1. In a set of data, such as a data field, block, or stream, being processed or transported, the part that represents user information and user overhead information, and may include user-requested additional information, such as network management and accounting information. Note: The payload does not include system overhead information for the processing or transportation system. Synonym mission bit stream. 2. The interface rate minus frame overhead. This is the act capability for information transfer provided for the next lower level of the hierarchy. 3. The 192 information bits of a DS1 frame.