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Physical Transmission Media Physical Transmission Media Electrical signals can be sent over a wire because wire is a good conductor of electricity. We are not restricted to just using wire, however. Other transmission media, such as radio and optical fibres, can also be used. Each of these media are used in different ways but they perform the same task: transmitting information from A to B. We need to know about physical transmission media in computer networks because these media form the links from one computer to the next. Electrical Media Broadly speaking, electrical media means wires. Most wires are made from copper because it is a particularly good conductor of electricity. The reason for this is that the copper atom has a free electrons in its outer shell that can jump easily from one atom to the next (see figure 3.1). By applying an electromotive force (measured in volts) along the media, the electrons will begin to flow. Just one electron in outer shell. This is a free electron. Figure : Diagram of a Just one copper atom and the free electron in outer shell. electron in its outer shell This is a free that moves easily electron. from one atom to the next under the influence of an electromotive force. Nucleus Physical Transmission Media The telegraph system used a single wire but you will not see single wires being used to transmit data these days. The reason is that a single wire is badly effected by noise. Noise can come from many sources, such as radio interference or physical damage to the wire. Most noise, however, is due to thermal noised (sometimes called Gaussian noise) which causes a constant hiss on the line. Thermal noise is due to electrons jostling around because of heat inside the wire. The only way to remove this noise is to super-cool the wire (i.e. bring its temperature down to near absolute zero - that is to say 273.15C). For most systems, this is impractical and we have to find other ways of coping with noise. Twisted Pair Cable To improve the quality of the simple telegraph system, a second wire was added to provide a non-ground return circuit. This second wire improves quality because any non-thermal interference that effects one wire usually effects the other in the same way. The signal is sent in the form of a voltage difference between the two wires. Since the effect of non-thermal noise on both wires is usually the same, the interference tends to leave the voltage difference unchanged. Site A Site B Figure 3.2: Twisted pair cables are less effected by non- thermal noise than single cables. Twisted Pair Cable Figure on previous slide shows a twisted pair cable connecting two sites. The common Category 3 twisted pair cable consists of two insulated 1mm thick copper wires gently twisted around each other. It is important for the wires to be twisted because otherwise they would form a simple antenna that would actually pick up noise. Category 3 twisted pair cables were widely used for telephone systems until 1988. In 1988, Category 5 twisted pair cables were introduced. These cables are similar to Category 3 twisted pair cables except that they use Teflon insulation and have more twists per centimetre – resulting in even less interference. Category 5 cables are widely used in computer networks.  By insulated we mean the wires are coated in plastic so that they will not be short circuited by touching each other or other metal objects. Twisted Pair Cable Twisted pair cables come in shielded and unshielded varieties. The shielded twisted pair cables (STP cables) are surrounded by a grounded metal sheath that provides extra protection against interference. This, however, makes them more expensive and bulkier than the more popular unshielded twisted pair cables (UTP cables). Twisted pair cables are typically bundled together to form 4 or 5- pair twisted pair cables (8 or 10 wires altogether). They are suitable for transmitting data at rates of about 1-100Mbps (a 100 million bits per second) over short distances (less than 100m). Higher data rates in twisted pair cables are limited by the effects of noise and a phenomenon called the skin effect.  As the data rate (and hence frequency) in a wire increases, electrons tend to flow only near the surface of the wire. This reduces the cross section of the wire carrying the signal. This means the resistance of the wire increases at higher frequencies and this causes signal attenuation (weakening). Coaxial Cable Coaxial cable consists of a copper core surrounded by a grounded sheath (usually a woven braided copper mesh). The sheath gives the copper core excellent protection from external noise and allows coaxial able to be used near machinery and other sources of electromagnetic radiation. It also reduces attenuation due to the skin effect because less energy can be radiated from the outer surface of the core. Broadband Coaxial Cable is usually used for carrying analogue transmissions whereas Baseband Coaxial Cable is commonly used for digital transmission. The only physical difference between the two cables is that the Baseband Coaxial Cable has a resistance of 50 Ohms/metre whereas the Broadband Coaxial Cable has a resistance of 75 Ohms/metre.  Broadband means the available bandwidth of the cable is divided up into several frequency channels.  Baseband means that all the available bandwidth is made available to a single channel. Physical Transmission Media Protective plastic coating The structure of coaxial cable. Plastic insulator Grounded copper sheath Copper wire Figure above shows the structure of coaxial cable. Broadband Coaxial Cable can carry frequencies of 350 MHz (millions of cycles per second) or more over nearly 100 km. The Baseband Coaxial Cable used in computer networks can be used to transmit digital data at bits rates of 10 Mbps (millions of bits per second) or more over several hundred metres. ElectroMagnetic Waves Rather than using electricity to transmit data, we can use electromagnetic radiation. Electromagnetic radiation includes light, microwaves and radio waves. Figure below shows the different types of electromagnetic radiation and their frequencies. 10 0 10 2 10 4 10 6 10 8 10 10 10 12 10 14 10 16 10 18 10 20 10 22 10 24 Hz Radio Microwave Infrared UV X-ray Gamma Ray Visible Light The electromagnetic wave spectrum. Physical Transmission Media This is because electromagnetic radiation travels as waves as can be seen below. The frequency of a wave is measure in Hertz (Hz) and is the number of wave cycles every second. The wavelength of a wave is the distance between two consecutive wave crests and is measured in metres (m). The frequency f and the wavelength Magnetic Field are related by equation (3.1). Electric Field (3.1) where c is the speed of light (299,792,458 m/s or approximately 3108m/s). Electromagnetic waves The amplitude of a wave is the height of its crest. In the case of an electromagnetic wave, amplitude means its intensity. By varying the amplitude of a wave we can send signals. This is type of signalling is called amplitude modulation. The bit rate that we can send using an electromagnetic wave depends heavily on its frequency. If we have a coherent wave, such as a radio wave, then we can calculate its bandwidth H from its frequency f using equation (3.2). Physical Transmission Media The bandwidth represents the range of signal frequencies that a medium can carry. In the case of a radio wave, only signal frequencies up to half the carrier frequency can be carried using amplitude modulation. The reason for this has something to do with the wagon wheel effect: in cowboy movies, the wagon wheels reach a certain speed before slowing down and even going backwards. The number of movie frames per second is not fast enough to show faster wheels. Similarly, the frequency of a radio wave is not high enough to represent signal frequencies more than half its own frequency. They just look like slower frequencies. Nyquists Limit The maximum data rate for any channel can be calculated from Nyquist’s Limit that states: (3.3) where H is the bandwidth of the channel and M is the number of signal levels used. For example, we might use 2 signal levels to represent 0s and 1s. In this case, max_bps=2 H since log2(2)=1. There is no reason why 4 signal levels could not be used to represent the bit combinations 00, 01, 10 and 11. Unfortunately, M cannot be made arbitrarily large in practice. Nyquist’s Limit does not take account of noise and noise is present, to a greater or lesser extent, in all communication systems. Noise is the second thing that limits the data rate. Optical Fiber Data can be transmitted using pulses of light. These pulses can be conveyed to their destination via optical fibers. An optical fiber is composed of two types of glass with different refractive indexes. Light pulses in the core are unable to escape because they are reflected back into the core by total internal reflection at the interface of the two glass types. Figure above illustrates the structure of the optical fibre. Currently optical fibres can carry data at the rate of about 1Gbps (billion bits per second). Optical Fiber The limiting factor in fiber optic transmission rates is the speed of the electronics used to convert light pulses to and from electrical signals. Transmission rates in excess of 50,000 Gbps (50 Tbps) are theoretically possible. Light pulses become distorted in long optical fibers because each photon takes slightly different path inside the fiber. One problem encountered with optical fibers is light pulses transmitted over long distances become distorted. This is due to an effect known as dispersion. Figure above shows why this effect occurs. Each of the photons that go to make up the light pulse takes a slightly different path inside the optical fiber. Microwave Transmission Microwaves form part of the electromagnetic spectrum, which includes light and radio waves. At high power, microwaves can be used for cooking (hence microwave ovens). At low power, microwaves are harmless and can be used for communication. Microwaves cannot travel through mountains or hills but they can travel through fog, trees and brick walls. This means that microwaves can be used to transmit data to locations that are in line-of-sight. Microwaves Receiver Transmitter Dish Dish Microwaves are transmitted to locations that are in line of sight. The microwaves are send and received using parabolic dishes. Microwave Transmission cont. Previous figure shows how microwaves are transmitted from site to site using parabolic dishes. Using microwaves is cheaper than using landlines are widely used for telecommunications. A single microwave link can cover a distance of up to 50km and transmit at a frequency of up to 10GHz. One problem is that microwaves are sometimes refracted by low level atmospheric conditions. This creates multiple paths for the microwave signal that may arrive at the receiver dish out of phase and cause severe interference. Microwave / Satellites Microwaves are also used to communicate with satellites. Satellites used for communication purposes are typically geostationary, meaning that they orbit the earth once every 24 hours and appear to remain in the same place in the sky (at a height of 35,880 km or 22,300 miles). Communication satellites have directional antenna and on- board circuits called transponders. Each transponder receives a particular band of microwave frequencies and relays (retransmits) the signals it receives to another location on the Earth’s surface. Figure below shows how a satellite can provide out of sight communication. Satellite Up link Antenna Down Link Ground Station Radio Wave Transmission Radio waves, like microwaves and light, are part of the electromagnetic spectrum. Data can be transmitted using a radio wave as the carrier wave. Longer wavelength radio waves can penetrate hills and mountains. Shorter radio waves must be transmitted in line-of-sight because the ground easily absorbs them (although they can travel through buildings). Some high frequency (HF) radio waves, however, are able to travel great distances by reflecting off the upper part of the Earth’s atmosphere (the ionosphere). All radio waves are subject to interference from machinery such as motors or electrical devices. Also the strength of a radio waves falls off quickly over distance (roughly at a rate proportional to 1/r3 for low frequencies where r is the distance from the transmitter). Modes of serial data transfer • Simplex communications – Unidirectional data path from transmitter to receiver in the manner of radio broadcasts • Half Duplex – Unidirectional at any one time in the manner of a conversation over radio link with change of direction signaled by ‘over’. • Full Duplex – two computers using two comms channels one for transmission and one for reception both working simultaneously. Parallel data transfer • Most data in the form of bytes or wider. – Transfer all of the bits at the same time however one conductor for each bit, more copper etc. suitable for short distances and very high data rates, used inside computer where groups of conductors are called busses . – synchronisation between each bit on different conductors becomes difficult specially as distance increases due to tiny differences between conductors and their environment. Serial Transmission Connectors and cables • D-type 25way used for RS232 serial links – consider computer- modem cable with straight through cable connecting DTE and DCE. • RJ45 - telephone type connectors. • Ribbon Cables and IDC connectors • Network connectors and cables Old Fashioned Coaxial connection Cables for data transmission Benefits of both…. • Common mode rejection. • Speeds of each (cat 5e 100m bits/sec) • Shielding against induced noise. Transatlantic Communications It was only 150 years ago since the first Atlantic submarine cable was completed, connecting Trinity Bay via Newfoundland to Valentia Island in Ireland. Although the subsea cable laying is still an arduous, expensive and sometimes dangerous task as it was then, the key differences in today’s cable is the quality and the sheer quantity of information that can be sent through links underwater. In August of 1858, the first transatlantic message was sent over the new link from Queen Victoria of the United Kingdom to the United States President James Buchanan, the 15th President of the United States, who served right before President Abraham Lincoln. Source: Dark Fibre Communications Magazine, Looking Back to Look Forward: 150 Years of Transatlantic Communications by Jaymie Scotto and Ilissa Miller, April 13, 2009 Transatlantic Communications The message was 99 words and took 12 hours to send. The cable that carried this message sent only 400 messages before it began to fail, just 23 days after it initially went live. Today, a privately held Trans-Atlantic submarine cable network could send the entire Library of Congress in only 63 milliseconds over one of their fiber optic submarine cable links and can carry 30 million simultaneous phone calls at a single time. Transatlantic Communications • The first transatlantic submarine cable system was born from a gentleman named Cyrus Field. He was born in 1819 in Stockbridge, Massachusetts and to this day is considered to be the ‘father’ of the Atlantic sub-sea cable systems. • Initially working in the paper industry with his family, he was fascinated with telegraphy, which is how he got the idea for a transatlantic telegraphic cable. • In 1854, with friends and associates, Cyrus Field formed the New York, New Foundland and London Telegraph Company that raised $1,500,000, a lot of money especially at that time. • The company secured landing rights for the American side of the Ocean and set out to install the first transatlantic sub-sea cable. Transatlantic Communications One of Cyrus Fields very close friends was Samuel Morse, who in 1832 had the first ideas of the electromagnetic telegraph with his Associate Dr. Charles T. Jackson. Just 4 years later, Mr. Morse demonstrated his recording Telegraph. The following year, in 1837, he successfully relayed a message through ten miles of wire, on reels. In 1842, after further developing telegraph communications, he began experiments with underwater transmissions on the two- mile stretch between the Battery in lower Manhattan and Governors Island in New York Harbor where he was successful at sending signals. In 1843, Congress voted to grant $30,000 to install an experimental telegraph line from Washington DC to Baltimore, Maryland. Unfortunately, the lead pipes that were installed did not work so they converted the telegraph lines to above ground poles. Transatlantic Communications By 1845, two years later, the Magnetic Telegraph Company was created to extend cables from Baltimore to Philadelphia and New York. By 1849, it was estimated that there was 12,000 miles of telegraph lines run by twenty different companies in the US. In 1856, the Western Union Telegraph Company was formed by a number of small companies. This brings us back to the years of 1854-1858, during the early times of transatlantic cabling. Samuel Morse was an electrician for Cyrus Field during the first attempts to lay the cable across the Atlantic. Transatlantic Communications An interesting turn of events occurred when Cyrus Field went to England to recruit assistance for the commission of the first transatlantic submarine cable. There he met Dr. Whitehouse, originally trained as a surgeon though by this time, his interests were more focused on the advancements of telegraphy. When planning to build the system, Samuel Morse and Dr. Whitehouse decided together that the cable should be constructed as thinly as possible. This was in opposition to others on the project that included Lord Kelvin and Charles Bright. Cyrus Field ‘broke the tie’ and set out to install a thin wire to lay the cable. Transatlantic Communications Others included Lord Kelvin and Charles Bright. Lord Kelvin introduced ‘kinetic energy’ in 1856 and joined the Cyrus Field project to lay the cable where he applied his analogy of heat flow to the flow of electricity. Lord Kelvin was heavily involved with the project. He assisted in improving the design of the cables. He also invented the mirror galvanometer to act as a long distance telegraph receiver and supervised the laying of the galvanometers. On the seas, Kelvin improved the way mariners worked with the invention of an improved gyrocompass, a new sounding equipment, and a tide-prediction, chart-recording machine. In 1866, Lord Kelvin was knighted for his achievements in submarine cable laying. When he died in 1907, he was buried next to Sir Isaac Newton in Westminster Abbey. Transatlantic Communications The other gentleman was Sir Charles Tilston Bright, who in 1856, at the age of 26, became the youngest person knighted at the time, Sir Bright was known in England as the man who first laid a complete system of wires under the streets of Manchester, at age 19. After this feat, he became the Chief Engineer of the Magnetic Telegraph Company where he extended these lines from Manchester throughout the United Kingdom. He later established the first connection from Great Britain to Ireland propelling Lord Kelvin to proclaim him as the ‘first to lay a cable in deep water.’ This was the feat, laying the first Atlantic cable that earned him his title as knight in 1856. Transatlantic Communications Sir Bright was an inventor, and one of his earliest inventions was as system that tested the insulated conductors to localize faults from a distance point, by means of a series of standard resistance coils of different values, brought into circuit successively by turning a connecting handle. This became the best way to test submarine telegraphs. After appointing Lord Kelvin as the Chairman of the Atlantic Telegraph Co, the ability to influence the development of transatlantic cable connectivity was realized. Transatlantic Communications As the Chief Engineer of the project, it was Charles Bright who suggested a much thicker line, that would weigh 460 tons and would have had 3.5 more power in conducting speed, be used; however, this idea was passed over for Mr. Whitehouse’s much thinner copper cable. So back in 1857-58, with a vote of three to two, these gentlemen set out to connect two continents with a thin cable. They began in Vanetia Harbor, Ireland on August 5, and six days later, with limited rate of descent and with just 3580 miles laid, the cable snapped. After returning to shore, an extra 700 miles of cable were made and the second attempt was made on June 25. This time two ships met each other in mid-Atlantic where they joined their respective ends— and the cable broke almost immediately. Transatlantic Communications Again the two ships made anther splice, and after another 40 miles, the cable broke again. The fourth time they had laid 145 miles before the cable snapped again. The ships returned to Ireland and the men decided that despite the loss, there was still enough cable for one more attempt. On July 29, the crew set out for one final, fifth attempt, starting from the midpoint. This time, it was a success - and on August 5, 1858, the two continents were connected. Transatlantic Communications Unfortunately, this success, which garnered much celebration and fanfare, was short-lived. Dr. Whitehouse overloaded the system by applying very high voltages rather than the very weak currents that had been tested during the cable laying. Within three weeks, the damage from the high voltages became apparent and the cable stopped working. In addition, the dots and dashes of Morse code ended up smearing out over such a long haul. The failure of this cable was so catastrophic that the creation of the Committee of Inquiry was formed in England to investigate the cause. As a result, Dr. Whitehouse lost credibility and his career spiraled downward. To successfully construct a second working transatlantic cable system, Cyrus Field had to employ others instead. Transatlantic Communications • After several other cable snaps and failed attempts, and an additional $2,500,000 funding, the Great Eastern cable was pulled ashore a tiny fishing village in Newfoundland on July 27, 1866. • The distance was 1868 nautical miles and the Great Eastern averaged 120 miles a day while paying out the cable. • Perhaps the story of the first cable system would have ended differently if Sir Charles Tilston Bright’s thicker, copper cable were installed. But nonetheless, the enhanced communications it provided between Europe and North America were unfounded at the time. • The transatlantic cable was considered ‘The Great Scientific Achievement of the Century’; the laying of cable in open sea was a feat of strength, endurance and wonder. Summary • Today, there are several cable systems interconnecting North America and Europe. • However, there is only one company that offers a similar route to the first transatlantic cable system, and that is Hibernia Atlantic. This cable connects Halifax, Nova Scotia directly to Dublin, Ireland. • In August 2009, the company finally connected Northern Ireland at Portrush onto the northern spur of the existing Hibernia Atlantic submarine system, providing the first modern submarine cable link connecting Northern Ireland directly to Europe and North America. • The name assigned to this undertaking is Project Kelvin, named after Lord Kelvin and his significant role in the first undertaking. Transatlantic Communications Only 150 years ago, the first transatlantic submarine cable system was painstakingly deployed. Today billions of bits of data are transmitted every minute between North America, New Foundland, Ireland and Europe, connecting the world with high-speed capacity.
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