The Low layer compatibility information element contains a negotiation indicator which may have one of the following values:
a) Out-band negotiation not possible (default) - Then the called user shall not invoke negotiation, according to J.3.
b) Out-band negotiation possible - The called user may then invoke low layer compatibility negotiation, as needed, according to J.3.
c) In-band negotiation possible -The called user may then invoke low layer compatibility negotiation using the supported in-band negotiation, according to service or application requirements.
d) Either in-band or out-band negotiation allowed - The called user may invoke one or the other low layer compatibility negotiation procedures according to its requirements. If the call is end-to-end ISDN, and the out-band low layer compatibility negotiation is supported by both parties, then this method of negotiation is preferred.