The Physical Layer

1. The Physical Layer Chapter 2

2. Physical layer deals with data communication.it decides way the other layers work. Example, when network connect, may have wired or wireless. If it is wired, go for error-checking at data link layer. On other hand of it is wireless, go for error handling mechanisms at data link layer. Duty of Physical layer :: • Carry bits form one end to another. Data communication happens in some form of electromagnetic (EM) wave transfer, either wired or wireless. • To see that only intended receives the data nobody else. • Ensure the sender and receiver are synchronized. • Multiplexing and demultiplexing the data. Telephone wires are connected to exchange where voice signals are digitized and multiplexed. After that, multiplexed signal is transmitted through fiber optic cables. Other end, reverse process takes place that separate signals and convert back to their original form.

3. The Theoretical Basis for Data Communication • Information can be transmitted on wires by varying some physical property such as voltage or current. • By representing the value of this voltage or current as a single-valued function of time f(t),we can model the behavior of a signal and analyze it mathematically. ∞ ∞ • Fourier Analysis g(t) = ½ C + ∑ an sin (2¶nft) + ∑ bn cos (2¶nft) where an is n=1 n=1 • an amplitude (the objective measurement of the degree of change (positive or negative) • Bandwidth-Limited Signals The range of frequency (measured in cycles/sec or Hertz (Hz) transmitted without being attenuated (loss of signal strength) is called bandwidth.

4. Quality of transmission depends on characteristics of medium (channel). Bandwidth and data rate are the important parameters that decide how data is transmitted from one place to another place. Bandwidth represents range of frequencies that can pass through medium. more the bandwidth of channel, more will be volume of data that can pass through given time. The amount of data passes through a medium per unit time(second) or the data rate is directly proportional to available bandwidth. Both are represented in bits/second. Data rate is actual data passing through the medium.it is varying quantity and may be zero. Bandwidth on other hand indicates the maximum data rate. Always fixed.

5. Maximum Data Rate of a Channel As early 1924, an AT & T engineer, Henry Nyquist, realized that even a perfect channel has a finite transmission capacity. He derived equation expressing the maximum data rate for a finite bandwidth noiseless channel. In 1948 , Claude Shannon carried Nyquist's work further and extended it to the case of channel subject to random noise. For Noiseless channel: maximum data rate = 2H log2 V bits/sec For Noisy channel : maximum data rate bits/sec = H log2 (1+ S/N) Here H = Bandwidth, V = Signal level , S & N = signal to noise ratio

6. Guided Transmission Media • Purpose of physical layer is to transport a raw bit stream from one machine to another. • Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics

7. Magnetic Media One of the most common way to transport data from one computer to another computer is to write them onto magnetic tape or media. then physically transport the tape from one destination to other. Bandwidth excellent Cost effective… Transportation cost is more.. Time delay.

8. Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP.

9. For many applications an on-line connection is needed. for that we using twisted pair cable as transmission media. It consists of two insulated copper wires, typically about 1 mm thick. Wires are twisted together like DNA molecule. Twisting is done because two parallel wires constitute a fine antenna. Common application is telephone system.. All telephones are connected to the telephone company office by a twisted pair cable.. For longer distance repeater are required. It is used to transmitting analog or digital signals. Bandwidth depend on thickness of wires and distance. Low cost, widely used. Cat 3 : consist of two insulated wires twisted together. Cat 5 : more twists per centimeter which results in less crosstalk and better quality signal.

10. Coaxial Cable A coaxial cable.

11. Better shielded than twisted pair, so it can span longer distances at higher speeds. Two kind of cable:- one kind 50 ohm cable, used for digital transmission from the start. Another kind 75 ohm cable, used for analog transmission and cable television. Coaxial cable consists of a stiff (operate easily, not moving, resistant) copper wire as core, surrounded by an insulating material. Insulator encased by cylindrical conductor. Outer conductor is covered in protective plastic sheath (close fitting). High Bandwidth and excellent noise immunity. Bandwidth depend on cable quality , length and signal to noise ratio of data signal. Modern cable have bandwidth of close to 1GHZ. Widely used in TV and MAN.

12. Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection.

13. Transmission of Light through Fiber Attenuation of light through fiber in the infrared region.

14. Fiber Cables Cladding is a process in which one material is used to cover another (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

15. Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources.

16. Fiber Optic Networks A fiber optic ring with active repeaters.

17. Fiber Optic Networks (2) A passive star connection in a fiber optics network.

18. Wireless Transmission/Unguided Media • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared and Millimeter Waves • Light wave Transmission

19. The Electromagnetic Spectrum • When Electron moves, they create electromagnetic waves that can propagate(Multiply or Increase) through space (even in vacuum).. • Waves are predicted by British Physicist James clerk Maxwell in 1865 and first observed by German Physicist Heinrich Hertz in 1887. • The number of oscillations(periodic fluctuation) per second of a wave is called frequency. Measured in Hz. • The Distance between two consecutive maxima or minima is called Wavelength (λ). • Principle of Wireless Communication:- When antenna of the appropriate size is attached to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by receiver some distance away. • In vacuum all electromagnetic waves travel at same speed usually called the speed of light, C (value Approx. 3 * 108 ) • Fundamental relation between f, λ, C is λf = C

20. The electromagnetic spectrum is shown in fig. The radio, Microwave, infrared and Visible light portions of the spectrum(signals through space) can all used for transmitting information by modulating the amplitude , frequency or phase of waves. • UV, X-rays and gamma rays would better , due to their high frequencies , but they are hard to produce and modulate, do not propagate well through building and are dangerous to living things. • Two types of Spread Spectrum : 1) In Frequency hopping spread spectrum, the transmitter hops from frequency hundreds of times per second. It is popular for military communication because it makes transmissions hard to detect and next to impossible jam. Ex: 802.11 and Bluetooth. 2) Direct sequence spread Spectrum, which spreads signal over a wide frequency band, is also gaining popularity in the commercial world. Ex: some second – third generation mobile phones

21. The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

22. Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere(region of the upper atmosphere.)

23. Radio waves are easy to generate , can travel long distances, and can implement in buildings easily, so they are widely used for communication , both indoors and outdoors. • Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.

24. Politics of the Electromagnetic Spectrum The ISM bands in the United States.

25. Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

26. Unguided optical signaling has been in use for centuries. A modern application is to connect the LANs in two building via lasers mounted on their rooftops. Coherent optical signaling using lasers in inherently unidirectional, so each buildings need its own laser and its own photo detector. This scheme offers very high bandwidth and very low cost. A disadvantage is that laser beams cannot penetrate rain or thick fog.. They normally work well on sunny days.

27. Communication Satellites • Geostationary Satellites • Medium-Earth Orbit Satellites • Low-Earth Orbit Satellites • Satellites versus Fiber

28. Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

29. Communication Satellites (2) The principal satellite bands.

30. Communication Satellites (3) VSATs using a hub.

31. Low-Earth Orbit SatellitesIridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth.

32. Globalstar (a) Relaying in space. (b) Relaying on the ground.

33. Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching

34. Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

35. Structure of the Telephone System (2) A typical circuit route for a medium-distance call.

36. Major Components of the Telephone System • Local loops • Analog twisted pairs going to houses and businesses • Trunks • Digital fiber optics connecting the switching offices • Switching offices • Where calls are moved from one trunk to another

37. The Politics of Telephones The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it.

38. The Local Loop: Modems, ADSL, and Wireless The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

39. Modems (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation

40. Modems (2) (a) QPSK. (b) QAM-16. (c) QAM-64.

41. Modems (3) (b) (a) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps.

42. Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.

43. Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation.

44. Digital Subscriber Lines (3) A typical ADSL equipment configuration.

45. Wireless Local Loops Architecture of an LMDS system.

46. Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.

47. Wavelength Division Multiplexing Wavelength division multiplexing.

48. Time Division Multiplexing The T1 carrier (1.544 Mbps).

49. Time Division Multiplexing (2) Delta modulation.

50. Time Division Multiplexing (3) Multiplexing T1 streams into higher carriers.