For this reason it is also known as "on-off keying". In clock language, a "one" transitions to or remains at a biased level on the trailing clock edge of the previous bit, while "zero" transitions to or remains at no bias on the trailing clock edge of the previous bit. Among the disadvantages of unipolar NRZ is that it allows for long series without change, which makes synchronization difficult, although this is not unique to the unipolar case. One solution is to not send bytes without transitions. More critically, and unique to unipolar NRZ, are issues related to the presence of a transmitted DC level – the power spectrum of the transmitted signal does not approach zero at zero frequency. This leads to two significant problems: first, the transmitted DC power leads to higher power losses than other encodings, and second, the presence of a DC signal component requires that the transmission line be DC-coupled. "One" is represented by one physical level (usually a positive voltage), while "zero" is represented by another level (usually a negative voltage). In clock language, in bipolar NRZ-level the voltage "swings" from positive to negative on the trailing edge of the previous bit clock cycle. An example of this is RS-232, where "one" is −12 V to −5 V and "zero" is +5 V to +12 V. "One" is represented by no change in physical level, while "zero" is represented by a change in physical level. In clock language, the level transitions on the trailing clock edge of the previous bit to represent a "zero". This "change-on-zero" is used by High-Level Data Link Control and USB. They both avoid long periods of no transitions (even when the data contains long sequences of 1 bits) by using zero-bit insertion. HDLC transmitters insert a 0 bit after 5 contiguous 1 bits (except when transmitting the frame delimiter "01111110"). USB transmitters insert a 0 bit after 6 consecutive 1 bits. The receiver at the far end uses every transition — both from 0 bits in the data and these extra non-data 0 bits — to maintain clock synchronization. The receiver otherwise ignores these non-data 0 bits. Non-return-to-zero, inverted (NRZI, also known as "Nonreturn to Zero IBM", "Inhibit code" or "IBM code") was devised by Bryon E. Phelps (IBM) in 1956.