smpp disconneted

hi all has anyone  ever had a GOT enquire_link error from a smsc, via SMPP. i am getting the following error

***
[1710] ERROR: SMPP: Unknown PDU type 0x00000015, ignored.
*****
i am getting disconected from my SMSC and i think it because of this error

this i a part of the error file/
2002-03-05 11:42:23 [22] DEBUG: SMPP: Waiting for data
2002-03-05 11:42:30 [20] DEBUG: SMPP: Got enquire_link PDU.**
2002-03-05 11:42:30 [20] DEBUG: SMPP PDU 00F45640 dump:
2002-03-05 11:42:30 [20] DEBUG:   type_name: enquire_link
2002-03-05 11:42:30 [20] DEBUG:   command_id: 21 = 0x00000015
2002-03-05 11:42:30 [20] DEBUG:   command_status: 0 = 0x00000000
2002-03-05 11:42:30 [20] DEBUG:   sequence_number: 3 = 0x00000003
2002-03-05 11:42:30 [20] DEBUG: SMPP PDU dump ends.
2002-03-05 11:42:30 [20] ERROR: SMPP: Unknown PDU type 0x00000015, ignored.*******
2002-03-05 11:42:30 [20] DEBUG: SMPP: Waiting for data
2002-03-05 11:42:33 [22] DEBUG: SMPP: Got enquire_link PDU.**
2002-03-05 11:42:33 [22] DEBUG: SMPP PDU 00F45640 dump:
2002-03-05 11:42:33 [22] DEBUG:   type_name: enquire_link
2002-03-05 11:42:33 [22] DEBUG:   command_id: 21 = 0x00000015
2002-03-05 11:42:33 [22] DEBUG:   command_status: 0 = 0x00000000
2002-03-05 11:42:33 [22] DEBUG:   sequence_number: 1 = 0x00000001
2002-03-05 11:42:33 [22] DEBUG: SMPP PDU dump ends.
2002-03-05 11:42:33 [22] ERROR: SMPP: Unknown PDU type 0x00000015, ignored*******

can anyone help.