Frequency justification and pointers:+/-ve Stuffing mechanism in SONET/SDH
When the input data has a rate lower than the output data rate of a multiplexer, the positive stuffing will occur. The input is stored in a buffer at a rate which is controlled by the WRITE clock. Since the output (READ) clock rate is higher than the WRITE clock rate, the buffer content will be depleted or emptied. To avoid this condition, the buffer fill is constantly monitored and compared to a threshold. If the the content fill is below a threshold, the READ clock is inhibited and stuffed bit is inserted to the output stream. Meanwhile, the input data stream is still filling the buffer. The stuffed bit location information must be transmitted to the receiver so that the receiver can remove the stuffed bit.
When the input data has a rate higher than the output data rate of a multiplexer, the negative stuffing will occur. If negative stuffing occur, the extra data can be transmitted through an other channel. The receiver must need to kown how to retrieve the data.
Positive Stuffing
If the frame rate of the STS SPE is too slow with respect to the frame rate then the alignment of the envelope should periodically slip back or the pointer should be incremented by one periodically. This operation is indicated by inverting the I bits of the 10 bit pointer. The byte right after the H3 byte is the stuff byte and should be ignored. The following frames should contain the new pointer. For example, the 10 bit of the H1 and H2 pointer bytes has the value of ‘0010010011’ for STS-1 frame N.
Frame # IDIDIDIDID
----------------------
N 0010010011
N+1 1000111001 <-- the I bits are inverted, positive stuffing
is required.
N+2 0010010100 <-- the pointer is increased by 1
Negative Stuffing
If the frame rate of the STS SPE is too fast with respect to the frame rate then the alignment of the envelope should periodically advance or the pointer should be decremented by one periodically. This operation is indicated by inverting the D bits of the 10 bit pointer. The H3 byte is containing actual data. The following frames should contain the new pointer. For example, the 10 bit of the H1 and H2 pointer bytes has the value of ‘0010010011’ for STS-1 frame N.
Frame # IDIDIDIDID
----------------------
N 0010010011
N+1 0111000110 <-- the D bits are inverted, negative stuffing
is required.
N+2 0010010010 <-- the pointer is decreased by 1
