本国际标准规定了程序、格式和 数据终端设备(DTE)分组层的设施 按照ITU-T建议X.25运行。 虚拟呼叫和永久虚拟电路模式 操作都包括在内。 这里指定的分组层协议可以用于 开放系统互连（OSI）和非OSI环境。 当在OSI的上下文中使用时，分组层协议 包含在OSI参考的网络层中 模型，ITU-T Rec。X.200 ISO/IEC 7498-1。 该国际标准涵盖分组处的DTE操作 访问公共或私有分组交换时的层 符合ITU-T建议X.25的网络 专用路径或电路交换连接。它还 涵盖两个所需的附加数据包层过程 符合本国际标准的DTEs进行通信 直接地（即，没有中间的分组交换网络） 通过专用路径、电路交换连接或本地 局域网（LAN）。 本国际标准还涵盖使用 ITU-T建议X.25连接到分组交换 公共数据网络，并且还可以提供X.25接口 DTE（见附件A）。 为了评估特定实现的一致性， 有必要说明哪些功能和选项 已实施。这种声明称为协议 实施一致性声明(PICS)，定义见 ITU-T Rec。X.290 ISO/IEC 9646-1。附件B提供了图片 根据ITUT给出的相关指南的形式 Rec.X.296 ISO/IEC 9646-7. 该国际标准的第一版基于 1984年CCITT红皮书建议X.25的文本。它还 包含与 1980年早些时候CCITT黄皮书建议X.25的文本。 第二版以1988年CCITT蓝皮书为基础 建议X.25的案文。第三版基于 1993年版本的X.25。这第四版是基于 1996年版本的X.25。第四版中保留的是 与1993年、1988年、1984年兼容的必要规定 和1980年版本的X.25。各种之间的差异 本国际标准的版本摘要见附录 C. CCITT建议X.613(1992)ISO/IEC 10588:1993， X.25数据包层协议在信息技术中的应用 结合X.21/X.21bis提供OSI连接模式 网络服务 CCITT建议X.614(1992)ISO/IEC 10732:1993， 信息技术使用X.25数据包层协议 提供OSI连接模式网络服务 电话网络

This International Standard specifies the procedures, formats and facilities at the Packet Layer for Data Terminal Equipment (DTE) operating in conformance with ITU-T Recommendation X.25. Both Virtual Call and Permanent Virtual Circuit modes of operation are covered. The Packet Layer protocol specified herein can be used in both Open Systems Interconnection (OSI) and non-OSI environments. When used within the context of OSI, the Packet Layer protocol is encompassed in the Network Layer of the OSI Reference Model, ITU-T Rec. X.200 | ISO/IEC 7498-1. This International Standard covers DTE operation at the Packet Layer when accessing a public or private packet-switched network conforming to ITU-T Recommendation X.25 by means of a dedicated path or a circuit-switched connection. It also covers the additional Packet Layer procedures necessary for two DTEs conforming to this International Standard to communicate directly (i.e., without an intervening packet-switched network) over a dedicated path, a circuit-switched connection, or a local area network (LAN). This International Standard also covers private networks that use ITU-T Recommendation X.25 to connect to packet-switched public data networks and that may also offer an X.25 interface to a DTE (see annex A). To evaluate conformance of a particular implementation, it is necessary to have a statement of which capabilities and options have been implemented. Such a statement is called a Protocol Implementation Conformance Statement (PICS), as defined in ITU-T Rec. X.290 | ISO/IEC 9646-1. Annex B provides the PICS proforma in accordance with the relevant guidance given in ITUT Rec. X.296 | ISO/IEC 9646-7. The first edition of this International Standard was based on the 1984 CCITT Red Book text of Recommendation X.25. It also contained the necessary provisions for compatibility with the earlier 1980 CCITT Yellow Book text of Recommendation X.25. The second edition was based on the 1988 CCITT Blue Book text of Recommendation X.25. The third edition is based upon the 1993 version of X.25. This fourth edition is based on the 1996 version of X.25. Retained within this fourth edition are the necessary provisions for compatibility with the 1993, 1988, 1984 and 1980 versions of X.25. The differences between various editions of this International Standard are summarized in annex C. CCITT Recommendation X.613 (1992) | ISO/IEC 10588 : 1993, Information technology Use of X.25 Packet Layer Protocol in conjunction with X.21/X.21bis to provide the OSI connectionmode Network Service CCITT Recommendation X.614 (1992) | ISO/IEC 10732 : 1993, Information technology Use of X.25 Packet Layer Protocol to provide the OSI connection-mode Network Service over the telephone network