Module 2
Module 2 Transmission media - Guided Transmission Media: Twisted pair, Coaxial cable, optical fiber, Wireless Transmission, Terrestrial microwave, Satellite microwave. Wireless Propagation: Ground wave propagation, Sky Wave propagation, LoS Propagation.
Transmission Media
Classes of transmission media
Guided Media
GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light.
Design Factors Bandwidth higher bandwidth gives higher data rate Transmission Impairments e.g. attenuation Interference Number of receivers in guided media more receivers introduces more attenuation
Electromagnetic Spectrum
Transmission Characteristics of Guided Media Twisted pair (with loading) Frequency Range Typical Attenuation 0 to 3.5 khz 0.2 db/km @ 1 khz Typical Delay 50 µs/km 2 km Repeater Spacing Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 db/km @ 1 khz Coaxial cable 0 to 500 MHz 7 db/km @ 10 MHz Optical fiber 186 to 370 THz 0.2 to 0.5 db/km 5 µs/km 2 km 4 µs/km 1 to 9 km 5 µs/km 40 km
Twisted Pair Cable
Twisted Pair Cable A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link.
Physical Characteristics 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. 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. Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath.
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. By twisting the pairs, a balance is maintained. For example, suppose in one twist, one wire is closer to the noise source and the other is farther; in the next twist, the reverse is true. 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. The unwanted signals are mostly canceled out. From the above discussion, it is clear that the number of twists per unit of length (e.g., inch) has some effect on the quality of the cable.
Transmission Characteristics Analog needs amplifiers every 5km to 6km Digital can use either analog or digital signals needs a repeater every 2-3km Compared to other guided medias, twisted pair has; limited distance limited bandwidth (1MHz) limited data rate (100MHz)
The attenuation for twisted pair is a very strong function of frequency. Other impairments are also severe for twisted pair. The medium is quite susceptible to interference and noise because of its easy coupling with electromagnetic fields. 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 crosstalk. For point-to-point analog signaling, a bandwidth of up to about 1 MHz is possible. For long-distance digital point-to-point signaling, data rates of up to a few Mbps are possible. For very short distances, data rates of up to 10 Gbps have been achieved in commercially available products.
Unshielded Versus Shielded Twisted-Pair Cable Unshielded Twisted Pair (UTP) ordinary telephone wire cheapest easiest to install suffers from external EM interference Shielded Twisted Pair (STP) metal braid or sheathing that reduces interference more expensive harder to handle (thick, heavy)
Applications It is the most commonly used medium in the telephone network (linking residential telephones to the local telephone exchange, or office phones to a PBX), and for communications within buildings (for LANs running at 10-100Mbps). Twisted pair is much less expensive than the other commonly used guided transmission media and is easier to work with. Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop-the line that connects subscribers to the central telephone office commonly consists of unshielded twisted-pair cables. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables.
Coaxial Cable
COAXIAL CABLE 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.
Physical Characteristics Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover
Categories of Coaxial Cables Coaxial cables are categorized by their radio government (RG) ratings
Applications Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including: Television distribution - aerial to TV & Cable TV systems Long-distance telephone transmission - traditionally used for inter-exchange links, now being replaced by optical fiber/microwave/satellite Local area networks
Optic Fiber Cable
Optical Fiber
Physical Characteristics 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 50 µm. 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 125 µm.
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.
Fiber-Optic Cable A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction.
Bending of light ray
Propagation Modes Current technology supports two modes (multimode and single mode) for propagating light along optical channels, each requiring fiber with different physical characteristics. Multimode can be implemented in two forms: Stepindex or Graded-index
Multimode step-index fiber Here, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change due to a lower density; this alters the angle of the beam's motion. The term step index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber.
Multimode graded-index fiber It decreases this distortion of the signal through the cable. The word index here refers to the index of refraction. As we saw above, the index of refraction is related to density. A graded-index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. It shows the impact of this variable density on the propagation of light beams.
Single-Mode Single-mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The single mode fiber itself is manufactured with a much smaller diameter than that of multimode fiber, and with substantially lower density (index of refraction). 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
Optical Fiber Transmission Modes
Advantages of Optical Fiber Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available. Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable. Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper. Light weight. Fiber-optic cables are much lighter than copper cables. Greater immunity to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antenna effects that can easily be tapped.
Disadvantages There are some disadvantages in the use of optical fiber. Installation and maintenance. Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere. Unidirectional light propagation. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed. Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified.
Unguided Media
UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. The electromagnetic spectrum, ranging from 3 khz to 900 THz, used for wireless communication.
Wireless Transmission Frequencies 2GHz to 40GHz microwave highly directional point to point Can also be used for satellite 30MHz to 1GHz omnidirectional broadcast radio 3 x 10 11 to 2 x 10 14 infrared Local point to point Multipoint application within confined areas.
Propagation methods Unguided signals can travel from the source to destination in several ways: ground propagation, sky propagation, and line-ofsight propagation
Wireless Propagation Ground Wave
Ground Propagation Below 2 MHz Ionosphere does not plays a vital role. The signal is propagating in all directions and signal is hugging the earth. As a result with along the ground it can propagate over long distances. Example: AM Radio
Wireless Propagation Sky Wave
Sky Propagation From 2 to 30 MHz The electromagnetic signal which will come out from the transmitter will go to the ionosphere. The signal gets reflected by the ionosphere and comes back to the receiving antenna. Ionosphere plays a very important role because as the reflection of signals by ionosphere helps sky propagation method to cover long distance. Example: Aircraft communication, BBC
Wireless Propagation Line of Sight
Line-of-sight Propagation Above 30 MHz Point to point communication And above 30 MHz the signal behaves somewhat light as the frequency is very high and the wavelength is small. Got two antennas both are communicating with each other with the help of line of sight communication. The antenna must be relatively quite so high so that the line of sight communication is visible Example: FM radio, television signals, cellular phone, terrestrial microwave, satellite microwave
Antennas For unguided media, transmission and reception are achieved by means of an 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. Transmission antenna radio frequency energy from transmitter converted to electromagnetic energy by antenna radiated into surrounding environment reception antenna electromagnetic energy impinging on antenna converted to radio frequency electrical energy fed to receiver Same antenna is often used for both purposes
Radiation Pattern Antenna will radiate power in all directions Does not perform equally well in all directions as seen in a radiation pattern diagram an isotropic antenna is a (theoretical) point in space radiates in all directions equally actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center.
Omnidirectional Antenna Also known as isotropic antenna Used mainly in broadcast radio Frequency range from 30 MHz -1 GHz Communication through lineof-sight Can penetrate through walls. Signal can travel long distance. Multipath interference.
Parabolic Reflective Antenna
Parabolic Reflective Antenna
Parabolic Reflective Antenna Used in terrestrial microwave and satellite applications such as headlights, optical and radio telescopes, and microwave antennas A parabola is the locus of all points equidistant from a fixed line (the directrix), and fixed point (the focus) not on the line. 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.
Antenna Gain Measure of directionality of antenna Defined as power output in particular direction verses that produced by an isotropic antenna or in any direction. measured in decibels (db) results in loss in power in another direction effective area relates to size and shape related to gain
Antenna Gain
Wireless transmission waves
Radio Waves Electromagnetic waves ranging in frequencies between 3 khz and 1 GHz are normally called radio waves Radio waves, for the most part, are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have 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.
Radio Waves Radio waves, particularly those waves that propagate in the sky mode, can travel long distances. This makes radio waves a good candidate for longdistance broadcasting such as AM radio. Radio waves, particularly those of low and medium frequencies, can penetrate walls. It is an advantage because, for example, an AM radio can receive signals inside a building. It is a disadvantage because we cannot isolate a communication to just inside or outside a building. suffers from multipath interference reflections from land, water, other objects
Microwaves Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas. Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.
Terrestrial Microwave used for long haul telecommunications and short point-to-point links requires fewer repeaters but line of sight use a parabolic dish to focus a narrow beam onto a receiver antenna 1-40GHz frequencies higher frequencies give higher data rates main source of loss is attenuation distance, rainfall also interference
Satellite Microwave satellite is relay station receives on one frequency, amplifies or repeats signal and transmits on another frequency eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz typically requires geo-stationary orbit height of 35,784km spaced at least 3-4 apart typical uses television long distance telephone private business networks global positioning
Satellite Point to Point Link
Satellite Broadcast Link
Characteristics of Microwave Transmission Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall. The curvature of the earth as well as other blocking obstacles do not allow two short towers to communicate by using microwaves. Repeaters are often needed for long distance communication. Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings. The microwave band is relatively wide, almost 299 GHz. Therefore wider sub bands can be assigned, and a high data rate is possible Use of certain portions of the band requires permission from authorities.
directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers
Infrared Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for shortrange communication. Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents interference between one system and another; a short-range communication system in one room cannot be affected by another system in the next room. When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors. However, this same characteristic makes infrared signals useless for long-range communication. In addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication.
Applications Signals for communication between devices such as keyboards, mice, PCs, and printers. For example, some manufacturers provide a special port called the IrDA port that allows a wireless keyboard to communicate with a PC.