EE354 : Communications System I

Lecture 13-15: Transmission media

Aliazam Abbasfar

OutlineTransmission media

Communications systemsWireline (wired)

Telephony (voice, fax, modem, DSL) Ethernet/LAN Cable TV Backplane copper links

Wireless (Electromagnetic) Over the air communication Radio and TV broadcast WLAN Cellular Radar

Fiber optics High speed long haul data communication High traffic data transfer

Transmission mediaOpen wire

Twisted pair

Coaxial cable

Optical fiber

Air

Open-wire

On utility poles share power line routes

Interference limited (EMI)Limits the BW too

Transposition to reduce interferenceEarly twisting scheme4 twists per Km

Environmental effects

Twisted pair cableUnshielded twisted pair (UTP)

Shielded twisted pair (STP)

Transmission parameters• Lumped model• for a unit length

• System response H(f) = e-gL = + g a j b =

Loss (dB) = 20 log10e a L = a’ L

phase change = b L

• Loss increases with frequency

))(( jCGjLR

Interference Interference is the main limitation (BW)

Twisting reduces interference Shielding further reduces interference

Cross talk (X-Talk) Near end cross talk (NEXT) Far end cross talk (FEXT)

NEXT FEXT

Cable length -- Space between pairs

NEXT and FEXT decreases with frequency f1.5 and f2

ApplicationsCommunication networks

Subscriber lines Analog (Voice) Digital (ADSL, HDSL, VDSL)

Links (E1,T1)

Computer networksLAN

Cat 3 < 10 Mbps Cat 4 < 100 Mbps Cat 5 < 150 Mbps Cat 6 < 350 Mbps

Coaxial cable

Low cross talk Increases with frequency

High speed/Long linksLong distance communication linksCable TVWAN/MAN

Wireline media comparisonMedium Freq. range Loss (dB/Km) @ freq

Open wire 0-150 KHz 0.030.2

1 KHz200 KHz

Twisted pair(Loaded)

0-1 MHz 0.7(0.2)

1 KHz(3.5 KHz)

Cat 6 1-250 MHz 21 60

1 MHz10 MHz

Coax 0-500 MHz 7 10 MHz

Optical FiberStructure

1. Core (8 µm)2. Cladding (125 µm)3. Buffer (250 µm)4. Jacket (400 µm)

Optical cable

Fiber opticsOptical propagation

Refraction indexncore > ncladding

Fiber typesStep indexGraded index

Single modeMulti mode

Optical fiber Loss

Absorption Scattering Connectors

Low loss : 0.2 dB/Km Color dependent

Dispersion Multi modes = different paths Refraction index is frequency (color) dependent

High bandwidth ( > 1 GHz)

Wavelength division multiplexing (WDM) DWDM Data rate > TB/s in a single fiber !

Wireless communication Frequency allocation needed in shared environment

To avoid interference Spectrum is a very valuable resource

Band allocation to applications Government regulations and policies ITU coordinates between nations

Freq band: 3-30KHz Very low freq. (VLF) 30-300KHz Low freq. (LF) 300K-3MHz Medium freq. (MF) 3-30MHz High freq (HF) 30-300MHz Very high freq (VHF) 300M-3G Ultra high freq (UHF) 3-3GHz Super high freq. (SHF)

Electromagnetic waves propagationsGround waves travels along the

surface of the earth ( freq < 2 MHz)

Sky waves reflected by ionosphere Very variable – seasonal

Angle and loss of reflection Freq < 30 MHz

Line of sight (LOS) No reflection or refraction

Non Line of sight Local reflections/refractions

Wireless issuesPath loss

Fading

Mobility

Interference

Satellite systems LEOs

Lower power Smaller delay Need many satellites

Shift towards LEOs in 1990 Global domination Compete with cellular systems Failed miserably (Iridium )

Big, power hungry mobile terminals

Global Positioning System (GPS) Satellite signals used to pinpoint location Popular in cars, cell phones, and navigation devices

Natural area for satellite systems is broadcasting Now operate in 12GHz band 100s of TV and radio channels All over the world

ReadingCarlson Ch. 1

Proakis Ch. 1