In both unipolar schemes (level and inverted), all signal levels are over or under the 0V line, never on it. That's actually the feature that gives them their name: 'Non-return to zero'.
In the unipolar non-return-to-zero level scheme, the signal is represented by two voltage levels: a positive voltage for a 0 and negative voltage for a 1. It's worth mentioning here that this is not a standard representation, and you may encounter the opposite polarity in some systems and reference manuals. In this website, we use the aforementioned representation consistently.
The unipolar non-return-to-zero level scheme is simple and easy to implement, but it has some drawbacks. Among them, one that stands out is the presence of synchronization issues. These issues arise when there are long sequences of identical bits. Whenever that happens, the line experiences a lack of transitions, making it difficult for the receiver to determine the timing of the bits.
Here's a couple of examples of the Unipolar non-return-to-zero level scheme.
