This page of SDH modulation covers ON OFF Keying modulation technique used in SDH/SONET optical communication network.
The basic modulation technique used in optic fibre is on off keying where in light is off when binary zero is to be transmitted and it is on when binary one is to be transmitted.
The simple method is to provide light emission for duration of "one" bit time and stop light emission for duration of "zero" bit time. This method is referred as NRZ or Non Return to Zero technique. Optical NRZ technique is different than electrical one as there is no negative light as was the case of negative voltage. In NRZ modulation, continuous long string of ones lead to light on for the entire duration of the bits and for longer period. Hence this cause receiver to lose timing synchronization and will lead to wrong decoding of bits.
To avoid above problem RZ technique is employed. But RZ modulation permits long string of zeros and reduces no. of photons seen by photo detector by half for each of the bit period. Due to this RZ is not used for long distance communication.
DC balance means average transmitted power is constant in optical communication domain. For long distance communication NRZ technique is used to minimize long string of ones and zeros to maintain DC balance. The best way to avoid long string of ones and zeros are line coding and scrambling.