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:

1. The bandwidth available
2. The level of the signals we use
3. 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.