A transmission medium (plural media) is one which carries a signal from one computer to another. It is also known as communication channel. Transmission medium can be wired or wireless. We also name them as Guided and Unguided Media respectively.
Wired transmission media includes twisted pair cable, Ethernet cable, coaxial cable and optical fibre whereas wireless transmission media includes microwave, radio wave, satellite, infrared, Bluetooth, WIFI etc.
Wired Transmission Media
The data signal physically gets transferred from the transmitting computer to the receiving computer through the wired transmission medium. Some of the wired transmission media are discussed below
1. Twisted Pair Cables
It is generally used for telephone communications in offices and also in modern Ethernet networks especially in LANs . For telephonic communication a Voice Grade Medium (VGM) cable is used but for LAN applications a higher quality cable known as Data Grade Medium (DGM) is used.
- It is capable of carrying a signal over long distances without amplification.
- It is simple, low weight, easy to install and easy to maintain.
- It is an adequate and least expensive medium for low speed (up to 10 mbps) applications where the distance between the nodes is relatively small.
- It can easily pick up noise signals.
- Being thin in size, it is likely to break easily.
- it is unsuitable for broadband applications.
Types of Twisted Pair Cables
There are two types of twisted pair cables available. These are:
- Shielded Twisted Pair(STP) Cable.
- Unshielded Twisted Pair(UTP) Cable
The STP cable comes with shielding of the individual pairs of wires, which further protects it from external interference and crosstalk. But STP is heavier and costlier than UTP and also requires proper grounding at both the ends.
2. Coaxial Cables
It is the most commonly used transmission media for LANs. It consists of solid wire cores surrounded by one or more foil or wire shields, each separated by some kind of plastic insulator.
- Data transmission characteristics are better than that of twisted pair.
- It can be used for broadband communication i.e. several channels can be transmitted simultaneously.
- It offers high bandwidth (up to 400 mbps)
- It can be used as the basis for shared cable network.
1. It is expensive as compared to twisted pair cables
Types of coaxial cables:
The two most common types of cables are Thicknet and Thinnet. Whereas thicknet is thicker and its cable segments can be up to 500 metres long , the thinnet on the other hand is thinner and it can have a maximum segment length of 185 metres.
3. Optical Fibres
These consists of thin strands of glass or glass like material which are so constructed that they carry light from a source at one end of the fibre to a detector at the other end. The light sources used are either light emitting diodes (LEDs) or laser diodes (LDs). The data to be transmitted is modulated onto a light beam using frequency modulation techniques. At the receiver’s end, the signals are demodulated.
Optical fibres offer a very high bandwidth and this makes it capable of multichannel communication.
The Optical fibre consists of three layers:
- Core -glass or plastic through which the light travels
- Cladding -covering of the core that reflects the light back to the core
- Protective (Buffer) coating-protects the fibre cable from hostile environments
- It is immune to electrical and magnetic interference.
- It is highly suitable for harsh industrial environments.
- It guarantees secure transmission and has a very high transmission capacity.
- It can be used for broadband transmission where several channels can be handled in parallel.
- It is difficult to install and maintain since they are quite fragile.
- It is most expensive of all cables.
- Connecting two fibres together or even connecting the light source with the cable is a difficult process. Hence connection loss is a common problem
- Light can reach the receiver out of phase.
Fibre Optic cable can be of two types:
- Single node fibre optic cable: It supports a segment length of up to 2kms and bandwidth of up to 100Mbps
- Multinode fibre optic cable: It has a segment length of 100kms and bandwidth of 2Gbps
Wireless Transmission Media
Wireless or unbounded or unguided media transport electromagnetic waves without using a physical conductor. The signals are broadcasted through air or water and thus are available to anyone that has a device capable of receiving them. Some of the wireless media are:
Infrared communication requires a transceiver (a combination of transmitter and receiver) in both devices that communicate. Infrared communication is playing an important role in wireless data communication due to the popularity of laptop computers , personal digital assistants(PDAs) , digital cameras , mobile phones , pagers and other devices but being a line-of-sight transmission , it is sensitive to fog and other atmospheric conditions.
- Since it is having short range of communication hence it is considered to be a secure mode of transmission.
- It is quite inexpensive transmission medium.
- It can only be used for short range communication
- Infrared wave transmission cannot pass through obstructions like walls, buildings etc.
In case of radiowave transmission, certain radio frequencies are allocated to private/government organizations for direct voice communications. Each radio signal uses a different frequency and this differentiates it from others. The transmitter takes some message, encodes it and then transmits it with radio wave. The receiver on the other hand
receives the radio waves and decodes it. Both the transmitter and the receiver use antennas to radiate and capture the radio signal. Radio transmission is widely used by delivery services, policemen, security personals etc.
- It is easy to communicate through radio waves in difficult terrains since there is no need of digging and laying cables.
- Radio waves can travel through long distances in all directions. Also they can easily pass through obstacles like a building so they can be used for both indoor and outdoor communication.
- It is susceptible to weather effects like rain, thunderstorm etc.
- Data transmitted through radiowaves is not secure.
This type of transmission uses high frequency radio signals to transmit data through space. Like radio waves, microwaves can pass through obstacles viz. buildings, mountains etc. Microwaves offer a line of sight method of communication. A transmitter and receiver of a microwave system are mounted on very high towers and both should be visible to each other (line of sight).It is generally used for long distance telephonic communications.
- Microwave transmission does not require the expense of laying cables
- It can carry 25000 voice channels at the same time.
- Since no cables are to be laid down so it offers ease of communication over difficult terrains like
- Signals become weak after travelling a certain distance and so require amplification. To overcome this roblem, repeaters are used at regular intervals (25-30 kms). The data signals are received, amplified and then retransmitted. This makes it a very expensive mode of communication Installation and maintenance of microwave links turns out be a very expensive affair.
- The transmission is affected by weather conditions like rain, thunderstorms etc.
Satellites are an essential part of telecommunications systems worldwide today. They can carry a large
amount of data in addition to TV signals.Satellite communication is a special use of microwave transmission system.Typical data transfer rates are 1 to 10 Mbps.
Satellites are especially used for remote locations, which are difficult to reach with wired
infrastructure. Also communication and data transfer on internet, is only possible through satellites.
- Satellite communication is very economical keeping in mind the fact that the area covered through satellite transmission is quite large. For e.g., satellites used for national transmission are visible from all parts of the country.
- Transmission and reception costs are independent of the distance between the two points.
- Placing the satellite into its orbit involves very high cost.
- Since signals sent to a satellite are broadcasted to all receivers, so necessary security measures have to be taken to prevent unauthorized tampering of data.
- Transmission is affected by weather conditions like rain, thunderstorm etc.