Chapter 4 Transmission Media TUNALI Data Communication

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

Design Factors Bandwidth Transmission impairments Interference 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

Electromagnetic Spectrum TUNALI Data Communication

Guided Transmission Media Twisted Pair Coaxial cable Optical fiber TUNALI Data Communication

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) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km Coaxial cable 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 40 km Point to point transmission TUNALI Data Communication

Twisted Pair TUNALI Data Communication

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

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

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

Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range TUNALI Data Communication

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

Attenuation of Guided Media TUNALI Data Communication

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

UTP Categories Cat 3 Cat 4 Cat 5 Cat 5E (Enhanced) –see tables Cat 6 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

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

Comparison of Shielded and Unshielded Twisted Pair   Attenuation (dB per 100 m) Near-end Crosstalk (dB) Frequency (MHz) Category 3 UTP Category 5 UTP 150-ohm STP 1 2.6 2.0 1.1 41 62 58 4 5.6 4.1 2.2 32 53 16 13.1 8.2 4.4 23 44 50.4 25 — 10.4 6.2 47.5 100 22.0 12.3 38.5 300 21.4 31.3 TUNALI Data Communication

Twisted Pair Categories and Classes   Category 3 Class C Category 5 Class D Category 5E Category 6 Class E Category 7 Class F Bandwidth 16 MHz 100 MHz 200 MHz 600 MHz Cable Type UTP UTP/FTP SSTP Link Cost (Cat 5 =1) 0.7 1 1.2 1.5 2.2 TUNALI Data Communication

Coaxial Cable TUNALI Data Communication

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

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

Optical Fiber TUNALI Data Communication

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

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

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

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

Optical Fiber Transmission Modes 1 TUNALI Data Communication

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

Frequency Utilization for Fiber Applications Wavelength (in vacuum) range (nm) Frequency range (THz) Band label Fiber type Application 820 to 900 366 to 333   Multimode LAN 1280 to 1350 234 to 222 S Single mode Various 1528 to 1561 196 to 192 C WDM 1561 to 1620 185 to 192 L TUNALI Data Communication

Attenuation in Guided Media TUNALI Data Communication

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

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

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

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

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

Parabolic Reflective Antenna TUNALI Data Communication

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

Antenna Gain 2 Relationship between antenna gain and effective area G= 4Ae /2 = 4f 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

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 = (3108)/(12109)=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

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

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

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 (4d/)2 dB where d is the distance  is the wave length Repeaters and amplifiers every 10 to 100 km   TUNALI Data Communication

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

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

Satellite Point to Point Link TUNALI Data Communication

Satellite Broadcast Link TUNALI Data Communication

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

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

Broadcast Radio Omnidirectional 30 MHz to 1 GHz Line of sight 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

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

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

Ground Wave Propagation TUNALI Data Communication

Sky Wave Propagation TUNALI Data Communication

Line of Sight Propagation TUNALI Data Communication

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

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

Optical and Radio Horizons TUNALI Data Communication

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

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

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 (3108 m/s) TUNALI Data Communication

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

Free Space Loss TUNALI Data Communication

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 (3108)/(4109)=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

Multipath Interference TUNALI Data Communication

Required Reading Stallings Chapter 4 TUNALI Data Communication