Chapter 4: Transmission Media COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal Chapter 4: Transmission Media

Agenda Overview Guided Transmission Media Wireless Transmission Twisted Pair Coaxial Cable Optical Fiber Wireless Transmission Antennas Terrestrial Microwaves Satellite Microwaves Broadcast Radio Infrared

Overview Media: Guided – wire or fiber Unguided - wireless Transmission characteristics and quality determined by: Signal Medium For guided, the medium is more important For unguided, the bandwidth provided by the antenna is more important

Design Issues Key communication objectives are: High data rate Low error rate Long distance Bandwidth economy: Tradeoff - Want larger BW for higher data rates: C  B - But limited by economy: Larger BW is costly e.g. Coaxial Vs TP Transmission impairments Attenuation: Twisted Pair > Cable > Fiber (best) Interference: Worse with unguided… (the medium is shared!) Number of receivers In multi-point links of guided media: More connected receivers introduce more attenuation

The Electromagnetic Spectrum Microwaves Light 10 GHz 10 KHz 100 MHz

Standard Multiplier Prefixes 1-18 to 10+18 exa- E 1018 = 1,000,000,000,000,000,000 peta- P 1015 = 1,000,000,000,000,000 tera- T 1012 = 1,000,000,000,000 giga- G 109 = 1,000,000,000 mega- M 106 = 1,000,000 kilo- K 103 = 1,000 milli- m 10-3 = 0.001 micro- 10-6 = 0.000,001 nano- n 10-9 = 0.000,000,001 pico- p 10-12 = 0.000,000,000,001 femto- f 10-15 = 0.000,000,000,000,001 atto- a 10-18 = 0.000,000,000,000,000,001

Electromagnetic Spectrum Ultra violet, X-Rays, Gamma-Rays Used for Communications

Study of Transmission Media Physical description Main applications Main transmission characteristics

Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

Transmission Characteristics of Guided Media: Overview   Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading coils) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pair (No loading coils) e.g. for ADSL 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km Up to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Larger Operating Frequencies Lower Attenuation Same delay (except with loading) Fewer Repeaters Required

Twisted Pair (TP) Effect of loading with coils in series at intervals But attenuation rises rapidly Outside this narrow band. No good for ASDL which tries to get 1 MHz BW from the TP line! Flat and low attenuation Over the telephone voice band (300-3400 Hz) (Passive Equalizer)

UTP Cables unshielded

Twisted Pair - Applications Most commonly used guided medium Telephone network (Analog Signaling) Between houses and the local exchange (subscriber loop) Originally designed for analog signaling. Digital data transmitted using modems at low data rates Within buildings (short distances): (Digital Signaling) To private branch exchange (PBX) (64 Kbps) For local area networks (LAN) (10-100Mbps) Example: 10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range Digital signal travels in its base band i.e. without modulating a carrier (short distances)

Twisted Pair - Pros and Cons Compared to other guided media Low cost Easy to work with (pull, terminate, etc.) Cons: Limited bandwidth Limited data rate Large Attenuation Limited distance range Susceptible to interference and noise (exposed construction)

Twisted Pair - Transmission Characteristics Analog Transmission For analog signals only Amplifiers every 5km to 6km Bandwidth up to 1 MHz (several voice channels): ADSL Digital Transmission For either analog or digital signals (carrying digital data) Repeaters every 2km or 3km Data rates up to few Mbps (1Gbps: very short distance) Impairments: Attenuation: A strong function in frequency ( Distortion, need for equalization) EM Interference: Crosstalk, Impulse noise, Mains interference, etc.

Attenuation in Guided Media Thinner Wires

Ways to reduce EM interference WK 7 Ways to reduce EM interference Shielding the TP with a metallic braid or sheathing Twisting also reduces low frequency interference Different twisting lengths for adjacent pairs help reduce crosstalk

STP: Metal Shield

Unshielded (UTP) and Shielded (STP) Unshielded Twisted Pair (UTP) Ordinary telephone wire: Abundantly available in buildings Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Shielded with foil, metal braid or sheathing: Reduces interference Reduces attenuation at higher frequencies (increases BW)  Better Performance: Increased data rates used Increased distances covered  But becomes: More expensive Harder to handle (thicker, heavier)

TP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data) Cat 3: Unshielded (UTP) Up to 16MHz Voice grade In most office buildings Twist length of 7.5 cm to 10 cm Cat 5: Unshielded (UTP) Up to 100MHz Data grade Pre-installed now in many new office buildings Twist length 0.6 cm to 0.85 cm (Tighter twisting increases cost but improves performance) Newer, shielded twisted pair: (150 W STP) Up to 300MHz

Near End Crosstalk (NEXT) Coupling of signal from one wire pair to another Coupling takes place when a transmitted signal entering a pair couples into an adjacent receiving pair at the same end i.e. near transmitted signal is picked up by near receiving pair Transmitted Power, P1 Disturbing pair Coupled Received Power, P2 Disturbed pair “NEXT” Attenuation = 10 log P1/P2 dBs The larger … the smaller the crosstalk (The better the performance) “NEXT” attenuation is a desirable attenuation- The larger the better!

Transmission Properties for Shielded & Unshielded TP Undesirable Attenuation- Smaller is better Desirable Attenuation- Larger is better! Signal Attenuation (dB per 100 m) Near-end Crosstalk Attenuation (dB) Frequency (MHz) Category 3 UTP Category 5 UTP 150-ohm STP 1 2.6 2.0 1.1 41 62 68? 4 5.6 4.1 2.2 32 53 58 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

Newer 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 UTP: Unshielded Twisted Pair FTP: Foil Twisted Pair SSTP: Shielded-Screen Twisted Pair

Coaxial Cable Physical Description: 1 - 2.5 cm Designed for operation over a wider frequency range

Physical Description

Coaxial Cable Applications Most versatile medium: Television distribution (FDM Broadband) Cable TV (CATV): 100’s of TV channels over 10’s Kms Long distance telephone transmission Can carry 10s of thousands of voice channels simultaneously (though FDM multiplexing) (Broadband) Now facing competition from optical fibers and terrestrial microwave links Local area networks, e.g. Thickwire Ethernet cable (10Base5): 10 Mbps, Baseband signal, 500m segment (5 time TP distance)

Coaxial Cable - Transmission Characteristics: Improvements over TP Extended frequency range Up to 500 MHz Reduced EM interference and crosstalk Due to enclosed concentric construction EM fields terminate within cable and do not stray outside causing interference Remaining limitations: Attenuation Thermal and inter modulation noise (FDM)

Attenuation in Guided Media

Coaxial Cable - Transmission Characteristics Analog Transmission Amplifiers every few kms Closer amplifier spacing for higher operating frequencies Digital Transmission Repeater every 1km Closer repeater spacing for higher data rates

Optical Fiber A thin (2-125 mm) flexible strand of glass or plastic Light entering at one end travels confined within the fiber until it leaves it at the other end As fiber bends around corners, the light remains within the fiber through multiple internal reflections Lowest losses (attenuation) with ultra pure fused silica glass… but expensive and more difficult to manufacture Reasonable losses with multi-component glass and with plastic Quality, Cost, Difficulty of Handling Attenuation (Loss) Pure Glass Multi-component Glass Plastic

Optical Fiber: Construction An optical fiber consists of three main parts Core A narrow cylindrical strand of glass/plastic, with refractive index n1 Cladding A tube surrounding each core, with refractive index n2 The core must have a higher refractive index than the cladding to keep the light beam trapped inside: n1 > n2 Protective outer jacket Protects against moisture, abrasion, and crushing Single Fiber Cable Individual Fibers: (Each having its core & Cladding) Multiple Fiber Cable Important: Each core surrounded by a cladding

Reflection and Refraction At a boundary between a denser (n1) and a rarer (n2) medium, n1 > n2 (e.g. water-air, optical fiber core-cladding) a ray of light will be refracted or reflected depending on the incidence angle Increasing Incidence angle, 1 Angles With the Normal rarer 2 v2 = c/n2 n2 denser 1 2 n1 critical n1 > n2 1 v1 = c/n1 Total internal reflection Refraction Critical angle

Optical Fiber Denser n1 n1 > n2 Refraction at boundary for . Escaping light is absorbed in jacket i < critical Rarer n2 Denser Denser n1 n1 Rarer i Total Internal Reflection at core-cladding boundary for i > critical n1 > n2

Attenuation in Guided Media Larger Frequency

Optical Fiber - Benefits Greater capacity Fiber: 100’s of Gbps over 10’s of Kms Cable: 100’s of Mbps over 1’s of Kms Twisted pair: 100’s of Mbps over 10’s of meters Lower/more uniform* attenuation (Fig. 4.3c) An order of magnitude lower Relatively constant over a larger range of frequencies* 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) * With careful selection of operating band

Optical Fiber – Benefits, Contd. Greater repeater spacing: Lower cost, Fewer Units Fiber: 10-100’s of Kms Cable, Twisted pair: 1’s Kms Smaller size and weight: An order of magnitude thinner for same capacity Useful in cramped places Reduced cost of digging in populated areas Reduced cost of support structures

Optical Fiber - Applications Long-haul trunks Telephone traffic over long routes between cities, and undersea: Fiber & Microwave now replacing coaxial cable  1500 km, Up to 60,000 voice channels Metropolitan trunks Joining exchanges inside large cities:  12 km, Up to 100,000 voice channels Rural exchange trunks Joining exchanges of towns and villages:  40-160 km, Up to 5,000 voice channels Subscriber loops Joining subscribers to exchange: Fiber replacing TP, allowing all types of data LANs, Example: 10BaseF 10 Mbps, 2000 meter segment Exchange City City Main Exchange

Optical Fiber - Transmission Characteristics Acts as a wave guide for light (1014 to 1015 Hz) Covers portions of infrared and visible spectrum Transmission Modes: Single Mode Multimode Step Index Graded Index

Optical Fiber Transmission Modes Dispersion: Spread in ray arrival time Refraction Deep reflection Shallow reflection n2 i < critical n1 Large Spread Core Cladding 2 ways: n1 n2 Curved path: n is not uniform- decreasing Smaller v = c/n n1 is made lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays Smallest Smaller spread  Narrower pulses Higher data rates supported

Optical Fiber – Transmission modes Spread of received light pulse in time (dispersion) is bad: Causes inter-symbol interference  bit errors (similar to delay distortion) Limits usable data rate and usable transmission distance Caused by propagation through multiple reflections at different angles of incidence Dispersion increases with: Larger distance traveled Thicker fibers with step index Less focused sources Can be reduced by: Limiting the distance Thinner fibers and a highly focused light source  Single mode (in the limit): High data rates, very long distances Or Graded-index multimode thicker fibers: The half-way (lower cost) solution

Optical Fiber Transmission System– Light Source + Fiber + Light Detector Light Sources Light Emitting Diode (LED) Incoherent light- More dispersion  Lower data rates Low cost Wider operating temp range Longer life Injection Laser Diode (ILD) Coherent light- Less dispersion  Higher data rate More efficient Faster switching  Higher data rate

Optical Fiber – Wavelength Division Multiplexing (WDM) A form of FDM (Channels sharing the medium by occupying different frequency bands) Multiple light beams at different light frequencies (wavelengths) transmitted on the same fiber Each beam forms a separate communication channel Separated at destination by filters Example: 256 channels @ 40 Gbps each  10 Tbps total data rate WDM

Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region Band selection is a system decision based on: Attenuation of the fiber Properties of the light sources Properties of the light receivers S C L Bandwidth, THz 33 12 4 7 Note: l in fiber = v/f = (c/n)/f = (c/f)/n = l in vacuum/n i.e. l in fiber < l in vacuum

Wireless Transmission Free-space is the transmission medium Need efficient radiators, called antennas Signal fed from transmission line (wireline) and radiated it into free-space (wireless) Popular applications Radio & TV broadcast Cellular Communications Microwave Links Wireless Networks

Wireless Transmission Frequency Ranges Radio: 30 MHz to 1 GHz Omni directional Broadcast radio Microwaves: 1 GHz to 40 GHz Highly directional beams Point to point (Terrestrial) Satellite Infrared Light: 0.3 THz to 20 THz (below light) Localized communications (confined spaces)

Antennas Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from surrounding space Transmission Radio frequency electrical energy from transmitter Converted into electromagnetic energy Radiated into surrounding space 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

Radiation Pattern Power radiated in all directions, but usually not with the same efficiency Isotropic antenna A hypothetical point source in space (Small dimensions relative to l) Radiates equally in all directions – A spherical radiation pattern Used as a reference for other antennae Directional Antenna Concentrates radiation in a given desired direction – hence point-to-point, line of sight communications Gives antenna ‘gain’ in that direction relative to isotropic for both TX and RX Larger dimensions relative to l  Greater directivity Radiation Patterns Isotropic Directional

Parabolic Reflective Antenna WK 8 Used for terrestrial and satellite microwave Source placed at the focal point will produce waves that get reflected from parabola parallel to the parabola axis Creates a (theoretically) parallel beam of light/sound/radio that does not spread (disperse) in space In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!) On reception, only signal from the axis direction is concentrated at focus, where detector is placed. Signals from other directions miss the focus  negligible O/P The larger the antenna (in wavelengths) the better the directionality  so, using Higher frequency is advantageous Focus Parabola

Parabolic Reflective Antenna WK 8 Axis

Antenna Gain, G A measure of antenna directionality Power output of the antenna in a particular direction compared to that produced by a perfect isotropic antenna Can be expressed in decibels (dB, dBi) (i = relative to isotropic) Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed) Effective area Ae: Related to size and shape of antenna Determines the antenna gain, Ae is the effective area

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 = p r2

Propagation Attenuation As signal propagates in space, its power drops with distance according to the inverse square law While with a guided medium, signal drops exponentially with distance… giving larger attenuation and lower repeater spacing d’ = distance in l’s i.e. loss in signal power over distance traveled, d Show that L increases by 6 dBs for every doubling of distance d. For guided medium, corresponding attenuation = a d dBs, a in dBs/km A disadvantage for operating at higher frequency?

Microwave Link Calculations TX-RX Net attenuation, A = L – G1 – G2, dBs S dBm = P dBm – A dBs

Terrestrial Microwave Parabolic dish Focused beam (with antenna gain) Line of sight requirement: Beam should not be obstructed Curvature of earth limits maximum range  Use relays to increase range (multi-hop link) Link performance sensitive to antenna alignment Applications: Long haul telecommunications Many voice/data channels over long distances between large cities, possibly through intermediate relays: Competes with coaxial cable and fiber Short wireless links between buildings: CCTV links Wireless links between LANs in close-by buildings Cellular Telephony

Terrestrial Microwave: Transmission Properties 1 - 40 GHz Higher f Advantages: Larger bandwidth, B  higher data rate (Table 4.6) Smaller l  smaller (lighter, cheaper) antenna for a required antenna gain (see gain eqn.) But Higher f  larger attenuation due propagation and absorption by rain So, Long-haul links (long distances) operate at lower frequencies (4-6 GHz,11 GHz) to avoid large attenuation Short links between close-by buildings operate at higher frequencies (e.g. 22 GHz) (Attenuation is not a big problem for the short distances, smaller antenna size)

Satellite Microwave Satellite is used as a relay station for the link Satellite receives on one frequency (uplink), amplifies or repeats signal and re-transmits it on another frequency (downlink) Spatial angular separation (e.g. 3) to avoid interference from neighboring TXs Require a geo-stationary orbit (satellite rotates at the same speed of earth rotation, so appears stationary): Height: 35,784km (long link, large transmission delays) Applications: Television direct broadcasting Long distance telephony Private business networks linking multiple company sites worldwide

a. Satellite Point to Point Link Relay Downlink Uplink Earth curvature Obstructs line of sight for large distances

b. Satellite Broadcast Link Direct Broadcasting Satellite

Transmission Characteristics 1-10 GHz Frequency Trade offs: Lower frequencies: More noise and interference Higher frequencies: Larger rain attenuation, but smaller antennas Downlink/Uplink frequencies recently going higher: 4/6 GHz  12/14  20/30 (better receivers becoming available) Delay = 0.25 s  noticeable for telephony Inherently a broadcasting facility

Broadcast Radio: 30 MHz – 1 GHz Omni directional (no need for antenna directionality horizontally) No dishes No line of sight requirement No antenna alignment requirement/problems Applications: FM radio UHF and VHF television Choice of frequency range: Reflections from ionosphere < 30 MHz -1 GHz < Rain Propagation attenuation: Lower than for Microwaves (as l is larger) Problems caused by omni directionality: Interference due to multi-path reflections e.g. TV ghost images

Multi-Path effects of omni-directionality Omni-Directional TV Broadcasting Antenna TV ghost images

Infrared Data Modulates a non coherent infrared light Relies on line of sight (or reflections through walls or ceiling) Blocked by walls (unlike microwaves) No licensing required for frequency allocation Applications: TV remote control Wireless LAN within a room