Transmission Media Basics PDF

Transmission Media Basics Transmission Media and Antenna System (EST 131) Prepared by: Engr. Rochelle M. Sabarillo Transmission Media The transmission media that are used to convey information can be classified as guided or unguided. Guided...

Transmission Media Basics Transmission Media and Antenna System (EST 131) Prepared by: Engr. Rochelle M. Sabarillo Transmission Media The transmission media that are used to convey information can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable, and optical fiber. Unguided media employ an antenna for transmitting through air, vacuum, or water. Traditionally, twisted pair has been the workhorse for communications of all sorts. Higher data rates over longer distances can be achieved with coaxial cable, and so coaxial cable has often been used for high-speed local area network and for high-capacity long-distance trunk applications. However, the tremendous capacity of optical fiber has made that medium more attractive than coaxial cable, and thus optical fiber has taken over much of the market for high-speed LANs and for long-distance applications. Unguided transmission techniques commonly used for information communications include broadcast radio, terrestrial microwave, and satellite. Infrared transmission is used in some LAN applications. In a data transmission system, the transmission medium is the physical path between transmitter and receiver. Department of Electrical Engineering & Technology 2 Guided Media vs. Unguided Media 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. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam. Department of Electrical Engineering & Technology 3 Data Rate and Distance 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. For example, twisted pairs are often bundled together and conduits often carry multiple cables. Interference can also be experienced from unguided transmissions. Proper shielding of a guided medium can minimize this problem. Number of receivers: A guided medium can be used to construct a point-to-point link or a shared link with multiple attachments. In the latter case, each attachment introduces some attenuation and distortion on the line, limiting distance and/or data rate. Department of Electrical Engineering & Technology 4 Types of Transmission Line The two primary requirements of a transmission line are that: 1. the line introduce minimum attenuation to the signal 2. the line not radiate any of the signal as radio energy Parallel-Wire Lines (Balanced Two-Wire Line, Open-Wire Line) - Parallel-wire line is made of two parallel conductors separated by a space of 1 ⁄2 in. to several inches. Parallel-wire lines are rarely used today. Coaxial Cable - The most widely used type of transmission line is coaxial cable, which consists of a solid center conductor surrounded by a dielectric material, usually a plastic insulator such as Teflon. Twisted-Pair Cable - Twisted-pair cable, as the name implies, uses two insulated solid copper wires covered with insulation and loosely twisted together. Optical Fiber - A fiber-optic cable is made up of incredibly thin strands of glass or plastic known as optical fibers; one cable can have as few as two strands or as many as several hundred. Each strand is less than a tenth as thick as a human hair Waveguides - Waveguides are a special category of transmission line that is used to guide (direct) the waves (radiation) along the length of the tube. Department of Electrical Engineering & Technology 5 Types of Transmission Line Department of Electrical Engineering & Technology 6 Department of Electrical Engineering & Technology 7 Guided Transmission Media For guided transmission media, 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. Table below indicates the characteristics typical for the common guided media for long-distance point-to-point applications: Department of Electrical Engineering & Technology 8 Guided Transmission Media Twisted Pair. The least expensive and most widely used guided transmission medium is twisted pair. Physical Description: A twisted pair consists of two insulated copper wire 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 Department of Electrical Engineering & Technology 9 thicknesses of from 0.4 to 0.9 mm. Guided Transmission Media Twisted Pair Applications: By far the most common guided transmission medium for both analog and digital signals is 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 or to a Centrex facility at the end office. Twisted pair is also the most common medium used for digital signaling. For connections to a digital data switch or digital PBX within a building, a data rate of 64 kbps is common. Twisted pair is also commonly used within a building for local area networks supporting personal computers. Data rates for such products are typically in the neighborhood of 100 Mbps. Cost: Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with. Department of Electrical Engineering & Technology 10 Guided Transmission Media Twisted Pair Transmission Characteristics: Twisted pair may be used to transmit both analog and digital transmission. For analog signals, amplifiers are required about every 5 to 6 km. For digital transmission (using either analog or digital signals), repeaters are required every 2 or 3 km. Compared to other commonly used guided transmission media (coaxial cable, optical fiber), twisted pair is limited in distance, bandwidth, and data rate. Figure here shows, the attenuation for twisted pair is a very strong function of frequency. Also, the medium is quite susceptible to interference and noise because of its easy coupling with electromagnetic fields. For example, a wire run parallel to an ac power line will pick up 60- Hz energy. Impulse noise also easily intrudes into twisted pair. Several measures are taken to reduce impairments. Shielding the wire with metallic braid or sheathing reduces interference. The twisting of the wire reduces low-frequency interference and the use of different twist lengths in adjacent pairs reduces Department of Electrical Engineering & Technology 11 crosstalk. Guided Transmission Media Twisted Pair: Unshielded and Shielded Twisted Pair Twisted pair comes in two varieties: unshielded and shielded. Unshielded twisted pair (UTP) is ordinary telephone wire. Office buildings, by universal practice, are prewired with excess unshielded twisted pair, more than is needed for simple telephone support. This is the least expensive of all the transmission media commonly used for local area networks and is easy to work with and easy to install. Unshielded twisted pair is subject to external electromagnetic interference, including interference from nearby twisted pair and from noise generated in the environment. A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference. This shielded twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more difficult to work with than unshielded twisted pair. Department of Electrical Engineering & Technology 12 Guided Transmission Media Twisted Pair: Category 3 and Category 5 UTP EIA-568-A recognizes three categories of UTP cabling: Category 3: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 16 MHz Category 4: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 20 MHz Category 5: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 100 MHz Of these, it is Category 3 and Category 5 cable that have received the most attention for LAN applications. Category 3 corresponds to the voice-grade cable found in abundance in most office buildings. Over limited distances, and with proper design, data rates of up to 16 Mbps should be achievable with Category 3. Category 5 is a data-grade cable that is becoming standard for preinstallation in new office buildings. Over limited distances, and with proper design, data rates of up to 100 Mbps are achievable with Category 5. Department of Electrical Engineering & Technology 13 Guided Transmission Media Twisted Pair: Category 3 and Category 5 UTP A key difference between Category 3 and Category 5 cable is the number of twists in the cable per unit distance. Category 5 is much more tightly twisted, with a typical twist length of 0.6 to 0.85 cm, compared to 7.5 to 10 cm for Category 3. The tighter twisting of Category 5 is more expensive but provides much better performance than Category 3. Department of Electrical Engineering & Technology 14 Guided Transmission Media Twisted Pair: Category 3 and Category 5 UTP Department of Electrical Engineering & Technology 15 Guided Transmission Media Coaxial Cable Physical Description: Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair. 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. Department of Electrical Engineering & Technology 1 6 Guided Transmission Media Coaxial Cable Applications: Coaxial cable is a versatile transmission medium, used 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 Department of Electrical Engineering & Technology 1 7 Guided Transmission Media Coaxial Cable Transmission Characteristics: Coaxial cable is used to transmit both analog and digital signals. As can be seen from figure below, coaxial cable has frequency characteristics that are superior to those of twisted pair and can hence be used effectively at higher frequencies and data rates. Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise, and intermodulation noise. For long-distance transmission of analog signals, amplifiers are needed every few kilometers, with closer spacing required if higher frequencies are used. For digital signaling, repeaters are needed every kilometer or so, with closer spacing needed for higher data rates. Department of Electrical Engineering & Technology 1 8 Guided Transmission Media Optical Fiber Physical Description: An optical fiber is a thin (2 to 125µm), flexible medium capable of guiding an optical ray. It is somewhat thicker than a human hair. It can function as a light pipe or waveguide to transmit light between the two ends of the fiber. Various glasses and plastics can be used to make optical fibers. The lowest losses have been obtained using fibers of ultrapure fused silica. Ultrapure fiber is difficult to manufacture; 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. Department of Electrical Engineering & Technology 1 9 Guided Transmission Media Optical Fiber Physical Description: 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; the core has a diameter in the range of 8 to 50µm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core and a diameter of 125µm. 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 Department of Electrical Engineering & Technology 2 moisture, abrasion, crushing, and other 0 Guided Transmission Media Optical Fiber Applications: 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 up to 100 Mbps to 10 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. For cramped conduits in buildings and underground along public rights-of-way, the advantage of small size is considerable. The corresponding reduction in weight reduces structural support requirements. Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair and is constant over a wide range. Department of Electrical Engineering & Technology 2 1 Guided Transmission Media Optical Fiber Applications: The following characteristics distinguish optical fiber from twisted pair or coaxial cable: Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do not radiate energy, so there is little interference with other equipment and there is a high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap. 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. Department of Electrical Engineering & Technology 2 2 Guided Transmission Media Optical Fiber Applications: Five basic categories of application have become important for optical fiber: Long-haul trunks. Long-haul routes average about 1500 km in length and offer high capacity (typically 20,000 to 60,000 voice channels). Metropolitan trunks. Metropolitan trunking circuits have an average length of 12 km and may have as many as 100,000 voice channels in a trunk group. Rural exchange trunks. Rural exchange trunks have circuit lengths ranging from 40 to 160 km and link towns and villages. Subscriber loops. Subscriber loop circuits are fibers that run directly from the central exchange to a subscriber. Local area networks. Standards have been developed and products introduced for optical fiber networks that have a total capacity of 100 Mbps to 10 Gbps and can support hundreds or even thousands of stations in a large office building or a complex of buildings. Department of Electrical Engineering & Technology 2 3 Guided Transmission Media Optical Fiber Transmission Characteristics: Optical fiber transmits a signal-encoded beam of light by means of total internal reflection. Total internal reflection can occur in any transparent medium that has a higher index of refraction than the surrounding medium. In effect, the optical fiber acts as a waveguide for frequencies in the range of about to Hertz; this covers portions of the infrared and visible spectra. Figure shows the principle of optical fiber transmission. Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material. This form of propagation is called step-index multimode, referring to the variety of angles that will reflect. Department of Electrical Engineering & Technology 2 4 Guided Transmission Media Optical Fiber Transmission Characteristics: By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass: the axial ray. Because there is a single transmission path with single-mode transmission, the distortion found in multimode cannot occur. Single-mode is typically used for long distance applications, including telephone and cable television. Finally, by varying the index of refraction of the core, a third type of transmission, known as graded-index multimode, is possible. This type is intermediate between the other two in characteristics. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Rather than zig-zagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and higher speed Department of Electrical Engineering & Technology 2 allows light at the periphery to arrive at a 5 Guided Transmission Media Optical Fiber Transmission Characteristics: Two different types of light source are used in fiber optic systems: the light emitting diode (LED) and the injection laser diode (ILD). Both are semiconductor devices that emit a beam of light when a voltage is applied. The LED is less costly, operates over a greater temperature range, and has a longer operational life. The ILD, which operates on the laser principle, is more efficient and can sustain greater data rates. Both single mode and multimode can support several different wavelengths of light and can employ laser or LED light sources. In optical fiber, based on the attenuation characteristics of the medium and on properties of light sources and receivers, four transmission windows are appropriate, shown in the table. Department of Electrical Engineering & Technology 2 6 Guided Transmission Media Optical Fiber Example: Figure here shows attenuation versus wavelength for a typical optical fiber. The unusual shape of the curve is due to the combination of a variety of factors that contribute to attenuation. The two most important of these are absorption and scattering. In this context, the term scattering refers to the change in direction of light rays after they strike small particles or impurities in the medium. More about optical fiber: of Electrical Engineering & Technology Department 2 https://www.youtube.com/watch?v=jZOg39v73c4&t=3s 7 Reminder Prepare for oral recitation. Department of Electrical Engineering & Technology 2 8 Thank You for Listening :) Do you have any questions? Email me at [email protected] or you may post a message in our Google Classroom.