Data and Computer Communications

1. Data and Computer Communications Chapter 4 –Transmission Media Eighth Edition by William Stallings Lecture slides by Lawrie Brown

2. Transmission Media Communication channels in the animal world include touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. —Mind of the Raven, Bernd Heinrich

3. Transmission Media - Overview • Transmission Medium • Physical path between transmitter and receiver • Guided Media • Waves are guided along a solid medium • e.g., copper twisted pair, copper coaxial cable, optical fiber • Unguided Media • Provides means of transmission but does not guide electromagnetic signals • Employ an antenna for transmission • e.g., atmosphere, outer space

4. Transmission Media - Overview • Characteristics and quality determined by medium and signal • For guided • Medium is more important • For unguided • Bandwidth produced by the antenna is more important • Key concerns are • Data rate and Distance • The greater the BETTER!

5. Design Factors • Bandwidth • higher bandwidth gives higher data rate • Transmission impairments • Attenuation limits distance. • Interference • Overlapping frequency bands in unguided media. • Nearby cables. • Number of receivers • In guided media • More receivers (multi-point) introduce more attenuation

6. Electromagnetic Spectrum

7. 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

8. Transmission Media • Guided media • Twisted-pair • Coaxial Cable • Optical Fiber • Unguided media • Satellites • Terrestrial Microwave • Broadcast Radio

9. Guided Transmission Media • Transmission Capacity • Either in terms of • Bandwidth, or • Data Rate • Depends critically on • Distance • Type of medium • Point-to-point • Mutipoint

10. Twisted Pair • Most common medium • Two insulated wires twisted together in a helical manner (like DNA) • Advantages • Cheap • Easy to work with • Disadvantages • Low data rate • Short range

11. Twisted Pair

12. Twisted Pair - Applications • Telephone network • Between house and local exchange • Within buildings • To private branch exchange (PBX) • For local area networks (LAN) • 10 Mbps or 100 Mbps

13. Twisted Pair - Transmission Characteristics • Analog • Amplifiers every 5 km to 6 km • Digital • Use either analog or digital signals • Repeater every 2 km or 3 km • Limited in • Distance • Bandwidth (1 MHz) • Data rate (100 Mbps) • Susceptible to interference and noise

14. Unshielded vs 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) • in a variety of categories - see EIA-568

15. Unshielded vs 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) • in a variety of categories - see EIA-568

16. UTP Categories • Cat 3 • Up to 16 MHz • 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 100 MHz • Commonly pre-installed in new office buildings • Twist length 0.6 cm to 0.85 cm

17. UTP Categories

18. Comparison of Shielded and Unshielded Twisted Pair

19. http://www.cabletesting.com Near End Crosstalk • coupling of signal from one pair to another • occurs when transmit signal entering the link couples back to receiving pair • ie. near transmitted signal is picked up by near receiving pair • The tighter the twist in the cable, the more effective the cancellation • Twisting is used to balance the noise.

20. Coaxial Cable • Most versatile medium *Braided shield is also referred to as the “outer conductor”

21. Coaxial Cable Applications • Television distribution • Cable TV • Long distance telephone transmission • Can carry 10,000 voice calls simultaneously • Being replaced by fiber optic • Short distance computer systems links • LANs

22. Coaxial Cable - Transmission Characteristics • superior frequency characteristics to TP • performance limited by attenuation & noise • analog signals • amplifiers every few km • closer if higher frequency • up to 500MHz • digital signals • repeater every 1km • closer for higher data rates

23. Optical Fiber • Greater capacity • Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • Electromagnetic isolation • Greater repeater spacing • 10s of km at least

24. Optical Fiber • System components: • Transmission medium - fiber optic cable • Light source - LED or laser diode • Detector - photodiode

25. Optical Fiber - Applications • Telephone Network Applications • Long-haul, metropolitan, rural, and subscriber loop circuits • Local Area Networks • Optical fiber networks • Data rates from 100 Mbps to 1 Gbps • Support hundreds (or even thousands) of stations

26. Optical Fiber - Transmission Characteristics • Light Sources • Light Emitting Diode (LED) • Cheaper • Wider operating temp range • Last longer • Injection Laser Diode (ILD) • More efficient • Greater data rate • Wavelength Division Multiplexing

27. Optical Fiber

28. Optical Fiber Transmission Modes • Three types of transmission modes: – Step-index multimode: • Rays are reflected, absorbed and propagated along the fiber. • Signal elements (light pulses) spread out in time. • Suited for short distance transmission. – Single mode: • Fiber core diameter is reduced to a single wavelength (3-10 µm). • Single transmission path. • Long distance application (telephone and cable TV). – Graded-index multimode: • Intermediate between single mode and step-index multimode. • Used in LAN.

29. Optical Fiber Transmission Modes

30. Optical Fiber Transmission Modes

31. Frequency Utilization for Fiber Applications

32. Attenuation in Guided Media

33. Wireless Transmission • Unguided media (transmission and reception via antenna). – Transmission: the antenna radiates electromagnetic energy into the medium (usually air). – Reception: the antenna pick up electromagnetic waves from the surrounding medium. • Two basic configurations: – Directional: • Focused electromagnetic beam. • Careful alignment required. – Omnidirectional: • Signal spreads in all directions. • Can be received by many antennas.

34. Wireless Transmission Frequencies • >1 GHz to 40GHz • microwave • highly directional • point to point • satellite • 30MHz to 1GHz (radio) • omnidirectional • broadcast radio • 3 x 1011 to 2 x 1014 • infrared • local

35. Antennas • electrical conductor used to radiate or collect electromagnetic energy • transmission antenna • radio frequency energy from transmitter • converted to electromagnetic energy by antenna • radiated into surrounding environment • reception antenna • electromagnetic energy impinging on antenna • converted to radio frequency electrical energy • fed to receiver • same antenna is often used for both purposes

36. Radiation Pattern • Away to characterize the performance of an antenna • A graphical representation of the radiation as a function of space coordinates • 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

37. Radiation Pattern

38. Parabolic Reflective Antenna • Used for terrestrial and satellite microwave • Parabola is locus of point equidistant from a line and a point not on that line • Fixed point is focus • Line is directrix • Revolve parabola about axis to get paraboloid • Cross section parallel to axis gives parabola • Cross section perpendicular to axis gives circle • Source placed at focus will produce waves reflected from parabola in parallel to axis • Creates (theoretical) parallel beam of light/sound/radio • On reception, signal is concentrated at focus, where detector is placed

39. Parabolic Reflective Antenna

40. Parabolic Reflective Antenna

41. Antenna Gain • measure of directionality of antenna • power output in particular direction verses that produced by an isotropic antenna • measured in decibels (dB) • results in loss in power in another direction • effective area relates to size and shape • related to gain

42. Antenna Gain G= Antenna gain • Ae = effective area (related to the physical size of the antenna and to its shape) F = carrier frequency C = speed of light • the effective area of an ideal isotropic antenna is with a power gain of 1 • effective area of a parabolic antenna with a face area of A is 0.56A

43. Example

44. Terrestrial Microwave • used for long haul telecommunications • alternative to coaxial cable or optical fiber • and short point-to-point links • closed-circuit TV or as a data link between local area networks • requires fewer repeaters but line of sight • use a parabolic dish to focus a narrow beam onto a receiver antenna

45. Terrestrial Microwave • Characteristics • 1-40GHz frequencies • higher frequencies give higher data rates • main source of loss is attenuation • distance, rainfall (above 10 GHz) • also interference • d and lamda have the same unit

46. Satellite Microwave • satellite is relay station • receives on one frequency, amplifies or repeats signal and transmits on another frequency • eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz • typically requires geo-stationary orbit • height of 35,784km • spaced at least 3-4° apart (angular displacement as measured from the earth) • typical uses • television • long distance telephone • private business networks • global positioning

47. Satellite Point to Point Link

48. Satellite Broadcast Link

49. Broadcast Radio • radio is 3kHz to 300GHz • use broadcast radio, 30MHz - 1GHz, for: • FM radio • UHF and VHF television • is omnidirectional • the ionosphere is transparent to radio waves above 30 MHz • still need line of sight • suffers from multipath interference • reflections from land, water, other objects

50. Infrared • modulate noncoherent infrared light • end line of sight (or reflection) • are blocked by walls • no licenses required • typical uses • TV remote control • IRD port