AHDLC engine type. Three main types are defined:
- a 'hardware' based engine requires the use of
AHDLC hardware assist. Each hardware based engine is
peer to one unique AHDLC hardware assist.
- a 'software' based engine is not peer to an AHDLC
hardware assist but it relies on the system cpu(s) to
perform AHDLC framing.
If the engine type cannot be determined, this object
value should return 'unknown'.