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

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

3 Overview Media:  Guided - wire  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

4 Design Issues Key communication objectives are:  High data rate  Low error rate  Long distance  Bandwidth economy: Tradeoff - Larger for higher data rates - But smaller for economy 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

5 The Electromagnetic Spectrum 10 KHz 100 MHz

6 Standard Multiplier Prefixes to exa- E = 1,000,000,000,000,000,000 peta- P = 1,000,000,000,000,000 tera- T = 1,000,000,000,000 giga- G 10 9 = 1,000,000,000 mega- M 10 6 = 1,000,000 kilo- K 10 3 = 1,000 milli- m = micro = 0.000,001 nano- n = 0.000,000,001 pico- p = 0.000,000,000,001 femto- f = 0.000,000,000,000,001 atto- a = 0.000,000,000,000,000,001

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

8 Study of Transmission Media Physical description Main applications Main transmission characteristics

9 Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

10 Transmission Characteristics of Guided Media: Overview 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/kmUp to 9 km Optical fiber186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km40 km Larger Operating Frequencies Lower Attenuation  Same attenuation (except with loading) Fewer Repeaters

11 Twisted Pair (TP)

12 UTP Cables unshielded

13 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)

14 Twisted Pair - Pros and Cons Compared to other guided media Pros: 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)

15 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)  EM Interference: Crosstalk, Impulse noise, Mains interference, etc.

16 Attenuation in Guided Media Thinner Wires

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 WK 7

18 STP: Metal Shield

19 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)

20 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  STP)  Up to 300MHz

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 (The better the performance) “NEXT” attenuation is a desirable attenuation- The larger the better!

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!

23 Newer 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) UTP: Unshielded Twisted PairFTP: Foil Twisted PairSSTP: Shielded-Screen Twisted Pair

24 Coaxial Cable Physical Description: Designed for operation over a wider frequency rage cm

25 Physical Description

26 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

27 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 Remaining limitations:  Attenuation  Thermal and inter modulation noise

28 Attenuation in Guided Media

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

30 Optical Fiber A thin (2-125  m) 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 remains within the fiber through multiple reflections  Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture  Reasonable losses with multi- component glass and with plastic Pure Glass Multi- component Glass Plastic Cost, Difficulty of Handling Attenuation (Loss)

31 Optical Fiber: Construction An optical fiber consists of three main parts  Core A narrow cylindrical strand of glass/plastic, with refractive index n 1  Cladding A tube surrounding each core, with refractive index n 2 The core must have a higher refractive index than the cladding to keep the light beam trapped in: n 1 > n 2  Protective outer jacket Protects against moisture, abrasion, and crushing Individual Fibers: (Each having its core & Cladding) Multiple Fiber Cable Single Fiber Cable

32 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 Angles With the Normal

33 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

34 Attenuation in Guided Media Larger Frequency

35 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)

36 Optical Fiber – Benefits, Contd. Greater repeater spacing: Lower cost, Fewer Units  Fiber: ’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

37 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:  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 City Exchange Main Exchange

38 Optical Fiber - Transmission Characteristics Acts as a wave guide for light (10 14 to Hz)  Covers portions of infrared and visible spectrum Transmission Modes: Single Mode Multimode Step Index Graded Index

39 Optical Fiber Transmission Modes Core Cladding n1n1 n2n2 n1n1 n2n2 Shallow reflection Deep reflection Dispersion: Spread in ray arrival time Large Smallest Smaller  i <  critical Refraction 2 ways: v = c/n n 1 is made lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays Curved path: n is not uniform- decreasing

40 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 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

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

42 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 Separated at destination by filters Example: Gbps each  10 Tbps total data rate

43 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 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

44 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

45 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  Localized communications (confined spaces)

46 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

47 Radiation Pattern Power radiated in all directions, but usually not with the same efficiency Isotropic 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 Isotropic Directional

48 Parabolic Reflective Antenna 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. The larger the antenna (in wavelengths) the better the directionality  so, using Higher frequency is advantageous Focus Parabola WK 8

49 Parabolic Reflective Antenna Axis WK 8

50 Antenna Gain, G A measure of antenna directionality Power output 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 A e :  Related to size and shape of antenna  Determines the antenna gain, A e is the effective area

51 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

52 Terrestrial Microwave Parabolic dish 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 sensitive to antenna alignment Applications:  Long haul telecommunications Many voice/data channels over long distances between large cities, possibly through intermediate relays: Competes with cable and fiber  Short wireless links between buildings: CCTV links Links between LANs in different buildings  Cellular Telephony

53 Terrestrial Microwave: Transmission Properties GHz Higher f Advantages:  Larger bandwidth, B  higher data rate (Table 4.6)  Smaller  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, smaller antenna size)

54 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 =  d dBs,  in dBs/km d’ = distance in ’s

55 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

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

57 b. Satellite Broadcast Link Direct Broadcasting Satellite

58 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

59 Broadcast Radio Omni directional (no need for antenna directionality horizontally)  No dishes  No line of sight requirement  No antenna alignment 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 is larger) Problems caused by omni directionality: Interference due to multi-path reflections  e.g. TV ghost images

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

61 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