COE 341: Data & Computer Communications (T081) Dr. Marwan Abu-Amara Chapter 4: Transmission Media

COE 341 – Dr. Marwan Abu-Amara 2 Agenda Overview Guided Transmission Media  Twisted Pair  Coaxial Cable  Optical Fiber Wireless Transmission  Antennas  Terrestrial Microwave  Satellite Microwave  Broadcast Radio  Infrared

COE 341 – Dr. Marwan Abu-Amara 3 Overview Media  Guided - wire  Unguided - wireless Transmission characteristics and quality determined by:  Medium  Signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance

COE 341 – Dr. Marwan Abu-Amara 4 Design Issues Key communication objectives are:  High data rate  Low error rate  Long distance  Bandwidth: Tradeoff - Larger for higher data rates - But smaller for economy Transmission impairments  Attenuation: Twisted Pair > Cable > Fiber (best)  Interference: Worse with unguided… (medium is shared!) Number of receivers  In multi-point links of guided media: More connected receivers introduce more attenuation

COE 341 – Dr. Marwan Abu-Amara 5 Electromagnetic Spectrum

COE 341 – Dr. Marwan Abu-Amara 6 Study of Transmission Media Physical description Main applications Main transmission characteristics

COE 341 – Dr. Marwan Abu-Amara 7 Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

COE 341 – Dr. Marwan Abu-Amara 8 Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz0.2 1 kHz 50 µs/km2 km Twisted pairs (multi-pair cables) 0 to 1 MHz0.7 1 kHz 5 µs/km2 km Coaxial cable0 to 500 MHz7 10 MHz 4 µs/km1 to 9 km Optical fiber186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km40 km

COE 341 – Dr. Marwan Abu-Amara 9 Twisted Pair

COE 341 – Dr. Marwan Abu-Amara 10 UTP Cables

COE 341 – Dr. Marwan Abu-Amara 11 UTP Connectors

COE 341 – Dr. Marwan Abu-Amara 12 Note: Pairs of Wires It is important to note that these wires work in pairs (a transmission line) Hence, for a bidirectional link  One pair is used for TX  One pair is used for RX

COE 341 – Dr. Marwan Abu-Amara 13 Twisted Pair - Applications Most commonly used guided medium Telephone network (Analog Signaling)  Between house and local exchange (subscriber loop) Within buildings (Digital Signaling)  To private branch exchange (PBX) For local area networks (LAN)  10Mbps or 100Mbps  Range: 100m

COE 341 – Dr. Marwan Abu-Amara 14 Twisted Pair - Pros and Cons Pros:  Cheap  Easy to work with Cons:  Limited bandwidth  Low data rate  Short range  Susceptible to interference and noise

COE 341 – Dr. Marwan Abu-Amara 15 Twisted Pair - Transmission Characteristics Analog Transmission  Amplifiers every 5km to 6km Digital Transmission  Use either analog or digital signals  Repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100Mbps) Susceptible to interference and noise

COE 341 – Dr. Marwan Abu-Amara 16 Attenuation in Guided Media

COE 341 – Dr. Marwan Abu-Amara 17 Ways to reduce EM interference Shielding the TP with a metallic braid or sheathing Twisting reduces low frequency interference Different twisting lengths for adjacent pairs help reduce crosstalk

COE 341 – Dr. Marwan Abu-Amara 18 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)

COE 341 – Dr. Marwan Abu-Amara 19 STP: Metal Shield

COE 341 – Dr. Marwan Abu-Amara 20 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

COE 341 – Dr. Marwan Abu-Amara 21 Near End Crosstalk (NEXT) Coupling of signal from one wire pair to another Coupling takes place when a transmitted signal entering a pair couples back to an adjacent receiving pair at the same end i.e. near transmitted signal is picked up by near receiving pair Disturbing pair Disturbed pair Transmitted Power, P 1 Coupled Received Power, P 2 “NEXT” Attenuation = 10 log P 1 /P 2 dBs The larger … the smaller the crosstalk (i.e. the better the performance) NEXT attenuation is a desirable attenuation - The larger the better!

COE 341 – Dr. Marwan Abu-Amara 22 Transmission Properties for Shielded & Unshielded TP Signal Attenuation (dB per 100 m)Near-end Crosstalk Attenuation (dB) Frequency (MHz) Category 3 UTP Category 5 UTP 150-ohm STP Category 3 UTP Category 5 UTP 150-ohm STP ? — — — — ——21.4——31.3 Undesirable Attenuation- Smaller is better Desirable Attenuation- Larger is better!

COE 341 – Dr. Marwan Abu-Amara 23 Twisted Pair Categories and Classes Category 3 Class C Category 5 Class D Category 5E Category 6 Class E Category 7 Class F Bandwidth16 MHz100 MHz 200 MHz600 MHz Cable TypeUTPUTP/FTP SSTP Link Cost (Cat 5 =1)

COE 341 – Dr. Marwan Abu-Amara 24 Coaxial Cable Physical Description:

COE 341 – Dr. Marwan Abu-Amara 25 Physical Description

COE 341 – Dr. Marwan Abu-Amara 26 Coaxial Cable Applications Most versatile medium Television distribution  Cable TV Long distance telephone transmission  Can carry 10,000 voice calls simultaneously (though FDM multiplexing)  Being replaced by fiber optic Short distance computer systems links Local area networks (thickwire Ethernet cable)

COE 341 – Dr. Marwan Abu-Amara 27 Coaxial Cable - Transmission Characteristics Analog  Amplifiers every few km  Closer if higher frequency  Up to 500MHz Digital  Repeater every 1km  Closer for higher data rates

COE 341 – Dr. Marwan Abu-Amara 28 Attenuation in Guided Media

COE 341 – Dr. Marwan Abu-Amara 29 Optical Fibers An optical fiber is a very thin strand of silica glass  It is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber it travels (confined within the fiber) until it leaves the fiber at the other end Two critical factors stand out:  Very little light is lost in its journey along the fiber  Fiber can bend around corners and the light will stay within it and be guided around the corners An optical fiber consists of three parts  The core Narrow cylindrical strand of glass with refractive index n 1  The cladding Tubular jacket surrounding the core with refractive index n 2 The core must have a higher refractive index than the cladding for the propagation to happen

COE 341 – Dr. Marwan Abu-Amara 30 Optical Fibers (Contd.)  Protective outer jacket Protects against moisture, abrasion, and crushing Individual Fibers: (Each having core & Cladding) Multiple Fiber CableSingle Fiber Cable

COE 341 – Dr. Marwan Abu-Amara 31 Reflection and Refraction At a boundary between a denser (n 1 ) and a rarer (n 2 ) medium, n 1 > n 2 (e.g. water-air, optical fiber core- cladding) a ray of light will be refracted or reflected depending on the incidence angle Total internal reflection Critical angle refraction Refraction denser rarer 11 22 n1n1 n2n2  critical 11 22 n 1 > n 2 Increasing Incidence angle, 11 v 1 = c/n 1 v 2 = c/n 2

COE 341 – Dr. Marwan Abu-Amara 32 Optical Fiber n1n1 n 1 > n 2 Denser Rarer n1n1 n2n2 ii Total Internal Reflection at boundary for  i >  critical Refraction at boundary for. Escaping light is absorbed in jacket  i <  critical

COE 341 – Dr. Marwan Abu-Amara 33 Attenuation in Guided Media

COE 341 – Dr. Marwan Abu-Amara 34 Optical Fiber - Benefits Greater capacity  Data rates of hundreds of Gbps Smaller size & weight Lower attenuation  An order of magnitude lower  Relatively constant over a larger frequency interval Electromagnetic isolation  Not affected by external EM fields: No interference, impulse noise, crosstalk  Does not radiate: Not a source of interference Difficult to tap (data security) Greater repeater spacing  10s of km at least

COE 341 – Dr. Marwan Abu-Amara 35 Optical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs

COE 341 – Dr. Marwan Abu-Amara 36 Optical Fiber - Transmission Characteristics Act as wave guide for light (10 14 to Hz)  Covers portions of infrared and visible spectrum Light Emitting Diode (LED)  Cheaper  Wider operating temp range  Last longer Injection Laser Diode (ILD)  More efficient  Greater data rate

COE 341 – Dr. Marwan Abu-Amara 37 Optical Fiber Transmission Modes Core Cladding n1n1 n2n2 n1n1 n2n2 Shallow reflection Deep reflection Dispersion: Spread in arrival time Large Smallest Smaller  i <  critical Refraction 2 ways: v = c/n n 1 lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays

COE 341 – Dr. Marwan Abu-Amara 38 Optical Fiber – Transmission modes Spread of received light pulse in time (dispersion) is bad:  Causes inter-symbol interference  bit errors  Limits usable data rate and usable distance Caused by propagation through multiple reflections at different angles of incidence Dispersion increases with:  Larger distance traveled  Thicker fibers with step index Can be reduced by:  Limiting the distance  Thinner fibers and a highly focused light source  Single mode: High data rates, very long distances  Graded-index thicker fibers: The half-way solution

COE 341 – Dr. Marwan Abu-Amara 39 Optical Fiber – Wavelength Division Multiplexing (WDM) A form of FDM (channels sharing the medium by occupying different frequency bands) Multiple light beams at different frequencies (wavelengths) transmitted on the same fiber Each beam forms a separate communication channel Example: Gbps each  10 Tbps total data rate

COE 341 – Dr. Marwan Abu-Amara 40 Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region Selection based on:  Attenuation of the fiber  Properties of the light sources  Properties of the light receivers L S C Note: in fiber = v/f = (c/n)/f = (c/f)/n = in vacuum/n i.e. in fiber < in vacuum Bandwidth, THz

COE 341 – Dr. Marwan Abu-Amara 41 Attenuation in Guided Media

COE 341 – Dr. Marwan Abu-Amara 42 Wireless Transmission Free-space is the transmission medium Need efficient radiators, called antenna, to take signal from transmission line (wireline) and radiate it into free-space (wireless) Famous applications  Radio & TV broadcast  Cellular Communications  Microwave Links  Wireless Networks

COE 341 – Dr. Marwan Abu-Amara 43 Wireless Transmission Frequencies Radio: 30MHz to 1GHz  Omni-directional  Broadcast radio Microwave: 2GHz to 40GHz  Microwave  Highly directional  Point to point  Satellite Infrared Light: 3 x to 2 x  Localized communications

COE 341 – Dr. Marwan Abu-Amara 44 Antennas Electrical conductor (or system of..) used to radiate/collect electromagnetic energy Transmission  Radio frequency electrical 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 TX and RX in 2- way communication systems

COE 341 – Dr. Marwan Abu-Amara 45 Radiation Pattern Power radiated in all directions Not same performance in all directions Isotropic antenna is (theoretical) point in space  Radiates in all directions equally  Gives spherical radiation pattern  Used as a reference for other antennae Directional Antenna  Concentrates radiation in a given desired direction Used for point-to-point, line of sight communications  Gives “gain” in that direction relative to isotropic Radiation Patterns Isotropic Directional

COE 341 – Dr. Marwan Abu-Amara 46 Parabolic Reflective Antenna Used for terrestrial and satellite microwave Source placed at focus will produce waves reflected from parabola parallel to axis  Creates (theoretical) parallel beam of light/sound/radio  In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!) On reception, signal is concentrated at focus, where detector is placed The larger the antenna (in wavelengths) the better the directionality

COE 341 – Dr. Marwan Abu-Amara 47 Parabolic Reflective Antenna Axis

COE 341 – Dr. Marwan Abu-Amara 48 Antenna Gain, G Measure of directionality of antenna Power output in particular direction compared with that produced by isotropic antenna Measured in decibels (dB) Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed) Effective area, A e, relates to size and shape of antenna  Determines antenna gain

COE 341 – Dr. Marwan Abu-Amara 49 Antenna Gain, G: Effective Areas An isotropic antenna has a gain G = 1 (0 dBi) i.e. A parabolic antenna has: Substituting we get: Gain in dBi = 10 log G Important: Gains apply to both TX and RX antennas A = Actual Area =  r 2

COE 341 – Dr. Marwan Abu-Amara 50 Terrestrial Microwave Parabolic dish Focused beam Line of sight  Curvature of earth limits maximum range  Use relays to increase range (multi-hop link) Long haul telecommunications Higher frequencies give higher data rates but suffers from larger attenuation

COE 341 – Dr. Marwan Abu-Amara 51 Terrestrial Microwave: Propagation Attenuation As signal propagates in space, its power drops with distance according to the inverse square law i.e. loss in signal power over distance traveled, d While with a guided medium, signal drops exponentially with distance… giving larger attenuation and lower repeater spacing Show that L increases by 6 dBs for every doubling of distance d. For guided medium, corresponding attenuation = a d dBs, a = dBs/km d’ = distance in ’s

COE 341 – Dr. Marwan Abu-Amara 52 Satellite Microwave Satellite is relay station Satellite receives on one frequency (uplink), amplifies or repeats signal and transmits on another frequency (downlink) Requires geo-stationary orbit  Height of 35,784km Applications  Television  Long distance telephone  Private business networks

COE 341 – Dr. Marwan Abu-Amara 53 Satellite Point to Point Link Relay Uplink Downlink

COE 341 – Dr. Marwan Abu-Amara 54 Satellite Broadcast Link

COE 341 – Dr. Marwan Abu-Amara 55 Broadcast Radio Omni-directional  No dishes  No line of sight requirement  No antenna alignment Applications  FM radio  UHF and VHF television Suffers from multipath interference  Reflections (e.g. TV ghost images)

COE 341 – Dr. Marwan Abu-Amara 56 Infrared Modulate non-coherent infrared light Line of sight (or reflection) Blocked by walls No licensing required for frequency allocation Applications  TV remote control  IRD port