Transmission Lines

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Metallic Cable Transmission Media

Transmission media
-It is included in the lowest layer of the OSI protocol hierarchy – the physical layer. -Transmission medium is simply the path between a transmitter and a receiver in a communications system. Guided Transmission media

-some form of conductor that provide conduit in which signals are contained -the conductor directs the signal
examples: copper wire, optical fiber
Unguided Transmission media
-wireless systems – without physical conductor
-signals are radiated through air or vacuum
-direction – depends on which direction the signal is emitted examples: air, free space
Cable transmission media
-guided transmission medium and can be any physical facility used to propagate EM signals between two locations e.g.: metallic cables (open wire, twisted pair), optical cables (plastic, glass core) Incident and Reflected wave

-Incident voltage is the voltage that propagates from sources towards the load -Reflected wave is the voltage that propagates from the load towards the source. [pic]

Transmission line classifications
1) Balanced Transmission line
- two wire balanced line.
- both conductors carry current. But only one conductor carries signals.


2) Unbalanced Transmission line
- One wire is at ground potential and the other wire is at signal potential - advantage – only one wire for each signal
- disadvantage – reduced immunity to noises
Balanced transmission lines connected to unbalanced transmission lines e.g.: coaxial cable to be connected to antenna

Metallic Transmission Line Types
1)Parallel conductors
2)Coaxial cable
1)Parallel conductors consists of two or more metallic conductors(copper) separated by insulator–air,rubber etc. Most common -- Open Wire
Twin lead
Twisted Pair (UTP & STP)
Open Wire
- two-wire parallel conductors
-Closely spaces by air
-Non conductive spaces support and constant distance between conductors (2-6 inches) -Adv – simple construction
-Disadv– no shielding, high radiation loss, crosstalk
application – standard voice grade telephone
Twin lead
-spacers between the two conductor are replaced with continuous dielectric – uniform spacing -application – to connect TV to rooftop antennas
-material used for dielectric – Teflon, polyethylene
Twisted pair--(UTP & STP)
-formed by twisting two insulated conductors around each other -Neighboring pairs is twisted each other to reduce EMI and RFI from external sources -reduce crosstalk between cable pairs
----Unshielded Twisted Pair
-two copper wire encapsulated in PVC
-twisted to reduce crosstalk and interference
-improve the bandwidth significantly
-Used for telephone systems and local area network

Level 1 (Category 1) -- ordinary thin cables
--for voice grade telephone and low speed data Level 2 (Category 2) -- Better than category 1
--For token ring LAN at txn. rate of 4 Mbps Category 3 -- more stringent requirement than level 1 and 2 -- more immunity than crosstalk
-- for token ring (16Mbps), 10Base T Ethernet (10Mbps) Category 4 -- upgrade version of category 3
-- tighter constraints for attenuation and crosstalk -- up to 100 Mbps
Category 5 -- better attenuation and crosstalk characteristics -- used in modern LAN. Data up to 100Mbps Category 5e -- enhanced category 5
-- data speed up to 350 Mbps
Category 6 -- data speed up to 550 Mbps
-- fabricated with closer tolerances and use more advance connectors...
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