Figure 7.1 Transmission medium and physical layer

1. Figure 7.1 Transmission medium and physical layer

2. Figure 7.2 Classes of transmission media

3. Overview • Guided - wire • Unguided - wireless • Characteristics and quality determined by medium and 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

4. Design Factors • Bandwidth • Higher bandwidth gives higher data rate • Transmission impairments • Attenuation • Interference • Number of receivers • In guided media • More receivers (multi-point) introduce more attenuation

5. Electromagnetic Spectrum

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

7. Figure 7.3 Twisted-pair cable

8. Table 7.1 Categories of unshielded twisted-pair cables

9. Twisted Pair

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

11. Twisted Pair - Pros and Cons • Cheap • Easy to work with • Low data rate • Short range

12. Twisted Pair - Transmission Characteristics • Analog • Amplifiers every 5km to 6km • Digital • Use either analog or digital signals • repeater every 2km or 3km • Limited distance • Limited bandwidth (1MHz) • Limited data rate (100MHz) • Susceptible to interference and noise

13. 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)

14. Figure 7.4 UTP and STP cables

15. STP • The metal casing prevents the penetration of EM noise • It also can eliminate a phenomenon called Crosstalk, which is the undesired effect of one circuit (or channel) on another circuit (or channel) • This effect can be experienced during telephone conversations when one can hear other conversations in the background • Shielding each pair of twisted pair can eliminate most crosstalk • STP is more expensive than UTP but is less susceptible to noise

16. UTP Categories • Cat 1 • Basic Twisted pair cabling used in Telephone system • Fine for voice but inadequate for all but low-speed data communication • Cat 2 • The next higher grade, suitable for voice and for data transmission of up to 4Mbps

17. UTP Categories • Cat 3 • up to 16MHz • Voice grade found in most offices • Twist length of 7.5 cm to 10 cm • Now the standard cable for most telephone lines • Cat 4 • up to 20 MHz • Must have at least 3 twists per foot • Cat 5 • up to 100MHz • Commonly pre-installed in new office buildings • Twist length 0.6 cm to 0.85 cm

18. Near End Crosstalk • Coupling of signal from one pair to another • Coupling takes place when transmit signal entering the link couples back to receiving pair • i.e. near transmitted signal is picked up by near receiving pair

19. Parallel Flat Wire

20. Noise Effect on Twisted-Pair

21. Coaxial Cable

22. Figure 7.7 Coaxial cable

23. Coaxial Cable Applications • Most versatile medium • Television distribution • Ariel to TV • Cable TV • Long distance telephone transmission • Can carry 10,000 voice calls simultaneously • Being replaced by fiber optic • Short distance computer systems links • Local area networks

24. Table 7.2 Categories of coaxial cables

25. Coaxial Cable Connectors • Over the years, a no. of connectors have been designed for use with coaxial cable • Most common of the connectors is called “BARREL connector” because of its shape • Of the barrel connectors, the most popular is the Bayonet Network Connector (BNC) • BNC connector pushes on and locks into place with half turn • Other types of barrel connectors either screw together and so require more effort to install or push on w/o locking which is less secure

26. Figure 7.8 BNC connectors

27. Figure 7.9 Coaxial cable performance

28. 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

29. Refraction • Light travels in a straight line as long as it is moving through a single uniform structure • If a ray of light traveling through one substance enters another (more or less dense) substance, its speed changes abruptly causing the ray to change direction, This phenomenon is called Refraction • When angle of incidence becomes greater than critical angle, reflection occurs • Light no longer passes into the less denser medium but is reflected back into the same medium • The Angle of Incidence (I) = Angle of Reflection ®

30. Figure 7.10 Bending of light ray

31. Optical Fiber

32. Optical Fiber • Optical fibers use Reflection to guide light through a channel • A glass or plastic CORE is surrounded by a CLADDING of less dense glass or plastic • The difference in the density of CORE and CLADDING is such that the beam of light moving through the core is reflected off the cladding • Information is encoded onto a beam of light as a series of ON-OFF flashes that represent 1 and 0 bits

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

34. Optical Fiber - Applications • Long-haul trunks • Metropolitan trunks • Rural exchange trunks • Subscriber loops • LANs

35. Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz • 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 • Wavelength Division Multiplexing

36. Figure 7.11 Optical fiber

37. Figure 7.12 Propagation modes

38. Multimode Step Index • Density of the CORE remains constant from the center to the edges • A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding • At the interface, there is an abrupt change to lower density, that alters the angle of the beam’s motion • Step Index Suddenness of this change • Some beams travel straight • Some strike the interface of core and cladding at an angle smaller than critical angle and penetrate cladding and are lost

39. Disadvantage of Multimode Step-Index Fiber • Each beams angle is equal to its angle of reflection • If I is small, R is small and the beam will require more bounces and it will take more time to reach the destination • If I is large, R is large and beam will reach destination quickly • In other words there is a difference is Path Lengths that results into a distortion at the receiver • This distortion limits the data rate and make Multimode Step index fiber inadequate for precise applications

40. Multimode Graded index • A grade index fiber is the one with varying densities • Density is highest at the center of the core and decreases gradually to its lowest at the edge •  The signal is introduced at the center of the core • Beams at other angles moves through the series of constantly changing densities • Each density difference causes each beam to refract into a curve • Signal can be reconstructed with far greater precision as all the beams reach the receiver at almost the same time

41. Figure 7.13 Modes

42. Table 7.3 Fiber types

43. Figure 7.14 Fiber construction

44. Figure 7.15 Fiber-optic cable connectors

45. Wireless Transmission • Unguided media • Transmission and reception via antenna • Directional • Focused beam • Careful alignment required • Omnidirectional • Signal spreads in all directions • Can be received by many antennae

46. Figure 7.18 Propagation methods

47. Table 7.4 Bands

48. Figure 7.19 Wireless transmission waves

49. Note Radio waves are used for multicast communications, such as radio and television, and paging systems.

50. Figure 7.21 Unidirectional antennas