As a network option, completion of the transmission path prior to receipt of a call acceptance indication may be provided in one of three ways:
a) on completion of successful channel negotiation at the destination interface; or
b) on receipt of a message containing an indication that in-band information is being provided; or
c) not at all, i.e. this option is not supported by the network.
When criteria a) is used to determine that transmission path should be established, the network shall connect, as a minimum, the backward side of the transmission path upon receipt of either aCALL PROCEEDING message or an ALERTING message containing an accepTable B-channel indication.
When criteria b) is used to establish the transmission path, the network shall connect, as a minimum, the backward side of the transmission path upon receipt of either an ALERTING message or a PROGRESS message containing progress indicator No. 8, in-band information or appropriate pattern now available, or progress indicator No. 1, call is not end.to-end ISDN; further call progress information may be available in-band, respectively.
The network providing the early completion of the transmission path in the backward direction may choose to support only one of methods a) or b) above. The network may choose to further restrict which message(s) will result in establishment of the transmission path. These restrictions may be imposed on a per interface basis to provide an administrative means for limiting potential misuse of the early connection capabilities.