Physical Communication Media Wire, cable, and other tangible (touchable) materials used to send communications signals, They are called physical communication media. The physical layer is concerned with network media and signaling. This layer produces the representation and groupings of bits as voltages, radio frequencies, or light pulses. Various standards organizations have contributed to the definition of the physical,electrical,and mechanical properties of the media available for different data communications.
GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. Topics discussed in this section: Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable
Twisted-pair cableA twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together.
Coaxial cableCoaxial cable (or coax) carries signals of higher frequencyranges than those in twisted- pair cable, in part because the two media are constructed quitedifferently. Instead of having two wires, coax has a central coreconductor of solid or stranded wire (usually copper) enclosed in an insulating sheath,
UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.Topics discussed in this section:Radio WavesMicrowavesInfrared
Electromagnetic spectrum for wireless communicationRadio, satellite microwave,, Bluetooth, and infrared light areall different forms of electromagnetic waves that are used totransmit data
1.Ground Propagation:Radio waves travel through the lowest portion of the atmosphere, hugging the earth.2.Sky Propagation:Higher frequency radio radiate upward into the ionosphere where they are reflected back to theearth.3.line-of-sight Propagation: Very high frequency signals are transmitted in straight lines directly from antenna to antenna.
Radio Waveselectromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radiowaves; Between 3 KHz – 1 GHz. Radio waves use omnidirectional antenna. Radio waves used for multicast communication, such as radio and television. Sky Propagation. This makes radio waves a good candidate for long-distance broadcasting such as AM radio.
Microwavewaves ranging in frequencies between 1 and 300 GHz are called microwaves.However, the behavior of the waves, rather than the frequencies, is a bettercriterion for classification. Unidirectional antennasMicrowaves are used for unicast communication such as cellular telephones,satellite networks, and wireless LANs.
Infrared waves Between 300 GHz-400 THz Used for short-range communication. Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer. Line-of-sight propagation.with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm),can be used for short-range communication. Infrared waves, having highfrequencies, cannot penetrate walls. This advantageous characteristic preventsinterference between one system and another; a short-range communicationsystem in one room cannot be affected by another system in the next room.When we use our infrared remote control, we do not interfere with the use ofthe remote by our neighbors.Infrared signals can be used for short-range communication in a closed areausing line-of-sight propagation.
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.
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