LP_SIZE_ERR is an alarm about wrong TU specifications. This alarm is reported when a board receives the TU service whose mapping structure is different from that set in the board. When this alarm occurs, the lower order services are interrupted, accompanied by TU_AIS alarm. LP_SIZE_ERR is reported when bits 5 and 6 of the pointer of the service of VC12 are not "1 0", that is, the pointer is deviated. Deviation of the pointer is most probably caused by the clock.
As something is wrong with the service of the extended subrack instead of the main subrack, the XCE board may be faulty. Actually, the XCE board has no clock module, and it is the main subrack that assigns a 38M clock to the board. The clock combines with the crystal oscillator on the XCE board to provide 8K and 2K frame headers to the service boards on the extended subrack. When the service boards lose clock signal, the TU_AIS, TU_LOP, and LP_SIZE_ERR alarms -are generated. Moreover, as the service on the main subrack has no problem, something must be wrong with the crystal oscillator of the XCE board.