The task of the justification opportunity bits (R bits) is to be available as extra bits that
can be used when the rate of the incoming tributaries is higher than its nominal value (within
the margin specified by ITU) by an amount that makes this necessary. In this case, the opportunity
bit is no longer mere stuffing but becomes an information bit instead.
In order for the device that receives the multiplexed signal to be able to determine whether a justification opportunity bit contains useful information (that is, information from a tributary), justification control bits (J bits) are included in the frame. Each group of control bits refers to one of the tributaries of the frame. All of them will be set to "0" if the associated opportunity bit is carrying useful information, otherwise they will be set to "1". Several bits are used instead of just one in order to provide protection against possible errors in transmission. On examining the control bits received, if they do not all have the same value it is decided that they were sent with the majority value (a "1" if there are more ones than zeros, for instance; it is assumed that there has been an error in the bits that are at zero).
It can be seen that there is a dispersion of the control bits referring to a tributary that causes them to be located in separate groups. By spreading out the J bits (control bits) the probability of errors occuring in them and a wrong decision being made as to whether or not they have been used as a useful data bit is reduced. If the wrong decision is made there is not only an error in the output data, but also a slip of one bit, that is, the loss or repetition of one bit of information.
