0x0000 Reserved
0x0002 SIGNAL Request
0x0003 Reserved
0x0004 Reserved
0x0005 Clear Request
0x0006 PROTOCOL PARAMETER Request

0x0000 Event Layer 3 Establish
0x0002 Reserved
0x0003 Event Layer 3 Signal
0x0004 Event Layer 3 Status
0x0005 Reserved
0x0006 Reserved
0x0007 Event Layer 3 Protocol parameter (only for AN side)
0x0014 Purge: PSTN initiated Purge for Userport when timeslot unknown
0x0015 Purge: PSTN returned to Idle state after purge
0x0016 L4 CONNECT REQ
0x0096 ERROR: No Channel Remove Acknowledge from BCC upon PURGE
0x0097 ERROR: No Disconnect Complete from L3 upon PURGE