以下标准通过专用电路规定了数据终端设备（DTE）和数据电路终端设备（DCE）之间的接口，用于在分组交换数据网络（PSDN）上以分组模式运行的终端，或两个DTE之间的接口:（1）CCITT建议X.25，数据终端设备（DTE）和数据电路终端设备（DCE）之间的接口，用于以分组模式运行并通过专用电路连接到公共数据网络的终端。（2） ISO 7776:1986，信息处理系统-数据通信-高级数据链路控制程序-X，25 LAPB兼容DTE数据链路程序的说明（3）ISO 8208: 1987年，信息处理系统-数据通信-数据终端设备的X.25包级协议CCITT建议X.25规定了DTE/DCE接口的DCE端。ISO 776:1989和ISO 8208:1987规定了DTE/DTE接口和DTE/DCE接口的DTE侧。本标准符合本节所列三个标准的要求，涵盖了DTE/DCE和DTE/DTE接口。

The following standards specify the interface between data terminating equipment (DTE) and data circuit-terminating equipment (DCE) for terminals operating in the packet mode on packet-switched data networks (PSDN), or between two DTEs, by dedicated circuit: (1) CCITT Recommendation X.25, interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to Public Data Networks by Dedicated Circuit. (2) ISO 7776:1986, Information Processing Systems - Data Communication - High-Level Data Link Control Procedures - Description of the X,25 LAPB-Compatible DTE Data Link Procedures (3) ISO 8208:1987, Information Processing Systems - Data Communications - X.25 Packet Level Protocol for Data Terminal Equipment CCITT Recommendation X.25 specifies the DCE side of the DTE/DCE interface. ISO 776:1989 and ISO 8208:1987 specify the DTE/DTE interface and the DTE side of the DTE/DCE interface. This standard confirms to the requirements of the three listed in this section and covers the DTE/DCE and DTE/DTE interfaces.