The most straightforward form of digital modulation is to use a positive voltage
to represent a 1 and a negative voltage to represent a 0. For an optical fiber,
the presence of light might represent a 1 and the absence of light might represent a
0. This scheme is called NRZ (Non-Return-to-Zero). The odd name is for historical
reasons, and simply means that the signal follows the data.
Once sent, the NRZ signal propagates down the wire. At the other end, the receiver converts it into bits by sampling the signal at regular intervals of time.
This signal will not look exactly like the signal that was sent. It will be attenuated and distorted by the channel and noise at the receiver. To decode the bits, the receiver maps the signal samples to the closest symbols. For NRZ, a positive voltage will be taken to indicate that a 1 was sent and a negative voltage will be taken to indicate that a 0 was sent.
NRZ is a good starting point for our studies because it is simple, but it is seldom used by itself in practice. More complex schemes can convert bits to signals that better meet engineering considerations. These schemes are called line codes. Below, we describe line codes that help with bandwidth efficiency, clock recovery, and DC balance.
Once sent, the NRZ signal propagates down the wire. At the other end, the receiver converts it into bits by sampling the signal at regular intervals of time.
This signal will not look exactly like the signal that was sent. It will be attenuated and distorted by the channel and noise at the receiver. To decode the bits, the receiver maps the signal samples to the closest symbols. For NRZ, a positive voltage will be taken to indicate that a 1 was sent and a negative voltage will be taken to indicate that a 0 was sent.
NRZ is a good starting point for our studies because it is simple, but it is seldom used by itself in practice. More complex schemes can convert bits to signals that better meet engineering considerations. These schemes are called line codes. Below, we describe line codes that help with bandwidth efficiency, clock recovery, and DC balance.
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