Proposes requirements for 64 k-bit/s circuit mode bearer services to complement and be compatible with the corresponding services for public ISDN specified by CCITT. It is one of a series of emerging Standards applicable to private integrated services digital networks (PISN). The series uses the ISDN concepts as developed by CCITT and is also within the framework of Standards for open systems interconnection (OSI) as defined by ISO. This Standard is a companion to the Standard dealing with inter-exchange signalling procedures and protocol (DR 91263:T) and was prepared in parallel with it. This draft is identical with and reproduced from ISO/IEC CD 11574:1991.