Describes the type of physical system used to carry acommunication signal from one system to another. Examples of transmission media include twisted-pair cable, coaxial cable, and fiber optic cable.

♥ Wire based media (hardwire, or guided), either : 

-electric, like twisted pair cable TP, coaxial cable 

-optic, like fiber optics 

♥ Wireless (softwire, or unguided), like infrared rays, radio waves, microwaves. 

Electromagnetic Spectrum & used frequencies by each media’s transmission technique

♥ Bandwidth

 Higher bandwidth gives higher data rate.

♥ Transmission impairments 

Attenuation limits possible covered distances (acts more for guided media) 

Interference (acts on both categories); for guided media use of proper shielding.

♥ Number of receivers

 In guided media: more receivers (multi-point transmissions) introduce more attenuation.

♥ Twisted pair 

Consists of two metallic copper wires, twisted after a given step. 

Coax cable:

 -base -band cable, 50 band cable Ω impedance, used in Ethernet LANs

     -thick Ethernet (RG213), difficult to install 

     -thin Ethernet (RG58), excellent versatility 

-broad -band cable, 75 cable Ω impedance, used less for LANs, more for CATV or long distance telephone transmissions 

Advantages: goes up to 500MHz, repeaters every 1-2 km Drawback: is a shared broadcast medium, not for full duplex (switched) transmissions => will be replaced by UTP (LANs) or by fiber optics => (long telephony trunks)

For unguided media: higher frequencies give higher transmission data rates Antenna based transmissions:

-directional, antenna-to-antenna focused beam, requiring antennas alignment 

-omnidirectional, beam spread and may be received by many antennas LANs using wireless media present flexibility, easiness in installing and maintenance 

-Terrestrial microwaves

      Use frequency domain of 2-40GHz, offers up to 500MHz analog signal bandwidth, up to 100Mbps digital signal data rate Use of parabolic ‘dish’ => ‘line-of-sight’ transmissions of a focused electro => magnetic beam => existence of a theoretical maximum distance betwe => en antennas:

      

D = 7.14 sqr(K•h),

 

Where h is antenna’s height and K an adjustment factor for waves reflection due to the earth curvature (a 4/3 value may be acceptable) For long hauls => a succession of relay towers

-Satellite microwaves 

Transmissions (directional, station – satellite – station(s)):

 -optimum frequencies domain: 1-10GHz, due to low natural noise interferences (solar, wind, human devices); the most are point-to-point transmissions, referred as 4/6GHz band (the uplink based on 6GHz, the downlink frequencies centred on 4GHz. Today in use 12/14GHz (especially by small earth-stations) and 19/29GHz, offer higher bandwidth, but is need for overcoming attenuation problems. 

Satellite: a microwave relay station, receiving on one frequency band (uplink) and retransmits on another (downlink), avoiding interferences. These frequency channels – transponders. 

Problem: satellite remains stationary with respect to the fixed (usually) earth-stations => equal rotation period as the earth’s (launched for 35,784km height) Satellites on the same orbit, need for an angular displacement of 4° (4/6GHz band) and 3° (12/14GHz band) for no interferences between.

-Broadcast radio waves 

Being omni-directional transmission, radio antennas are not dish-shaped and may be mobile; generally radio waves use frequencies in the range of 3kHz – 300GHz; broadcast radio (telecomms radio) covers VHF and part of UHF band: 30MHz – 1GHz. 

Advantages: -good wave propagation, low reflection and refraction due to ionosphere -line-of-sight transmission obeys same law as terrestrial microwave; an usual value for radio repeaters: 20km 

Drawbacks: -multipath interference, due to reflections from land, water, natural and human-made objects. -radio transmissions allow up to 20Mhz analog signal bandwidth and up to 10Mbps digital signal data rate

-Infrared rays

Infrared comms are based on modulated infrared light, using transceivers; use of THz frequency range; only line-of-sight transmissions => rigid station alignment or passive => surface reflections => no interferences, due to impossibility to penetra => te surfaces => good enough analog signals bandwidth or digital data rate (LANs at 16Mbps). No licences for use of infrared channels.