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Explore how data is transmitted as energy across mediums, covering analog and digital signals, wave properties, electromagnetic spectrum, and transmission media evaluation criteria.
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CSC 335Data CommunicationsandNetworking Lecture 2: Transmission Fundamentals Dr. Cheer-Sun Yang
Data Communication Examines how data, in the form of energy, travel across some medium from a source to a destination.
Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic representations of data • Transmission • Communication of data by propagation and processing of signals
Terminology (1) • Source – also called sender • Destination – also called receiver • Medium • Guided medium • e.g. twisted pair, optical fiber • Unguided medium • e.g. air, water, vacuum
Terminology (2) • Direct link • No intermediate devices • Point-to-point • Direct link • Only 2 devices share link • Multi-point • More than two devices share the link
How can data be transmitted? This question will be our focus in the next couple of weeks. First, we’ll introduce the concept of electrical signal. Then, we’ll focus on the concept of communication media and how data can be transferred across such media. Finally, we’ll explain how transmission forms the basis of data networking.
Analog and Digital Signals • Signal – electrical energy measured by the unit of voltage. • Digital signals – a sequence of voltage levels. Graphically, they are represented as a square wave. • Analog signals – continuously varying voltage levels; used in the communication over phone lines. • Refer to Fig 2.1 for examples of digital and analog signals
Signal Wave • Peak Amplitude (A) • maximum strength of signal • volts • Frequency (f) • Rate of change of signal • Hertz (Hz) or cycles per second or (1/second = Hz) • Period = time for one repetition (T) • f = 1/T ( so T = 1/f ) • Phase () • Relative position in time
Some Examples • If T = 0.5 ms, what is the frequency? (NOTE: 1/second = Hz) • If ƒ = 1 MHz, T = ?
Some Units • Some units: • Kilo = • Mega = • Giga =
Some Other Units • Millisecond (ms) = • Microsecond (µs) = • Nanosecond (ns) =
Theoretical Basis for Data Transmission • Information can be transmitted through a medium by varying some physical property. • The physics of the universe (noise, distortion, attenuation) places some limits on what can be sent over a channel. • Purpose of physical layer – to transport a raw bit stream from one machine to another.
Electromagnetic Spectrum • Electromagnetic energy – waves created by moving electrons. • Electromagnetic wave – a large family of waves consisting of electric and magnetic fields that vibrate ate high angles to each other, both vibrating at the same frequency • Pertinence to media – asset and hindrance
Electromagnetic Spectrum • James Maxwell 1865 – predict the existence of electromagnetic waves • Heinrich Hertz 1887 – first produced and observed these waves. That’s why frequency is measured in Hertz(Hz) – the number of oscillations per second of an electromagnetic wave.
How do we transmit these waves? • Feed an electrical signal to the antenna of a transmitter • The signal makes the atoms of the antenna vibrate (changing energy levels). • This change causes the antenna to emit electromagnetic waves.
voltage +1 0 -1 1 0 1 0 0 1 Sample Data Representation • Bits can be sent as a voltage or current through a wire • For example: • zero = +1 volts • one = -1 volts
Bandwidth A given transmission medium can accommodate signals within a given frequency range. The bandwidth is equal to the difference between the highest and the lowest frequencies that may be transmitted. For example, a telephone signal can handle frequencies between 300 Hz and 3300 Hz, giving it a bandwidth of 3000 Hz. This means, very high- or low-pitched sound cannot pass through the telephone system. Sometimes, bandwidth is used to denote the number of bits that can be transmitted.
Criteria for Media Evaluation • Bandwidth – difference between highest and lowest frequencies that may be transmitted. • Bit rate – expresses the data rate capacity of a network system. • Delay – the time period required to send a signal across a network. • Cost of medium material. • Ease of installation and maintenance.
Transmission Media • Copper Wires • Glass Fibers • Radio • Satellites • Geosynchronous Satellites • Low Orbit Satellites • Low Orbit Satellite Arrays • Microwaves • Infrared • Laser Lights
Copper Wires • Why copper? – low resistance to electrical current; signal travels farther; low cost. • Guided medium • Bandwidth – depends on the thickness of the wire and the distance traveled; typical is several megabits/second. • Interference – the twist helps reduce interference. • Two main types: twisted pair and coaxial cable.
Twisted Pair • Insulated Copper wires, about 1mm thickTwisted, to avoid forming an antenna: reduces interference • Two major kinds • Cat 3 (1988 and earlier) • four pairs: (allows four telephone lines) • Cat 5: (new installations) • more twists per centimeter, and Teflon insulation • more suitable for high speed networks. • Shielded vs. Unshielded: • shielded twisted pair (STP) • (shield serves as ground, some applications in business use this, but becoming more rare) • unshielded twisted pair (telco local loop to home is usually UTP)
More about Twisted Pair • Bandwidth = 250 kHz for analog signals • Bandwidth varies for carrying digital signals. For example, a local area network can use twisted pair to operate at 100 Mbps over a segment length of 100 meters. • Twisted pair can also support a 2400 bps rate for up to 10 miles.
Signal Distortion • attenuation - when a signal is transmitted over a copper wire, it will distort and lose strength. This situation is called attenuation. • Repeater – the device connecting two sections of twisted pairs. A repeater removes distortion, amplifies and receives received signal.
Copper Wire Insulation Copper Mesh Outside Insulation Coaxial Cable • Provides more protection from interference • Single wire – surrounded by a heavier metal shield which protects from incoming electromagnetic waves • Bandwidth – depends on cable length; typical data rate of 1 to 2 Gbps for 1 Km cable. • Cost higher than TP • Installation – heavy and unwieldy.
More about Coaxial Cable Coaxial cable typically transmits information in one of two modes: baseband or broadbandmode. • Baseband mode - the cable’s bandwidth is devoted to a single stream of data. • Broadband mode - the bandwidth is divided into ranges. Each range typically carries separate coded information.
Optical Fiber • Very prevalent • Medium – glass fiber • Energy – light pulses • Three components of a fiber system: • Light source: Light-emitting diode(LED) or laser (Light Amplification by Simulated Emission of Radiation) • Glass fiber • Detector: transforms the light to electrical pulses at the receiving end. • Cost – higher than copper • Installation – requires specialized technicians. • Bandwidth - huge
Laser • Unguided medium (fiber was guided). • Technology uses a laser beam of light to carry data through the air • 2 sites: transmitter and receiver • Equipment is fixed • Beam is unidirectional, traveling in a straight line.
Advantages of Fiber over Copper • Interference – does not cause interference; is not susceptible to interference. • Bandwidth – handles much higher bandwidth than copper • Low attenuation – requires fewer repeaters and amplifiers (every 30 km vs. 5 km, or 20 miles vs. 4 miles) • Immune to power surges, failures, and other electromagnetic interference • Thin and lightweight • Don’t leak light; tough to tap into, thus more secure.
Why no leakage? • Property of refraction – a light ray reflects when passing from one medium to another. Some will cross the boundary into the other. It is called refraction. When is less than a certain angle, there is no refrected light. (Fig 2.6) • This is what makes fiber optics work.
Fiber Cables • Similar to coax • 3 parts: core (glass), cladding (glass), and jacket (plastic). The cladding has a lower index of refraction than the core to keep the light in. • Where are they? • Terrestrial – within 1 meter of surface • Transoceanic fibers – buried in trenches by sea plows • Deep water – just lie on the bottom
Fiber Optics - Disadvantages • Inherently unidirectional – For two-way communication, two fibers are required. • Costly – fiber interfaces are more expansive than copper or coax. • Modal Dispersion - as distance increases, the difference between modes of the lights becomes bigger. (mode: path of light) Solution: graded-index for MM;step-index for SM
Radio • Electromagnetic spectrum: 102 – 1010 Hz. • Using radio waves(RF) of the spectrum to transmit computer data • Radio waves are omni directional • No physical connection required – unguided • Each computer attaches to an antenna which both transmits and receives
Radio (cont’d) • Antennas • Sizes of antenna depends on distance of communication to be performed • Communication of several miles: antenna should be about two meters high mounted on building top • Communication within same building: antenna can be small enough to fit inside a portable computer.
Microwave • Uses electromagnetic waves in the range of • Used for localized, small areas • Transmitter pointed directly at receiver • No antenna needed • Repeaters may be needed.(Fig. 2.13, 2.14) • Others: infrared • See Fig. 2.10
Satellites • Satellite – an object launched to orbit a celestial body • Orbit – the path of a satellite as it revolves around another body • Geostationary orbit – a path of a satellite that coincides with the revolution of the earth such that the satellite remains seemingly fixed at the same point above the equator from the perspective of a person standing on earth • Geostationary orbit for earth – 22300 miles (36000km) above the equator
Satellite Transmission System • Uplink earth station – • Takes baseband signals as inputs • Modulates a high frequency radio frequency • Satellite (receiver, transponder, transmitter) • Receives the radiated signal • Shifts its frequency using a transdponder to avoid interference • Amplifies the signal • Retransmits the signal back to earth where it can be received by downlink earth stations in the coverage area • Downlink earth station – • Receives and demodulates the radiated signal • Transmits the information to local receivers
Satellite Transmission System • Geosynchronous Satellites – According to Kepler’s Law, at the orbit height of 22,300 miles above the equator, a satellite can appear stationary to a ground observer. • Low Earth Orbit Satellites – Military surveillance require that a satellite not remain in a fixed position. LEO allows the satellite to move relative to the earth’s surface and scan different areas.. LEO requires less powerful rocket. However, since it keeps moving, eventually it may move out of the range of a ground station. A row of LEO may be required. (Fig 2.21)
Satellite Transmission System • Transponder - a device that accepts a signal within a specified frequency range and rebroadcast it over a different frequency • Each satellite has several transponders. • A ground based transmitter sends a signal (uplink) to a satellite, where one of the transponders relays the signal back down to earth (downlink) to a different location. • Satellite communications are now commonly used to transmit telephone and television signals. • Satellite dish - a private receiver for cable television reception.
Satellite Frequency Bands • L band: (uplink)1.6465-1.66GHz; (downlink) 1.545-1.5585 GHz • C band: (uplink)5.925-6.425GHz;(downlink) 3.7-4.2GHz • Ku band: (uplink)14-14.5GHz; (downlink) 11.7-12.2 GHz • Ka band: (uplink)27.5-30.5GHz; (downlink) 17.7-21.7 GHz
Problems • How can a satellite discriminate signals that were not meant for it? - FCC defines US satellite positions. • How do you prevent unauthorized reception of signals? • How do you prevent unauthorized transmission via satellite?
Wireless LAN • Allows PC and other Local Area Network (LAN) to communicate without physical link. • Many applications can take advantage of this kind of systems. For example, medical personnel can use notebook computer to connect to a wireless LAN. • Fiber optic and microwave systems installed at Edwards Air Force Base is another example. • Disadvantage: data rate is low.
Reading Assignment • Section 2.1
