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

Remaining five chapters: Chapter 7: Data Link: Flow and Error control, Link management Data Link Chapter 8: Improved utilization: Multiplexing Physical Layer Chapter 6: Data Communication: Synchronization, Error detection and correction Chapter 4: Transmission Media Transmission Medium Chapter 5: Encoding: From data to signals Chapter 3: Signals, their representations, their transmission over media, Impairments

Agenda Overview Guided Transmission Media Twisted Pair Coaxial Cable Optical Fiber Unguided (open space, wireless) Transmission Antennas Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared

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

Design Issues Key communication objectives are: High data rate Low error rate Long distance Bandwidth: Tradeoff - Larger for higher data rates - But smaller for low link cost Transmission impairments Attenuation: Twisted Pair > Cable > Fiber (best) Interference and Cross talk: Twisted Pair > Cable > Fiber (best) Worse with unguided… (the medium is shared!) Number of receivers In multi-point links of guided media: Attenuation increases with increased number of connected receivers

Part of the Electromagnetic Spectrum 1 KHz 1 MHz 1 GHz 1 THz Guided Unguided l V = l f

Study of Transmission Media Physical description Main transmission characteristics Main applications

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) 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 3 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  

Twisted Pair (TP)

UTP Cables unshielded

Twisted Pair - Applications The most commonly used guided medium Telephone network (Analog Signaling) Analog Data (original purpose) : Between houses and the local exchange, e.g. 5 km (subscriber loop) Digital Data: Transmitted using modems, low data rates Within buildings (short distances) (Digital Signaling) To private branch exchange (PBX) (64 Kbps) For local area networks (LAN) (10-100Mbps) Example, Ethernet: 10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range

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 Limited distance range (due to large attenuation) Susceptible to interference and noise

Twisted Pair - Transmission Characteristics Analog Transmission Mode For analog signals only Amplifiers every 5km to 6km Bandwidth up to 1 MHz (several voice channels): ADSL (Ch 8) Digital Transmission Mode Using either analog or digital signals Repeaters every 2km or 3km Data rates up to few Mbps (1Gbps: over very short distances) Impairments: Attenuation: A strong function in frequency, can be modified with loading coils EM Interference: Crosstalk, Impulse noise, Mains interference

Attenuation in Guided Media

Attenuation in Twisted pairs (unloaded) Thinner Wires Wire Gauge Wire Diameter 300 1 KHz 3400 1 MHz Telephone Voice

Ways to reduce EM interference Shielding the TP with a metallic braid or sheathing Twisting reduces low frequency interference Tighter twisting  Better performance Using different twisting lengths for adjacent pairs to 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 metal braid or sheathing: Reduces interference Reduces attenuation at higher frequencies  Better Performance: Increases data rates used Increases distances covered But becomes: More expensive Harder to handle (thicker, heavier) Transmit faster and go further!

UTP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data) Up to 16MHz Voice grade In most office buildings Twist length: 7.5 cm to 10 cm Cat 4 Up to 20 MHz Cat 5 Up to 100MHz Data grade Pre-installed in many new office buildings Twist length: 0.6 cm to 0.85 cm (Tighter twisting increases cost but improves performance)

Near End CrossTalk (NEXT) Coupling of signal from a wire pair to an adjacent pair Coupling takes place when a transmitted signal entering a pair couples (leaks) into an adjacent receiving pair on the same (near) end of the link 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 Better Better Better Better Not Usable Not Usable

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 rage

Physical Description

Coaxial Cable Applications Frequency Division Multiplexing 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

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 out Remaining limitations: Attenuation Thermal and noise Inter modulation noise (especially for broadband operation) Because of FDM

Attenuation in Guided Media

Coaxial Cable - Transmission Characteristics Analog Transmission Amplifiers every few kms Closer spacing for higher frequency 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 at the other end As fiber bends around corners, the light stays within the fiber Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture Reasonable losses and performance with multi-component glass and with plastic Cost, Difficulty of Handling Attenuation (Loss) Pure Glass Multi-component Glass Plastic Best Performance

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 core & Cladding) Multiple Fiber Cable (Note multiple cladding)

Reflection and refraction of light 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, Shallower Incidence 1 rarer 2 v2 = c/n2 n2 denser 1 2 n1 critical n1 > n2 1 v1 = c/n1 Total internal reflection Refraction Critical angle refraction

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 boundary for i > critical n1 > n2

Attenuation in Guided Media Which side is the IR? Compare attenuation ranges!

Optical Fiber - Benefits Greater channel capacities over larger distances 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 frequency interval Electromagnetic isolation Fiber is not affected by external EM fields: No interference, impulse noise, crosstalk Fiber does not radiate (light ray trapped inside): Not a source of interference Difficult to tap (data security) But what could happen at the repeater?

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

Optical Fiber - Applications Long-haul trunks between cities 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 traffic LANs, Example: 10BaseF 10 Mbps, 2000 meter segment Exchange City City Competition: Fiber, Coaxial, m Waves Main Exchange Compare segment length with twisted pair and coaxial!

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 Single ray Step Index Graded Index Multiple rays

Optical Fiber Transmission Modes Dispersion: Spread in ray arrival time Refraction Deep reflection Shallow reflection n2 i < critical n1 Large Core n1 > n2 Cladding 2 ways to reduce dispersion: n1 n2 Smaller v = c/n Make n1 lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays Smallest

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 Dispersion can be reduced by: Limiting the distance Thinner fibers and a highly focused light source  In the limit: Single mode: High data rates, very long distances Graded-index thicker fibers: The half-way solution

The transmission system is not just the medium (fiber) The transmission system is not just the medium (fiber)! We have also light Sources and detectors… Light Sources Light Emitting Diode (LED) Lower cost, longer life Wider operating temp range Injection Laser Diode (ILD) More efficient (more light power for same electric power input) Faster switching  Higher data rate

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: 256 channels @ 40 Gbps each  10 Tbps total data rate

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 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 l values shown are in vacuum

Wireless Transmission Free-space is the transmission medium Need efficient radiators, called antennas / aerials Signal fed from transmission line (wireline) and radiated 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 (just below visible light) Localized communications (confined spaces)

Antennas Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from the environment (TX/RX operation) Transmission Radio frequency electrical energy obtained from transmitter Converted into electromagnetic energy 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

Radiation Pattern Power radiated in different directions, usually not with the same efficiency: Isotropic (Omni-directional) antenna A hypothetical point source in space 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 ‘gain’ in that direction relative to isotropic Radiation Patterns Point Source Isotropic Directional

Parabolic Reflective Antenna Focus Parabola (The Dish!) Axis

Parabolic Reflective Antenna (The Dish!) Used for terrestrial and satellite microwave On Transmission: Source placed at the focus will produce waves that get reflected from parabola parallel to the parabola axis Creates a (theoretically) parallel beam to the parabola axis that does not spread (disperse) in space ( Zero radiation off axis) 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 ( Zero o/p off axis) i.e. Directionality in both TX, RX operation The larger the antenna (in wavelengths) the better the directionality  High frequency is advantageous Focus Parabola

Antenna Gain, G A measure of directionality of the antenna Power output in a given 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 other directions (Total power is fixed) Gain Gq depends on the effective area (Ae) of the antenna: Depends on size and shape of the antenna The Radiation pattern

Antenna Gain, G in the direction of maximum radiation 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 of antenna circle = p r2 Higher frequencies  higher gains

Terrestrial Microwave Parabolic dish i.e. Focused beam 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 affected by antenna alignment (effect of wind!) Applications: Long haul telecommunications Many channels over long distances between large cities, possibly through intermediate relays:Competes with coaxial cable and fiber Short links between buildings: CCTV links Links between LANs in different buildings Cellular Telephony

Terrestrial Microwave: Transmission Properties 1 - 40 GHz: f range used depends on application Advantages of higher frequencies : Larger bandwidth, B  higher data rate (Table 4.6) Smaller l  smaller (lighter, cheaper) antenna for a given antenna gain (see gain eqn) But Higher f  larger attenuation due 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 (22 GHz) (Attenuation is not a big problem for the short distances, reduces antenna size)

Terrestrial Microwave: 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 = dBs/km

Satellite Microwave Satellite is used as a relay station for the link to avoid limitation on distance due to earth curvature 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

Satellite Point to Point Link Relay Downlink Uplink Earth curvature (overcome)

Satellite Broadcast Link Direct Broadcasting Satellite

Transmission Characteristics 1-10 GHz Frequency Trade offs: Lower frequencies: Noise and interference Higher frequencies: Smaller antenna, but rain attenuation, Downlink/Uplink frequencies recently going higher: 4/6 GHz  12/14  20/30 (due to better receivers) Delay = 0.25 s  noticeable for telephony Inherently a broadcasting facility

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

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

Infrared Non coherent infrared light used as carrier 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