If the negotiation indicator (see 4.5) of the Low layer compatibility information element included in the SETUP message is set to "Out-band negotiation is possible (octet 3a, bit 7)", then one or more of the low layer protocol attribute(s) may be negotiated. In this case,the called user responding positively to the call may include a Low layer compatibility information element in the CONNECT message. This element will be conveyed transparently by the network and delivered to the calling user in the CONNECT message.
Note - Only the low layer protocol attributes may be negotiated and therefore the information transfer attributes (octets 3to 4), if returned by the called user in the CONNECT message, will be identical to the ones received in the Low layer compatibility information element contained in the SETUP message.
If, for any reason, the network is unable to convey this information element, it shall act as described in 5.8.7.1 (unrecognized information element). Users are advised not to include in the Low layer compatibility information element sent from the called user to the calling user, attributes which would have the same value as the ones contained in the Low layer compatibility information element received from the calling party.
If bearer capability selection applies, and if a Low layer compatibility information element is
returned by the called user in the CONNECT message, then the called user shall ensure that the
information transfer attributes in that returned Low layer compatibility information element is the
same as the information transfer attributes of the selected bearer capability.