Abort Sequence Basic Link Service Accept Basic Link Service Reject Remove Dedicated Connection (Class 1 or 6) Pre-empt Dedicated Connection (Class 1 or 6)
NOP performs no specific operation but can be used to carry control information in the header, such as initiating a Class 1 or Class 6 connection.
ABTS is used to abort the current sequence or an entire exchange. Only the ABTS command has a reply: BA_ACC if accepted and BA_RJT if rejected.