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Transmission Media: Overview and Guided Media

This chapter provides an overview of transmission media and focuses on guided transmission media such as twisted pair, coaxial cable, and optical fiber. It also discusses the key concerns and design issues related to transmission media.

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Transmission Media: Overview and Guided Media

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  1. COE 341: Data & Computer Communications (T061)Dr. Marwan Abu-Amara Chapter 4: Transmission Media

  2. Agenda • Overview • Guided Transmission Media • Twisted Pair • Coaxial Cable • Optical Fiber • Wireless Transmission • Antennas • Terrestrial Microwave • Satellite Microwave • Broadcast Radio • Infrared COE 341 (T061) – Dr. Marwan Abu-Amara

  3. Overview • Media • Guided - wire • Unguided - wireless • Transmission characteristics and quality determined by: • Medium • Signal • For guided, the medium is more important • For unguided, the bandwidth produced by the antenna is more important • Key concerns are data rate and distance COE 341 (T061) – Dr. Marwan Abu-Amara

  4. Design Issues • Key communication objectives are: • High data rate • Low error rate • Long distance • Bandwidth: Tradeoff - Larger for higher data rates - But smaller for economy • Transmission impairments • Attenuation: Twisted Pair > Cable > Fiber (best) • Interference: Worse with unguided… (medium is shared!) • Number of receivers • In multi-point links of guided media: More connected receivers introduce more attenuation COE 341 (T061) – Dr. Marwan Abu-Amara

  5. Electromagnetic Spectrum COE 341 (T061) – Dr. Marwan Abu-Amara

  6. Study of Transmission Media • Physical description • Main applications • Main transmission characteristics COE 341 (T061) – Dr. Marwan Abu-Amara

  7. Guided Transmission Media • Twisted Pair • Coaxial cable • Optical fiber COE 341 (T061) – Dr. Marwan Abu-Amara

  8. Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Transmission Characteristics of Guided Media COE 341 (T061) – Dr. Marwan Abu-Amara

  9. Twisted Pair COE 341 (T061) – Dr. Marwan Abu-Amara

  10. UTP Cables COE 341 (T061) – Dr. Marwan Abu-Amara

  11. UTP Connectors COE 341 (T061) – Dr. Marwan Abu-Amara

  12. Note: Pairs of Wires • It is important to note that these wires work in pairs (a transmission line) • Hence, for a bidirectional link • One pair is used for TX • One pair is used for RX COE 341 (T061) – Dr. Marwan Abu-Amara

  13. Twisted Pair - Applications • Most commonly used guided medium • Telephone network (Analog Signaling) • Between house and local exchange (subscriber loop) • Within buildings (Digital Signaling) • To private branch exchange (PBX) • For local area networks (LAN) • 10Mbps or 100Mbps • Range: 100m COE 341 (T061) – Dr. Marwan Abu-Amara

  14. Twisted Pair - Pros and Cons • Pros: • Cheap • Easy to work with • Cons: • Limited bandwidth • Low data rate • Short range • Susceptible to interference and noise COE 341 (T061) – Dr. Marwan Abu-Amara

  15. Twisted Pair - Transmission Characteristics • Analog Transmission • Amplifiers every 5km to 6km • Digital Transmission • Use either analog or digital signals • Repeater every 2km or 3km • Limited distance • Limited bandwidth (1MHz) • Limited data rate (100Mbps) • Susceptible to interference and noise COE 341 (T061) – Dr. Marwan Abu-Amara

  16. Attenuation in Guided Media COE 341 (T061) – Dr. Marwan Abu-Amara

  17. Courtesy of Dr. Radwan Abdel-Aal Ways to reduce EM interference • Shielding the TP with a metallic braid or sheathing • Twisting reduces low frequency interference • Different twisting lengths for adjacent pairs help reduce crosstalk COE 341 (T061) – Dr. Marwan Abu-Amara

  18. Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) • Ordinary telephone wire • Cheapest • Easiest to install • Suffers from external EM interference • Shielded Twisted Pair (STP) • Metal braid or sheathing that reduces interference • More expensive • Harder to handle (thick, heavy) COE 341 (T061) – Dr. Marwan Abu-Amara

  19. STP: Metal Shield COE 341 (T061) – Dr. Marwan Abu-Amara

  20. UTP Categories • Cat 3 • up to 16MHz • Voice grade found in most offices • Twist length of 7.5 cm to 10 cm • Cat 4 • up to 20 MHz • Cat 5 • up to 100MHz • Commonly pre-installed in new office buildings • Twist length 0.6 cm to 0.85 cm • Cat 5E (Enhanced) –see tables • Cat 6 • Cat 7 COE 341 (T061) – Dr. Marwan Abu-Amara

  21. Courtesy of Dr. Radwan Abdel-Aal Transmitted Power, P1 Disturbing pair Coupled Received Power, P2 Disturbed pair Near End Crosstalk (NEXT) • Coupling of signal from one wire pair to another • Coupling takes place when a transmitted signal entering a pair couples back to an adjacent receiving pair at the same end • i.e. near transmitted signal is picked up by near receiving pair “NEXT” Attenuation = 10 log P1/P2 dBs The larger … the smaller the crosstalk (i.e. the better the performance) NEXT attenuation is a desirable attenuation - The larger the better! COE 341 (T061) – Dr. Marwan Abu-Amara

  22. Courtesy of Dr. Radwan Abdel-Aal Signal Attenuation (dB per 100 m) Near-end Crosstalk Attenuation (dB) Frequency (MHz) Category 3 UTP Category 5 UTP 150-ohm STP Category 3 UTP Category 5 UTP 150-ohm STP 1 2.6 2.0 1.1 41 62 68? 4 5.6 4.1 2.2 32 53 58 16 13.1 8.2 4.4 23 44 50.4 25 — 10.4 6.2 — 41 47.5 100 — 22.0 12.3 — 32 38.5 300 — — 21.4 — — 31.3 Transmission Properties for Shielded & Unshielded TP Desirable Attenuation- Larger is better! Undesirable Attenuation- Smaller is better COE 341 (T061) – Dr. Marwan Abu-Amara

  23. Category 3 Class C Category 5 Class D Category 5E Category 6 Class E Category 7 Class F Bandwidth 16 MHz 100 MHz 100 MHz 200 MHz 600 MHz Cable Type UTP UTP/FTP UTP/FTP UTP/FTP SSTP Link Cost (Cat 5 =1) 0.7 1 1.2 1.5 2.2 Twisted Pair Categories and Classes COE 341 (T061) – Dr. Marwan Abu-Amara

  24. Coaxial Cable Physical Description: COE 341 (T061) – Dr. Marwan Abu-Amara

  25. Physical Description COE 341 (T061) – Dr. Marwan Abu-Amara

  26. Coaxial Cable Applications • Most versatile medium • Television distribution • Cable TV • Long distance telephone transmission • Can carry 10,000 voice calls simultaneously (though FDM multiplexing) • Being replaced by fiber optic • Short distance computer systems links • Local area networks (thickwire Ethernet cable) COE 341 (T061) – Dr. Marwan Abu-Amara

  27. Coaxial Cable - Transmission Characteristics • Analog • Amplifiers every few km • Closer if higher frequency • Up to 500MHz • Digital • Repeater every 1km • Closer for higher data rates COE 341 (T061) – Dr. Marwan Abu-Amara

  28. Attenuation in Guided Media COE 341 (T061) – Dr. Marwan Abu-Amara

  29. Optical Fibers • An optical fiber is a very thin strand of silica glass • It is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber it travels (confined within the fiber) until it leaves the fiber at the other end • Two critical factors stand out: • Very little light is lost in its journey along the fiber • Fiber can bend around corners and the light will stay within it and be guided around the corners • An optical fiber consists of three parts • The core • Narrow cylindrical strand of glass with refractive index n1 • The cladding • Tubular jacket surrounding the core with refractive index n2 • The core must have a higher refractive index than the cladding for the propagation to happen COE 341 (T061) – Dr. Marwan Abu-Amara

  30. Optical Fibers (Contd.) • Protective outer jacket • Protects against moisture, abrasion, and crushing Individual Fibers: (Each having core & Cladding) Single Fiber Cable Multiple Fiber Cable COE 341 (T061) – Dr. Marwan Abu-Amara

  31. Courtesy of Dr. Radwan Abdel-Aal Reflection and Refraction • At a boundary between a denser (n1) and a rarer (n2) medium, n1 > n2 (e.g. water-air, optical fiber core-cladding) a ray of light will be refracted or reflected depending on the incidence angle Increasing Incidence angle, 1 2 rarer v2 = c/n2 n2 denser 2 1 n1 critical 1 n1 > n2 v1 = c/n1 Total internal reflection Critical angle refraction Refraction COE 341 (T061) – Dr. Marwan Abu-Amara

  32. Optical Fiber Refraction at boundary for . Escaping light is absorbed in jacket i < critical n2 Rarer Denser Denser n1 n1 Rarer i Total Internal Reflection at boundary for i > critical n1 > n2 COE 341 (T061) – Dr. Marwan Abu-Amara

  33. Attenuation in Guided Media COE 341 (T061) – Dr. Marwan Abu-Amara

  34. Optical Fiber - Benefits • Greater capacity • Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • An order of magnitude lower • Relatively constant over a larger frequency interval • Electromagnetic isolation • Not affected by external EM fields: • No interference, impulse noise, crosstalk • Does not radiate: • Not a source of interference • Difficult to tap (data security) • Greater repeater spacing • 10s of km at least COE 341 (T061) – Dr. Marwan Abu-Amara

  35. Optical Fiber - Applications • Long-haul trunks • Metropolitan trunks • Rural exchange trunks • Subscriber loops • LANs COE 341 (T061) – Dr. Marwan Abu-Amara

  36. Optical Fiber - Transmission Characteristics • Act as wave guide for light (1014 to 1015 Hz) • Covers portions of infrared and visible spectrum • Light Emitting Diode (LED) • Cheaper • Wider operating temp range • Last longer • Injection Laser Diode (ILD) • More efficient • Greater data rate COE 341 (T061) – Dr. Marwan Abu-Amara

  37. Courtesy of Dr. Radwan Abdel-Aal i < critical n2 n1 Dispersion: Spread in arrival time Optical Fiber Transmission Modes Refraction Shallow reflection Deep reflection n2 n1 Large Core Cladding 2 ways: Smaller • v = c/n • n1 lower away from center…this speeds up deeper rays • and compensates for their larger distances, arrive together with shallower rays Smallest COE 341 (T061) – Dr. Marwan Abu-Amara

  38. Courtesy of Dr. Radwan Abdel-Aal Optical Fiber – Transmission modes • Spread of received light pulse in time (dispersion) is bad: • Causes inter-symbol interference  bit errors • Limits usable data rate and usable distance • Caused by propagation through multiple reflections at different angles of incidence • Dispersion increases with: • Larger distance traveled • Thicker fibers with step index • Can be reduced by: • Limiting the distance • Thinner fibers and a highly focused light source  Single mode: High data rates, very long distances • Graded-index thicker fibers: The half-way solution COE 341 (T061) – Dr. Marwan Abu-Amara

  39. Courtesy of Dr. Radwan Abdel-Aal Optical Fiber – Wavelength Division Multiplexing (WDM) • A form of FDM (channels sharing the medium by occupying different frequency bands) • Multiple light beams at different frequencies (wavelengths) transmitted on the same fiber • Each beam forms a separate communication channel • Example: 256 channels @ 40 Gbps each  10 Tbps total data rate COE 341 (T061) – Dr. Marwan Abu-Amara

  40. Courtesy of Dr. Radwan Abdel-Aal Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region • Selection based on: • Attenuation of the fiber • Properties of the light sources • Properties of the light receivers S L C Bandwidth, THz 33 12 4 7 Note: l in fiber = v/f = (c/n)/f = (c/f)/n = l in vacuum/n i.e. l in fiber < l in vacuum COE 341 (T061) – Dr. Marwan Abu-Amara

  41. Attenuation in Guided Media COE 341 (T061) – Dr. Marwan Abu-Amara

  42. Wireless Transmission • Free-space is the transmission medium • Need efficient radiators, called antenna, to take signal from transmission line (wireline) and radiate it into free-space (wireless) • Famous applications • Radio & TV broadcast • Cellular Communications • Microwave Links • Wireless Networks COE 341 (T061) – Dr. Marwan Abu-Amara

  43. Wireless Transmission Frequencies • Radio: 30MHz to 1GHz • Omni-directional • Broadcast radio • Microwave: 2GHz to 40GHz • Microwave • Highly directional • Point to point • Satellite • Infrared Light: 3 x 1011 to 2 x 1014 • Localized communications COE 341 (T061) – Dr. Marwan Abu-Amara

  44. Antennas • Electrical conductor (or system of..) used to radiate/collect electromagnetic energy • Transmission • Radio frequency electrical energy from transmitter • Converted to electromagnetic energy by antenna • Radiated into surrounding environment • Reception • Electromagnetic energy impinging on antenna • Converted to radio frequency electrical energy • Fed to receiver • Same antenna often used for both TX and RX in 2-way communication systems COE 341 (T061) – Dr. Marwan Abu-Amara

  45. Radiation Pattern • Power radiated in all directions • Not same performance in all directions • Isotropic antenna is (theoretical) point in space • Radiates in all directions equally • Gives spherical radiation pattern • Used as a reference for other antennae • Directional Antenna • Concentrates radiation in a given desired direction • Used for point-to-point, line of sight communications • Gives “gain” in that direction relative to isotropic Radiation Patterns Isotropic Directional COE 341 (T061) – Dr. Marwan Abu-Amara

  46. Parabolic Reflective Antenna • Used for terrestrial and satellite microwave • Source placed at focus will produce waves reflected from parabola parallel to axis • Creates (theoretical) parallel beam of light/sound/radio • In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!) • On reception, signal is concentrated at focus, where detector is placed • The larger the antenna (in wavelengths) the better the directionality COE 341 (T061) – Dr. Marwan Abu-Amara

  47. Parabolic Reflective Antenna Axis COE 341 (T061) – Dr. Marwan Abu-Amara

  48. Antenna Gain, G • Measure of directionality of antenna • Power output in particular direction compared with that produced by isotropic antenna • Measured in decibels (dB) • Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed) • Effective area, Ae, relates to size and shape of antenna • Determines antenna gain COE 341 (T061) – Dr. Marwan Abu-Amara

  49. Courtesy of Dr. Radwan Abdel-Aal Antenna Gain, G: Effective Areas • An isotropic antenna has a gain G = 1 (0 dBi) • i.e. • A parabolic antenna has: • Substituting we get: • Gain in dBi = 10 log G • Important: Gains apply to both TX and RX antennas A = Actual Area = p r2 COE 341 (T061) – Dr. Marwan Abu-Amara

  50. Terrestrial Microwave • Parabolic dish • Focused beam • Line of sight • Curvature of earth limits maximum range  Use relays to increase range (multi-hop link) • Long haul telecommunications • Higher frequencies give higher data rates but suffers from larger attenuation COE 341 (T061) – Dr. Marwan Abu-Amara

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