* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Fiber-Optic Cable
Survey
Document related concepts
Standing wave ratio wikipedia , lookup
Electrical connector wikipedia , lookup
Direction finding wikipedia , lookup
Cellular repeater wikipedia , lookup
Microwave transmission wikipedia , lookup
Opto-isolator wikipedia , lookup
Telecommunication wikipedia , lookup
History of telecommunication wikipedia , lookup
Loading coil wikipedia , lookup
Index of electronics articles wikipedia , lookup
Submarine communications cable wikipedia , lookup
Transcript
Transmission Medium • Media for serial Data transmission Selection Criteria • When selecting which medium is suitable, several factors should be kept in mind: - costs and installation effort, - transmission safety - susceptibility to tapping, interference susceptibility, error probability, etc. - maximum data rate, - distances and topological position of the participants, etc. Good Signal Quality and Low Interference Susceptibility • No medium has all the optimum properties so that the signals are more or less attenuated with increasing distance. To achieve high data rates, the transmission medium must fulfill specific requirements. • Another negative effect is the risk of data being corrupted by interference signals. Common Types of Physical Cable Straight Cable • This is the simplest type of cable. It consists of copper wires surrounded by an insulator. The wire comes in bundles or as flat “ribbon” cables and is used to connect various peripheral devices over short distances. Cables for internal disk drives are typically flat cables with multiple transmission wires running in parallel. Properties of Wired Transmission Media • Bbbb Twisted-pair Cable • This cable consists of copper-core wires surrounded by an insulator. Two wires are twisted together to form a pair, and the pair forms a balanced circuit (voltages in each pair have the same amplitude but are opposite in phase). • The twisting protects against EMI (electromagnetic interference) and RFI (radio frequency interference). A typical cable has multiple twisted pairs, each color-coded to differentiate it from other pairs. • UTP (unshielded twisted-pair) has been used in the telephone network and is commonly used for data networking in the United States. • STP (shielded twisted-pair) cable has a foil shield around the wire pairs in a cable to provide superior immunity to RFI. Traditional twisted-pair LANs use two pairs, one for transmit and one for receive, but newer Gigabit Ethernet networks use four pairs to transmit and receive simultaneously. UTP and STP are constructed of 100-ohm, 24-AWG solid conductors. Coaxial Cable • This cable consists of a solid copper core surrounded by an insulator, a combination shield and ground wire, and an outer protective jacket. • In the early days of LANs, coaxial cable was used for its high bit rates. An Ethernet Thinnet (10Base-2) network has a data rate of 10Mbits/sec and implements a bus topology in which each station is attached to a single strand of cable. • Today, hierarchical wiring schemes are considered more practical, and even though more twisted pair wire is required to cable such a network, cost has dropped, making such networks very practical. Fiber-optic Cable • This cable consists of a center glass core through which light waves propagate. This core is surrounded by a glass cladding that basically reflects the inner light of the core back into the core. A thick plastic outer jacket surrounds this assembly, along with special fibers to add strength. • Fiber-optic cable is available with a metal core for strength if the cable will be hung over distances. Electric Lines • A great advantage of electric lines is their simple and cost-effective preparation (cutting to length and termination). However, there are some disadvantages which include the attenuation of signals and interference susceptibility. • These drawbacks are not only influenced by the type of cable used - twisted-pair, coaxial, etc. but also by the interface specification Transmission Behavior of Electric Lines • To be able to determine the electric properties of a cable, the line is described as a sequence of sub-networks consisting of resistors, capacitors, and inductors (See Figure on the next slide). While the resistors change the static signal level, capacitors and inductors create low passes which have a negative effect on the edge steepness. Equivalent Circuit Diagram of a Transmission Cable • Equivalent Circuit Transmission Cable Characteristics • Bbb Attenuation and Signal Distortion • The cable must therefore be selected to meet the following criteria: - The line resistance must be low enough so that a sufficiently high signal amplitude can be guaranteed on the receiver side. - The cable capacitances and inductances must not distort the signal edges to an extent that the original information is lost. Both criteria are influenced by the electric line parameters and the influence increases with the length of the line as well as with the number of participants connected. As a result, each cable type is limited in its line length and maximum number of participants. The higher the signal frequency, the stronger the effect the capacitances and inductances have on the signal. An increasing transmission frequency has therefore a limiting effect on the maximum line length. To limit the signal distortion occurring in long-distance lines and at high data rates, such applications frequently use low-inductance and low-capacitance cables, e.g. Ethernet with coaxial cable. Interference Caused by Line Reflection • Signals transmitted over electric lines are subject to yet another phenomenon, which is important to be aware of when installing a line. The electric properties of a line can be influenced by - changing the cable type, - branching the cable, - connecting devices or - a line that is not terminated at the beginning or at the end. • This causes so-called line reflections. The term means that transient reactions take place on the line, that are caused by the finite signal propagation speed. Since transient reactions distort the signal levels, a signal can only be read accurately, when - the transient reactions have largely died out or - the effects of the transient reactions are small. Terminating Resistors • To enable the use of long lines even for high data rates, the formation of line reflections must be prevented. This is achieved when the electric properties remain constant across the entire line. The line properties must be imitated as precisely as possible at the beginning and at the end of the line by connecting a terminating resistor. • The line properties are described by means of the socalled characteristic wave impedance of the cable. Typical values for the characteristic wave impedance and, hence, the terminating resistor are as follows: - twisted-pair line: 100 to 150 ohms - coaxial cable (RG 58): 50 ohms Terminating Resistors for Different Lines • a) twisted two-wire line • b) RS 485 standard • c) IEC 61158-2 Fiber Optics • An optical fiber consists of a light-transmitting core fiber embedded in a glass cladding and an external plastic cladding. When light hits the boundary layer in a small angle of incidence, the different densities of the core and the glass cladding cause total reflection. The light beam is reflected almost free of any loss and transmitted within the core fiber only. • The diameter of an optical fiber is approx. 0.1 mm. Depending on the version, the diameter of the lighttransmitting core lies between 9 µm and 60 µm. Usually, several - up to a thousand - of such fibers and a strain relief are grouped into a cable. • Multimode and monomode optical fiber Cross Section of a Fiber-Optic Cable • A typical cable Profiles and refractive indices of optical fibers • Bbbb • 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. • When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass: the axial ray. This single-mode propagation provides superior performance for the following reason. Because there is a single transmission path with single-mode transmission, the distortion found in multimode cannot occur. Singlemode is typically used for long-distance applications, including telephone and cable television. • 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. Sizes • A fiber is thinner than a human hair but stronger than a steel fiber of similar thickness. The sizes of the fiber have been standardized nationally and internationally. For example, when expressed as 62.5/125, the first number is the core diameter and the second number is the cladding diameter in microns or μm. Types of material make up fiberoptic cables • • • • Glass Plastic Plastic-clad silica (PCS) These three cable types differ with respect to attenuation. Attenuation is principally caused by two physical effects: absorption and scattering. Absorption removes signal energy in the interaction between the propagating light (photons) and molecules in the core. Scattering redirects light out of the core to the cladding. Glass Fiber-Optic Cable • Glass fiber-optic cable has the lowest attenuation. A pure-glass, fiber-optic cable has a glass core and a glass cladding. This cable type has, by far, the most widespread use. It has been the most popular with link installers, and it is the type of cable with which installers have the most experience. The glass used in a fiber-optic cable is ultra-pure, ultra-transparent, silicon dioxide, or fused quartz. During the glass fiber-optic cable fabrication process, impurities are purposely added to the pure glass to obtain the desired indices of refraction needed to guide light. • Germanium, titanium, or phosphorous is added to increase the index of refraction. Boron or fluorine is added to decrease the index of refraction. Other impurities might somehow remain in the glass cable after fabrication. These residual impurities can increase the attenuation by either scattering or absorbing light. Plastic Fiber-Optic Cable • Plastic fiber-optic cable has the highest attenuation among the three types of cable. Plastic fiber-optic cable has a plastic core and cladding. This fiber-optic cable is quite thick. • Typical dimensions are 480/500, 735/750, and 980/1000. The core generally consists of polymethylmethacrylate (PMMA) coated with a fluropolymer. Plastic fiber-optic cable was pioneered principally for use in the automotive industry. The higher attenuation relative to glass might not be a serious obstacle with the short cable runs often required in premise data networks. The cost advantage of plastic fiber-optic cable is of interest to network architects when they are faced with budget decisions. • Plastic fiber-optic cable does have a problem with flammability. Because of this, it might not be appropriate for certain environments and care has to be taken when it is run through a plenum. Otherwise, plastic fiber is considered extremely rugged with a tight bend radius and the capability to withstand abuse. Plastic-Clad Silica (PCS) FiberOptic Cable • The attenuation of PCS fiber-optic cable falls between that of glass and plastic. PCS fiber-optic cable has a glass core, which is often vitreous silica, and the cladding is plastic, usually a silicone elastomer with a lower refractive index. • PCS fabricated with a silicone elastomer cladding suffers from three major defects. First, it has considerable plasticity, which makes connector application difficult. Second, adhesive bonding is not possible. And third, it is practically insoluble in organic solvents. These three factors keep this type of fiber-optic cable from being particularly popular with link installers. However, some improvements have been made in recent years. • The light signals are usually supplied to the fiber via a laser LED and analyzed by photo-sensitive semiconductors on the receiver side. Since signals transmitted in optical fibers are resistant to electromagnetic interferences and only slightly attenuated, this medium can be used to cover extremely long distances and achieve high data rates. The advantages of optical data transmission are summarized in the following: - suitable for extremely high data rates and very long distances, - resistant to electromagnetic interference, - no electromagnetic radiation, - suitable for hazardous environments and - electrical isolation between the transmitter and receiver stations Monomode fibers • Monomode fibers help achieve the best pulse repeat accuracy. The core diameter of these fibers is so small that only the paraxial light beam (mode 0) can be formed. The small diameter, however, requires particularly high precision when the light beam is supplied to the fiber. Multimode Fibers • If multimode fibers with a larger diameter are used, the number of possible propagation paths increases and, hence, the distortion of the pulses. However, this effect can be reduced by using specially manufactured fibers. These special fibers do not have a step index profile, i.e. a constant refractive index, but a so-called grade index profile. In this case, the refractive index of the core increases with the radius. The propagation rate which changes with the refractive index largely compensates for the different propagation times in the core, thus enabling higher pulse accuracy. Cross Section • mmm Digital Data Transmission • High Speed Transmission Over Optical Fiber Fiber-Optic Communications System • Information (voice, data, and video) from the source is encoded into electrical signals that can drive the transmitter. Optical fiber communications link • Simplex optical fiber communications link Source Analog or digital interface Transmitter Voltage-tocurrent converter Light source Source-tofiber interface Optical fiber cable Signal regenerato r Optical fiber cable Fiber-tolight detector interface Light detector Receiver Current-tovoltage converter Analog-todigital interface Destination Applications • Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in military applications is growing. The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local area networking. • Characteristics: • 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. 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. Modern optical fiber communications systems are capable of transmitting several gigabits per second over hundreds of miles, allowing literally millions of individual voice and data channels to be combined and propagated over one optical fiber cable. Applications (Cont.) • 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. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for miles without requiring regeneration. • Electromagnetic isolation: Because optical fiber cables are nonconductors of electrical current, they are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. Applications (Cont.) • 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. Disadvantages • Cost. Fiber-optic cable is expensive. Because any impurities or imperfections in the core can throw off the signal, manufacturing must be painstakingly precise. Also, a laser light source can cost thousands of dollars, compared to hundreds of dollars for electrical signal generators. • Installation/maintenance. Any roughness or cracking in the core of an optical cable diffuses light and alters the signal. All splices must be polished and precisely fused. All connections must be perfectly aligned and matched for core size, and must provide a completely light-tight seal. Metallic media connections, on the other hand, can be made by cutting and crimping using relatively unsophisticated tools. • Fragility. Glass fiber is more easily broken than wire, making it less useful for applications where hardware portability is required. Unguided Media • Unguided or wireless, media transport electromagnetic waves without using a physical conductor. Instead, signals are broadcast either through air (or, in a few cases, water), and thus are available to anyone who has a device capable of receiving them. Wireless Data Transmission • Wireless transmission in communications systems is well-suited to extremely long distances (radio relay systems, satellite technology, etc.) and remote controlled and/or mobile applications. • When the participants communicate while in sight of each other and when the distances to be covered are small and the data rates low, the comparably simple optical transmission via infrared radiation can be used successfully. • Radio-based communication can be used for a lot more applications. In everyday life, mobile phones are a good example of the widespread use of radio-based communication. Radio communications extend not only to the field of telecommunications. Types of Propagation Telecommunication Link • Wireless communication is usually combined with wired communication. Antennas • 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 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. Antennas (Cont.) • In two-way communication, the same antenna can be and often is used for both transmission and reception. This is possible because any 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. Put another way, antenna characteristics are essentially the same whether an antenna is sending or receiving electromagnetic energy. Parabolic Reflective Antenna • An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave and satellite applications. You may recall from your precollege geometry studies that a parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. Microwave Antenna • The most common type of microwave antenna is the parabolic “dish.” A typical size is about 3 m in diameter. The antenna is fixed rigidly and focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna. • Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and to be able to transmit over intervening obstacles. • To achieve long-distance transmission, a series of microwave relay towers is used, and point-to-point microwave links are strung together over the desired distance. Microwave Antenna • To increase the distance served by terrestrial microwave, a system of repeaters can be installed with each antenna. Earth Line of sight to the horizon • Microwave is commonly used for both voice and television transmission. The transmit station must be in visible contact with the receive station Atmosphere 50 km Earth Transmission Characteristics • Microwave transmission covers a substantial portion of the electromagnetic spectrum. Common frequencies used for transmission are in the range 1 to 40 GHz. Band (GHz) Bandwidth (MHz) Data Rate (Mbps) 2 7 12 6 30 90 11 40 135 18 220 274 Satellite Microwave • The communication satellite Transponders Satellite Solar panel 35 863 km orbit Antenna Downlink Uplink Earth Stations Footpoint Downlink Geosynchronous Satellites • To remain stationary, the satellite must have a period of rotation equal to the earth’s period of rotation. This match occurs at a height of 35 863 km at the equator North Pole Equator Satellite 35 863 km orbit