08-08-2014, 11:25 AM
A Industrial training Report On TELECOMMUNICATION
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Introduction
Since its invention in the early 1970s, the use of and demand for optical fiber have grown tremendously. The uses of optical fiber today are quite numerous. With the explosion of information traffic due to the Internet, electronic commerce, computer networks, multimedia, voice, data, and video, the need for a transmission medium with the bandwidth capabilities for handling such vast amounts of information is paramount. Fiber optics, with its comparatively infinite bandwidth, has proven to be the solution. Companies such as AT&T, MCI, and U.S. Sprint use optical fiber cable to carry plain old telephone service (POTS) across their nationwide networks. Local telephone service providers use fiber to carry this same service between central office switches at more local levels, and sometimes as far as the neighborhood or individual home. Optical fiber is also used extensively for transmission of data signals. Large corporations, banks, universities, Wall Street firms, and others own private networks. These firms need secure, reliable systems to transfer computer and monetary information between buildings, to the desktop terminal or computer, and around the world. The security inherent in optical fiber systems is a major benefit. Cable television or community antenna television (CATV) companies also find fiber useful for video services. The high information-carrying capacity, or bandwidth, of fiber makes it the perfect choice for transmitting signals to subscribers.
The fibering of America began in the early 1980s. At that time, systems operated at 90 Mb/s. At this data rate, a single optical fiber could handle approximately 1300 simultaneous voice channels. Today, systems commonly operate at 10 Gb/s and beyond. This translates to over 130,000 simultaneous voice channels. Over the past five years, new technologies such as dense wavelength-division multiplexing (DWDM) and erbium-doped fiber amplifiers (EDFA) have been used successfully to further increase data rates to beyond a terabit per second (>1000 Gb/s) over distances in excess of 100 km. This is equivalent to transmitting 13 million simultaneous phone calls through a single hair-size glass fiber. At this speed, one can transmit 100,000 books coast to coast in 1 second!
The growth of the fiber optics industry over the past five years has been explosive. Analysts expect that this industry will continue to grow at a tremendous rate well into the next decade and beyond. Anyone with a vested interest in telecommunication would be all the wiser to learn more about the tremendous advantages of fiber optic communication.
Fiber-Optic Applications
FIBRE OPTICS : The use and demand for optical fiber has grown tremendously and optical-fiber applications are numerous. Telecommunication applications are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs.
Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service between central office switches at local levels, and sometimes as far as the neighborhood or individual home (fiber to the home [FTTH]).
Optical fiber is also used extensively for transmission of data. Multinational firms need secure, reliable systems to transfer data and financial information between buildings to the desktop terminals or computers and to transfer data around the world. Cable television companies also use fiber for delivery of digital video and data services. The high bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as high-definition television (HDTV) telecasts. Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated tollbooths, and changeable message signs, also use fiber-optic-based telemetry systems.
Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used in most modern telemedicine devices for transmission of digital diagnostic images. Other applications for optical fiber include space, military, automotive, and the industrial sector
BENEFITS OF FIBER OPTICS
Optical fiber systems have many advantages over metallic-based communication systems. These advantages include:
• Long-distance signal transmission
The low attenuation and superior signal integrity found in optical systems allow much longer intervals of signal transmission than metallic-based systems. While single-line, FIBER OPTIC TELECOMMUNICATION voice-grade copper systems longer than a couple of kilometers (1.2 miles) require in-line signal for satisfactory performance, it is not unusual for optical systems to go over 100 kilometers (km), or about 62 miles, with no active or passive processing.
• Large bandwidth, light weight, and small diameter
Today’s applications require an ever-increasing amount of bandwidth. Consequently, it is important to consider the space constraints of many end users. It is commonplace to install
cabling within existing duct systems or conduit. The relatively small diameter and light weight of optical cable make such installations easy and practical, saving valuable conduit space in these environments.
• Nonconductivity
Another advantage of optical fibers is their dielectric nature. Since optical fiber has no metallic components, it can be installed in areas with electromagnetic interference (EMI), including radio frequency interference (RFI). Areas with high EMI include utility lines,
power-carrying lines, and railroad tracks. All-dielectric cables are also ideal for areas of high lightning-strike incidence.
• Security
Unlike metallic-based systems, the dielectric nature of optical fiber makes it impossible to remotely detect the signal being transmitted within the cable. The only way to do so is by accessing the optical fiber. Accessing the fiber requires intervention that is easily detected by security surveillance. These circumstances make fiber extremely attractive to government bodies, banks, and others with major security concerns.
• Designed for future applications needs
Fiber optics is affordable today, as electronics prices fall and optical cable pricing remains low. In many cases, fiber solutions are less costly than copper. As bandwidth demands increase rapidly with technological advances, fiber will continue to play a vital role in the long-term success of telecommunication.
FIBER OPTIC LOSS CALCULATIONS
Loss in a system can be expressed as the following:
Loss = Pout/ Pin
Where P in is the input power to the fiber and Pout is the power available at the output of the fiber.
For convenience, fiber optic loss is typically expressed in terms of decibels (dB) and can be calculated using Equation 8-2a.
Loss dB = 10 log ( Pout/Pin)
Oftentimes, loss in optical fiber is also expressed in terms of decibels per kilometer (dB/km)
Optical power in fiber optic systems is typically expressed in terms of dB m, which is a decibel term that assumes that the input power is 1 m watt .If P is in mil li watts, Equation 8-3 gives the power in dB m, referenced to an input of one mil li watt:
P( dB m) = 10log ( P/ 1 MW)
With optical power expressed in dB m, output power anywhere in the system can be determined simply by expressing the power input in dB m and subtracting the individual component losses, also expressed in dB . It is important to note that an optical source with a power input of 1 Mw can be expressed as 0 dB m. For every 3-dB loss, the power is cut in half. Consequently, for every 3-dB increase, the optical power is doubled. For example, a 3-dBm optical source has a P of 2 m W, whereas a –6-dBm source has a P of 0.25 m W,
P.D.H SYSTEM
With the introduction of PCM technology in the 1960s, communications networks were gradually converted to digital technology over the next few years. To cope with the demand for ever higher bit rates, a multiplex hierarchy called the plesiochronous digital hierarchy (PDH) evolved. The bit rates start with the basic multiplex rate of 2 Mbit/s with further stages of 8, 34 and 140 Mbit/s. In North America and Japan, the primary rate is 1.5 Mbit/s. Hierarchy stages of 6 and 44 Mbit/s developed from this. Because of these very different developments, gateways between one network and another were very difficult and expensive to realize. PCM allows multiple use of a single line by means of digital time-domain multiplexing. The analog telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and encoded and then transmitted at a bit rate of 64 k bit/s. This so-called primary rate is used throughout the world. Only the USA, Canada and Japan use a primary rate of 1544 k bit/s, formed by combining 24 channels instead of 30. The growing demand for more bandwidth meant that more stages of multiplexing were needed throughout the world. A practically synchronous (or, to give it its proper name: plesiochronous) digital hierarchy is the result. Slight differences in timing signals mean that justification or stuffing is necessary when forming the multiplexed signals. Inserting or dropping an individual 64 k bit/s channel to or from a higher digital hierarchy requires a considerable amount of complex multiplexer equipment.
PULSE CODE MODULATION
Pulse code modulation (PCM) is the process of converting an analog signal into a 2n-digit binary code. An analog signal is placed on the input of a sample and hold. The sample and hold circuit is used to “capture” the analog voltage long enough for the conversion to take place. The output of the sample and hold circuit is fed into the analog-to-digital converter (A/D). An A/D converter operates by taking periodic discrete samples of an analog signal at a specific point in time and converting it to a 2n-bit binary number. For example, an 8-bit A/D converts an analog voltage into a binary number with 28 discrete levels (between 0 and 255). For an analog voltage to be successfully converted, it must be sampled at a rate at least twice its maximum frequency. This is known as then N Quist sampling rate. An example of this is the process that takes place in the telephone system. A standard telephone has a bandwidth of 4 kHz. When you speak into the telephone, your 4-kHz bandwidth voice signal is sampled at twice the 4-kHz frequency or 8 kHz. Each sample is then converted to an 8-bit binary number. This occurs 8000 times per second.
Thus, if we multiply
8 k samples/s × 8 bits/sample = 64 k bits/s
we get the standard bit rate for a single voice channel in the North American DS1 System, which is 64 k bits/s. The output of the A/D converter is then fed into a driver circuit that contains the appropriate circuitry to turn the light source on and off. The process of turning the light source on and off is known as modulation and will be discussed later in this module. The light then travels through the fiber and is received by a photo detector that converts the optical signal into an electrical current. A typical photo detector generates a current that is in the micro- or nanoamp range, so amplification and/or signal reshaping is often required. Once the digital signal has been reconstructed, it is converted back into an analog signal using a device called a digital-to-analog converter or DAC. A digital storage device or buffer may be used to temporarily store the digital codes during the conversion process. The DAC accepts an n-bit digital number and outputs a continuous series of discrete voltage “steps.” All that is needed to smooth the stair-step voltage out is a simple low-pass filter with its cutoff frequency set at the maximum signal frequency .
Fiber Optic Sources
Two basic light sources are used for fiber optics: laser diodes (LD) and light-emitting diodes (LED). Fiber optic sources must operate in the low-loss transmission windows of glass fiber. LEDs are typically used at the 850-nm and 1310-nm transmission wavelengths, whereas lasers are primarily used at 1310 nm and 1550 nm. LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. They are used in applications in which data rates are in the hundreds of megahertz as opposed to GHz data rates associated with lasers. Two basic structures for LEDs are used in fiber optic systems: surface-emitting and edge emitting.
In surface-emitting LEDs the radiation emanates from the surface. An example of this is the Burris diode .LEDs typically have large numerical apertures. Burris diode makes light coupling into single-mode fiber difficult due to the fiber’s small N.A. and core diameter. For this reason LEDs are most often used with multimode fiber. LEDs are used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. The output spectrum of a typical LED is about 40 nm, which limits its performance because of severe chromatic dispersion. LEDs operate in a more linear fashion than do laser diodes. This makes them more suitable for analog modulation. Note that the LED has a more linear output power, which makes it more suitable for analog modulation. Often these devices are pigtailed, having a fiber attached during the manufacturing process. Some LEDs are available with connector-ready housings that allow a connect to fiber to be directly attached. They are also relatively inexpensive. Typical applications are local area networks, closed-circuit TV, and transmitting information in areas where EMI may be a problem
FIBER BRAGG GRATINGS
Fiber Bragg gratings are typically used to separate very closely spaced wavelengths in a DWDM system (< 0.8 nm). Erbium-doped fiber amplifiers (EDFA)—The EDFA is an optical amplifier used to boost the signal level in the 1530-nm to 1570-nm region of the spectrum. When it is pumped by anexternal laser source of either 980 nm or 1480 nm, signal gain can be as high as 30 dB (1000 times). Because EDFAs allow signals to be regenerated without having to be converted back to electrical signals, systems are faster and more reliable. When used in conjunction with wavelength-division multiplexing, fiber optic systems can transmit enormous amounts of information over long distances with very high reliability.
Fiber Bragg gratings—Fiber Bragg gratings are devices that are used for separating wavelengths through diffraction, similar to a diffraction grating (see Figure 8-40). They are of critical importance in DWDM systems in which multiple closely spaced wavelengths require separation. Light entering the fiber Bragg grating is diffracted by the induced period variations in the index of refraction. By spacing the periodic variations at multiples of the half-wavelength of the desired signal, each variation reflects light with a 360° phase shift causing a constructive interference of a very specific wavelength while allowing others to pass. Fiber
Transportation of PDH and ATM signals by SDH
The heterogeneous nature of modern network structures has made it necessary that all PDH and ATM signals are transported over the SDH network. The process of matching the signals to the network is called mapping. The container is the basic package unit for tributary channels. A special container (C-n) is provided for each PDH tributary signal. These containers are always much larger than the payload to be transported. The remaining capacity is used partly for justification (stuffing) in order to equalize out timing inaccuracies in the PDH signals. Where synchronous tributaries are mapped, fixed fill bytes are inserted instead of justification bytes. A virtual container (VC-n) is made up from the container thus formed together with the path overhead (POH). This is transmitted unchanged over a path through the network. The next step towards formation of a complete STM-N signal is the addition of a pointer indicating the start of the POH. The unit formed by the pointer and the virtual container is called an administrative unit (AU-n) or a tributary unit (TU-n). Several TUs taken together form a tributary unit group (TUG-n); these are in turn collected together into a VC. One or more AUs form an administrative unit group (AUG). Finally, the AUG plus the section overhead (SOH) forms the STM-N. ATM signals can be transported in the SDH network in C11, C12, C3 and C4 containers. Since the container transport capacity does not meet the continually increasing ATM bandwidth requirement, methods have been developed for transmitting the ATM payload in a multiple (n) C-4 (virtual or contiguous concatenation). As an example, a quadruple C-4 can be transmitted in a STM-4 (see the section on ªContiguous concatenation).
Conclusion
Over the past five years, major breakthroughs in technology have been the impetus for tremendous growth experienced by the fiber optic industry. The development of EDFAs, fiber Bragg gratings and DWDM, as well as advances in optical sources and detectors that operate in the 1550-nm range, have all contributed to advancing the fiber optics industry to one of the fastest growing and most important industries in telecommunication today. As the industry continues to grow, frustrating bottlenecks in the “information superhighway” will lessen, which will in turn usher in the next generation of services, such as telemedicine, Internet telephony, distance education, e-commerce, and high-speed data and video. More recent advances in EDFAs that operate at 1310-nm and 1590-nm technology will allow further enhancement in fiber optic systems. The future is bright. Just remember, the information superhighway is paved with glass!