Reasons the pending transaction is not available include:
DSR's Tx sender buffer is filling up causing connection congestion.
PAT expiry or total transaction life-time expiry is causing transaction timeout.
The associated measurement tag for this event is RxAnswerUnexpected (10008), which is the number of times that the DRL receives an answer message event from DCL/RCL with a valid Connection ID for which a pending transaction cannot be found.
Recovery: