# Modem - Questions and answers

A MODEM has a delay equalizer for receiving a trellis-coded test signal transmitted from an opposite MODEM and compensating for delay distortion of this test signal on the transmission path. The output of this delay equalizer and the input test signal are selectively entered into a demodulator. The output of this demodulator is assigned to predetermined signal point coordinates by a decision circuit. A Viterbi decoder computes a branch metric representing the distance between each assigned point and receives signal point from the output signals of the decision circuit and the demodulator, and figures out the pass metrics of accumulated values based on the branch metrics so computed. A difference signal represents the difference between the maximum and minimum values of the pass metrics from the Viterbi decoder. A first difference signal corresponds to the pass metrics when the delay equalizer is not set for the transmission path, and a second difference signal corresponds to the pass metrics when the delay equalizer is set for the transmission path. A control circuit compares the first and second difference signals and sets the delay equalizer out of the transmission path if the first difference signal is greater than the second, or sets the delay equalizer in the transmission path if the second difference signal is greater than the first. Thus the MODEM automatically decides whether the delay equalizer is to be inserted in the transmission path.

According to SyQuest theorem the sampling rate of frequency is always twice of the maximum frequency Ices=2( fax)

2nd thing is the signal which is sampled, must be have limited bandwidth.

Band width of optical fiber and copper wire is finite so these transmitting media can follow the nicest theorem.

Whenever the signal travels from sender to recover the there is modulating technique are used to convert their signal format . when the recover receives the incoming signal of data

Then there is another kind of signal coupled with original one.the seare the noise signal which causes due to atmospheric damage. Noises are in form of amplitude variation I transmitting signal so in frequency modulation, the resistance to noise is more.

When a voltage level of digital data get constant while, there is a maintain a very low frequency (the frequency is near about zero) , is called as DC component(direct current )

In many kind of encoding scheme , if there is sender send a long string of 0s or long string of 1s(in NRZ-L and UNIPOLAR-NRZ) the receiver difficult to identify the initial of bits and beginning of bit i.e. there is arises a problem of synchronization . Hence we should select the encoding system which responds as a change in every kind of bit pattern to avoid DC component and synchronization

Multiplexing is a technique which allows to simultaneous transmission of multiple signals across the single data link. For creating multiplexing environment and combining the multiple signal, we must follow the facts that the bandwidth of transmitting channel must be greater than connecting devices. On the basis of which we have design following techniques of multiplexing—

1 frequency division multiplexing

2 wavelength division multiplexing

3 time division multiplexing

### MULTIPLEXING TECHNIQUE(COMBINATION)

Each source generate a signal of similar frequency range . inside the multiplexer these similar signal modulates the different carrier frequency(F1.F2.F3) the resulting modulating signal are combined into single composite signal that is sand out over data link that has enough bandwidth to accommodate it.