Transmission Media Used in Computer Networks


A small description of the various types of Physical medias used in the computer networks. Emphasis is laid on the structure and applications of the transmission medias.

In computer networks we have various network architectures which allow the flow of data and information from one node to another. OSI Reference model, TCP/IP Reference model are such network architectures. In both these models the actual communication takes place through the physical communication media located below the physical layer of each model. Various physical media can be used for the actual transmissions which are described as follows:



Magnetic Media



It is one of the most common and convenient ways to transfer the data from one machine to another. The data from one computer is written on a floppy disk or a magnetic tape and then transferred to the destination machine where it is read back.


Twisted Pair



It consists of two stiff copper wires each of which is about 1mm thick. The copper wires are twisted together in the helical form giving them the appearance like a DNA molecule. The objective of this twisting is to reduce the electric interference between two or more twisted pairs which are located close to each other. Twisted pairs can run several kilometers without amplification. Twisted pairs are used for both analog and digital transmission. The twisted pairs are used in the telephone systems. All the telephones are connected to the telephone exchange by means of twisted pairs. Due to the adequate performance of the twisted pairs they are widely used.

There are further two types of twisted pair viz. Category 3 and Category 5. Category 3 twisted pair consisted of two copper wires twisted together. Four such pairs are grouped together in this type in a plastic sheath. While in case of Category 5 twisted pair, the structure is similar to Category 3 but there are more twists per centimeter. There is a Teflon insulation that reduces cross talk.


Coaxial Cables



Coaxial cables provide better shielding than the twisted pair therefore they are used for communication over long distances that too at a high speed. There are two types of coaxial cables viz 50 ohm cable and 75 ohm cable. 50 ohm cable is used for the digital transmission and 75 ohm cable is used for the analog transmission. A coaxial cable consists of a stiff copper wire which is surrounded by an insulating material. The insulator is covered by a cylindrical conductor in the form of braided mesh. This entire structure is enclosed in a protective plastic sheath. Coaxial cables are widely used in the cable television and local area networks.


Fiber Optic Cables



In case of fiber optics there are three components viz. light source, transmission medium and detector. The pulse of light indicates 1 bit while absence of light indicates 0 bit. The Light Emitting Diodes and Semiconductor Lasers acts as the source of light. The transmission medium is a thin glass fiber. Photo conductor acts as the detector.

The fiber optic cables are based on the principle of total internal reflection. Fiber optic cables have a glass core surrounded by a glass cladding. The refractive index of the glass cladding is smaller than the glass core so that the entire light propagates through the core. The glass cladding is surrounded by plastic jacket for the protection.


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