ITS323 Transmission Media Design Factors Guided Media Transmission Media Wireless Transmission Wireless Media ITS323: Introduction to Data Communications Sirindhorn International Institute of Technology Thammasat University Prepared by Steven Gordon on 5 August 2014 ITS323Y14S1L04, Steve/Courses/2014/s1/its323/lectures/media.tex, r3262
ITS323 Contents Transmission Media Design Factors Design Factors Guided Media Wireless Transmission Wireless Media Guided Media Wireless Transmission Wireless Media
ITS323 Design Factors Transmission Media ◮ Key concerns are data rate and distance: maximise both ◮ Design factors: Design Factors Guided Media ◮ Bandwidth Wireless ◮ Transmission impairments Transmission ◮ Interference Wireless Media ◮ Number of receivers
ITS323 Electromagnetic Spectrum for Transmission Telecommunications Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Contents Transmission Media Design Factors Design Factors Guided Media Wireless Transmission Wireless Media Guided Media Wireless Transmission Wireless Media
ITS323 Electrical Cables Transmission Media ◮ Transmit electrical signals on a conductor, e.g. copper ◮ Cable carrying electrical current radiates energy, and Design Factors can pick-up energy from other sources Guided Media Wireless ◮ Can cause interference on other cables Transmission ◮ Other sources can cause interference on the cable Wireless Media ◮ Interference results in poor quality signals being received ◮ To minimise interference: ◮ Keep the cable lengths short ◮ Keep the cables away from other sources ◮ Design the cables to minimise radiation and pick-up ◮ Use materials to shield from interference ◮ Organise multiple wires so they dont interfere with each other
ITS323 Twisted Pair Transmission Media ◮ Two insulated copper wires arranged in spiral pattern ◮ Most commonly used and least expensive medium Design Factors Guided Media ◮ Used in telephone networks and in-building Wireless communications Transmission ◮ Telephone networks designed for analog signalling (but Wireless Media supporting digital data) ◮ Also used for digital signalling ◮ Two varieties of twisted pair: shielded (STP) and unshielded (UTP); also multiple categories (CAT5)
ITS323 Coaxial Cable Transmission Media ◮ Two conductors, one inside the other ◮ Provide much more shielding from interference than Design Factors twisted pair: Higher data rates; More devices on a Guided Media Wireless shared line; Longer distances Transmission ◮ Widely used for cable TV, as well as other audio/video Wireless Media cabling ◮ Used in long-distance telecommunications, although optical fibre is more relevant now
ITS323 Optical Fibre Transmission Media ◮ Light (optical rays) is guided within glass or plastic fibres Design Factors ◮ Used in long-distance telecommunications, as well as Guided Media Wireless telephone systems, LANs, and city-wide networks Transmission ◮ Advantages of optical fibre over electrical cables: Wireless Media 1. Lower loss: can transfer larger distances 2. Higher bandwidth: a single fibre is equivalent to 10’s or 100’s of electrical cables 3. Small size, light weight: lowers cost of installation 4. Electromagnetic isolation
ITS323 Comparison of Guided Media Transmission Media Electrical Cables Design Factors ◮ Moderate data rates: 1Gb/s Guided Media Wireless ◮ Maximum distance: 2km (twisted pair); 10km (coaxial) Transmission ◮ Cheapest for low data rates Wireless Media ◮ UTP: easy to install, susceptible to interference ◮ STP, Coaxial Cable: rigid, protection against interference Optical Cables ◮ Very high data rates: 100Gb/s+ ◮ Maximum distance: 40km ◮ Expensive equipment, but cost effective for high data rates ◮ Difficult to install
ITS323 Point-to-Point Transmission Characteristics of Transmission Guided Media Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Attenuation of Typical Guided Media Transmission Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Contents Transmission Media Design Factors Design Factors Guided Media Wireless Transmission Wireless Media Guided Media Wireless Transmission Wireless Media
ITS323 Wireless Transmission Model Transmission Media Common wireless systems for communications include: Design Factors ◮ Terrestrial microwave, e.g. television transmission Guided Media ◮ Satellite microwave, e.g. IPstar Wireless Transmission ◮ Broadcast radio, e.g. IEEE 802.11 WiFi (wireless LAN) Wireless Media ◮ Infrared, e.g. in-home communications Transmit Receive Antenna Antenna Signal Transmitter Receiver
ITS323 Antennas Transmission Media ◮ Antenna converts between electrical current and electromagnetic waves Design Factors Guided Media ◮ Waves are within the Radio Frequency (RF) band of 3 Wireless kHz to 300 GHz Transmission ◮ Antenna characteristics are same whether sending or Wireless Media receiving ◮ Direction and propagation of a wave depends on antenna shape ◮ Isotropic antenna: power propagates in all directions equally (spherical pattern, ideal) ◮ Omni-directional antenna: power propagates in all directions on one plane (donut) ◮ Directional antenna: power concentrated in particular direction ◮ Power output in particular direction compared to power produced by isotropic antenna is antenna gain [dBi]
ITS323 Antenna Examples Transmission Media See http://www.cisco.com/en/US/products/hw/ wireless/ps469/ Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Parabolic Reflective Antenna Transmission Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Calculating Antenna Gain Transmission Media ◮ Relationship between effective area of antenna and its gain: Design Factors G = 4 π A e Guided Media λ 2 Wireless Transmission where λ is signal carrier wavelength Wireless Media ◮ Effective area is related to physical size, but differs among antenna designs ◮ E.g. parabolic antenna may have effective area of 0 . 5 × physical area
ITS323 Wireless Propagation Transmission Media ◮ Ground Wave Propagation (below 2 MHz): signal follows contour of Earth, e.g. AM radio Design Factors Guided Media ◮ Sky Wave Propagation (2–30 MHz: signal reflected Wireless between ionosphere and Earth, e.g. amateur radio, Transmission international radio stations Wireless Media ◮ Line-of-Sight Propagation (above 30 MHz): signal not reflected; antennas must be in effective line-of-sight; used for most communications
ITS323 Different Propagation Characteristics Transmission Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Frequency Bands Transmission Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Wireless Transmission Impairments Transmission Media Free Space Loss: Signal disperses with distance; therefore signal attenuates over distance Design Factors Guided Media Atmospheric Absorption: Water vapour and oxygen in Wireless atmosphere may attenuate signals; only Transmission significant for specific frequencies (e.g. 22GHz, Wireless Media > 30 GHz) Multipath: Signals reflect off obstacles; multiple copies of signal arrive at receiver with varying delays causing reinforcement or cancellation Refraction: Signals are refracted through atmosphere; only part of wave received
ITS323 Examples of Multipath Interference Transmission Media Design Factors Guided Media Wireless Transmission Wireless Media
ITS323 Free Space Loss Model Transmission Media ◮ Ideal model to determine amount of power loss between transmitter and receiver Design Factors Guided Media = (4 π d ) 2 P t Wireless Transmission G t G r λ 2 P r Wireless Media ◮ Assumes no obstacles, operating in vacuum and perfect antennas ◮ Other models: Okumura-Hata (urban, suburban); Longley-Rice (TV broadcast); Log-distance (indoor)
ITS323 Example of Path Loss Transmission Media Two parabolic antennas with diameter 1 metre; frequency 5 GHz; transmit power 1 W; distance 1 km. What is required Design Factors receive power threshold of receiver? Guided Media Wireless Transmission Wireless Media
ITS323 Contents Transmission Media Design Factors Design Factors Guided Media Wireless Transmission Wireless Media Guided Media Wireless Transmission Wireless Media
ITS323 Terrestrial Microwave Transmission Media ◮ Parabolic antenna (usually 1-3 m) used to transmit point-to-point to another antenna Design Factors Guided Media ◮ Line-of-sight communications; often antennas are Wireless placed high (towers, buildings) to avoid obstacles Transmission ◮ Long-distance telecommunications (alternative to Wireless Media optical fibre, coaxial cable), e.g. voice and TV transmission ◮ Short communications between buildings (e.g. office buildings in city) ◮ Mobile telephone systems (GSM, CDMA, 3G)
Recommend
More recommend
