non return to zero (NRZ)
Non Return to Zero (NRZ) is an extremely trivial coding method for pulse code modulated data. The method corresponds to Non Return to Zero Level ( NRZ-L). In both methods, NRZ and NRZ-L, the binary signals are mapped directly. A 1-bit signal is represented by a high level, and a 0-bit signal is represented by a low or negative level. Non Return to Zero can be used with the single stream method and with the double stream method.
The bit sequences 10 and 01 can thus be represented by a single frequency cycle. Two bits are thus encoded in one hertz transmission frequency. For successive bits of the same logical value, the signal level remains unchanged. The method is not self- clocking, i.e. it does not support clock recovery. The NRZ method allows highest data rates, but is only suitable for short transmission distances.
If long 0- or 1-bit sequences occur with NRZ coding, this can cause fluctuations in the DC voltage level on the transmission line and impair transmission and switching levels. In addition, the method has no clock recovery for synchronization purposes and requires a relatively high bandwidth, about half the bandwidth to be transmitted. For example, a transmission rate of 100 Mbit/s requires a bandwidth of 50 MHz.
The various NRZ processes
In addition to the classic NRZ method, there are also Non Return to Zero Inverted( NRZ-I), which uses an inverted NRZ signal, Non Return to Zero Mark( NRZ-M), which corrects possible DC fluctuations of NRZ and enables clock recovery, and Non Return to Zero Space( NRZ-S), which avoids long 0 sequences by means of state changes.