PHYSICAL AND WIRELESS TRANSMISSION MEDIA


PHYSICAL TRANSMISSION MEDIA
Physical transmission media used in communications include twisted-pair cable, coaxial cable, and fiber-optic cable. These cables typically are used within or underground between buildings. Ethernet and token ring LANs often use physical transmission media.
Twisted-Pair Cable
One of the more commonly used transmission media for network cabling and telephone systems is twisted-pair cable. Twisted-pair cable consists of one or more twisted-pair wires bundled together (Figure 8-24). Each twisted-pair wire consists of two separate insulated copper wires that are twisted together. The wires are twisted together to reduce noise. Noise is an electri- cal disturbance that can degrade communications.


 Coaxial Cable
Coaxial cable, often referred to as coax (pronounced KO-ax), consists of a single copper wire surrounded by at least three layers: (1) an insulating material, (2) a woven or braided metal, and (3) a plastic outer coating (Figure 8-25).
Cable television (CATV) network wiring often uses coaxial cable because it can be cabled over longer distances than twisted-pair cable. Most of today’s computer networks, however, do not use coaxial cable because other transmission media such as fiber-optic cable transmit signals at faster rates. 

Fiber-Optic Cable
The core of a fiber-optic cable consists of dozens or hundreds of thin strands of glass or plastic that use light to transmit signals. Each strand, called an optical fiber, is as thin as a human hair. Inside the fiber-optic cable, an insu- lating glass cladding and a protective coating surround each optical fiber

Fiber-optic cables have the following advantages over cables that use wire, such as twisted-pair and coaxial cables:
  • Capability of carrying significantly more signals than wire cables
  • Faster data transmission
  • Less susceptible to noise (interference) from other devices such as a copy machine
  • Better security for signals during transmission because they are less susceptible to noise
  • Smaller size (much thinner and lighter weight)

Disadvantages of fiber-optic cable are it costs more than twisted-pair or coaxial cable and can be difficult to install and modify. Despite these limitations, many local and long- distance telephone companies are replacing existing telephone lines with fiber-optic cables, enabling them to offer fiber Internet access to home and business users. 



WIRELESS TRANSMISSION MEDIA


WAVES:Electromagnetic waves are formed when an electric field comes in contact with a magnetic field. They are hence known as ‘electromagnetic’ waves. 


Many users go for wireless transmission media because it is more convenient than installing cables. In addition, businesses use wireless transmission media in locations where it is impossible to install cables. Types of wireless transmission media used in communications include infrared, broadcast radio, cellular radio, microwaves, and communications satellites.


(a) Microwave: Is a form of high frequency radio communication beam that is used to transmit data signals from source to destination focused at a direct line of sight between the receiving and sending dishes.



Microwaves can transmit data at rates up to 4,500 times faster than a dial-up modem.microwave stations or antennas are placed 20 to 30 miles to each other.no buildings or mountains between these stations.


(b) Satellite transmission system: Is a high frequency radio transmission that is geostationary in the orbit. The satellite receives or captures data signals from the earth satellites (Uplink) and transmits them to receiving earth satellite (downlink). satellite communication is placed about 22,300 miles above the earth.
 


(c) Radio transmission: Is a wireless channel that broadcasts data signals in all directions and can be received by anyone with a suitable receiver.

Broadcast radio is a wireless transmission medium that distributes radio signals through the air over long distances such as between cities, regions, and countries and short distances such as within an office or home. Bluetooth, UWB, Wi-Fi, and WiMAX communications technologies discussed earlier in this chapter use broadcast radio signals.

(d) Cellular Radio
Cellular radio is a form of broadcast radio that is used widely for mobile communications, specifically wireless modems and cell phones. A cell phone is a telephone device that uses high-frequency radio waves to transmit voice and digital data messages.
Some mobile users connect their notebook computer or other mobile computer to a cell phone to access the Web, send and receive e-mail, enter a chat room, or connect to an office or school network while away from a standard telephone line. .

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