Key to the robust operation of the retransmission protocol is the correct generation and detection of a valid sequence discontinuity. A valid sequence discontinuity is the mechanism the TX uses to indicate that it has begun retransmission and the mechanism the RX uses to identify the start of retransmission.
A valid sequence discontinuity is defined as follows:
"A sequence discontinuity is an unexpected change in primary sequence numbers between Burst Control Words without any CRC24 errors in between."
For example, suppose a burst with a primary sequence number of 157 is received, followed by numerous Idle Control Words without CRC24 errors, followed by another burst with a primary sequence number of 123. This is a sequence discontinuity. Zero or more Idle Control Words without CRC24 errors can appear between the pair of Burst Control Words. If one or more of the Idle Control Words in between the pair of Burst Control Words has a CRC24 error, the pair is not recognized as a valid sequence discontinuity.
This definition of a valid sequence discontinuity is unambiguous and ensures robust operation. If the transmitter follows the preceding rule, it will generate sequence discontinuities that can be positively identified.
If a valid sequence discontinuity cannot be positively identified, the RX will again request retransmission if configured to do so.