Carriers and human communications media (e.g Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), radio, television, cable television, interactive media)
Communications
For communications to be effective, there needs to be a source, a means of transport of data and a receiver.
The "Source" is the place from which the communications starts. This can be a person speaking, a space probe sending data to earth from a million kilometers out in space or a computer sending data over a network cable at 1 billion bits a second.
The "Means of Transport" can be the air that allows the transmission of sound by compression and decompression of air, the electromagnetic radiation that supports radio waves through space, or the voltage differences in a wire that represents the binary values of 0 and 1.
The "Receiver" is the opposite of "Source". It needs to translate the signals being sent from the source via the transport mechanism. The receiver must be able to determine when data is sent and if the received data is correct.
Carriers
To allow the transmission of Data, a carrier is required. For sound, air is the carrier. It is compressed and decompressed to allow sound to travel to the receiver. The receiver has a facility to detect these changes on pressure (ear) which is then transformed into a perceived sound. For radio, the carrier is an electromagnetic wave onto which the data is "attached" usually my modulating the Frequency or the Amplitude of the wave. For a digital network system, the wire is the carrier and the change in the voltage applied to the wire allows for data transfer.
Within the Public Switched Telephone Network (PSTN), the carrier is usually a twisted wire pair that transmits sound waves to, and recieves sound waves from, two communications devices. To carry digital data, one device sends a carrier frequency to the other devices and then the frequency is modulated in relation to the binary value of the data to affect the transmission of data.
When an Integrated Services Digital Network (ISDN) is used, the data is transmitted in digital form between the two devices by changing the voltage on the line to represent the transmission of binary data. This allows a faster data rate of transfer than an analogue system. The transmission medium can be twisted pair and/or fiber optics. (Ref Page 166 Capron)
Radio transmission requires no wires between sender and receiver. Older systems relied on the analogue transmission of data with similar traits to that of the analogue Plain old telephone system (POTS). New digital services are rapidly taking over the transmission of data using broad spectrum and microwave radio transmission systems.
Television systems that rely on cable technology also offer a carrier system that enables the transmission of data. These cables are similar in construction to UTP cables and normally provide television services to the home.
Mike Leishman