ES-385
Communication Systems
22ES Sec-I/II
ES-385
Communication Systems
22ES Sec-I/II
Transmission Media (CLO-1)
Lecture-2
B.P. Lathi, “Modern Digital & Analog Communication Systems”, 4e
Freeman, R. “Telecommunication Transmission Handbook.” New York: Wiley, 1998.
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References
Guided Transmission Media
Twisted Pair
Coaxial Cable
Optical Fiber
Wireless Transmission
Antennas
Terrestrial Microwave
Satellite Microwave
Broadcast Radio
Wireless Propagation
Ground Wave Propagation
Sky Wave Propagation
Line-of-Sight Propagation
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Contents
The transmission medium is the physical path between transmitter and receiver.
For guided media, electromagnetic waves are guided along a solid medium, such as copper twisted pair, copper, coaxial cable, and optical fiber.
For unguided media, wireless transmission occurs through the atmosphere, outer space, or
water.
The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal.
In the case of guided media, the medium itself is more important in determining the limitations of transmission.
For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics.
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Introduction
In considering the design of data transmission systems, key concerns are data rate and distance: the greater the data rate and distance the better.
A number of design factors relating to the transmission medium and the signal determine the data rate and distance:
Bandwidth:
All other factors remaining constant, the greater the bandwidth of a signal, the higher the data rate that can be achieved.
Transmission impairments:
Impairments, such as attenuation, limit the distance. For guided media, twisted pair generally suffers
more impairment than coaxial cable, which in turn suffers more than optical fiber.
Interference:
Interference from competing signals in overlapping frequency bands can distort or wipe out a signal. Interference is of particular concern for unguided media but is also a problem with guided media. For guided media, interference can be caused by emanations from nearby cables.
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Introduction
Introduction
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The transmission capacity, in terms of either data rate or bandwidth, depends critically on the distance and on whether the medium is point-to-point or multipoint.
The three guided media commonly used for data transmission are twisted pair, coaxial cable, and optical fiber.
Twisted Pair
The least expensive and most widely used guided transmission medium is twisted pair.
A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link.
Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough
protective sheath.
Over longer distances, cables may contain hundreds of pairs. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable.
Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.
Guided Transmission Media
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Twisted Pair
It is the most commonly used medium in the telephone network and is the workhorse for communications within buildings.
In the telephone system, individual residential telephone sets are connected to the local telephone exchange, or “end office,” by twisted-pair wire. These are referred to as subscriber loops.
Within an office building, each telephone is also connected to a twisted pair, which goes to the in-house private branch exchange (PBX) system.
Twisted-pair installations were designed to support voice traffic using analog signaling. However, by means of a modem, these facilities can handle digital data traffic at modest data rates.
Data rates for such products are typically in the neighborhood of 10 Mbps. However, twisted- pair networks with data rates of to 1 Gbps have been developed, although these are quite limited
Guided Transmission Media
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Coaxial Cable
Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor.
The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield.
A single coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.
Guided Transmission Media
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Coaxial Cable
Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use in a wide variety of applications. The most important of these are
Television distribution
Long-distance telephone transmission
Short-run computer system links
Local area networks
Coaxial cable is widely used as a means of distributing TV signals to individual homes—cable TV. A cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few tens of kilometers.
Coaxial cable has traditionally been an important part of the long-distance telephone network.
Today, it faces increasing competition from optical fiber, terrestrial microwave, and satellite.
Guided Transmission Media
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Optical Fiber
An optical fiber is a thin, flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers.
The lowest losses have been obtained using fibers of ultrapure fused silica. higher-loss
multicomponent glass fibers are more economical and still provide good performance.
Plastic fiber is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.
An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket. The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic; Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core.
The interface between the core and cladding acts as a reflector to confine light that would otherwise escape the core. The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket.
The jacket is composed of plastic and other material layered to protect against moisture,
abrasion, crushing, and other environmental dangers.
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Guided Transmission Media
Optical Fiber
Guided Transmission Media
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Optical Fiber
One of the most significant technological breakthroughs in data transmission has been the development of practical fiber optic communications systems. Optical fiber already enjoys considerable use in long-distance telecommunications.
The following characteristics distinguish optical fiber from twisted pair or coaxial cable:
Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or upto 100 Mbps to 1 Gbps over a few tens of meters for twisted pair.
Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or bundled twisted- pair cable—at least an order of magnitude thinner for comparable information transmission capacity.
Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair and is constant over a wide range.
Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. Fibers do not radiate energy, so there is little interference with other equipment.
Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. Repeater spacing in the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers have been demonstrated. Coaxial and twisted-pair systems generally have repeaters every few kilometers.
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Guided Transmission Media
Optical Fiber
Guided Transmission Media
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Three general ranges of frequencies are of interest in our discussion of wireless transmission.
Frequencies in the range of about 1 GHz (gigahertz ) to 40 GHz are referred to as microwave frequencies. At these frequencies, highly directional beams are possible, and microwave is quite suitable for point-to-point transmission. Microwave is also used for satellite communications.
Frequencies in the range of 30 MHz to 1 GHz are suitable for omnidirectional applications. We refer to this range as the radio range.
Another important frequency range, for local applications, is the infrared portion of the spectrum. This covers, roughly, from to Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room.
For unguided media, transmission and reception are achieved by means of an Antenna.
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Wireless Media
An antenna is 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 from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water).
For reception of a signal, electromagnetic energy impinging on the antenna is converted into electrical energy and fed into the receiver.
In two-way communication, the same antenna can be and often is used for both transmission and reception.
The antenna transfers energy from the surrounding environment to its input receiver terminals with the same efficiency that it transfers energy from the output transmitter terminals into the surrounding environment, assuming that the same frequency is used in both directions.
Antenna characteristics are essentially the same whether an antenna is sending or receiving electromagnetic energy.
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Wireless Media - Antenna
An antenna will radiate power in all directions but, typically, does not perform equally well in all directions.
A common way to characterize the performance of an antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates.
The simplest pattern is produced by an idealized antenna known as the isotropic antenna.
Isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center.
Wireless Media - Antenna
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Parabolic Reflector Antenna (Dish Antenna)
Parabolic reflective antenna is used in terrestrial microwave and satellite applications.
A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directrix. If a parabola is revolved about its axis, the surface generated is called a paraboloid.
Such surfaces are used in headlights, optical and radio telescopes, and microwave antennas because of the following property:
If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid;
In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point.
The larger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.
Wireless Media - Antenna
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Yagi Uda Antenna
Yagi antenna, is a directional antenna consisting of multiple parallel elements in a line,usually half-wave dipoles made of metal rods.
Yagi–Uda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional "parasitic elements" which are not connected to the transmitter or receiver: a so-called reflector and one or more directors.
The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter.
The parasitic elements absorb and reradiate the radio waves from the driven element with a different phase, modifying the dipole's radiation pattern.
Waves from the multiple elements superpose and interfere to enhance radiation in a single direction, achieving a very substantial increase in the antenna's gain compared to a simple dipole
Wireless Media - Antenna
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Antenna gain is defined as the power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna).
For example, if an antenna has a gain of 3 dB, that antenna improves upon the isotropic antenna in that direction by 3 dB, or a factor of 2.
The increased power radiated in a given direction is at the expense of other directions. In effect, increased power is radiated in one direction by reducing the power radiated in other directions.
Antenna gain does not refer to obtaining more output power than input power but rather to directionality.
A concept related to that of antenna gain is the effective area of an antenna. The effective area of an antenna is related to the physical size of the antenna and to its shape. The relationship between antenna gain and effective area is:
Wireless Media -Antenna Gain
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The primary use for terrestrial microwave systems is in long haul telecommunications service, as an alternative to coaxial cable or optical fiber.
The microwave facility requires far fewer amplifiers or repeaters than coaxial cable over the same distance but requires line-of-sight transmission. Microwave is commonly used for both voice and television transmission.
Another common use of microwave is for short point-to-point links between buildings. This can be used for closed-circuit TV or as a data link between local area networks. Short-haul microwave can also be used for the so-called bypass applications.
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Wireless Media- Terrestrial Microwave
Among the most important applications for satellites are: Television distribution, Long-distance, telephone transmission, and Private business networks
Because of their broadcast nature, satellites are well suited to television distribution and are being used extensively throughout the world for this purpose.
In its traditional use, a network provides programming from a central location. Programs are transmitted to the satellite and then broadcast down to a number of stations, which then distribute the programs to individual
Wireless Media- Satellite Microwave
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The principal difference between broadcast radio and microwave is that the former is omnidirectional and the latter is directional.
Thus broadcast radio does not require dish-shaped antennas, and the antennas need not be rigidly mounted to a precise alignment.
Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz.
We are using the informal term broadcast radio to cover the VHF and part of the UHF band: 30 MHz to 1 GHz.
This range covers FM radio and UHF and VHF television. This range is also used for a number of data networking applications.
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Wireless Media- Radio Broadcast
A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight (LOS).
Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz.
Several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth’s curvature.
One factor is that the electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth’s curvature.
Another factor is diffraction, which is a phenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles.
Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The best-known example of ground wave communication is AM radio.
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Wireless Propagation- Ground Wave Propagation
Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America.
With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth.
Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused by refraction.
A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth’s surface.
With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter.
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Wireless Propagation- Sky Wave Propagation
Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight.
For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond the horizon.
For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other.
The term effective is used because microwaves are bent or refracted by the atmosphere.
The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight.
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Wireless Propagation- Line of Sight (LOS) Propagation
Wireless Propagation- Propagation Modes
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Wireless Propagation- Propagation Modes
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