本标准详细说明了C12之间通信所需的标准。21个设备和一个C12。21通过调制解调器连接到交换电话网络的客户端。C12。21客户端可以是笔记本电脑或便携式计算机、主站系统或其他电子通信设备。本标准未规定电话交换网络对调制解调器的实施要求，也未包括建立通信信道的定义。本文件提供了根据ISO/IEC 7498-1实施开放系统互连（OSI）七层模型的详细信息。本标准中规定的协议旨在以表格格式传输数据。ANSI C12的定义见表。19和本文件附件D。 本标准规定了ANSI C12之间的差异。 18-2006和ANSI C12。19-1997以及描述电话调制解调器通信协议规范所需的功能和服务。

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.