Aircraft Data Communications & Networking PDF

Aircraft Data Communications & Networking Topic 2: Basic of Data Communications 2.1 Components of Data Communications……………………………………2 2.2 Signals – Analog and Digital……………………………………………...3 2.3 Bandwidth of Signals and Media……………………………….…………4 2.3.1 Signal Bandwidth……...

Aircraft Data Communications & Networking Topic 2: Basic of Data Communications 2.1 Components of Data Communications……………………………………2 2.2 Signals – Analog and Digital……………………………………………...3 2.3 Bandwidth of Signals and Media……………………………….…………4 2.3.1 Signal Bandwidth……………………………………………….…. 4 2.3.2 Media Bandwidth…………………………………………………...4 2.3.3 Data Transmission Rate and Bandwidth……………………………5 2.4 Types of transmission (Analog & Digital transmission) …………………6 2.4.1 Analog Transmission of Analog and Digital signals………………6 2.4.2 Digital Transmission of Analog and Digital signals……………….8 2.5 Types of transmission medium……………………………………………8 September 2024 Page 1 Aircraft Data Communications & Networking 2.1 Components of Data Communications Data communications is about the exchange of data between two devices (e.g. computers) using an electronic transmission system. For occurrence of data communication, communicating devices must be a part of communication system made up of a combination of hardware or software devices and programs. There are four components which are essential in a data communication system as shown in Fig 2.1. - Message - Transmitting device - Receiving device - Transmission Medium Fig 2.1 Components of a data communication system Message is the data or information to be transmitted to the receiving device. They could be in the form of text, audio, video or a combination of them. There are different types of data representation formats, such as groups of bits or bytes, ASCII (American Standard Code for information Interchange) for character codes, JPEG (Joint Picture Expert Group) for digital still pictures, etc. A transmitting device is an electronic device that sends out messages. Examples of transmitting devices are computers, telephones, mobiles, video cameras, or workstations, etc. September 2024 Page 2 Aircraft Data Communications & Networking A receiving device receives the message and processes the information in the message. Examples are computers, telephones, mobiles, workstations, televisions, etc. Transmission medium is the physical channel where a message travels from transmitting device to the receiving device. Transmission medium could be guided (with wires) or unguided (without wires), for example, twisted pair cable, fiber optic cable, radio waves, microwaves, etc. 2.2 Signals – Analog and Digital An electromagnetic current that carries data from the source to the receiver is termed a signal. Data or message is the first component in a data communications system. They can be in the form of analog or digital. The data carried through the signal can be in the form of voice, music, video, document, or a picture. Example of analog data is the human voice, which is a continuous sine wave. Example of digital data: digital file (in ‘1’ or ‘0’) stored in computer. Fig 2.2 Example of Analog and Digital Signals Data (be it analog or digital) are then transformed into analog or digital signals respectively before transmission, as shown in above Figure 2.2. Both signal types are aperiodic signals as they do not display any repetitive pattern over time. On the other hand, a periodic signal has a recognizable pattern for a fixed period and the pattern repeats over the subsequent identical fixed periods of time. Example for a Periodic Analog Signal is the sine wave over time; and for a Periodic Digital Signal is the square wave over time. September 2024 Page 3 Aircraft Data Communications & Networking 2.3 Bandwidth of Signals and Media 2.3.1 Signal Bandwidth Baseband is the frequency range associated with a message signal when it is first generated. The bandwidth of signal may be the same as or different form from the baseband. For example, the original signal may be processed before transmission and therefore the bandwidth may be smaller or greater than the baseband. Analog signal is best described in frequency domain. For non-complex signals such as sine wave which has only one frequency component (excluding DC component), the bandwidth of the signal is equal to the signal frequency. For complex analog signals, the range of the frequency components will determine the required bandwidth of transmission medium. Hence bandwidth can be measured as Bandwidth = f₂ – f₁ (where f₂ is the upper frequency and f₁ is the lower frequency in a band). Bandwidth generally refers to the span of frequencies in a band used to transmit a signal through a particular medium. Example of a calculation using the bandwidth formula: The bandwidth of an analog signal is 3.4 kHz. The frequency spectrum has a highest frequency component at 10.2 kHz. What is the lowest component of the signal? (Hints: Bandwidth given (3.4 kHz), f2 given too (10.2 kHz), then use the above formula) Digital signals typically have higher bandwidth than Analog signals because they use more advanced modulation and encoding techniques that allow for more efficient use of the available frequency spectrum. For example: A square wave can be obtained from a sum of many sine waves of different frequencies and amplitudes. This is known as Fourier synthesis or analysis. In other words, requiring more sine waves to approximate a square wave can lead to higher bandwidth requirements due to additional frequency components needed to accurately represent the waveform. 2.3.2 Media Bandwidth Bandwidth of a transmission medium is the minimum passband (a range of frequencies) required to propagate the source information from transmitting device to the receiving device. Some frequency components of a signal will be passed through the medium and some will be blocked September 2024 Page 4 Aircraft Data Communications & Networking or weakened. To have an identical signal at the receiving end, all frequency components of a signal must be transmitted, and their amplitudes and phases are not distorted. For example: if a transmission medium can pass signal components which are in the range of 3000 Hz to 5000 Hz, what is the bandwidth of the medium? Answer is 5000 – 3000 = 2000 Hz. 2.3.3. Data Transmission Rate and Bandwidth In digital signal transmission, there are two important new terms associated with the digital signal: Bit Interval: The time interval required to send one bit of information. Bit Rate: The data transmission rate in bits per sec (bps) or number of bits that can be sent out in 1 second. The below figure shows the relationship between the bit interval and the bit rate of a transmission system. In other words, their relationship can be expressed as follows: For transmission of digital data over a transmission medium, the required bandwidth of the medium must be known. In a digital signal, 1-bit is used to represent each data level (For example, a data level of 5V is represented by binary ‘1’ and 0V by binary ‘0’, the minimum bandwidth requirement is assumed as follows: September 2024 Page 5 Aircraft Data Communications & Networking Bit rate and the bandwidth are proportional to each other for 1-bit digital signal Another term you will come across very often is BAUD RATE when each data level contains more than 1-bit digital signal. Please watch the 3rd video from Bright Space or Highlight of Topic 2 explaining about 3-bit digital signal representing each signal level change. Summary of bandwidth as follows: - Analog bandwidth of a transmission medium in Hertz determines the range of signal frequencies that can be passed through. - Digital bandwidth of a transmission medium in bits per sec (bps), and it sets the maximum allowable data bit rate can be passed through. 2.4 Types of the transmission (Analog and Digital Transmission) We will examine the two types of transmission with both having the analog and digital signals. 2.4.1 Analog Transmission of Analog and Digital signals An analog system is designed for transmitting analog signals (continuously variable along amplitude and time). It can be made to transmit digital input (having discrete pulses) if it can be converted to analog signal using a modem. Fig 2.4.1 Analog Transmission of Analog and Digital signals September 2024 Page 6 Aircraft Data Communications & Networking A basic analog communication system consists of: - A transmission channel/medium which provides the physical link between the communicating parties. - A modulator that takes the source signal and transforms it so that it is physically suitable for the transmission channel. - A transmitter that amplifies the modulated signal before it is fed to the transmission channel. - A receiver that detects the transmitted signal on the channel and usually amplifies it before it goes to the demodulator. - A demodulator that extracts the original source signal and sends it to the destination. Analog modulation describes the modulation of an analog input (input data) onto the analog carrier. The analog modulation methods are: - Amplitude Modulation (AM) - Double Side Band Suppressed Carrier (DSBSC) - Single Side Band (SSB) - Phase Modulation (PM) - Frequency Modulation (FM) A modem (contraction of modulator-demodulator) allows digital input (input data) from the computer to be converted to the analog voice grade telephone lines. Techniques for converting digital information to analog signals are: - Amplitude shift keying (ASK) - Frequency shift keying (FSK) - Phase shift keying (PSK) - Binary phase shift keying (BPSK) - Quadrature phase shift keying (QPSK) - Quadrature amplitude modulation (QAM) Applications: - Analog data over an analog medium such as AM and FM radio. - Digital data over an analog medium such as Local Area Network (LAN) communication. September 2024 Page 7 Aircraft Data Communications & Networking 2.4.2 Digital Transmission of Analog and Digital signals Digital transmission is the transmittal of digital pulses between two points in a communication system. The information at the source may be a digital or an analog signal. If it is analog, then devices like codec (coder and decoder) are used to convert the analog input to digital pulses before transmission and convert it back to the analog form at the destination. Fig 2.4.2 Digital Transmission of Analog and Digital signals Techniques for converting analog input to digital pulses are: - Pulse modulation methods: o PAM – pulse amplitude modulation o PWM – pulse width modulation o PPM – pulse position modulation - Digital modulation methods: o PCM – pulse code modulation o DM – delta modulation There are many advantages of digital transmission over the analog one but some disadvantages too for digital transmission over the analog one. Subsequent topics of this module will be touching based on digital transmission method. 2.5 Types of the transmission medium A transmission medium refers to the base material that helps transmit or transfer the data from one device to another. Signal waves can be transmitted through any material substance, such as September 2024 Page 8 Aircraft Data Communications & Networking fibre-optic cable, twisted-wire pair cable, coaxial cable, waveguide, and free space, that can be used for the propagation of signals, usually in the form of modulated radio, light, between the transmitter and receiver. Free space can be considered a transmission medium for electromagnetic waves, although it is not a material medium. Points to note for transmission medium: - Physical path between transmitter and receiver - Guided (wired) or unguided (wireless) - Communication is in the form of electromagnetic waves - Characteristics and quality of data transmission depends on the characteristics of the medium and signal - In the guided medium, medium characteristics are more important, whereas in the unguided medium, bandwidth and frequency are important considerations Choice of medium depends on: - Data rate - Distance - Bandwidth – high bandwidth allows higher data rate - Attenuation - Interference – can be minimized by proper shielding in guided media - Number of shared receivers – each attachment introduces attenuation and distortion The transmission media can be classified as the following based on transmitting the signals. The two broad groups of transmission media are: - Guided media or Wired type such as Twisted-pair cable and Fiber optic cable. - Unguided or Wireless type such as Radio wave, Microwave and Infrared wave, etc. Guided Transmission media: the data are transmitted through physical wires directed to a particular receiver. These are divided into 3 types. September 2024 Page 9 Aircraft Data Communications & Networking - Fiber Optic cable: transmits light waves to transfer data. The fiber optics are packed as a bundle covered by a protective sheath. You may refer to Experiment Lab#1 for details of its construction and the tools and tester needed to test the cable. The benefits of using Fiber Optic cable are: o Capable of transmitting data at very high speeds, o Having a larger bandwidth as compared to copper cables, o Immune to electromagnetic interference, o Can transmit data over longer distances without loss of signal quality. - Twisted cable: contain a bunch of two coiled wires that are insulated separately in a protective sheath. It can be divided into two types. - Unshielded twisted pair (UTP) is a pair of two insulated copper wires that inhibit interference. Thus, it is most used in telecommunications. - Shielded twisted pair (STP) has a foil covering the twisted pair of wires to block the external interference during transmission. It is mostly used in transmitting data through telephone cables and on the Ethernet. You may refer to Experiment Lab#1 on the UTP cable making and testing for Ethernet. - Coaxial cable: consists of two insulated parallel conductors. ========== Further Reads on Coaxial cable: 1st layer – The innermost copper wire layer. 2nd layer – An insulation layer around the copper wire. 3rd layer is made up of the parallel conductor, which acts as a second conductor and a noise reducer. 4th layer – another insulator is protecting the parallel secondary conductor. This whole 4-layered set-up covers the cable with a plastic tube that protects it. September 2024 Page 10 Aircraft Data Communications & Networking Fig 2.5.1 Coaxial cable & connectors The coaxial cable is of two types: - Baseband uses digital signals so it can be used to deliver large amounts of data at high speed. It is used in LANs. - Broadband uses analog signals allowing multiple waves of varied frequencies to pass through. Television cables use such a medium. ======== Unguided Transmission media: is a wireless medium that requires no physical bound to transmit data. The transmission occurs through the air in the atmosphere. It can be of 3 different types. - Radio wave: The radio wave frequency range is around 3KHz to 1GHz. It can pass through the buildings and is simple to set up. Radio waves either propagate through the troposphere or the ionosphere. - Microwave: The microwave frequency range is around 1GHz to 300GHz. It has a higher frequency than radio waves. It needs the sending and receiving antennae to be positioned accurately. The higher the antennae, the more efficient the signaling is. - Infrared wave: The frequency range of infrared waves is around 300GHz to 400THz, so the distance travelled by the waves is shorter. It is generally used in Remotes and wireless gadgets. September 2024 Page 11 Aircraft Data Communications & Networking Fig 2.5.2 Electromagnetic Spectrum (Wikimedia Commons) --- END--- September 2024 Page 12