Transmission medium is the physical layer or path or link or way through which the data and information are transmitted over the networks. A transmission medium can be broadly defined as anything that can carry information from a source to a destination. It is usually a free space, metallic cable or fiber optic cable.
In telecommunication, transmission media can be divided into two broad categories;
- Guided Media
- Unguided Media
>>> Short description of above points <<<
I).Guided Media
Guided media are those that provide the conduit from one device to another and includes twisted pair cable, co-axial cable, fiber optic cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium.
There are several guided media available in the markets given as below;
1). Twisted pair cable
Twisted pair cable consists of two insulted strands of copper wire twisted around each other to form a pair. One of the wire is used to carry signals to the receiver and the other is used only as a ground reference. The receiver uses the difference between the two.
In addition to the signal sent by the sender on one of the wires, interference and crosstalk many affect both wires and creates unwanted signal.
The purpose of twisting wire is to eliminate electrical interference fro mother wires and out side resources. Twisting the wire cancels any electrical noise from the adjacent pair. The more twist per linear foot, the greater the effect.
There are two types of twisted pair cable.
a). Shielded twisted pair (STP)
The only difference between STP and UPT is that STP has a foil or wired braid wrapped around the individual wires of the pair. The shielding designed to minimized electromagnetic interference radiation and succeptibility to crosstalk. The STP cable sues a woven copper braided jackets which is a higher quality more protective jacket than UTP.
b). Unshielded twisted pair (UTP)
As the name implies UPT cabling is twisted pair cabling that contains no shielding. It is the most common twisted pair cable used in communication.
2)Co-axial Cable
It is made up of two conductors that share the same axis, the center is a copper wire that is insulated by a plastic coating and then wrapped with an outer conductor usually a wire braid. This outer conductor around the insulation serves as electrical shielding for the signal being carried out by the inner conductor. A tough insulating plastic tube outside the outer conductor provides physical and electrical protection. It carries the signal of high frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently.
Categories of co-axial cable
| Categories | Impedence | Use |
| RJ-59 | 75 Ohm | Cable TV |
| RJ-58 | 50 Ohm | Thin Ehternet |
| RJ-11 | 50 Ohm | Thick Ethernet |
3)Fiber Optics
A fiber optic cable is made up of light conducting glass or plastic fibers. It can carry data signals in the form of the pulses of light. The plastic core cables are easy to install but do not carry signals as far as glass core cables. Optical fibers use total internal reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead being refracted into it.
#Propagation Mode of fiber optics
Current technology supports two modes of propagation: multimode and single mode for propagating light along optical channels, each requiring fiber with different physical characteristics. Multimode can be implemented in two forms: step-index or graded index.
1).Multi mode propagation:
It is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the structure of the core. It is further divided into two categories;
i) Multi-mode step-index fiber
In this mode, the density of the core remains constant form the center to the edges. A beam of light moves through this constant density in a straight line until ti reaches the interface of the core and cladding. At the interface, there is an abrupt change due to the lower density; this alters the angle of the beams motion. The term step-index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber.
ii) Multi-mode graded-index fiber
It decreases this distortion of the input through the cable. The word index here refers to the index of refraction. We know that the index of refraction is related to the density. A graded index fiber, therefore, is one with varying density. Density is highest at the center of the core and decreases gradually to its lowest at the edge.
2). Single mode propagation
Single mode use step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The single mode fiber itself is manufactured with a much smaller diameter than that of multi-mode fiber and with substantially lower density. The decrease in density results in a critical angle that is close enough to 90 degree to make the propagation of beams almost horizontal. In this phase, propagation of different beams is almost identical and delays are negligible. All the beam arrive at the destination together and can be recombined with little distortion to the signal. | 
Types