With NRZ, the signal may cycle between the positive and negative levels up
to every 2 bits (in the case of alternating 1s and 0s). This means that we need a
bandwidth of at least B/2 Hz when the bit rate is B bits/sec. This relation comes
from the Nyquist rate . It is a fundamental limit, so we cannot run NRZ
faster without using more bandwidth. Bandwidth is often a limited resource, even
for wired channels, Higher-frequency signals are increasingly attenuated, making
them less useful, and higher-frequency signals also require faster electronics.
One strategy for using limited bandwidth more efficiently is to use more than two signaling levels. By using four voltages, for instance, we can send 2 bits at once as a single symbol. This design will work as long as the signal at the receiver is sufficiently strong to distinguish the four levels. The rate at which the signal changes is then half the bit rate, so the needed bandwidth has been reduced.
We call the rate at which the signal changes the symbol rate to distinguish it from the bit rate. The bit rate is the symbol rate multiplied by the number of bits per symbol. An older name for the symbol rate, particularly in the context of devices called telephone modems that convey digital data over telephone lines, is the baud rate. In the literature, the terms ‘‘bit rate’’ and ‘‘baud rate’’ are often used incorrectly.
Note that the number of signal levels does not need to be a power of two. Often it is not, with some of the levels used for protecting against errors and simplifying the design of the receiver.
One strategy for using limited bandwidth more efficiently is to use more than two signaling levels. By using four voltages, for instance, we can send 2 bits at once as a single symbol. This design will work as long as the signal at the receiver is sufficiently strong to distinguish the four levels. The rate at which the signal changes is then half the bit rate, so the needed bandwidth has been reduced.
We call the rate at which the signal changes the symbol rate to distinguish it from the bit rate. The bit rate is the symbol rate multiplied by the number of bits per symbol. An older name for the symbol rate, particularly in the context of devices called telephone modems that convey digital data over telephone lines, is the baud rate. In the literature, the terms ‘‘bit rate’’ and ‘‘baud rate’’ are often used incorrectly.
Note that the number of signal levels does not need to be a power of two. Often it is not, with some of the levels used for protecting against errors and simplifying the design of the receiver.
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