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Chapter 4: Transmission Media

COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal. Chapter 4: Transmission Media. Remaining five chapters:. Chapter 7: Data Link: Flow and Error control, Link management. Data Link. Chapter 8: Improved utilization: Multiplexing. Physical Layer.

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Chapter 4: Transmission Media

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  1. COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal Chapter 4: Transmission Media

  2. Remaining five chapters: Chapter 7: Data Link: Flow and Error control, Link management Data Link Chapter 8: Improved utilization: Multiplexing Physical Layer Chapter 6: Data Communication: Synchronization, Error detection and correction Chapter 4: Transmission Media Transmission Medium Chapter 5: Encoding: From data to signals Chapter 3: Signals, their representations, their transmission over media, Impairments

  3. Agenda • Overview • Guided Transmission Media • Twisted Pair • Coaxial Cable • Optical Fiber • Unguided (open space, wireless) Transmission • Antennas • Terrestrial Microwave • Satellite Microwave • Broadcast Radio • Infrared

  4. Overview • Media: • Guided - wire • Unguided - wireless • Transmission characteristics and quality determined by: • Signal • Medium • For guided, the medium is important • For unguided, the antenna is important

  5. Design Issues • Key communication objectives are: • High data rate • Low error rate • Long distance • Bandwidth: Tradeoff - Larger for higher data rates - But smaller for low link cost • Transmission impairments • Attenuation: Twisted Pair > Cable > Fiber (best) • Interference and Cross talk: Twisted Pair > Cable > Fiber (best) Worse with unguided… (the medium is shared!) • Number of receivers • In multi-point links of guided media: Attenuation increases with increased number of connected receivers

  6. Part of the Electromagnetic Spectrum f 1 KHz 1 MHz 1 GHz 1 THz Guided Unguided l V = l f

  7. Study of Transmission Media • Physical description • Main transmission characteristics • Main applications

  8. Guided Transmission Media • Twisted Pair • Coaxial cable • Optical fiber

  9. 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 3 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: Overview

  10. Twisted Pair (TP)

  11. UTP Cables unshielded

  12. Twisted Pair - Applications • The most commonly used guided medium • Telephone network (Analog Signaling) • Analog Data (original purpose) : Between houses and the local exchange, e.g. 5 km (subscriber loop) • Digital Data: Transmitted using modems, low data rates • Within buildings (short distances) (Digital Signaling) • To private branch exchange (PBX) (64 Kbps) • For local area networks (LAN) (10-100Mbps) Example, Ethernet: 10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range

  13. Twisted Pair - Pros and ConsCompared to other guided media Pros: • Low cost • Easy to work with (pull, terminate, etc.) Cons: • Limited bandwidth  Limited data rate • Limited distance range (due to large attenuation) • Susceptible to interference and noise

  14. Twisted Pair - Transmission Characteristics • Analog Transmission Mode • For analog signals only • Amplifiers every 5km to 6km • Bandwidth up to 1 MHz (several voice channels): ADSL (Ch 8) • Digital Transmission Mode • Using either analog or digital signals • Repeaters every 2km or 3km • Data rates up to few Mbps (1Gbps: over very short distances) • Impairments: • Attenuation: A strong function in frequency, can be modified with loading coils • EM Interference: Crosstalk, Impulse noise, Mains interference

  15. Attenuation in Guided Media

  16. Attenuation in Twisted pairs (unloaded) Thinner Wires Wire Gauge Wire Diameter 1 MHz 300 1 KHz 3400 Telephone Voice

  17. Ways to reduce EM interference • Shielding the TP with a metallic braid or sheathing • Twisting reduces low frequency interference Tighter twisting  Better performance • Using different twisting lengths for adjacent pairs to reduce crosstalk

  18. STP: Metal Shield

  19. Unshielded (UTP) and Shielded (STP) • Unshielded Twisted Pair (UTP) • Ordinary telephone wire: Abundantly available in buildings • Cheapest • Easiest to install • Suffers from external EM interference • Shielded Twisted Pair (STP) • Shielded with metal braid or sheathing: • Reduces interference • Reduces attenuation at higher frequencies  Better Performance: • Increases data rates used • Increases distances covered • But becomes: • More expensive • Harder to handle (thicker, heavier) Transmit faster and go further!

  20. UTP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data) • Cat 3 • Up to 16MHz • Voice grade • In most office buildings • Twist length: 7.5 cm to 10 cm • Cat 4 • Up to 20 MHz • Cat 5 • Up to 100MHz • Data grade • Pre-installed in many new office buildings • Twist length: 0.6 cm to 0.85 cm (Tighter twisting increases cost but improves performance)

  21. Near End CrossTalk (NEXT) • Coupling of signal from a wire pair to an adjacent pair • Coupling takes place when a transmitted signal entering a pair couples (leaks) into an adjacent receiving pair on the same (near) end of the link Transmitted Power, P1 Disturbing pair Coupled Received Power, P2 Disturbed pair “NEXT” Attenuation = 10 log P1/P2 dBs The larger … the smaller the crosstalk (The better the performance) NEXT attenuation is a desirable attenuation- The larger the better!

  22. 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 Undesirable Attenuation- Smaller is better Desirable Attenuation- Larger is better! Better Better Better Better Not Usable Not Usable

  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 Newer Twisted Pair Categories and Classes UTP: Unshielded Twisted Pair FTP: Foil Twisted Pair SSTP: Shielded-Screen Twisted Pair

  24. Coaxial Cable Physical Description: 1 - 2.5 cm Designed for operation over a wider frequency rage

  25. Physical Description

  26. Coaxial Cable Applications Frequency Division Multiplexing • Most versatile medium • Television distribution (FDM Broadband) • Cable TV (CATV): 100’s of TV channels over 10’s Kms • Long distance telephone transmission • Can carry 10s of thousands of voice channels simultaneously (though FDM multiplexing) (Broadband) • Now facing competition from optical fibers and terrestrial microwave links • Local area networks, e.g. Thickwire Ethernet cable (10Base5): 10 Mbps, Baseband signal, 500m segment

  27. Coaxial Cable - Transmission CharacteristicsImprovements over TP • Extended frequency range • Up to 500 MHz • Reduced EM interference and crosstalk • Due to enclosed concentric construction • EM fields terminate within cable and do not stray out • Remaining limitations: • Attenuation • Thermal and noise • Inter modulation noise (especially for broadband operation) Because of FDM

  28. Attenuation in Guided Media

  29. Coaxial Cable - Transmission Characteristics • Analog Transmission • Amplifiers every few kms • Closer spacing for higher frequency • Digital Transmission • Repeater every 1km • Closer repeater spacing for higher data rates

  30. Optical Fiber • A thin (2-125 mm) flexible strand of glass or plastic • Light entering at one end travels confined within the fiber until it leaves at the other end • As fiber bends around corners, the light stays within the fiber • Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture • Reasonable losses and performance with multi-component glass and with plastic Cost, Difficulty of Handling Attenuation (Loss) Pure Glass Multi-component Glass Plastic Best Performance

  31. Single Fiber Cable Optical Fiber: Construction • An optical fiber consists of three main parts • Core • A narrow cylindrical strand of glass/plastic, with refractive index n1 • Cladding • A tube surrounding each core, with refractive index n2 • The core must have a higher refractive index than the cladding to keep the light beam trapped inside: n1 > n2 • Protective outer jacket • Protects against moisture, abrasion, and crushing Individual Fibers: (Each having core & Cladding) Multiple Fiber Cable (Note multiple cladding)

  32. Reflection and refraction of light • 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 Shallower Incidence 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

  33. 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

  34. Which side is the IR? Attenuation in Guided Media Compare attenuation ranges!

  35. Optical Fiber - Benefits • Greater channel capacities over larger distances • Fiber: 100’s of Gbps over 10’s of Kms • Cable: 100’s of Mbps over 1’s of Kms • Twisted pair: 100’s of Mbps over 10’s of meters • Lower/moreuniform attenuation (Fig. 4.3c) • An order of magnitude lower • Relatively constant over a larger frequency interval • Electromagnetic isolation • Fiber is not affected by external EM fields: • No interference, impulse noise, crosstalk • Fiber does not radiate (light ray trapped inside): • Not a source of interference • Difficult to tap (data security) But what could happen at the repeater?

  36. Optical Fiber – Benefits, Contd. • Greater repeater spacing: Fewer Units, Lower cost • Fiber: 10-100’s of Kms • Cable, Twisted pair: 1’s Kms • Smaller size and weight: • An order of magnitude thinner for same channel capacity • Useful in cramped places • Reduced cost of digging in populated areas • Reduced cost of cable support structures

  37. Optical Fiber - Applications • Long-haul trunks between cities • Telephone traffic over long routes between cities, and undersea: • Fiber & Microwave now replacing coaxial cable •  1500 km, Up to 60,000 voice channels • Metropolitan trunks • Joining exchanges inside large cities: •  12 km, Up to 100,000 voice channels • Rural exchange trunks • Joining exchanges of towns and villages: •  40-160 km, Up to 5,000 voice channels • Subscriber loops • Joining subscribers to exchange: • Fiber replacing TP, allowing all types of traffic • LANs, Example: 10BaseF 10 Mbps, 2000 meter segment Exchange City City Competition: Fiber, Coaxial, m Waves Main Exchange Compare segment length with twisted pair and coaxial!

  38. Optical Fiber - Transmission Characteristics • Acts as a wave guide for light (1014 to 1015 Hz) • Covers portions of infrared and visible spectrum • Transmission Modes: Multimode Single Mode Single ray Graded Index Step Index Multiple rays

  39. i < critical n2 n1 Dispersion: Spread in ray arrival time Optical Fiber Transmission Modes Refraction Shallow reflection Deep reflection n2 n1 Large Core n1 > n2 Cladding 2 ways to reduce dispersion: Smaller • v = c/n • Make n1 lower away from center…this speeds up deeper rays • and compensates for their larger distances, arrive together with shallower rays Smallest

  40. 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 • Dispersion can be reduced by: • Limiting the distance • Thinner fibers and a highly focused light source  In the limit: Single mode: High data rates, very long distances • Graded-index thicker fibers: The half-way solution

  41. The transmission system is not just the medium (fiber)! We have also light Sources and detectors… Light Sources • Light Emitting Diode (LED) • Lower cost, longer life • Wider operating temp range • Injection Laser Diode (ILD) • More efficient (more light power for same electric power input) • Faster switching  Higher data rate

  42. 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

  43. 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 l values shown are in vacuum

  44. Wireless Transmission • Free-space is the transmission medium • Need efficient radiators, called antennas / aerials • Signal fed from transmission line (wireline) and radiated into free-space (wireless) • Popular applications • Radio & TV broadcast • Cellular Communications • Microwave Links • Wireless Networks

  45. Wireless Transmission Frequency Ranges • Radio: 30 MHz to 1 GHz • Omni directional • Broadcast radio • Microwaves: 1 GHz to 40 GHz • Highly directional beams • Point to point (Terrestrial) • Satellite • Infrared Light: 0.3 THz to 20 THz (just below visible light) • Localized communications (confined spaces)

  46. Antennas • Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from the environment (TX/RX operation) • Transmission • Radio frequency electrical energy obtained from transmitter • Converted into electromagnetic energy • 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

  47. Radiation Pattern • Power radiated in different directions, usually not with the same efficiency: • Isotropic (Omni-directional) antenna • A hypothetical point source in space • Radiates equally in all directions – A spherical radiation pattern • Used as a reference for other antennae • Directional Antenna • Concentrates radiation in a given desired direction – hence point-to-point, line of sight communications • Gives ‘gain’ in that direction relative to isotropic Radiation Patterns Point Source Isotropic Directional

  48. Parabola Focus Parabolic Reflective Antenna (The Dish!) Axis

  49. Parabola Focus Parabolic Reflective Antenna (The Dish!) • Used for terrestrial and satellite microwave • On Transmission: Source placed at the focus will produce waves that get reflected from parabola parallel to the parabola axis • Creates a (theoretically) parallel beam to the parabola axis that does not spread (disperse) in space ( Zero radiation off axis) • In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!) • On reception: Only signal from the axis direction is concentrated at focus, where detector is placed. Signals from other directions miss the focus ( Zero o/p off axis) • i.e. Directionality in both TX, RX operation • The larger the antenna (in wavelengths) the better the directionality  High frequency is advantageous

  50. Antenna Gain, G • A measure of directionality of the antenna • Power output in a given direction compared to that produced by a perfect isotropic antenna • Can be expressed in decibels (dB, dBi) (i = relative to isotropic) • Increased power radiated in one direction causes less power radiated in other directions (Total power is fixed) • Gain Gq depends on the effective area (Ae) of the antenna: • Depends on size and shape of the antenna • The Radiation pattern

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