Guide to Wireless Communications

1. Guide to Wireless Communications

2. How Wireless Works Chapter 2

3. Learning Objectives • Explain how network data is represented using binary notation • List and explain the two types of wireless transmission • Describe the different ways in which data can be transmitted by radio waves

4. Mind-boggling… • Making sense of the number of parts that make up a modern communication system can be confusing • The “bottom-up” method lets us look first at the individual elements that make up a system and then tie them together to see how it all works

5. How Data Is Represented • Digital wireless data is represented the same way that standard computer data is • Binary or Base 2 numbering system has only two states, using the digits 0 and 1 • Eight of these binary digits (bits) are grouped together to form a byte • Each character or symbol is assigned an arbitrary number based on a specific coding scheme, such as the American Standard Code for Information Interchange (ASCII)

6. ASCII and Unicode • ASCII has a possible 256 different codes, represented by the decimal values 0-255 • Each character or symbol is assigned a particular number based on the coding scheme • Table 2-1 shows part of the ASCII code • Not enough codes in ASCII for all symbols in foreign languages such as Chinese • Another coding scheme called Unicode has a possible 65,535 different characters using 16-bits for each one

7. Partial ASCII Code Table

8. Wireless Signals • Wireless communications use waves that travel through space • The electromagnetic spectrum or electromagnetic waves require no medium for movement • Waves travel freely through space at speed of light or 186,000 miles per second • Figure 2-1 shows the electromagnetic spectrum • Wireless signals use infrared light and radio waves

9. Electromagnetic Spectrum

10. Infrared Light • Easy to send information with light using a binary code with only two digits • A 1 in binary flashes a light on, while a 0 signal leaves the light off, as seen in Figure 2-2 • Visible light, making up only small part of spectrum, is not reliable for data transport • Infrared light, although invisible, is a better way of transmitting wireless data

11. Light Transmission

12. Infrared Wireless Transmission • Wireless devices must have two components: • An emitter, usually a laser diode or light-emitting diode, transmits a signal • A detector receives the signal and produces a proportional electrical current, as shown in Figure 2-3

13. Light Pulses

14. Directed Wireless Transmission • Directed transmission requires emitter and detector to be directly aimed at one another • Figure 2-4 shows line of sight transmission

15. Diffused Wireless Transmission • Diffused transmission relies on reflected light • Emitter points at the ceiling of room and uses it as a reflection point • The detector, aimed at same reflection point, detects the reflected signal, as shown in Figure 2-5

16. Diffused Transmission

17. Advantages No interference with or effect from other communication signals Does not penetrate walls, so signals kept inside room Impossible for someone elsewhere to “listen in” on signals Limitations Lack of mobility Limited range of coverage Limited transmission speed, only up to 4Mbps for diffused infrared Advantages and Limitations of Infrared Wireless

18. Specialized Applications of Infrared Wireless • Used for data transfer between devices such as notebook computers, digital cameras, PDAs, and similar mobile devices • Wireless local areas networks using infrared signals • Common in areas where radio signals would interfere, such as hospital operating rooms or secure government buildings

19. Infrared Wireless Device

20. Limited Movement of Infrared Transmission • Light waves and heat waves cannot penetrate materials • Infrared waves travel only a limited distance • See Figure 2-7

21. Radio Waves • More common and effective means of wireless communications • Similar to water waves from garden hose in Figure 2-8 • Radiotelephone (radio) waves created by electrical current passing through wire and radiating outward in all directions

22. Advantages of Radio Waves • Not limited like light or heat waves • Can travel great distances • Can penetrate nonmetallic objects • See Figure 2-9

23. Radio Waves Without Boundaries

24. How Radio Data Is Transmitted • To understand how radio waves transmit data over long distances, one must first understand several concepts, including: • Analog and Digital • Frequency • Transmission Speed • Analog Modulation • Digital Modulation • Spread Spectrum

25. Analog Signals • Continuous signal with no breaks • No individual element can be uniquely identified from another element of signal • Audio, video, and light all use analog signals • See Figure 2-10

26. Digital Signals • Has discrete or separate on/off activity • Squirting water through a garden hose is an example, as seen in Figure 2-11 • Morse code with series of dots and dashes is another digital signal

27. Digital Signals • Computers use digital signals • Digital signal must be converted to analog to be transmitted over analog medium like a telephone line • A modem (for Modulator/ DEModulator) converts from analog to digital, or vice versa

28. Frequency • Using the garden hose example, you can move your hand slowly to create long waves, as seen in Figure 2-13 • If you move your hand quickly, you create short waves, as seen in Figure 2-14 • Rate at which an event occurs results in a different number of radio waves or the frequency of the radio wave

29. Long and Short Waves Long waves Short waves

30. Frequency • Frequencies refer to how frequently an event occurs • A cycle is changing the event to create different radio frequencies • Radio transmissions send a carrier signal that changes based on electrical pressure (voltage) and direction of the signal • Sine wave or oscillating wave demonstrates changing signals, as seen in Figure 2-15

31. Sine Wave • Wave starts at zero, moves up to maximum voltage, then down to minimum voltage, and finally back to starting point • This is one cycle • Frequency is number of times a wave completes a cycle

32. Different Frequencies • Figure 2-16 illustrates two different frequencies, a lower one and a higher one • A change in voltage does not create a change in frequency • A change in frequency results in how long it takes to reach maximum, fall back to minimum, and then return to neutral to complete a cycle

33. Lowest and Highest Frequencies

34. Measuring Frequencies • Frequencies are measured by number of cycles per second • Hertz (Hz) is used instead of “cycle” • Kilohertz (KHz) is a thousand hertz • Megahertz (MHz) is a million hertz • Gigahertz (GHz) is a billion hertz • In electrical terms, the cycle produces alternating current (AC) because it flows between positive and negative

35. Antenna • An antenna, usually a copper wire, allows radio waves to be transmitted and received • During transmission, the radio wave strikes wire and sets up an electrical pressure (voltage) along wire, causing small electrical current to flow into it • Receiving antenna moves back and forth in response to radio signals reaching it, creating a voltage into receiver • See Figure 2-17

36. Radio Antennas Transmit and Receive

37. Transmission Speed • Speed of data transmitted by radio waves is measured in bits per second (bps) • Baud refers to a change in signal or the number of times signal changes per second • Sometimes bps and baud are incorrectly used interchangeably • Originally one bit was transmitted per baud • Now, more than one bit can be transmitted per baud • See Table 2-2

38. Bit Representation of Four Signal Changes

39. Multiple Bits per Baud • More than one bit may be transmitted per baud • A dibit is two bits per baud • A tribit is three bits per baud • A quadbit is four bits per baud • See Table 2-3

40. Signaling Techniques

41. Bandwidth • Bandwidth is another term for measuring transmission speed • Bandwidth is the range of frequencies that can be transmitted or the difference between the higher frequency and the lower frequency

42. Analog Modulation • A carrier signal is a continuous electrical signal that carries no information • Three types of modulation or changes to the signal enable it to carry information • The height of the signal • The frequency of the signal • The relative starting point • These three are sometimes called the “three degrees of freedom”

43. Amplitude • Amplitude refers to the height of the carrier wave • See Figure 2-18

44. Amplitude Modulation (AM) • Amplitude modulation (AM) is changing the height of the carrier so that a higher wave represents a 1 and a lower wave represents a 0, as shown in Figure 2-19 • Radio stations frequently use AM, transmitting between 535 KHz and 1,700 KHz • AM is very susceptible to interference from outside sources, such as lightning • AM is not suitable for data transmission

45. Amplitude Modulation (AM)

46. Frequency Modulation (FM) • Frequency Modulation (FM) changes the number of waves used to represent one cycle • More waves represent a 1 bit while fewer waves represent a 0 bit, as shown in Figure 2-20 • Radio stations frequently broadcast in FM since it is not as susceptible to interference from outside sources • FM stations broadcast between 88 MHz and 108 MHz

47. Frequency Modulation (FM)

48. Phase Modulation (PM) • Phase Modulation (PM) changes the starting point of the cycle • A change in starting point that goes down represents a change in the bit transmitted (0 to 1) • The starting point is the complete opposite of what is being sent, or 180 degrees • See Figure 2-21

49. Phase Modulation (PM)

50. Phase Modulation (PM) • PM has eight different starting points of a carrier signal • Each dot in Figure 2-20 represents a starting point • A tribit requires eight signals