Nyquist bandwidth criterion

According to Nyquist bandwidth constraint for transmission without ISI , the minimum channel bandwidth should be R/2 symbols /sec, where R symbols /sec is the transmisson rate.
If this is correct then i want to know that if this channel cannot support full transmission rate then do we use this model ??

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What is ISI?

Nyquist says that if you have a discrete signal that should transfer all relevant frequencies, you should use a sample frequency that is at least double the max relevant frequency.

ISI is INTERSYMBOL INTERFERENCE

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Ah, okay. I was not aware of that.
My remark remains the same though.
If you have a transfer rate (as a frequency) of R, you can only transfer frequencies up to R/2 without loss of information.
Or am I misunderstanding your question?

ok
and what can be the bandwidth of the channel in terms of R to support the transmission ?

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R/2 as Nyquist states.

okay ..
So we are sampling a signal whose frequency is R at a frequency of R/2 and which should be the minimum bandwidth of channel...
AM I RIGHT ?

Is this statement true ?
For ideal Nyquist pulses (sinc), the required bandwidth is only Rs/2 to support an Rs symbol rate

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I'm not sure what you're asking, so I'll just state what I know.

When you want to transmit a signal using a bandwidth signal at some frequency R, what you do is, you put your signal through a low-pass filter, filtering out all frequencies above R/2.
And then you transmit the signal.
Otherwise you lose information.

okay