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ITGN 235: Principles of Networking ITGN 225: Networking Fall 2007/2008 1 Chapter 4 Transmission Media 2 Topics Covered • • • • • • • • • • • • 4.1 Introduction 4.2 Copper Wires 4.3 Glass Fibers 4.4 Radio 4.5 Satellites 4.6 Geosynchronous Satellites 4.7 Low Earth Orbit Satellites 4.8 Low Earth Orbit Satellite Arrays 4.9 Microwave 4.10 Infrared 4.11 Light From A Laser 4.12 Selecting the proper cabling 3 4.1 Introduction • At the lowest level, all computer communication involves – encoding data in a form of energy – and sending the energy across a transmission medium • HW devices attached to a computer perform the encoding and decoding of data • This part covers the basics of data transmission: – This chapter examines the media that are used for transmission in modern network systems – The next two chapters explain how data can be transferred across such media – Later sections explain how transmission forms the basis of data NW • Transmission Media = the physical path between transmitter (Tx) and receiver (Rx). 4 Transmission Media • two major classes – conducted (wireline) or guided media • use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver – wireless or unguided media • use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals 5 Transmission Media • design factors – – – – Transmission capacity (throughput) attenuation: weakening of signal over distances interference number of receivers 6 Impairments and Capacity Impairments exist in all forms of data transmission Analog signal impairments result in random modifications that impair signal quality Digital signal impairments result in bit errors (1s and 0s transposed) 7 Attenuation Attenuation refers to the loss of signal strength as the signal travels further along a cable. 8 Throughput • Throughput is amount of data the medium can transmit during a given period of time – Also called capacity 9 Throughput Throughput measures 10 Transmission Flaws • Electromagnetic interference (EMI) – Interference that may be caused by motors, power lines, television, copiers, fluorescent lights, or other sources of electrical activity • Radiofrequency interference (RFI) – Interference that may be generated by motors, power lines, televisions, copiers, fluorescent lights, or broadcast signals from radio or TV towers 11 Transmission Flaws Figure 4-11: An analog signal distorted by noise 12 Transmission Flaws Figure 4-12: A digital signal distorted by noise 13 Transmission Flaws • Attenuation – Loss of signal strength as transmission travels away from source – Analog signals pass through an amplifier, which increases not only voltage of a signal but also noise accumulated Figure 4-13: An analog signal distorted by noise, and then amplified 14 Transmission Flaws • Regeneration – Process of retransmitting a digital signal • Repeater – Device used to regenerate a signal Figure 4-14: A digital signal distorted by noise, and then repeated 15 Media Characteristics • Throughput – Perhaps most significant factor in choosing a transmission medium is throughput • Cost – – – – – Cost of installation Cost of new infrastructure versus reusing existing infrastructure Cost of maintenance and support Cost of a lower transmission rate affecting productivity Cost of obsolescence 16 Media Characteristics • Size and scalability – Specifications determining size and scalability: • Maximum nodes per segment • Maximum segment length • Maximum network length – Latency is the delay between the transmission of a signal and its receipt 17 Media Characteristics • Connectors – Connects wire to network device • Noise immunity – Thicker cables are generally less susceptible to noise – Possible to use antinoise algorithms to protect data from being corrupted by noise – Conduits can protect cabling from noise 18 Conducted (guided) Media 19 4.2 Copper Wires (1) • Conventional computer NW use wires as the primary medium – NW use copper wire almost exclusively because its low resistance • The type of wiring used for NW is chosen to minimize interference • Interference arises because an electrical signal traveling across a wire acts like a miniature radio station – the wire emits a small amount of electromagnetic (EM) energy, which can travel through the air • Whenever it encounters another wire, an EM wave generates a small electric current in the wire. – The amount of current generated depends on the strength of the EM wave and the physical position of the wire • When two wires are placed close together and in parallel, a strong signal sent on one wire will generate a similar signal on the other 20 4.2 Copper Wires (2) • Problem of interference is severe – because wires that comprise a NW often are placed in parallel with many other wires • To minimize interference, networks use one of three basic wiring types: – Unshielded Twisted Pair (UTP) – Shielded Twisted Pair (STP) – Coaxial Cable 21 4.2 Copper Wires (2) • See Figure 4.1 for an illustration of twisted-pair cables • Twists change the electrical properties of the wire: – First, they limit the EM energy the wire emits: • So they help prevent electric currents on the wire from radiating energy that interferes with other wires – Second, they make the pair of wires less susceptible to EM energy: • They help prevent signals on other wires from interfering with the pair 22 23 Twisted-Pair (TP) Cable • • Color-coded pairs of insulated copper wires twisted around each other and encased in plastic coating Twists in wire help reduce effects of crosstalk – Number of twists per meter or foot known as twist ratio • Alien Crosstalk – When signals from adjacent cables interfere with another cable’s transmission Figure 21: Twisted-pair cable 24 Unshielded Twisted-Pair • Consists of one or more insulated wire pairs encased in a plastic sheath • Does not contain additional shielding Figure 4-23: UTP cable 25 Unshielded Twisted-Pair • To manage network cabling, it is necessary to be familiar with standards used on modern networks, particularly Category 3 (CAT3) and Category 5 (CAT5) Figure 4-24: A CAT5 UTP cable 26 UTP Categories • Category 1 - traditional telephone cable; can carry voice but not data • Category 2 - consists of 4 twisted-pairs and is rated for data transmissions up to 4 Mbps. • Category 3 - consists of 4 twisted-pairs and 3 twists per foot; rated for 10 Mbps • Category 4 - 4 TP, rated for 16 Mbps • Category 5 - 4 TP, rated for 100 Mbps » More tightly twisted than category 3 cables » More expensive, harder to work with There is also now CAT5E (1Gbps) & CAT6. 27 3 twists/foot) 5 twists/foot) Figure 1.10 Copper wire transmission media: (a) unshielded twisted pair (UTP) 28 Shielded Twisted-Pair (STP) • STP cable consists of twisted wire pairs that are individually insulated and surrounded by shielding made of metallic substance (plastic casting) (thin metallic shielding: Aluminum foil) Figure 4-22: STP cable 29 STP (Shielded Twisted-Pair) Figure 3-18: STP cable 30 Shielded Twisted-Pair (STP) Shielded Twisted Pair (STP) – A shielded twisted pair cable consists of a pair of wires surrounded by a metal shield • Each wire is coated with an insulating material, – so the metal in one wire does not touch the metal in another • The shield merely forms a barrier that prevents EM radiation from entering or escaping • The additional shielding provided by coaxial or shielded twisted pair cabling is often used – when wires from a NW pass near equipment that generates strong electric or magnetic fields 31 10BaseT • Popular Ethernet networking standard Figure 4-25: A 10BaseT Ethernet network 32 10BaseT • Enterprise-wide network – Spans entire organization – Often services needs of many diverse users Figure 4-26: Interconnected 10BaseT segments 33 100BaseT • Enables LANs to run at 100-Mbps data transfer rate • Also known as Fast Ethernet • Two 100BaseT specifications have competed for popularity as organizations move to 100-Mbps technology: – 100BaseTX (using 2 pairs of UTP category 5 cable) – 100BaseT4 (using 4 pairs of UTP category 3 or above cable) http://en.wikipedia.org/wiki/100BaseTX 34 Comparing STP and UTP • Throughput – Both can transmit up to 100 Mbps • Cost – Typically, STP is more expensive • Connector – Both use RJ-45 connectors and data jacks • Noise immunity – STP is more noise-resistant • Size and scalability – Maximum segment length for both is 100 meters RJ-11 for telephone 35 Installing Cable Figure 4-36: A typical UTP cabling installation 36 Installing Cable • Straight-through cable – Terminations at both ends are identical • Crossover cable – Terminations locations of transmit and receiver wires on one end of cable are reversed Figure 4-37: RJ-45 terminations on a crossover 37 Coaxial Cables • Coaxial cables provides even more protection from interference than twisted pair – Instead of twisting wires around one another to limit interference, • a coaxial cable consists of a single wire surrounded by a metal shield • It’s shown in Figure 4.2 • Metal shield in a coaxial cable forms a flexible cylinder around the inner wire to provide a barrier for EM radiation – The barrier isolates the inner wire in two ways: • it protects the wire from incoming EM energy • and keeps signals on the inner wire from radiating EM energy • The cable can be placed parallel to other cables or bent and twisted around corners 38 39 Coaxial Cable • Consists of central copper core surrounded by an insulator, braiding, and outer cover called a sheath Figure 4-15: Coaxial cable BNC connector) 40 Types of Coaxial Cable • Thin (thinnet) • Thick (thicknet) 41 Thinnet Coax Thinnet is a flexible coaxial cable about 0.25 inch thick. Thinnet coaxial cable can carry a signal up to approximately 185 meters. 42 Thicknet Coax Thicknet is relatively rigid coaxial cable about 0.5 inch in diameter. The copper core is thicker than a thinnet core. Because of the thicker copper core, thicknet can carry a signal for up to 500 meters. 43 Coaxial Cable Table 4-2: Some types of coaxial cable 44 Thicknet vs Thinnet Important Characteristics of Coaxial Cable Thinnet Thicknet Maximum Cable Length 185 meters (607 feet) 500 meters (1640 feet) Bandwidth 10 Mbps 10 Mbps Bend Radius 360 degrees/foot 30 degrees/foot Cost More than twisted pair More than thinnet, less than fiber Interference Susceptibility Good, better than twisted pair Best of all electrical cable 45 4.3 Glass Fibers (1) • NW also use flexible glass fibers to transmit data – known as an optical fiber • Medium uses light to transport data • The miniature glass fiber is encased in a plastic jacket – which allows the fiber to bend without breaking • A transmitter at one end of a fiber uses – a light emitting diode (LED) or a laser to send pulses of light • A receiver at the other end uses – a light sensitive transistor to detect the pulses 46 Fiber-Optic Cable • Contains one or several glass fibers at its core – Surrounding the fibers is a layer of glass called cladding Figure 4-28: A fiber-optic cable 47 Figure 1.11 Optical fiber transmission media: (a) cable structures 48 Fiber-Optic Cable • Single-mode fiber – Carries light pulses along single path • Multimode fiber – Many pulses of light generated by LED travel at different angles Core = 9 microns in diameter Core = 50 or 62.5 microns in diameter Single-mode and multimode fiber-optic cables 49 Fiber-Optic Cable • Two popular connectors used with fiber-optic cable: – ST connectors – SC connectors ST and SC fiber connectors 50 4.3 Glass Fibers (2) Main advantages fiber optics over wires: • First, because they use light, – neither cause electrical interference in other cables – nor are they susceptible to electrical interference • Second, because glass fibers can be manufactured to reflect most of the light inward – a fiber can carry a pulse of light much farther than a copper wire can carry a signal • Third, because light can encode more information than electrical signals – an optical fiber can carry more information than a wire • Fourth, unlike electricity, which always requires a pair of wires connected into a complete circuit, – light can travel from one computer to another over a single fiber 51 4.3 Glass Fibers (3) Optical fibers do have some disadvantages • First, installing a fiber requires special equipment – that polishes the ends to allow light to pass through • Second, if a fiber breaks inside the plastic jacket: – finding the location of the problem is difficult • Third, repairing a broken fiber is difficult – special equipment is needed to join two fibers 52 10BaseF and 100BaseFX • 10BaseF – Physical layer standard for networks specifying baseband transmission, multimode fiber cabling, and 10-Mbps throughput • 100BaseFX – Physical layer standard for networks specifying baseband transmission, multimode fiber cabling, and 100-Mbps throughput 53 Submarine Optical Cable 54 Broken Submarine Cable 55 Physical Layer Networking Standards Voice-grade Table 4-3: Physical layer networking standards 56 Wireless Media 57 4.4 Radio • In Radio Frequency (RF) transmissions – each participating computer attaches to an antenna – Antenna can both transmit and receive RF • Physically, the antennas used with RF networks may be large or small, depending on the range desired: – An antenna designed to propagate signals several miles • may consist of a metal pole approximately two meters long that is mounted vertically on top of a building – An antenna designed to permit communication within a building • may be small enough to fit inside a portable computer (e.g., less than twenty centimeters) 58 microwave 4.4 Radio 300Mhz 59 The Wireless Spectrum Figure 3-37: The wireless spectrum 60 Characteristics of Wireless Transmission Figure 3-38: Wireless transmission and reception 61 Wireless LAN Architecture (continued) Figure 3-41: An infrastructure WLAN 62 Wireless LAN Architecture (continued) Figure 3-42: Wireless LAN interconnection 63 4.5 Satellites • RF technology can be combined with satellites – to provide communication across longer distances • Figure 4.3 illustrates a satellite in orbit • The satellite contains a transponder – that consists of a radio receiver and transmitter • The transponder – accepts an incoming radio transmission – amplifies it – and transmits the signal back toward the ground at a slightly different angle than it arrived • A single satellite usually contains multiple transponders – Each transponder uses a different radio frequency (i.e., channel) 64 65 Satellite Point to Point Link 66 Satellite Broadcast Link 67 Satellite systems: (a) broadcast television; (b) data communications 68 4.6 Geostationary Satellites (1) • Communication satellites can be grouped into categories according to the height at which they orbit: – The easiest is geosynchronous or geostationary satellites – The name arises because a geosynchronous satellite is placed in an orbit that is exactly synchronized with the rotation of the earth. – Such an orbit is classified as a Geostationary Earth Orbit (GEO) – When viewed from the ground, • satellite appears to remain at exactly the same point in the sky at all times • Laws of physics determine the exact distance from the earth that a satellite must orbit to remain synchronized with the earth's rotation – See Kepler's Law for details – The distance is 35,785 kilometers or 22,236 miles • An antenna can point in a fixed direction and maintain a link with the satellite 69 4.6 Geosynchronous Satellites (2) • GEO is about one tenth of the distance to the moon – Engineers refer the distance as “high earth orbit” • There is a limited amount of ``space'' available in the GEO above the equator – because satellites using a given frequency must be separated from one another to avoid interference • The minimum separation depends on the power of the transmitters 70 • Sample geostationary satellite coverage 71 • Orbital positions of the 165 geostationary satellites orbit Earth today 72 4.7 Low Earth Orbit Satellites (1) • Second category of satellites operate in what is called Low Earth Orbit (LEO) – which means that they orbit a few hundred miles above the earth (typically 200 to 400 miles) • The chief disadvantage of a LEO lies in the rate at which a satellite must travel – Their period of rotation is faster than the rotation of the earth • LEOs do not stay above a single point on the earth's surface • An observer, who stands on the earth looking upward through a telescope, sees LEOs move across the sky • A single satellite can complete an entire orbit in approximately 1.5 hours 73 4.7 Low Earth Orbit Satellites (2) From a communication provider's point of view: • having a satellite that does not appear to remain stationary causes problems: – First, the satellite can only be used during the time • that its orbit passes between two ground stations – Second, maximal utilization requires complex control systems • that continuously move the ground stations so they point directly at the satellite International space station 74 4.8 Low Earth Orbit Satellite Arrays (1) • A group of LEO satellites working in concert is known as a satellite constellation. • Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronised so that they overlap well in coverage and complement rather than interfere with other satellites' coverage. The GPS constellation calls for 24 satellites to be distributed equally among six circular orbital planes 75 4.8 Low Earth Orbit Satellite Arrays (2) • Instead of focusing on one satellite, – the scheme requires a communication company to launch a set of satellites into low earth orbits • Although a given satellite orbits quickly, – the set of orbits is chosen so that each point on the ground has at least one satellite overhead at any time – sixty-six (66) satellites are required to provide service over the entire surface of the earth • From the point of view of an observer on earth, – it appears that a satellite emerges from a point on the horizon – flies overhead – and then disappears into a point on the opposite horizon • The key to the scheme lies in the set of orbits – guarantees at least one satellite is available at any time 76 4.8 Low Earth Orbit Satellite Arrays (3) • In addition to transponders used to communicate with ground stations – an array of satellites in low earth orbit contains radio equipment used to communicate with other satellites in the array • As they move through their orbits – the satellites communicate with one another and agree to forward data 77 Satellite Advantages • can reach a large geographical area • high bandwidth • cheaper over long distances 78 Satellite Disadvantages • high initial cost • susceptible to noise and interference • propagation delay 79 4.9 Microwave • Many long-distance telephone companies use microwave (MW) to carry telephone conversations – A few large companies have also installed MW systems as part of the company's NW • MW are merely a higher frequency (300 megahertz and 300 gigahertz) version of radio waves, but they behave differently – Instead of broadcasting in all directions, • a MW transmission can be aimed in a single direction, preventing others from intercepting – In addition, MW transmission can carry more information than lower frequency RF transmissions 80 4.9 Microwave • MW cannot penetrate metal structures: – transmission works best when a clear path exists between two parties – most MW installations consist of two towers • that are taller than the surrounding buildings and vegetation – each MW transmitter aimed directly at a MW receiver on the other 81 4.9 Microwave Frequency Bands Designation Frequency range L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz 82 4.10 Infrared • Infrared is limited to a small area (e.g., a single room) • Usually requires that the transmitter be pointed toward the receiver • Infrared HW – is inexpensive compared to other mechanisms, – and does not require an antenna • It is possible to equip a large room with a single infrared connection – that provides NW access to all computers – computers can remain in contact with the NW while they are moved within the room • Infrared NW are especially convenient for small, portable computers 83 4.11 Light From A Laser (FSO: free space optic) • A beam of light can also be used to carry data through the air • A communication link that uses light consists of two sites that each have a transmitter and receiver – equipment is mounted in a fixed position, often on a tower – aligned so the transmitter at one location sends its beam of light directly to the receiver at the other • The transmitter uses a laser to generate the beam of light – because a coherent laser beam will stay focused over a long distance • Light from a laser must travel in a straight line and must not be blocked • A laser beam cannot penetrate vegetation or weather conditions such as snow and fog: – Thus, laser transmission has limited use 84 4.11 Light From A Laser (FSO: free space optic) 85 4.11 Light From A Laser (Applications) – LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds. – LAN-to-LAN connections in a city. example, Metropolitan area network. – To cross a road or other barriers. – Speedy service delivery of high bandwidth access to fiber networks. – Converged Voice-Data-Connection. – Two solar-powered satellites communicating optically in space via lasers. – Temporary network installation (for events or other purposes). – Reestablish high-speed connection quickly (disaster recovery). – As an alternative or upgrade add-on to existing wireless technologies. – As a safety add-on for important fiber connections (redundancy). – For communications between spacecraft, including elements of a satellite constellation. – The light beam can be very narrow, which makes FSO hard to intercept, improving security. FSO provides vastly improved EMI behavior using light instead of microwaves. 86 Choosing the Right Transmission Media • • • • • • Areas of high EMI or RFI Corners and small spaces Distance Security Existing infrastructure Growth 87 Selecting Cabling • • • • • • • • • • • How heavy will the network traffic be? What are the security needs of the network? What are the distances that the cable must cover? What are the cable options? What is the budget? installation logistics (how easy to install?) shielding (what level?) noise and crosstalk (any power lines? motors?) transmission speed (how fast?) cost (how much?) attenuation 88 Cable Comparison Summary • • • • • • Cable cost Usable length Transmission Rates Flexibility Ease of installation Susceptibility to interference 89