A transmission medium is required to carry information from a source to a destination. The information is usually a signal that has to travel a long distance. For this, the transmission media can be wired or wireless. In wired transmission, the signal travels along the cable from one device to another. But in wireless transmission the electromagnetic waves are transmitted without using a physical conductor. There are three types of cables such as.

1)Twisted pair cable
2) Coaxial cable
3) Fiber optic cable

Twisted-Pair Cable

A twisted pair cable consists of two copper conductors, each one with its own plastic insulation and twisted together. One wire carries the signal and other is used as ground reference. The advantage of twisting is that both wires are equally affected by external influences. So the unwanted signals are canceled out as the receiver calculates the difference between signals in two wires.

This cable is of two types such as.
1- UTP (unshielded twisted pair)
2- STP (shielded twisted pair)

STP cable has one extra metal shield covering the insulated twisted pair conductors. But this is absent in UTP cables. The most common UTP connector is RJ45.

The unshielded twisted pair cable is classified into seven categories based on cable quality. Category 1 of cables is used in telephone lines with data rate around 0.1 Mbps. Whereas Category 5 used in LANs having 100 Mbps data rate.

Performance of twisted-pair cable is measured by comparing attenuation versus frequency. Attenuation increases with frequency above 100 kHz.
These cables are used in telephone lines to provide voice and data channels. DSL lines and Local area networks also use twisted pair cables.

Coaxial Cable

Coaxial cable (coax) carries high frequency signals than twisted-pair cables. Coax has a central core conductor of solid wire enclosed in an insulator, which is covered by an outer conductor of metal foil. This outer conductor completes the circuit. Outer conductor is also enclosed in an insulator, and the whole cable is protected by a plastic cover.

These cables are categorized by RG (radio government) ratings. RG-59 used for Cable TV, RG-58 for thin Ethernet and RG-11 for thick Ethernet. The connector used in these cables is called BNC connector; it is used to connect the end of the cable to a device. 
Though the coaxial cable has higher bandwidth, but its attenuation is much higher compare to twisted-pair cables. It is widely used in digital telephone networks where a single cable can carry data up to 600 Mbps. Cable TV networks use RG-59 coaxial cable. Traditional Ethernet LANs also use this cable.

Fiber-Optic Cable

A fiber optic cable transmits signals in the form of light. Optical fiber use reflection to guide light through a channel. It consists of two main parts: core and cladding. Core is denser compare to cladding and is made up of plastic or glass. Cladding acts as a protective cover to core. The difference in density of core and cladding is such that a beam of light moving through the core is reflected off the cladding, instead of being refracted into it.

Two modes of propagation of light are possible in optical fiber such as: multimode and single mode. Multimode fiber allows multiple beams from a light source move through the core. In multimode step-index fiber, the core density remains constant from the center to the edges. But in multimode graded-index fiber, core density gradually decreases from the center of the core to its edge. Graded-index fiber creates less distortion in the signal compare to step-index.

There are two types of connectors for fiber optic cables. The SC connector is used for cable TV, and ST connector used for connecting cable to networking devices. Attenuation in fiber optic cable is very low compare to other two types of cable. It provides very high bandwidth and immunity to electromagnetic interference. Light weight and greater immunity to tapping makes it more preferable cable.

Fiber optic cable is often used in backbone networks because of its wide bandwidth and cost effectiveness. Local area networks such as 100Base-FX network and 100Base-X use this cable. Also it is used by cable TV companies.

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