Transmission medias

1. Transmission medias NET 205: Data Transmission and Digital Communication 1st semester 1440-1439

2. 205NET LOC • 1-Introduction to Communication Systems and Networks architecture OSI Reference Model. • 2- Data Transmission Principles • 3- Transmission medias

3. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

4. Transmission Medias • A transmission media is the channel that provides the connection between the transmitter and the receiver. • physical or non-physical link • moves electromagnetic energy from one or more source to one or more receiver. • Which medium should be used?

5. Types of Transmission Medias The Transmission media or channels can be classified as : • Analog Channels:These channels can carry analog signals.Examples: telephone system, commercial radio system • Digital Channels:These channels can carry digital signals.Example: computer communications

6. Types of Transmission Medias Medias also can be classified as: • Bounded (guided) medias: signals are confined to the medium and do not leave it.Examples: electrical cables: twisted pair and coaxial cable and optical fiber • Unbounded (unguided) medias: signals travel freely and spread throughout the medium. • Unguided media employ an antenna for transmitting through air or water

7. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

8. Guided Media

9. Electrical Cables • Transmit electrical signals on a conductor, e.g. copper • To minimize interference: • Keep the cables away from other sources • Design the cables to minimize radiation and pick-up

10. Twisted Pair Cable • A twisted pair consists of two insulated copper wires twisted together in a helical form

11. Twisted Pair Cable • Most commonly used and least expensive medium • Used in telephone networks and in-building communications • Telephone networks designed for analog signaling (but supporting digital data) • Also used for digital signaling • Two varieties of twisted pair: shielded (STP) and unshielded (UTP); also multiple categories (CAT5)

12. Coaxial Cable • Coaxial cable consists of two conductors. • The inner conductor is held inside an insulator • the other conductor woven around it providing a shield. • An insulating protective coating called a jacket covers the outer conductor.

13. Coaxial Cable • Provide much more shielding from interference than twisted pair: • Higher data rates; • more devices on a shared line; • Longer distances. • Widely used for cable TV, as well as other audio/video cabling. • Used in long-distance telecommunications, although optical fiber is more relevant now

14. Fiber Optic Cables • These cables carry the transmitted information in the form of a fluctuating beam of light in a glass fiber.

15. Fiber Optic Cables • Used in long-distance telecommunications, as well as telephone systems, LANs, and city-wide networks • Advantages of optical fiber over electrical cables: 1. Lower loss: can transfer larger distances 2. Higher bandwidth: a single fiber is equivalent to 10's or 100's of electrical cables 3. Small size, light weight: lowers cost of installation 4. Electromagnetic isolation

16. Comparison of Guided Media Optical Cables

17. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

18. Wireless Transmission Model • Three general ranges of frequencies are of interest in discussion of wireless transmission. • For unguided media, transmission and reception are achieved by means of an antenna

19. Antenna • An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. • For transmission of a signal, electrical energy  electromagnetic energy • For reception of a signal, electromagnetic energy  electrical energy • Direction and propagation of a wave depends on antenna type: Isotropic, Omni-directional , and Directional.

20. Antenna Types • Isotropic antenna radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center.(ideal) • Omni-directional antenna radiates power in all directions on one plane (circle , donut). • Directional antenna: radiatespower in particular direction. Dish and Yagi are two common types.

21. Antenna Patterns

22. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

23. Wireless Propagation • A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight (LOS).

24. Wireless Propagation

25. Multipath Propagation • In unguided channels, signals are not only transmitted directly from source to destination but also a lot of paths from source to destination by reflection, diffraction , …etc. • So the receiver receive multiple copies (components) of transmitted signal. • Line of sight (LOS) is the fastest component reaching to destination.

27. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

28. Electromagnetic Spectrum • Electromagnetic spectrum is used by many applications • International and national authorities regulate usage of spectrum • Aim: minimize interference between applications/users, while allowing many applications/users

30. Microwave and Radio Signals • Microwave signals are higher frequency signals. • Higher frequency  carry large quantities of information. • It is highly directional so it follow LOS propagation. • The required antenna is smaller due to shorter wavelength (due to higher frequencies) ( the size of the antenna required to transmit a signal is proportional to the wavelength (λ) of the signal).

31. Microwave and Radio Signals • Microwave is quite suitable for point-to-point transmission and it is also used for satellite communications. • Radio frequency is lower frequency signalssuitable for omnidirectional applications. • It follow ground or sky wave propagation

32. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

33. Channel Impairments • Imperfect characteristics of the channel a signal is subjected to different types of impairments. • As a consequence, the received and the transmitted signals are not the same. • These impairments introduce • random modifications in analog signals leading to distortion. • error in the bit values in digital signals.

34. Channel Impairments

35. Attenuation • Irrespective of whether a medium is guided or unguided, the strength of a signal falls off with distance. • When a signal travels through a medium it loses energy overcoming the resistance of the medium • Attenuation means loss of energy  weaker signal. • The attenuation leads to several problems:

36. Attenuation 1- Attenuated signal cannot be detected and interpreted properly  Amplifier 2. Attenuation Distortion

37. Delay Distortion • Means that the signal changes its form or shape • How this happen • A composite signal made of different frequencies components • Each signal component has its own propagation speed through a medium and, therefore, its own delay in arriving at the final destination. • Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. • In other words, signal components at the receiver have phases different from what they had at the sender. • The shape of the composite signal is therefore not the same.

39. Noise • As signal is transmitted through a channel, noise gets mixed up with the signal, along with the distortion introduced by the transmission media. • Noise is any unwanted energy tending to interfere with the signal to be transmitted. • The noise either be: • External Noise. This is noise originating from outside the communication system • Internal Noise: This is noise originating from within the communication system.

40. Some Examples of Noise • Thermal Noise: This noise is due to the random and rapid movement of electrons in any resistive component. Electrons “bump” with each other. • Impulse noise is irregular pulses or noise spikes of short duration • Cross talk is a result of bunching several conductors together in a single cable. Signal carrying wires generate electromagnetic radiation, which is induced on other conductors because of close proximity of the conductors

41. Signal-to-Noise Ratio • In the study of noise, it is not important to know the absolute value of noise. • Even if the power of the noise is very small, it may have a significant effect if the power of the signal is also small. • What is important is a comparison between noise and the signal. • The signal-to-noise ratio (SNR) is the ratio of signal power to noise power.

42. Signal-to-Noise Ratio SNR = Ps / Pn

43. Signal-to-Noise Ratio • Ideally, SNR = ∞ (when Pn= 0).In practice, SNR should be high as possible. • A high SNR ratio means a good-quality signal. • A low SNR ratio means a low-quality signal. • The SNR is normally expressed in decibels, that is: SNR = 10 log10 (Ps / Pn) dB

44. Figure 3.30 Two cases of SNR: a high SNR and a low SNR

45. Example • The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB SNR = 10 × 10-3 / 10-6 = 10,000 SNRdB = 10 log10 (10 × 10-3 / 10-6) = 10 log10 (10,000) = 40 dB

46. Outline • Transmission Media • Guided Media • Wireless Transmission and Antennas • Wireless Propagation • Electromagnetic Spectrum • Channel Impairments • Channel Capacity

47. Channel Capacity • It is the maximum rate at which data can be correctly communicated over a channel in presence of noise and distortion. • The capacity of an analog channel is its bandwidthis the difference between the lowest and highest frequency an analog channel can convey to a receiver • The capacity of a digital channel is the number of digital values the channel can convey in one second (bps).

48. Relationship Between Bandwidth and Datarate • Digital signals consist of a large number of frequency components. If digital signals are transmitted over a channel with a limited bandwidth, only those components that are within the bandwidth of the transmission medium are received. • The faster the data rate of a digital signal, the higher the bandwidth will be required since the frequency components will be spaced farther apart. • Therefore, a limited bandwidth will also limit the data rate that can be used for transmission

50. Noiseless Channel: Nyquist Bit Rate For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate of a transmission medium as a function of its bandwidth C = 2 x BW x log2 m bits/sec, C is known as the channel capacity, BW is the bandwidth of the channel and m is the number of signal levels used.