16. Transmission Media - Unguided (cont'd)

Introduction to Data Communications
16. Transmission Media - Unguided (cont'd)

16b. Radio Frequencies
(see table below)

Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

16c. Microwave

Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

(see image below)

Radio frequencies

The frequency spectrum operates from 0 Hz (DC) to Gamma Rays (1019 Hz).

Name	Frequency (Hertz)	Examples
Gamma Rays	10^19 +
X-Rays	10^17
Ultra-Violet Light	7.5 x 10^15
Visible Light	4.3 x 10^14
Infrared Light	3 x 10^11
EHF - Extremely High Frequencies	30 GHz (Giga = 10^9)	Radar
SHF - Super High Frequencies	3 GHz	Satellite & Microwaves
UHF - Ultra High Frequencies	300 MHz (Mega = 10^6)	UHF TV (Ch. 14-83)
VHF - Very High Frequencies	30 MHz	FM & TV (Ch2 - 13)
HF - High Frequencies	3 MHz2	Short Wave Radio
MF - Medium Frequencies	300 kHz (kilo = 10^3)	AM Radio
LF - Low Frequencies	30 kHz	Navigation
VLF - Very Low Frequencies	3 kHz	Submarine Communications
VF - Voice Frequencies	300 Hz	Audio
ELF - Extremely Low Frequencies	30 Hz	Power Transmission