1. A codec is a device or computer program capable of encoding and/or decoding a digital data stream or signal.Coder part of the codec encodes the value i.e it converts from analog to digital signals whereas demodulator part decodes the value i.e converts back the digital signal to analog. Therefore,these two parts are different in codec. Codec does the job opposite to modems i.e convert analog to digital first with help of coder and then back to analog from digital with demodulator.
2. The Nyquist theorem is not applicable to copper wire.Because it is applicable to noiseless channel. The bandwidth of a copper wire depends on the cable quality, length, and signal to noise ratio of data transmitted. As we see copper bandwidth is directly depends upon the signal to noise rationwhich concept is given by Shannon'scapacity formula relating to SNR in case of noisy channels.
Also it is not applicable to fibre optic cable because in fiber optic cable we transmit the pulse of light and the light is produceddue to the motion of molecules throughout the system that creates much noise. To calculate the noise created per signal we uses the concept of signal to noise ratio that is given by shanon;s not by nyquiest.
3. Noise can easily change the amplitude of the signal rather than frequancy . So, Noise affects more AM because amplitude of carrier wave will change easily and so will the signal.But in FM frequency of carrier wave is not changed easily and therefore not affected much. In PM phase changes but not the amplitude therefore it can be affected by noise but not more as in case of AM.
5. DC component contains all positive voltages which do not get cancelled. They cannot be sent over long distances and even cannot work in transformers. In unipolar coding, it contains dc component and therefore cannot be used for communication over large distances.It has an unbounded DC component, gives too many errors on systems. But in other codings like NRZL and NRZI , Manchester and differential Manchester, Bipolar schemes dc component is not present. We can reduce DC components by taking one side voltage positive and other side negative.
1: signal to noise ratio: SNR = signal power/noise power.
2 : Bit error rate: BER = (number of improperly detected bits during time t) /(total number of bits detected during time t )
7. For copper wires most suitable multiplexing techneque is Frequency division because signal of different frequencies will be sent easily and will be demodulated easily.
For optical fibre multiplexing involved is Wavelength-division multiplexing (WDM) because it capable of multiplying the available capacity of an optical fiber by adding new channels. Each channel on a new wavelength of light. Also, optical fibre transmits light signal therefore different wavelength lights can be sent easily.
8. Guided media are more secure than unguided media. Also in guided media the coaxial wires are most safest because they are shielded twice and no noise or other inference is possible. Optical fibres and twisted cables are less secure.
But in unguided media , we transfer data through infrared raya or microwaves by 3 ways :-
(a) Line of sight
Thus,there are chances of interference and these are not secure compared to guided media.