Data Rate Limits

A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors:

•     The bandwidth available
•     The level of the signals we use
•     The quality of the channel (the level of noise)

Topics discussed in this section:

•     Noiseless Channel: Nyquist Bit Rate
•     Noisy Channel: Shannon Capacity
•     Using Both Limits
  

Noiseless Channel
For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate:

BitRate=2×Bandwidth×log2L

Where, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and BitRate is the bit rate in bits per second.

Note:
Increasing the levels of a signal may reduce the reliability of the system.

 

Noisy Channel-Shannon Capacity

    In reality, we cannot have a noiseless channel; the channel is always noisy.
    In 1944,Claude Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a noisy channel:

    Capacity=Bandwidth×log2(1+SNR)

    Where, bandwidth is the bandwidth of the channel,SNR is the signal-to-noise ratio, and capacity is the capacity of the channel in bits per second.

 

Note:
The Shannon capacity gives us the upper limit; the Nyquist formula tells us how many signal levels we need.

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