This document is the baseline of a series of standards that define a modular family of rates and formats available for use in interfaces generally referred to as SONET. This series of documents is identified by the T1.105 prefix. This document (T1.105-2001) describes a base rate and format along with a multiplexing scheme. Other characteristics described in this standard are: layering of overhead@ definitions of function and position of overhead@ frequency justification@ scrambling@ conditions for setting overhead values@ and a standardized set of payload carrying envelopes. As an aid to the reader@ a mapping is provided between SONET and SDH terminology. Any differences between the SDH and SONET specifications at the time of approval of this standard that affect interworking are highlighted. The differences are relative to the SDH Recommendations under review in SG 15 at that time. Other T1.105.xx standards are listed in clause 2 and describe such items as payload mappings@ protection switching@ data communications interfaces@ tandem connection@ and physical layer specifications.