Transmission Media

1. Transmission Media

3. Guided Transmission Data • Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics

4. Magnetic Media • Most common way to transport data from one computer to another is to write them onto Magnetic Tape or removable media. • Example: • An industry-standard Ultrium tape can hold 800gigabytes. • A box of size 60x60x60cm can hold about 1000 of such tapes • Total capacity of box: 800terabytes. • If this box needs to delivered by road to a destination which is only on hour away • bandwidth comes out to be 1700Gbps. • No computer network can even approach this.

5. Twisted Pair • A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. • One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two.

6. Twisted Pair • In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. • If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise. This results in a difference at the receiver.

7. Twisted Pair By twisting the pairs, a balance is maintained. Twisting makes it probable that both wires are equally affected by external influences (noise or crosstalk). This means that the receiver, which calculates the difference between the two, receives no unwanted signals.

8. CAT5 replaced earlier CAT3 cables with a similar cable that uses the same connected but has more twists per meter. • More twists result in less crosstalk and a better quality signal over longer distances making the cables more suitable for high speed communication.

11. The most common UTP connector is RJ45 (RJ stands for registered jack). The RJ45 is a keyed connector, meaning the connector can be inserted in only one way.

12. Coaxial Cable • Shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. • Coaxial cables used to be widely used within the telephone system for long-distance lines but have now largely been replaced by fiber optics. • Coax is still widely used for cable television and metropolitan area networks.

13. Fiber Optics • A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. • Light travels in a straight line as long as it is moving through a single uniform substance. • If a ray of light travelling through one substance suddenly enters another substance (of a different density), the ray changes direction.

14. Fiber Optics • If the angle of incidence I (the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. • If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. • Critical angle is a property of the substance, and its value differs from one substance to another.

15. Figure 7-13 Refraction The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

16. Figure 7-15 Reflection The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

17. Fiber Optics • A glass or plastic core is surrounded by a cladding of less dense glass or plastic. • The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

18. Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

19. Propagation Modes in Fiber-Optic Cable

21. The single mode fiber itself is manufactured with a much smaller diameter than that of multimode fiber, and with substantially lower density. • The decrease in density results in a critical angle that is close enough to 90° to make the propagation of beams almost horizontal. • In this case, propagation of different beams is almost identical, and delays are negligible. All the beams arrive at the destination "together" and can be recombined with little distortion to the signal

22. Advantages: # Higher bandwidth # Less signal attenuation – run for 50 km w.r.t 5 km in other cables # Immunity to electromagnetic interferences # Resistance to corrosive material # Light weight # More immune/protected to tapping Disadvantages: # Installation/maintenance # Unidirectional # Cost : If the demand for bandwidth is not high, often the use of optical fiber Can not be justified

23. Unguided Media: Wireless Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.

24. Unguided Media: Wireless Unguided signals can travel from the source to destination in several ways: ground propagation, sky propagation and line-of-sight propagation.

25. In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. • These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. • Distance depends on the amount of power in the signal. • The greater the power, the greater the distance. • In sky propagation, higher-frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particles exist as ions) where they are reflected back to earth. • This type of transmission allows for greater distances with lower output power. • In line-or-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. (Antennas must be directional, facing each other)

26. Unguided media: Wireless Electromagnetic spectrum defined as radio waves and microwaves is divided into eight ranges, called bands, each regulated by government authorities. These bands are rated from VLF to EHF.

27. Unguided media: Wireless Wireless transmission waves

28. Unguided media: Wireless Radio Waves Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are called radio waves. Waves between 1 and 300 GHz are called microwaves. Note: Radio waves are used for multicast communications, such as radio and television.

29. Radio waves, for the most part, are omni-directional(When an antenna transmits radio waves, they are propagated in all directions) • Sending and receiving antennas need not to be aligned. A sending antenna sends waves that can be received by any receiving antenna. • The omnidirectional property has a disadvantage, too. The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band. • The omnidirectional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. AM and FM radio, television, are examples of multicasting.

30. Unguided media: Wireless Microwaves Microwaves: frequencies between 1 and 300 GHz # Unidirectional : When an antenna transmits microwave waves, they can be narrowly focused. # Sending and Receiving antennas need to be aligned. # Propagation is line-of-sight # Very high frequency microwaves cannot penetrate walls.

31. Unguided media: Wireless Microwaves Note: Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

32. Unguided media: Wireless Infrared 300GHzto 400THz • Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. • Infrared waves, having high frequencies, cannot penetrate walls. This characteristic prevents interference between one system and another. • When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors. • We cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication. • The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate. • Some manufacturers provide a special port called the IrDA port that allows a wireless keyboard to communicate with a PC.

33. Satellite Communication

34. What is a satellite? • A satellite is simply any body that moves around another (usually much larger) one in a mathematically predictable path called an orbit • A communication satellite is a microwave repeater station in space that is used for telecommunication , radio and television signals • The first man made satellite with radio transmitter was in 1957 • There are about 750 satellite in the space, most of them are used for communication

35. Working… • Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means • The two stations can use a satellite as a relay station for their communication • One Earth Station transmits the signals to the satellite. Up linkfrequency is the frequency at which Ground Station is communicating with Satellite • The satellite Transponder converts the signal and sends it down to the second earth station. This frequency is called a Downlink frequency

36. Applications… General applications • Weather forecasting • Radio and TV broadcast broadcasting • Military • Satellites for navigation and localization (e.g. GPS) In context of mobile communication • Global telephone backbones (get rid of large cables) • Connections for communication in remote places or developing areas (for researchers in Antarctica) • Global mobile communication (satellites with lower orbits are needed like LEO’s)

37. Communication Satellites • In its simplest form, a communication satellite can be thought of as a big microwave repeater in the sky. • It contains several transponders, each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal. • The downward beams can be broad, covering a substantial fraction of the earth's surface, or narrow, covering an area only hundreds of kilometers in diameter. This mode of operation is known as a bent pipe.

38. Communication Satellites • According to Kepler's law, the orbital period of a satellite varies as the radius of the orbit to the 3/2 power. • The higher the satellite, the longer the period. • Near the surface of the earth, the period is about 90 minutes. • At an altitude of about 35,800 km, the period is 24 hours. • At an altitude of 384,000 km, the period is about one month • A satellite's period is important, but it is not the only issue in determining where to place it. • Another issue is the presence of the Van Allen belts, layers of highly charged particles trapped by the earth's magnetic field. • Any satellite flying within them would be destroyed fairly quickly by the highly-energetic charged particles trapped there by the earth's magnetic field.

39. Communication Satellites • Geostationary Satellites (GEO) • Medium-Earth Orbit Satellites (MEO) • Low-Earth Orbit Satellites (LEO)

40. Communication Satellites • The invention of the transistor changed all that, and the first artificial communication satellite, Telstar, was launched in July 1962 • To prevent total chaos in the sky, orbit slot allocation is done by ITU. • Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

41. Geostationary Earth Orbit (GEO) • Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. • Means GEO satellites remain in the same position relative to the surface of earth. • Because of the long distance from earth it gives a large coverage area, almost a fourth of the earth’s surface • This distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication. • High transmit power needed and launching of satellites to orbit are complex and expensive. • Not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission

42. Communication Satellites (3) Very Small Aperture Terminals VSATs using a hub.

43. Communication Satellites • In many VSAT (Very Small Aperture Terminals) systems, the microstations do not have enough power to communicate directly with one another (via the satellite, of course). • Instead, a special ground station, the hub, with a large, high-gain antenna is needed to relay traffic between VSATs, . In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier

44. Medium Earth Orbit (MEO) • MEO satellites have a larger coverage area than LEO satellites • A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network • Used for navigation systems • A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite. • Because they are lower than the GEOs, they have a smaller footprint on the ground and require less powerful transmitters to reach them.

45. Low Earth Orbit (LEO) • LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface • LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass • A network of LEO satellites is necessary for LEO satellites to be useful • Handover necessary from one satellite to another • Need for routing

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

47. Teledesic for internet users with to provide the high downward link • The original design was for a system consisting of 288 small-footprint satellites arranged in planes just below the lower Van Allen belt at an altitude of 1350 km. • This was later changed to 30 satellites with larger footprints. Transmission occurs in the relatively uncrowded and high-bandwidth band. • The system is packet-switched in space, with each satellite capable of routing packets to its neighboring satellites. When a user needs bandwidth to send packets, it is requested and assigned dynamically in about 50 msec.

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

49. Satellites versus Fiber • Broadcasting the data use satellite communication (because it is cheaper as compare to fiber) • For mobile users (when driving and moving from one place to other place) use satellite • For getting the good bandwidth use fiber • Good infrastructure is not there use satellite communication • Rapid development was not possible like in war military communication