Topic 2 Notes on Data Communications PDF

Topic 2 2.1 Components of Data Communications Data communications: the exchange of data between two devices (e.g. computers) using an electronic transmission system. 4 components of it: -Message (data transmitted to receiving device) - Transmitting device (sends out message) - Receiving device...

Topic 2 2.1 Components of Data Communications Data communications: the exchange of data between two devices (e.g. computers) using an electronic transmission system. 4 components of it: -Message (data transmitted to receiving device) - Transmitting device (sends out message) - Receiving device (receives it) - Transmission Medium (channel where message travels from transmitting to receiving) 2.2 Signals – Analog and Digital Signal: an electromagnetic current that carries data from the source to the receiver. - Can be analog/digital. Analog signal - in frequency domain e.g. human voice = continuous sine wave. Digital signal - have higher bandwidth than Analog signals cuz use more advanced modulation and encoding techniques  more efficient use of the available frequency spectrum. e.g. ‘1’/’0’ in computer = discrete pulse wave. Aperiodic: signals do not display any repetitive pattern over the time. Periodic: recognizable pattern for a fixed period of time and the pattern repeats over the subsequent identical fixed periods of time. - e.g.s Period analog = sine, Period digital = square 2.3 Bandwidth of Signals and Media 2.3.1 Signal Bandwidth Baseband: frequency range associated with a message signal when it is first generated. Bandwidth: refers to the span of frequencies in a band used to transmit a signal through a particular medium  - Formula- bandwidth = f2 – f2 (all in hertz (Hz)) - bandwidth of signal maybe the same as or different form from the baseband 2.3.2 Media Bandwidth Bandwidth of a transmission medium: the minimum passband (a range of frequencies) required to propagate the source information from transmitting device to the receiving device. - For identical signal at the receiving end, all frequency components of a signal must be transmitted and their amplitudes and phases are not distorted. - e.g. transmission medium can pass signal components which are in the range of 3000 Hz to 5000 Hz, what is the bandwidth of the medium? 5000 – 3000 = 2000 Hz bandwidth of cable must be wider that OG signal else, signal distorted, cannot all go thru 2.3.3. Data Transmission Rate and Bandwidth 2 terms associated with digital. - Bit Interval: the time interval required to send one bit of information. - Bit Rate: number of bits that can be sent out in 1 second. Formulas: o Bit Rate = 1/Bit Interval bps o Bandwidth = Bit Rate/2 bps (bps: bits per sec- for data transmission) Bandwidth summary: - Analog bandwidth (Hz): determines the range of signal frequencies that can be passed through. - Digital bandwidth (bps): sets the maximum allowable data bit rate can be passed through. 2.4 Types of the transmission (Analog and Digital Transmission) 2.4.1 Analog Transmission of Analog and Digital signals Analog system: 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. (the analog transmitted signal is sine wave) Basic analog comm. System: 1. Transmission channel- physical link between communicating parties 2. Modulator- takes source signal and transforms it so that it is physically suitable for the transmission channel 3. Transmitter- actually amplifies the modulated signal before it is fed to the transmission channel 4. Receiver- detects transmiited signal on channel and amplifies it before entering demodulator 5. Demodulator- extracts original source and send it to destination Analog method (analog  modulated analog): - Amplitude Modulation (AM) - Double Side Band Suppressed Carrier (DSBSC - Single Side Band (SSB) - Phase Modulation (PM) - Frequency Modulation (FM) Modem (digital  modulated analog): - 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: - AM, FM radio (analog input) - Local Area Network (LAN) (digital input) 2.4.2 Digital Transmission of Analog and Digital signals Digital transmission: 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. Analog  digital conversion techniques: Pulse modulation methods: PAM – pulse amplitude modulation PWM – pulse width modulation PPM – pulse position modulation Digital modulation methods: PCM – pulse code modulation DM – delta modulation Encoder: converts a set of binary inputs into a unique binary code. Decoder: converts unique binary code into a set of binary outputs. 2.5 Types of the transmission medium Transmission medium: refers to base material that helps transmit/transfer data from one device to another. - Signal can be transmitted through any material e.g.s fibre-optic cable, twisted-wire pair cable, coaxial cable, waveguide, and free space - in the form of modulated radio, light, between the transmitter and receiver - free space: transmission medium for electromagnetic waves, but its not 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 Determining factors for medium choice: - data rate - distance - bandwidth, high bandwidth allows higher data transmission rate. - attenuation- reduction of signal strength (amplitude) - interference, minimised by proper shielding in guided media. - no. of shared receivers, introduces attenuation and distortion. Transmission media groups: 1) guided/wired: data transmitted through physical wires directed to a particular receive. a. Fiber-optic: transmits light waves to transfer data i. bundle-packed by protective sheath. ii. benefits: - transmit data at high speeds. - larger bandwidth than copper cables - immune to EMI - transmit data over long distances w/o loss of signal quality. b. Twisted: bunch of two coiled wires insulated separately in a protective sheath  i. Unshielded (UTP): pair of 2 insulated copper wires that inhibit interference  for telecommunications. ii. Shielded (STP): foil covering the twisted pair of wires to block the external interference during transmission  for data transmission in telephone cables; ethernet. c. Coaxial: 2 insulated // conductors. i. 1. Innermost copper wire 2. Insulation 3. // conductor: 2nd conductor & noise reducer. 4. Outer jacket ii. Coaxial types: - Baseband: digital signals to deliver large amounts of data at high speed  LANs. - Broadband: analog signals for multiple waves of diff frequencies to pass through  television cables as medium. 2) unguided/wireless: no physical bound to transmit data, occurs in atmosphere. a. Radio: radio wave frequency range is around 3KHz – 1GHz. i. Pass through buildings. ii. Simple to set up. iii. Propagate either through troposphere/ionosphere. b. Microwave: 1GHz – 300GHz. i. Higher frequency than radio. ii. Sending and receiving antennae to be positioned accurately. iii. Higher antennae, more efficient. c. Infrared: 300GHz – 400THz i. Shorter distance travelled by the waves. ii. Remotes, wireless gadgets. Qns: 1) What is a complex analog signal? More of equations combined… 2) Is there a distinct difference between signal and media bandwidth? Media as in the medium for transmission like cable?? 3) So in real world context we just assume that analog is continuous wave while digital is pulse wave, both aperiodic unless specified to be periodic, in which case they respectively will be sine wave and square wave? 4) Are modulator = transmitter and receiver = demodulator? So what’s an analog carrier? 5) What does it exactly mean when signal is modulated or demodulated? 6) Is the number of frequencies of a wave of a modulated/transmitted wave still the same after its been modulated/transmitted? 7) Ask why in the OAL, its digital became a sine wave and not a continuous one?