This standard details the criteria required for communication between a C12.21 Device and a C12.21 Client via a modem connected to the switched telephone network. The C12.21 Client could be a laptop or portable computer, a master station system, or some other electronic communications device. This standard does not specify the implementation requirements of the telephone switched network to the modem, nor does it include definitions for the establishment of the communication channel. This document provides details for an implementation of the Open Systems Interconnection (OSI) seven-layer model in accordance with ISO/IEC 7498-1. The protocol specified in this standard was designed to transport data in Table format. The Table definitions are in ANSI C12.19 and in Annex D of this document. This standard specifies the differences between ANSI C12.18-2006 and ANSI C12.19-1997 and the features and services required to describe a protocol specification for telephone modem communications.