Maximum date rate=2hlog 2 V bits/sec. Maximum number of bits/sec=hlog 2 (1+S/N)

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1 Basics Data can be analog or digital. The term analog data refers to information that is continuous, digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values. Frequency is the rate of change with respect to time. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Bandwidth is a range of frequencies within a given band, in particular that used for transmitting a signal. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise. Q-1 Explain Maximum Data Rate of channel OR Explain the Nyquist and Shannon data rate capacity theory for noiseless and noisy channel respectively. Nyquist proved that if an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless because the higher frequency components that such sampling could recover have already been filtered out. If the signal consists of V discrete levels, Nyquist's theorem states: Maximum date rate=2hlog 2 V bits/sec For example, a noiseless 3-kHz channel cannot transmit binary (i.e., two-level) signals at a rate exceeding 6000 bps. So far we have considered only noiseless channels. If random noise is present, the situation deteriorates rapidly. And there is always random (thermal) noise present due to the motion of the molecules in the system. The amount of thermal noise present is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio. Shannon's major result is that the maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by Maximum number of bits/sec=hlog 2 (1+S/N) For example, a channel of 3000-Hz bandwidth with a signal to thermal noise ratio of 30 db (typical parameters of the analog part of the telephone system) can never transmit much more than 30,000 bps, no matter how many or how few signal levels are used and no matter how often or how infrequently samples are taken. Q-2 Define Guided transmission media and explain following a) Magnetic media b) Twisted pair c) Coaxial cable d) Fiber optics A transmission medium can be defined as anything that can carry information from a source to a destination. [Prof. Rupesh G Vaishnav] Page 1

2 Transmission Media Guided Media Unguided Media Cable Figure: Classification Transmission Media Guided Media Guided media, which are those that provide a channel from one device to another, include twistedpair cable, coaxial cable, and fiber-optic cable. A signal travelling along any of these media is directed and contained by the physical limit of the medium. a) Magnetic Media One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or removable media (e.g., recordable DVDs), physically transport the tape or disks to the destination machine, and read them back in again. Although this method is not as sophisticated as using a geosynchronous communication satellite, it is often more cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor. b) Twisted Pair A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form, just like a DNA molecule. Twisted- Pair Cable Coaxial Cable Fiber Optic Radio Wave Microwave Infrared Wave Twisting is done because two parallel wires constitute a fine antenna. When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively. Figure: Twisted Pair Cable Why cable is twisted? 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 relatives to the noise or crosstalk sources. This results in a difference at the receiver. By twisting the pair, a balance is maintained. Types of Twisted-Pair Cable 1) Unshielded twisted-pair (UTP) Twisted pair cabling comes in several varieties, two of which are important for computer networks. Category 3 twisted pairs consist of two insulated wires gently twisted together. Most office buildings had one category 3 cable running from a central wiring closet on each floor into each office. Category 5 is the more advanced twisted pairs were introduced. They are similar to category 3 pairs, but with more twists per centimeter, which results in less crosstalk and a better-quality signal over longer distances, making them more suitable for highspeed computer communication. [Prof. Rupesh G Vaishnav] Page 2

3 Up-and-coming categories are 6 and 7, which are capable of handling signals with bandwidths of 250 MHz and 600 MHz, respectively (versus a mere 16 MHz and 100 MHz for categories 3 and 5, respectively). 2) Shielded twisted-pair (STP). STP cable has a metal foil or braided mesh covering that encases each pair of insulated conductors. Metal casing improves the quality of cable by preventing the penetration of noise or crosstalk. It is bulkier and more expensive. Applications Used in telephone lines to provide voice and data channels. The DSL lines uses by telephone companies use the high-bandwidth capability of UTP cables. LANs, such as 10Base-T, 100Base-T, also uses twisted-pair cables. c) Coaxial Cable It has better shielding than twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television but is becoming more important with the advent of Internet over cable. A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely-woven braided mesh. The outer conductor is covered in a protective plastic sheath. The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable quality, length, and signal-to-noise ratio of the data signal. Modern cables have a bandwidth of close to 1 GHz. Coaxial cables used to be widely used within the telephone system for long-distance lines but have now largely been replaced by fiber optics on long-haul routes. Figure: Coaxial Cable d) Fiber Optics A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. 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 density of the two materials must be such that a beam of light moving through a core is reflected off the cladding instead of being refracted into it. [Prof. Rupesh G Vaishnav] Page 3

4 Figure: Fiber Optic Cable Fiber optic cables are similar to coax, except without the braid. Figure shows a single fiber viewed from the side. At the center is the glass core through which the light propagates. The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Next comes a thin plastic jacket to protect the cladding. Fibers are typically grouped in bundles, protected by an outer sheath. Figure shows a sheath with three fibers. Q-4 Define Unguided(Wireless) transmission media and explain following a) Radio Transmission b) Microwave Transmission c) Infrared d) Light wave Transmission Unguided transmission Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. a) Radio Transmission Radio waves are easy to generate, can travel long distances, and can penetrate 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. The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source, roughly as 1/r 2 in air. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, radio waves are subject to interference from motors and other electrical equipment. In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. In the HF they bounce off the ionosphere b) Microwave Transmission Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way.consequently, repeaters are needed periodically. Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. In [Prof. Rupesh G Vaishnav] Page 4

5 addition, even though the beam may be well focused at the transmitter, there is still some divergence in space. Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio. Disadvantages: Do not pass through buildings well. Multipath fading problem (the delayed waves cancel the signal). Absorption by rain above 8 GHz. Severe shortage of spectrum. Advantages: No right way is needed (compared to wired media). Relatively inexpensive. Simple to install. c) Infrared Unguided infrared and millimeter waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. They are relatively directional, cheap, and easy to build but have a major drawback: they do not pass through solid objects (try standing between your remote control and your television and see if it still works). In general, as we go from long-wave radio toward visible light, the waves behave more and more like light and less and less like radio. On the other hand, the fact that infrared waves do not pass through solid walls well is also a plus. It means that an infrared system in one room of a building will not interfere with a similar system in adjacent rooms or buildings. Furthermore, security of infrared systems against eavesdropping is better than that of radio systems precisely for this reason. Therefore, no government license is needed to operate an infrared system, in contrast to radio systems, which must be licensed outside the ISM bands. d) Light wave Transmission The laser's strength, a very narrow beam, is also its weakness here. Aiming a laser beam 1-mm wide at a target the size of a pin head 500 meters away requires the marksmanship of a latter-day Annie Oakley. Usually, lenses are put into the system to defocus the beam slightly. A disadvantage is that laser beams cannot penetrate rain or thick fog, but they normally work well on sunny days. [Prof. Rupesh G Vaishnav] Page 5

6 Q-5 What is the purpose of physical layer? Explain multimode fiber and single mode fiber. Explain the transmission of light through fiber. Propagation Modes 1) Single Mode Fiber If the fiber's diameter is reduced to a few wavelengths of light, the fiber acts like a wave guide, and the light can propagate only in a straight line, without bouncing, yielding a single-mode fiber. 2) Multimode fiber Any light ray incident on the boundary above the critical angle will be reflected internally, many different rays will be bouncing around at different angles. Each ray is said to have a different mode, so a fiber having this property is called a multimode fiber. Propagation of light through wires When a light ray passes from one medium to another, for example, from fused silica to air, the ray is refracted (bent) at the silica/air boundary, as shown in Figure (a). Here we see a light ray incident on the boundary at an angle a 1 emerging at an angle b 1. The amount of refraction depends on the properties of the two media (in particular, their indices of refraction). For angles of incidence above a certain critical value, the light is refracted back into the silica; none of it escapes into the air. Thus, a light ray incident at or above the critical angle is trapped inside the fiber, as shown in Figure (b), and can propagate for many kilometers with virtually no loss. Figure: Propagation of light through fiber optic [Prof. Rupesh G Vaishnav] Page 6

7 Q-6 Comparison of Guided medias Sr Twisted-Pair Cable Coaxial Cable Fiber Optic Cable (FOC) 1. It uses electrical signals for transmission. It uses electrical signals for transmission. It uses optical form of signal (i.e. light) for transmission. 2. It uses metallic conductor to carry the signal. It uses metallic conductor to carry the signal. It uses glass or plastic to carry the signal. 3. Noise immunity is low. Therefore more distortion. Higher noise immunity than twisted-pair cable due to the presence of shielding conductor. Highest noise immunity as the light rays are unaffected by the electrical noise. 4. Affected due to external magnetic field. Less affected due to external magnetic field. Not affected by the external magnetic field. 5. Cheapest Moderately costly Costly 6. Can support low data rates. Moderately high data rates. Very high data rates. 7. Power loss due to conduction and radiation. Power loss due to conduction. Power loss due to absorption, scattering, dispersion. 8. Short circuit between two conductors is possible. Short circuit between two conductors is possible. Short circuit is not possible. 9. Low bandwidth. Moderately high bandwidth. Very high bandwidth. Q-7 Comparison of Wired and Wireless Media Sr No Wired Media Wireless Media 1. The signal energy is contained and guided within a solid medium. The signal energy propagates in the form of unguided electromagnetic waves. 2. Used for point-to-point communication Used for broadcasting Twisted-pair cable, coaxial cable, fiber optical cables are example of wired media. Attenuation depends exponentially on the distance. Radio and infrared light are the examples of wireless media. Attenuation is proportional to square of distance. [Prof. Rupesh G Vaishnav] Page 7