In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which 1's are represented by one significant condition (usually a positive voltage) and 0's are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition. The pulses have more energy than a RZ code. Unlike RZ, NRZ does not have a rest state. NRZ is not inherently a self-synchronizing code, so some additional synchronization technique (for example a run length limited constraint, or a parallel synchronization signal) must be used to avoid bit slip.
For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the bandwidth required by the Manchester code.
When used to represent data in an asynchronous communication scheme, the absence of a neutral state requires other mechanisms for data recovery, to replace methods used for error detection when using synchronization information when a separate clock signal is available.
NRZ-Level itself is not a synchronous system but rather an encoding that can be used in either a synchronous or asynchronous transmission environment, that is, with or without an explicit clock signal involved. Because of this, it is not strictly necessary to discuss how the NRZ-Level encoding acts "on a clock edge" or "during a clock cycle" since all transitions happen in the given amount of time representing the actual or implied integral clock cycle. The real question is that of sampling--the high or low state will be received correctly provided the transmission line has stabilized for that bit when the physical line level is sampled at the receiving end.
However, it is helpful to see NRZ transitions as happening on the trailing (falling) clock edge in order to compare NRZ-Level to other encoding methods, such as the mentioned Manchester code, which requires clock edge information (is the XOR of the clock and NRZ, actually) and to see the difference between NRZ-Mark and NRZ-Inverted.
Unipolar Non-Return-to-Zero Level
"One" is represented by one physical level (such as a DC bias on the transmission line).
"Zero" is represented by another level (usually a positive voltage).
In clock language, "one" transitions or remains high on the trailing clock edge of the previous bit and "zero" transitions or remains low on the trailing clock edge of the previous bit, or just the opposite. This allows for long series without change, which makes synchronization difficult. One solution is to not send bytes without transitions.Disadvantages of on-off keying are the wastage of power due to the transmitted DC level and also the power spectrum of the transmitted signal does not approach to zero at zero frequency. see RLL
Bipolar Non-Return-to-Zero Level
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