Shanon capacity in computer network

            Shanon capacity 

Data rate governs the speed of data transmission. A very important consideration in data communication is how fast we can send data, in bits per second, over a channel. Data rate depends upon 3 factors:

• The bandwidth available
• Number of levels in digital signal
• The quality of the channel – level of noise

Two theoretical formulas were developed to calculate the data rate: 

one by Nyquist for a noiseless channel, another by Shannon for a noisy channel.

Noisy Channel : Shannon Capacity –
In reality, we cannot have a noiseless channel; the channel is always noisy. 

Shannon capacity is
used, to determine the theoretical highest data rate for a noisy channel.

Shannon capacity is
used, to determine the theoretical highest data rate for a noisy channel:

Capacity = bandwidth * log2(1 + SNR)

In the above equation, 

Bandwidth is the bandwidth of the channel

SNR is the signal- to- noice ratio

Capacity is the capacity of the channel in bits per second.

Bandwidth is a fixed quantity, so it cannot be changed. Hence, the channel capacity is directly proportional to the power of the signal, as SNR = (Power of signal) / (power of noise).
The signal-to-noise ratio (S/N) is usually expressed in decibels (dB) given by the formula:

10 * log10(S/N)

so for example a signal-to-noise ratio of 1000 is commonly expressed as:

10 * log10(1000) = 30 dB.