Transmission Media

TTrraannssmmiissssiioonn MMeeddiiaa Raj Jain Washington University Saint Louis, MO 63131 [email protected]

These slides are available on-line at: http://www.cse.wustl.edu/~jain/cse473-05/

ß Electromagnetic Spectrum ß Transmission Media: Twisted Pair, Coax, fiber, wireless ß Unshielded Twisted Pair (UTP) categories ß Reflection and Refraction ß Antennas: Isotropic, directional, omni-directional ß Terrestrial and Satellite Microwave

ß Infrared light is used for optical communication

ß Guided: ß Twisted Pair ß Coaxial cable ß Optical fiber ß Unguided: ß Microwave ß Satellite ß Wireless

ß Twists decrease the cross-talk ß Neighboring pairs have different twist length ß Most of telephone and network wiring in homes and offices is TP.

ß Unshielded Twisted Pair (UTP) ß Ordinary telephone wire ß Cheap, Flexible Ω Easiest to install ß No shielding Ω Suffers from external EM interference ß Used in Telephone and Ethernet ß Shielded Twisted Pair (STP) ß Metal braid or sheathing that reduces interference ß More expensive ß Harder to handle (thick, heavy) ß Used in token rings

ß Cat 3 ß Up to 16MHz ß Voice grade found in most offices ß Twist length of 7.5 cm to 10 cm ß Cat 4 ß Up to 20 MHz. Not used much in practice. ß Cat 5 ß Up to 100MHz ß Used in 10 Mbps and 100 Mbps Ethernet ß Twist length 0.6 cm to 0.85 cm ß Cat 5E (Enhanced), Cat 6, Cat 7

ß Higher bandwidth than UTP. Up to 500 MHz. ß Used in cable TV

ß Index of Refraction = Speed of light in Vacuum/Speed in glass = 300 m/µs / 200 m/µs =1.5

Index = 1.48 Refraction

Index = 1.5 Reflection

ß Refracted light bends towards the higher index medium

Dispersion 0

ß A cylindrical mirror is formed by the cladding ß The light wave propagate by continuous reflection in the fiber ß Not affected by external interference Ω low bit error rate ß Fiber is used in all long-haul or high-speed communication ß Infrared light is used in communication

ß Multimode Fiber: Core Diameter 50 or 62.5 µm Wide core Ω Several rays (mode) enter the fiber Each mode travels a different distance ß Single Mode Fiber: 10-µm core. Lower dispersion.

Cladding

Core

ß Step Index: Index takes a step jump ß Graded Index: Core index decreases parabolically

Washington University in St. Louis CSE473s ©2005 Raj Jain 4-15 TTyyppeess ooff FFiibbeerrss IIII ß Dispersion-Shifted Fiber: Zero dispersion at 1310nm EDFAs/DWDM systems operate at 1550 nm Special core profile Ω zero dispersion at 1550 nm ß Dispersion Flattened Fiber: 3 ps/nm/km 1300-1700nm Use 1300 nm now and 1550 in future Low dispersion causes four-wave mixing Ω DSF/DFF not used in multi-wavelength systems

Dispersion Standard DSF 0 DFF

ß 2GHz to 60GHz ß Terrestrial Microwave, Satellite Microwave ß Highly directional ß Point to point ß 30MHz to 1GHz ß Omni-directional ß Broadcast radio ß 3 x 1011 to 2 x 1014 ß Infrared ß Short distance

ß Transmitter converts electrical energy to electromagnetic waves ß Receiver converts electromagnetic waves to electrical energy ß Same antenna is used for transmission and reception ß Omni-Directional: Power radiated in all directions ß Directional: Most power in the desired direction ß Isotropic antenna: Radiates in all directions equally ß Antenna Gain = Power at particular point/Power with Isotropic Expressed in dBi

Omni-Directional Directional Isotropic

Focus

ß Used in Terrestrial microwaves ß Line of sight communication ß 10-60 GHz ß Higher frequencies for higher data rates

ß Parabolic dish ß Focused beam ß Line of sight ß Long haul telecommunications ß Higher frequencies give higher data rates

ß Relay station Ω Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency ß Geo-stationary orbit: Height of 35,784km ß Point to Point or Direct broadcast satellite

ß Omni-directional ß FM radio, UHF and VHF television ß Line of sight ß Suffers from multi-path interference (Reflections)

ß Used in TV remote control IRD port of computers ß Modulate infrared light ß Line of sight (or reflection) ß Blocked by walls

ß Ground wave: Follows contour of earth. Up to 2MHz. AM radio ß Sky wave: Signal reflected (Actually refracted) from ionosphere layer of upper atmosphere. Amateur radio, BBC world service, Voice of America ß Line of sight: Above 30MHz. Density of atmosphere decreases with height. Results in radio waves bending towards earth

Washington University in St. Louis CSE473s ©2005 Raj Jain 4-24 LLiinnee ooff SSiigghhtt TTrraannssmmiissssiioonn ß Free space loss: Signal disperses with distance ß Greater for lower frequencies (longer wavelengths) ß Atmospheric Absorption: Water vapour and oxygen ß Water greatest at 22GHz, less below 15GHz ß Oxygen greater at 60GHz, less below 30GHz ß Rain and fog scatter radio waves ß Multipath: Signal can be reflected causing multiple copies to be received. May be no direct signal at all. May reinforce or cancel direct signal

ß Unshielded twisted-pair (UTP) vs STP ß Single mode and multimode optical fiber ß Optical communication wavelengths ß Isotropic vs omni directional vs directional antennas ß Parabolic antenna for microwave ß Ground wave, sky wave, line of sight

ß Read Chapter 4 of Stallings 7th edition.

ß Submit answer to exercise 4.3 (length of transmission media) of 7th edition by Stallings.