Transcript Chapter_4
Chapter 4 Transmission Media TUNALI Data Communication 1 Overview • Guided - wire • Unguided – wireless • In both media, communication is in the form of electromagnetic waves • Characteristics and quality determined by medium and signal • For guided, the medium is more important • For unguided, the bandwidth of the signal produced by the antenna is more important • Key concerns are data rate and distance TUNALI Data Communication 2 Design Factors • Bandwidth —Higher bandwidth gives higher data rate • Transmission impairments —Attenuation —Twisted pair suffers more impairment than coaxcial cable and optical fibre • Interference • Number of receivers —In guided media —More receivers (multi-point) introduce more attenuation TUNALI Data Communication 3 Electromagnetic Spectrum TUNALI Data Communication 4 Guided Transmission Media • Twisted Pair • Coaxial cable • Optical fiber TUNALI Data Communication 5 Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) Coaxial cable 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Point to point transmission TUNALI Data Communication 6 Twisted Pair TUNALI Data Communication 7 Twisted Pair - Description • There are two insulated cupper wires. Many pairs are bundled into a single cable. • Twisting reduces crosstalk interference between adjacent pairs of cable. —Neighboring pairs in a bundle typically have different twist lenghts • On long distance links, twist length varies from 5 to 15 cm. TUNALI Data Communication 8 Twisted Pair – Applications 1 • Most common medium —For both analog and digital systems • Telephone network —Between house and local exchange (subscriber loop) • Within buildings —Each telephone is connected to a twisted pair, which goes to private branch exchange (PBX) • Designed to support voice traffic using analog signalling —Also can handle digital data traffic at modest data rates (modem) TUNALI Data Communication 9 Twisted Pair – Applications 2 • Mostly used in telephone lines. • Compared to coaxial cable and optical fiber, twisted pair is limited in distance, bandwidth and data rate. • Twisted pair is also used within a building for LAN supporting personal computers. • For long distance digital point-to-point signaling a data rate around a few Mbps is possible. • For a very short distances data rates of up to 1 Gbps have been achieved. TUNALI Data Communication 10 Twisted Pair - Pros and Cons • • • • Cheap Easy to work with Low data rate Short range TUNALI Data Communication 11 Twisted Pair - Transmission Characteristics • Analog —Amplifiers every 5km to 6km • Digital —Use either analog or digital signals —repeater every 2km or 3km • • • • Limited distance Limited bandwidth Limited data rate Susceptible to interference and noise TUNALI Data Communication 12 Attenuation of Guided Media TUNALI Data Communication 13 Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) —Ordinary telephone wire —Cheapest —Easiest to install —Suffers from external EM interference • Shielded Twisted Pair (STP) —Metal braid or sheathing that reduces interference —More expensive —Harder to handle (thick, heavy) UTP is cheap but subject to electromagnetic interference. STP is expensive but better. TUNALI Data Communication 14 UTP Categories • Cat 3 — up to 16MHz — Voice grade found in most offices — Twist length of 7.5 cm to 10 cm • Cat 4 — up to 20 MHz • Cat 5 — up to 100MHz — Commonly pre-installed in new office buildings — Twist length 0.6 cm to 0.85 cm • Cat 5E (Enhanced) –see tables • Cat 6 • Cat 7 TUNALI Data Communication 15 Near-End Crosstalk • Coupling of signal from one pair of conductors to another pair —Metal pins on a connector —Wire pairs in a cable • Coupling takes place when transmit signal entering the link couples back to receiving pair at the same end of the link • i.e. near transmitted signal is picked up by near receiving pair TUNALI Data Communication 16 Comparison of Shielded and Unshielded Twisted Pair Attenuation (dB per 100 m) Frequency (MHz) Category 3 UTP Category 5 UTP 1 2.6 2.0 4 5.6 16 13.1 150-ohm STP Near-end Crosstalk (dB) Category 3 UTP Category 5 UTP 150-ohm STP 1.1 41 62 58 4.1 2.2 32 53 58 8.2 4.4 23 44 50.4 25 — 10.4 6.2 — 41 47.5 100 — 22.0 12.3 — 32 38.5 300 TUNALI — — 21.4 Data Communication — — 31.3 17 Twisted Pair Categories and Classes Category 3 Class C Category 5 Class D Bandwidth 16 MHz 100 MHz Cable Type UTP Link Cost (Cat 5 =1) 0.7 TUNALI Category 5E Category 6 Class E Category 7 Class F 100 MHz 200 MHz 600 MHz UTP/FTP UTP/FTP UTP/FTP SSTP 1 1.2 1.5 2.2 Data Communication 18 Coaxial Cable TUNALI Data Communication 19 Coaxial Cable Applications • Most versatile medium • Television distribution —Cable TV can carry hundreds of TV channels • Long distance telephone transmission —Can carry 10,000 voice calls simultaneously (using FDM) —Being replaced by fiber optic • Short distance computer systems links • Local area networks —Using digital signaling, coax cable can be used to provide high speed channels TUNALI Data Communication 20 Coaxial Cable - Transmission Characteristics • Can be used effectively higher frequencies and data rates • Analog — Amplifiers every few km — Closer if higher frequency — Usable spectrum is up to 500MHz • Digital — Repeater every 1km — Closer for higher data rates • Performance Constraints — Attenuation — Thermal noise — Intermodulation noise TUNALI Data Communication 21 Optical Fiber TUNALI Data Communication 22 Optical Fiber - Benefits • Greater capacity — Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • Electromagnetic isolation — Are not affected by external electromagnetic fields (thus not vulnarable to interference, impulse noise, crosstalk) — Little interference with other equipment — High degree of security from eavesdropping — inherently difficult to tap • Greater repeater spacing — 10s of km at least TUNALI Data Communication 23 Optical Fiber – Applications (1) • Long-haul trunks —1500 km in length —20000-60000 voice channels —Telephone network, undersea • Metropolitan trunks —12 km —100,000 voice channels —Joining telephone exchanges in a metropolitan or city area TUNALI Data Communication 24 Optical Fiber – Applications (2) • Rural exchange trunks —40-160 km —5000 voice channels —Links towns and villages • Subscriber loops —Run directly central exchange to a subscriber —Handling not only voice and data, but also image and video • LANs —100 Mbps to 10 Gbps TUNALI Data Communication 25 Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz — Portions of infrared and visible spectrum • Two different types of ligth of source are used: • Light Emitting Diode (LED) — Cheaper — Wider operating temp range — Last longer • Injection Laser Diode (ILD) — More efficient — Greater data rate • Wavelength Division Multiplexing TUNALI Data Communication 26 Optical Fiber Transmission Modes 1 TUNALI Data Communication 27 Optical Fiber – Transmission Modes 2 • Step-index multimode — Rays at shallow angles are reflected and propagated along the fiber — Multiple propagation paths exist • Pulses spread out in time • The need to leave spacing between the pulses limits data rate — Suitable for short distance transmission • Single-mode — By reducing the radius of the core — No distortion (because there is a single transmission path) — Suitable for long distance transmission • Graded-index multimode — Suitable for LANs TUNALI Data Communication 28 Frequency Utilization for Fiber Applications Wavelength (in vacuum) range (nm) Frequency range (THz) 820 to 900 366 to 333 1280 to 1350 234 to 222 1528 to 1561 1561 to 1620 TUNALI Band label Fiber type Application Multimode LAN S Single mode Various 196 to 192 C Single mode WDM 185 to 192 L Single mode WDM Data Communication 29 Attenuation in Guided Media TUNALI Data Communication 30 Wireless Transmission • Unguided media • Transmission and reception via antenna • Directional —Focused beam —Careful alignment required • Omnidirectional —Signal spreads in all directions —Can be received by many antennae TUNALI Data Communication 31 Wireless Transmission Frequencies • 1GHz to 40GHz —Microwave —Highly directional —Point to point —Satellite • 30MHz to 1GHz —Omnidirectional —Broadcast radio • 3 x 1011 to 2 x 1014 Hz —Infrared portion of the spectrum —Local point to point and multipoint applications within confined areas TUNALI Data Communication 32 Antennas • Electrical conductor (or system of..) used to radiate electromagnetic energy or collect electromagnetic energy • Transmission — Radio frequency energy from transmitter — Converted to electromagnetic energy — By antenna — Radiated into surrounding environment • Reception — Electromagnetic energy impinging on antenna — Converted to radio frequency electrical energy — Fed to receiver • Same antenna often used for both TUNALI Data Communication 33 Radiation Pattern • Power radiated in all directions • Not same performance in all directions • A way to characterize the performance of an antenna • Graphical representation of the radiation propoerties of an antenna as a function of space coordinates • Isotropic antenna is (theoretical) point in space —Radiates in all directions equally —Gives spherical radiation pattern TUNALI Data Communication 34 Parabolic Reflective Antenna • Used for terrestrial and satellite microwave • Parabola is locus of all points equidistant from a fixed line and a fixed point not on that line — Fixed point is focus — Line is directrix • Revolve parabola about axis to get paraboloid — Cross section parallel to axis gives parabola — Cross section perpendicular to axis gives circle • Source placed at focus will produce waves reflected from parabola in parallel to axis — Creates (theoretical) parallel beam of light/sound/radio • On reception, signal is concentrated at focus, where detector is placed TUNALI Data Communication 35 Parabolic Reflective Antenna TUNALI Data Communication 36 Antenna Gain 1 • Measure of directionality of antenna • Power output in particular direction compared with that produced by isotropic antenna • Measured in decibels (dB) • Results in loss in power in another direction • Effective area relates to size and shape —Related to gain TUNALI Data Communication 37 Antenna Gain 2 • Relationship between antenna gain and effective area • G= 4Ae /2 = 4f 2Ae / c2 — G = antenna gain — Ae effective area — f carrier frequency — c speed of light — carrier wavelength • The effective area of — an isotropic antenna : 2 / 4 (power gain of 1) — a parabolic antenna with face area of A: 0.56A (power gain of 7A/2) TUNALI Data Communication 38 Antenna Gain 3 • For a parabolic reflective antenna with a diameter of 2m, operating at 12 GHz, what is the effective area and antenna gain? —A = r 2 = —Effective area for parabolic antenna is known as Ae = 0.56 A yielding Ae = 0.56 —The wavelength is = c / f = (3108)/(12109)=0.025m —The power gain of parabolic antenna is known as 7A/2 yielding (7 ) / (0.025)2 = 35,186 which is 45.46dB TUNALI Data Communication 39 Terrestrial Microwave – Physical Description • Parabolic dish sizing 3 m in diameter • Focuses narrow beam • Line of sight transmission to the receiving antenna • To achieve long distance transmissions, a series of microwave relat towers is used TUNALI Data Communication 40 Terrestrial Microwave Applications • Long haul communications systems —Alternative to coax cable or optical fiber —Requires far fewer amplifiers or repeaters than coax but requires line of sight transmission —Used for both voice and television transmission • Short point to point links between buildings • Cellular systems TUNALI Data Communication 41 Terrestrial Microwave – Transmission Characteristics • Common frequencies: 1 to 40 GHz —The higher the frequency used, the higher the potential bandwidth • The loss in communications is computed by — L= 10 log10 (4d/)2 dB where —d is the distance — is the wave length • Repeaters and amplifiers every 10 to 100 km TUNALI Data Communication 42 Terrestrial Microwave Transmission Characteristics • Attenuation increases with rainfall • Interference might be a problem —Most common bands for long haul communication are 4 GHz to 6 GHz —11 GHz band is coming to use • Higher microwave frequencies —Increased attenuation —Used in shorter distances —Antennas are smaller and cheaper TUNALI Data Communication 43 Satellite Microwave – Physical Description • Satellite is relay station • Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency —Operate on a number of frequency bands: transponder channels • Requires geo-stationary orbit to remain stationary —Height of 35,784km TUNALI Data Communication 44 Satellite Point to Point Link TUNALI Data Communication 45 Satellite Broadcast Link TUNALI Data Communication 46 Satellite Microwave Applications • Television — Public Broadcasting Service (PBS) — Programs are transmitted to the satellite, broadcast down to the stations, individual viewers — Direct Broadcast Satellite (DBS) — Direct to the home user • Long distance telephone • Private business networks — Very Small Aperture Terminal (VSAT) System — A powerful hub station acts as a relay for communication of subscribers • Global Positioning Systems TUNALI Data Communication 47 Satellite Microwave – Transmission Characteristics • To avoid interference, between satellites 4 spacing required — This limits the number of satellites • For continuous operation without interference, satellite cannot transmit and receive on the same frequency • Inherently a broadcast medium • There is 270 msec propagation delay • Operation is in the range of 1 -10 GHz TUNALI Data Communication 48 Broadcast Radio • Omnidirectional • 30 MHz to 1 GHz —FM radio —UHF and VHF television • Line of sight • Less sensitive to attenuation from rainfall • Suffer relatively less attenuation —Because of the longer wavelength • Suffers from multipath interference —Reflection from land, water or objects TUNALI Data Communication 49 Infrared • Modulate noncoherent infrared light • Tranceivers must be within the line of sight (directly or via reflection) • Blocked by walls —No security problems —No frequency allocation • e.g. TV remote control TUNALI Data Communication 50 Wireless Propagation • Signal travels along three routes — Ground wave • Follows contour of earth • Up to 2MHz • AM radio — Sky wave • • • • Amateur radio, BBC world service, Voice of America 2 MHz to 30 MHz Signal reflected from ionosphere layer of upper atmosphere (Actually refracted) — Line of sight • Above 30Mhz because it is not reflected by the ionosphere • May be further than optical line of sight due to refraction TUNALI Data Communication 51 Ground Wave Propagation TUNALI Data Communication 52 Sky Wave Propagation TUNALI Data Communication 53 Line of Sight Propagation TUNALI Data Communication 54 Refraction • Velocity of electromagnetic wave is a function of density of material — ~3 x 108 m/s in vacuum, less in anything else • As wave moves from one medium to another, its speed changes — Causes bending of direction of wave at boundary — Towards more dense medium • Index of refraction (refractive index) is — Sin(angle of incidence)/sin(angle of refraction) — Varies with wavelength • May cause sudden change of direction at transition between medium • May cause gradual bending if ref. idx. gradually changes — Density of atmosphere decreases with height — Results in bending towards earth of radio waves 55 Optical and Radio Line of Sight • Optical line of sight d = 3.57 ( h )1/2 • Radio line of sight d = 3.57 (K h )1/2 where d is the distance between an antenna and the horizon in kilometers h is the antenna height in meters K is the adjustment factor (microwaves are bent with the curvature of the earth) K is approximately 4/3 TUNALI Data Communication 56 Optical and Radio Horizons TUNALI Data Communication 57 Example • The maximum distance between two antennas for LOS transmission if one antenna is 100m high and the other is at ground level is d = 3.57 (Kh) ½ = 3.57 (133) ½ = 41 km Now suppose that the receiving antenna is 10m high. To achieve the same distance, how high must the transmitting antenna be? The result is 41 = 3.57 ((Kh1) ½ + (13.3) ½ ) h1 = 46.2 m • Savings over 50 m in the height of transmission antenna — Raising receiving antennas reduces the necessary height of the transmitter TUNALI Data Communication 58 Line of Sight Transmission • Free space loss — Signal disperses with distance — Greater for lower frequencies (longer wavelengths) • Atmospheric Absorption — Water vapor and oxygen absorb radio signals — Water greatest at 22GHz, less below 15GHz — Oxygen greater at 60GHz, less below 30GHz — Rain and fog scatter radio waves • Multipath — Better to get line of sight if possible — Signal can be reflected causing multiple copies to be received — May be no direct signal at all — May reinforce or cancel direct signal • Refraction — May result in partial or total loss of signal at receiver 59 Free Space Loss for Isotropic Antennas Pt / Pr = (4 d ) 2 / 2 = ( 4 f d ) 2 / c 2 Pt = signal power at the transmittin antenna Pr = signal power at the receiving antenna = carrier wavelength (m) d = propagation distance between antennas (m) c = speed of light (3108 m/s) TUNALI Data Communication 60 Free Space Loss for Other Antennas Pt / Pr = (4 )2 (d)2 / GrGt 2 = (d )2/Ar At = (cd )2 / f 2 ArAt Gt = gain of the transmitting antenna Gr = gain of the receiving antenna At = effective area of the transmitting antenna Ar = effective area of the receiving antenna TUNALI Data Communication 61 Free Space Loss TUNALI Data Communication 62 Example • Determine the isotropic free space loss at 4 GHz for the shortest path to a synchronous satellite from earth (35,863 km). — At 4 GHz, the wavelength is (3108)/(4109)=0.075m — LdB=195.6 dB (computed by loss in isotropic antenna) • Now consider the antenna gain of both satellite-and ground-based antennas. Typical values are 44 dB and 48 dB, respectively. The free space loss is — LdB = 195.6 - 44 - 48 = 103.6 dB • Now assume a transmit power of 250 W at the earth station. What is the power received at the satellite antenna? — A power of 250 W translates into 24 dBW, so the power at the receiving antenna is 24 – 103.6 = -79.6 dBW. TUNALI Data Communication 63 Multipath Interference TUNALI Data Communication 64 Required Reading • Stallings Chapter 4 TUNALI Data Communication 65