in kilometers between antennas Pg 76

27. RF Math • No need for LOG on test. • Rules of 10s and 3s • Provide for approximate values • For every 3 dB of gain (relative), double the absolute power (mW). • For every 3 dB of loss (relative), halve the absolute power (mW). • For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10. • For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10. Pg 77

28. RF Math • For every 3 dB of gain (relative), double the absolute power (mW). • For example, if your access point is configured to transmit at 100 mW and the antenna is rated for 3 dBi of passive gain, the amount of power that will radiate out of the antenna (EIRP) will be 200 mW Pg 77

29. RF Math • For every 3 dB of loss (relative), halve the absolute power (mW). • Conversely, if your access point is configured to transmit at 100 mW and is attached to a cable that introduces 3 dB of loss, the amount of absolute amplitude at the end of the cable will be 50 mW Pg 77

30. RF Math • For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10. • In another example, if your access point is configured to transmit at 40 mW and the antenna is rated for 10 dBi of passive gain, the amount of power that radiates out of the antenna (EIRP) will be 400 mW Pg 77

31. RF Math • For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10. • Conversely, if your access point is configured to transmit at 40 mW and is attached to a cable that introduces 10 dB of loss, the amount of absolute amplitude at the end of the cable will be 4 mW. Pg 77

32. RF Math • dBm is a measure of power • dB is a unit of change • dB can be applied to dBm • So, if you have +10dBm and increase by 3 dB, you have +13 dBm Pg 77

33. Step by Step Exercise Pg 78

34. RF Math Summary • Log Functions • dBm =10 × log10(mW) • mW = log–1 (dBm ÷ 10) = 10(dBm ÷ 10) • Rules of 10 and 3 • 3 dB gain = mW × 2 • 3 dB loss = mW ÷ 2 • 10 dB gain = mW × 10 • 10 dB loss = mW ÷ 10 Pg 85

35. RF Math Summary Pg 86

36. Received Signal Strength Indicator(RSSI) • Receive sensitivity is the power level of an RF signal required to be successfully received by the receiver • The lower this level, the more sensitive the receiver. • For 802.11 receive sensitivity is often defined as a function of network speed • In order to use a certain speed, you must have a certain level of loss • More loss, less speed. Pg 86

37. Sensitivity Thresholds Pg 87

38. Received Signal Strength Indicator(RSSI) • 802.11-2007 standard defines received signal strength indicator as a relative metric used to measure amplitude. • 0 to 255 • Usually mapped to receive sensitivity thresholds in dBm Pg 87

39. Received Signal Strength Indicator(RSSI) Pg 87

40. Received Signal Strength Indicator(RSSI) • 802.11-2007 also defines Signal Quality (SQ) • Singal quality as it affects coding techniques like barker or complementary code keying • Anything that affects bit error rate (BER) will trigger SQ metrics • RSSI and SQ are often refered to together as RSSI metrics Pg 88

41. Received Signal Strength Indicator(RSSI) • Signal to Noise Ration (SNR) is not signal quality • It is a measure of the difference between the received signal and background noise (noise floor) • Noise is -100 dBm and radio receives -85dBm the SNR is 15 dB • 25dB or greater is considered good • Vendors get to choose how to map RSSI • proprietary Pg 88

42. Received Signal Strength Indicator(RSSI) • Most vendors use RSSI for decisions on roaming and dynamic rate switching • Roaming is when a client switches from one AP to another • Dynamic Rate Switching is when 802.11 radios switch between data rates • Often due to reduced signal quality or loss Pg 88

43. Link Budget • Sum of all gains and losses from transmitting radio, through the RF medium to the receiver radio • Calculated to make sure the final received signal is about the sensitivity threshold Pg 90

44. Link Budget • Link budget calculations include • original transmit gain • passive antenna gain • active gain from RF amplifiers. • All gain must be accounted for, including RF amplifiers and antennas, and all losses must be accounted for, including attenuators, FSPL, and insertion loss. Pg 90

45. Link Budget • Loss in many places Pg 90

46. Link Budget Pg 91

47. Link Budget • Simpler example • +20 dBm + 5 dBi – 73.98 dB + 2.14 dBi = –46.84 dBm Pg 92

48. Fade Margin/System Operating Margin • Level of desired signal above what is required. • The buffer, or comfort zone • Plan for 10 to 25 dB above the receive sensitivity • 10 dB is minimum • Higher if more MILEs away • Fade Margin buffer is also known as the System Operating Margin (SOM) Pg 92

49. Exam Essentials • Understand the RF components. • Know the function of each of the components and which components add gain and which components add loss. • Understand the units of power and comparison. • Make sure you are very comfortable with the difference between units of power (absolute) and units of comparison (relative). Know all of the units of power and comparison, what they measure, and how they are used. • Be able to perform RF mathematics. • There will be no logarithms on the test; however, you must know how to use the rule of 10s and 3s. You will need to be able to calculate a result based on a scenario, power value, or comparative change.

50. Exam Essentials • Understand the practical uses of RF mathematics. • When all is said and done, the ultimate question is, Will the RF communication work? This is where an understanding of RSSI, SOM, fade margin, and link budget is important. • Define RSSI. • Understand that RSSI metrics are used by radios to interpret signal strength and quality. 802.11 radios use RSSI metrics for decisions such as roaming and dynamic rate switching. Understand the necessity of a link budget and fade margin. A link budget is the sum of all gains and losses from the transmitting radio, through the RF medium, to the receiver radio. The purpose of link budget calculations is to guarantee that the final received signal amplitude is above the receiver sensitivity threshold of the receiver radio. Fade margin is a level of desired signal above what is required.