According to the Shannon-Hartley theorem, what factor dictates that a noisier environment lowers the maximum achievable data rate for a fixed bandwidth?

Answer

Signal-to-Noise Ratio (SNR).

The Shannon-Hartley theorem formula shows that channel capacity is directly related to the signal power to noise power ratio ($S/N$); thus, increasing noise (lowering SNR) reduces the maximum achievable rate.

According to the Shannon-Hartley theorem, what factor dictates that a noisier environment lowers the maximum achievable data rate for a fixed bandwidth?
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