Transmission Media Unguided Media 1 Unguided Media n

Transmission Media Unguided Media 1

Unguided Media n n n wireless communication No physical conductor Signals are broadcast through air – Available to anyone who has a device capable of receiving them n Radio Frequency Allocation – Eight ranges n VLF (very low frequency) to Extremely high frequency (EHF) 2

Propagation of Radio Waves n Radio wave transmission utilizes 5 types of propagation – Surface – Tropospheric n aprox. 30 miles from the earth’s surface – Ionospheric n above Tropospheric, below the space – Line-of-sight – Space 3

Radio Frequency Allocation 4

Types of Propagation Surface Propagation Ionosphere Propagation Troposphere Propagation Line-of-sight Propagation Space Propagation 5

Surface Propagation n n Radio waves travel through the lowest portion of the atmosphere Signals emanate in all directions – From transmitting antenna – Follow the curvature of the planet n Distance depends on the amount of power in signal 6

Troposhperic Propagation n Works in two ways – A signal is sent from antenna to antenna (line-ofsight) – A signal is broadcast at an angle into the upper layers of the troposphere, then it is reflected back down to the earth’s surface n Longer distance to cover 7

Ionosphere Propagation n Signals radiate upward into the ionosphere Reflected back to earth The density difference between the troposphere and the ionosphere cause each radio wave to speed up and change direction, bending back to earth 8

Line-of-Sight Propagation n n Very high frequency signals are transmitted in straight lines directly from antenna to antenna Antennas facing each other, tall enough 9

Space Propagation n Use satellite relays instead of antenna Signals are received by satellite Satellite rebroadcasts the signal back to earth 10

Radio Frequency Ranges n Very Low Frequency (VLF) waves – – – Propagated as surface wave, through air Do not suffer much attenuation in transmission Susceptible to the high levels of atmosphere noise (heat and electricity) – Used mostly for long-range radio navigation – Frequency range : 3 -30 KHz 11

Radio Frequency Ranges n Low Frequency (LF) waves – – – Propagated as surface waves Used for long-range radio navigation Attenuation is greater during the daytime n (absorption of waves by natural obstacles increases) – Frequency range : 30 -300 KHz 12

Radio Frequency Ranges n Middle Frequency (MF) waves – – – – Propagated in the troposphere These frequencies are absorbed by the ionosphere Absorption increases during the daytime Most of MF transmission rely on line-of-sight antennas to increase control and avoid absorption problem Used in AM radio, maritime radio Frequency range : 300 KHz – 3 MHz (AM Radio : 535 KHz – 1605 MHz) 13

Radio Frequency Ranges n High Frequency (HF) waves – Use ionosphere propagation – Reflected back to earth – Used in n amateur radio (ham radio), n international broadcasting , n military communication , n Long-distance aircraft n Ship communication n Telephone, telegraph 14

Radio Frequency Ranges n Others – – VHF (Very High Frequency) UHF (Ultra High Frequency) SHF (Super High Frequency) EHF (Extremely High Frequency) 15

Terrestrial Microwave n n n Do not follow the curvature of the earth Require line-of-sight signal transmission and reception equipment Distance depends of the height of the antennas – The taller antenna, the longer distance n Microwave signals propagate in one direction at a time – Two frequencies are required for two-way communication 16

Repeaters n n n Repeaters are used to increase the distance by terrestrial microwave Signal received by one antenna is relayed to the next antenna Used in telephone system worldwide 17

Antennas n Two types – Parabolic dish – Horn n Parabolic dish antenna n Horn antenna 18

Satellite Communication n n Similar to line-of-sight microwave transmission Satellites acting as super tall antennas and repeaters 19

Cellular Telephony n n n Designed to provide stable communications connections between tow moving devices, Or between one mobile unit and one stationary (land) unit A service provider must be able to – – – track a caller Assign a channel to the call Transfer the signal from channel to channel as the caller moves out the range of one channel into the range of another 20

Cellular Telephony n To make tracking possible – Cellular service area is divided into small regional called cells 21

Transmission Impairment impairment attenuation distortion noise 22

Transmission Impairment n Attenuation – Loss of energy n Some of the energy is converted to heat – Amplifiers are used to amplify the signal n Distortion – Signal changes its form or shape n Frequencies change n Noise – – – Thermal noise Induced noise Crosstalk, etc. 23

Performance n Three concepts to measure performance – Throughput – Propagation speed – Propagation time n Throughput – How fast data can pass through a point n Propagation speed – The distance a signal or bit can travel through a medium in one second – Depends on the medium and the frequency of the signal 24

Performance n Propagation time – Measure the time required for a signal to travel from one point of the transmission medium to another – Calculated by dividing the distance by the propagation speed Propagation time = distance/propagation speed 25