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Chapter 2

Chapter 2. The Physical Layer. The Physical Layer. It defines the mechanical, electrical, and timing interfaces to the network. Purpose: To transport a raw bit stream Each one has its own function in terms of bandwidth, delay, cost, ease of installation and maintenance.

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Chapter 2

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  1. Chapter 2 The Physical Layer

  2. The Physical Layer • It defines the mechanical, electrical, and timing interfaces to the network. • Purpose: To transport a raw bit stream • Each one has its own function in terms of • bandwidth, • delay, • cost, • ease of installation and maintenance

  3. Max Data Rate of a Channel Max Data Rate = 2Hlog2V bits/sec H : Bandwidth V : V discrete levels of a signal For binary = 2 levels Ex: for 3KHz channel, Max data rate – 6000 bps Signal to Noise Ratio (SNR) SNR = Signal Power / Noise Power Measured in decibels (db) – 10log10 S/N For Noisy channel Maximum number of bits/sec = Hlog2(1+S/N)

  4. Physical Media Groups • Roughly grouped into • Guided media, • copper wire and fiber optics, • Unguided media, • radio and lasers

  5. Guided Transmission Media • Magnetic Media • Twisted Pair • Co-axial cable • Fiber optics

  6. Guided Transmission Media Magnetic media • It is the most common way of transferring data • Transmission time is measured in minutes or hours • Used • Where very less frequent transportation is needed • Where amount of data is very high • Cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor. • Ex.Ultrium Tape-200 Gigabytes.

  7. Twisted Pair • Transmission time is measured in milliseconds. • Consisting of two insulated copper wires. • Thickness = 1 mm • Wires are twisted together in a helical form like DNA molecule. • To remove electro-magnetic effecton data

  8. Twisted-Pair Cable

  9. Twisted Pair • The most common application is telephone system • Can transfer data for several kilometers without amplification • But for very long distances amplification is needed • Repeaters are used • Many TP cables grouped together and covered by protected material. • They can be used for digital as well as analog transmission. • The bandwidth depends on the thickens of the wire and the distance traversed. • It is the cheapest solution

  10. Twisted Pair - Types • Two types • Category 3,4 pairs • 16 MHz • Gently twisted • Category 5,4 pairs • 100 MHz • More twists • Category 6 (250 MHz) and 7 (600 MHz) are also coming • It is also called UTP (Unshielded Twisted pair) cable.

  11. Figure 7-8 Twisted-Pair Cable

  12. A Coaxial Cable

  13. Coaxial cable • Construction • stiff copper wire as the core • surrounded by an insulating material • The insulator is encased by a cylindrical conductor • a closely-woven braided mesh • The outer conductor is covered in a protective plastic sheath

  14. Coaxial Cable • Advantages • better shielding than twisted pairs • High bandwidth (1 GHz) [600 MHz] • Excellent noise immunity • Use • Within the telephone system for long-distance lines • For cable television • For metropolitan area networks

  15. Coaxial cable • Two kinds of coaxial cable • 50-ohm cable • used for digital transmission • 75-ohm cable • used for analog transmission and cable television

  16. Fiber Optics • It transmit data by pulses of light • A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit • Optical transmission system has three components • The light source • The transmission medium • The detector

  17. Working of Fiber Optics • Light source is either LED or a laser diode. • The transmission medium is ultra thin fiber of glass • The detector is a Photodiode which emits electric pulse when light falls on it. • Attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end. • Data rate = 10 Gbps

  18. Example a) Three examples of light rays from inside a silica fiber impinging on the air/silica boundary at different angles b) light trapped by total internal reflection

  19. The Single Mode Fiber • Fiber with core diameter less than about ten times the wavelength of light then it is known as single mode fiber • Single mode fiber acts like a wave guide, and the light propagates in a straight line • They require expensive laser diodes but are more efficient and run for longer distances • It can transmit data at 50 Gbps for 100 km without amplification • The most common type of single-mode fiber has a core diameter of 8 to 10 μm

  20. The Single Mode Fiber • Single-mode fibers are more expensive but are widely used for longer distances. • Fiber with large (greater than 10 μm  ) core diameter called multimode fiber. • A fiber can pass more than one rays at a time, at different angles then it is known as multimode fiber.

  21. (a) Multimode fiber: multiple rays follow different paths reflected path direct path (b) Single mode: only direct path propagates in fiber

  22. Transmission of Light Through Fiber • Glass used is Very transparent! • Attenuation of light passing thru glass depends on the wavelength of it • Attenuation = reduction in power

  23. Attenuation of Light Thru Fiber in the Infrared Region

  24. The Spectrum Used • Three wavelengths are used for optical communication. • 0.85 micron, 1.30, 1.55 microns are centers • Later two have less then 5% loss per km

  25. Dispersion and Solution • Light pulses spread out in length as they propagate, This spreading is known as chromatic dispersion • The amount of dispersion is Wavelength dependent. • Dispersion results in overlapping of light waves in multimode fiber • To stop spread out pulses overlapping, is to increase the distance between them. This can be done only by reducing the signaling rate.

  26. The Fiber Cable • At the center is the glass core through which the light propagates. • In multimode fiber, the core is about 50 microns thick,( thickness of human hair) • In single mode fiber, the core is 8 to 10 microns wide

  27. The Fiber Cable • The core is surrounded by a glass cladding with a lower index of refraction • Next comes a thin plastic jacket to protect the cladding • Fibers are typically grouped together in bundles, protected by an outer sheath

  28. Fiber Cables • a) Side view of a single fibre • b) End view of a sheath with three fibres

  29. Light Sources A Comparison of LED and Semiconductor diodes as Light sources

  30. Interfaces (With Computers) • The connector is very difficult to make and substantial light is lost • Two type of interfaces are used • first one is called the Passive Interface • second one is called Active repeater • Both of them, at each computer, serves as a T junction to allow the computer to send and accept messages, and pass data through

  31. A Fiber optic Ring With Active repeaters

  32. The Active Repeater • In the Active repeater the incoming light is converted to an electric signal • It is regenerated to the full strength and retransmitted as light • connector is a simple copper wire • If an active repeater fails, the ring gets broken and the network goes down • There is no virtual limit on the size of ring

  33. Passive Interfaces: • Passive interface consists of two tapes fused onto main fiber • One tap has LED or Laser diode at the end of it and the other has the Photodiode • It is extremely reliable because a broken LED or photodiode does not break the ring. It just takes one computer off-line.

  34. A Passive Star Connection

  35. Comparing Fiber and Copper • High bandwidth with min loss of power • Not affected by power line surges, Electromagnetic interference, power failure • Repeaters are needed every 50km compared to 5 km in copper wire • They are very thin and light weight

  36. Comparing Fiber and Copper • One thousand twisted pairs 1 km long weigh 8000 kg. Two fibers have more capacity and weigh is only 100 kg. • Fibers do not leak light and are quite difficult to tap • Since optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber • It is an less familiar technology for most Engineers. • Can be damaged easily by being bent too much • Fiber interfaces cost more than electrical interfaces

  37. Wireless Transmission • For people who need to be on-line all the time • For mobile users. • Running a fiber to a building is difficult due to the terrain (mountains, jungles, etc.)

  38. The Electromagnetic Waves • When electrons move, they create EM waves that can propagate thru free space • Frequency( f ) • The number of oscillations per second of wave • measured in Hz • Wavelength (λ) • The distance between consecutive maxima or minima • By attaching an antenna to an Electric Circuit, the EM Wave can be broadcasted and can be received by receiver some distance away.

  39. The Properties of EMWs In Vacuum all EMWs travel at the same speed even though different frequency- 3 * 108 m/sec or 30 cm/nano sec In copper or fiber, it slows about 2/3rd of above value and become slightly freq dependent. 39

  40. The EM Spectrum

  41. The EM Spectrum • The Radio, microwave, infrared and visible light, all can be used for transmission • Transmission can be done by modulating either amplitude, frequency or phase • UV, X-rays, and gamma rays are hard to produce, do not propagate well thru buildings and are dangerous to living things

  42. The Capacity of Transmission • The amount of info an EMW can carry is related to its bandwidth • It is possible to encode few bits per Hz at lower freqs but nearly 8 at high freqs

  43. Transmission Methods • Direct Sequence spread spectrum & • Frequency hopping spread spectrum The transmitter and receiver hops from frequency to frequency hundreds of times per second • makes transmissions hard to detect • which spreads the signal over a wide frequency band • good efficiency • high noise immunity • Used in military and commercial world

  44. Radio Waves • They are easy to generate, can travel long distances, penetrate buildings easily • They are omni directional ( can travel in all directions) • so transmitter and receiver are not needed to be aligned • Radio waves are frequency dependent • At low freq, power falls off sharply with distance from the source. • At higher freq, they tend to travel in straight lines and bounce of obstacles

  45. Continue… • In VLF, LF and MF bands radio waves follow the ground • Easily pass through buildings • These bands offer relative low bandwidth for data communication

  46. Continue… • In HF and VHF bands, the ground waves tends to be absorbed by earth • The waves that reach ionosphere, are refracted by it and sent back to earth • military operates on these bands for long distance talks

  47. Transmission of Radio Waves In the VLF, LF and MF bands, radio waves follow the Curvature of the earth In the HF, and VHF they bounce of The ionosphere

  48. Microwave Transmission • Microwave = Wave above 100 MHz • Travel in Straight line • Transmitting and receiving antennas must be accurately aligned with each other • Repeaters are needed • If towers are too far, the earth will get in the way • Height : distance Ratio = r:r2 • Higher the tower, farther apart they can be.

  49. Terrestrial Microwave

  50. Microwave Transmission • To achieve high data rate -10GHz, microwave is in routine use but • At about 4 GHz, MW absorbed by water and generate hit • These waves are only a few centimeters long and are absorbed by rain • Soln :- Shut off links where rain is falling & take another route

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