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. However, in wireless communication, 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
A twisted pair cable consisted of two copper conductors, each one with its own plastic insulation and twisted together. One wire carries the signal and other is used as a ground reference.
The advantage of twisting is that both cables are equally affected by external influences. So the unwanted signals are cancelled 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. However, 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 are 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 a 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 (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. The outer conductor is also enclosed in an insulator, and the whole cable is protected by a plastic cover.
These cables are categorised 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, its attenuation is much higher compared 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 fibre optic cable transmits signals in the form of light. Optical fibre use reflection to guide light through a channel. It consists of two main parts: core and cladding. The core is denser compared to cladding and is made up of plastic or glass.
Cladding acts as a protective cover to the 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 fibre such as multimode and single mode. Multimode fibre allows multiple beams from a light source move through the core. In multimode step-index fibre, the core density remains constant from the centre to the edges.
However, in multimode graded-index fibre, core density gradually decreases from the centre of the core to its edge. Graded-index fibre creates less distortion in the signal compared to step-index.
There are two types of connectors for fibre optic cables. The SC connector is used for cable TV, and ST connector used for connecting cable to networking devices.
Attenuation in fibre optic cable is very low compared to other two types of cable. It provides very high bandwidth and immunity to electromagnetic interference. Lightweight and higher resistance to tapping make it preferable cable.
Fiber optic cable is often used in backbone networks because of its extensive 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.