This standard provides the physical layer (Layer 1) specifications for DS1 User-Network Interfaces (UNIs) that are used to transport asynchronous transfer mode (ATM) cells in the DS1 payload. The term User-Network Interface (UNI) is used in this standard to refer to both the Network Interface (NI) and interfaces within the Customer Installation (CI). DS1-ATM interfaces include@ but are not limited to@ DS1 access to a Broadband - Integrated Services Digital Network (B-ISDN). For information concerning the four B-ISDN reference points (UB@ TB@ SB@ and SSB)@ see T1.646-1995. This standard addresses synchronization@ line code@ framing format@ ATM mapping@ and connectors associated with the DS1-ATM interface. All requirements are applicable at all UNIs unless specifically identified as being applicable to the NI or the IA and IB interfaces. T1.646-1995 defines interface points IA and IB as physical locations designating the boundary of functional groupings. Interface point IA is adjacent to the B-TE@ B-NTA@ and B-NT1 on their network side; interface point IB is adjacent to the B-NT2 and B-NT1 on their user side (refer to figures 7 and 8 of T1.646-1995 for a graphical representation. Specifications for processing the header error check (HEC) and for maintenance and performance monitoring of a DS1 signal that transports ATM cells are also provided@ by reference@ in this document. This standard is a revision of the portions of T1.646-1995 that refer@ in the generic sense@ to the transport of ATM cells in the DS1 payload. Portions of T1.646-1995 that relate specifically to the use of DS1 transport for broadband integrated services digital network (B-ISDN) applications will remain in the revision of T1.646- 1995.The provisions of this standard are intended to be consistent with applicable requirements concerning safety and environmental conditions. NOTE?C The user's attention is called to the possibility that compliance with this standard may require use of an invention covered by patent rights. By publication of this standard@ no position is taken with respect to the validity of this claim or of any patent rights in connection therewith. The patent holder has@ however@ filed a statement of willingness to grant a license under these rights on reasonable and nondiscriminatory terms and conditions to applicants desiring to obtain such a license. Details may be obtained from the publisher.