GB/T 16974-2009 in English

《GB/T 16974-2009信息技术 数据通信 数据终端设备用X.25包层协议》由TC28（全国信息技术标准化技术委员会）归口，主管部门为国家标准化管理委员会。
标准规定的包层协议能用于开放系统互连（OSI）和非OSI两种情况。在OSI语境中使用时，包层协议包融在OSI参考模型的网络层中，即GB/T 9387.1-1998中。
当借助于专用路径或电路交换连接访问符合ITU-T X.25的公用或专用包交换网络时，本标准包括包层上的DTE操作。本标准还包括两个符合本标准的DTE，经专用路径、电路交换连接或局域网（LAN）直接通信（没有包交换网络介入）时所必需的附加包层规程。
本标准也包括使用ITU-T X.25连接到包交换公用数据网，而且也能对DTE提供X.25接口的专用网络（见附录A）。
为评估特定实现的一致性，需有所实现的那些能力和选项的声明。这样的声明称之为“协议实现一致性声明（PICS）”，在GB/T 17178.1-1997中定义。根据ISO/IEC 9646-7:1997给出的相关导则，附录B提供了PICS形式表。
相应国际标准的第一版是以1984 CCITT X.25的红皮书文本为基础的，也包括与早期1980 CCITT X.25的1993版为基础的，这个四版是以X.25的1996版为基础的。其包括了与1988、1984和1980 X.25版本兼容的必要条款。本标准不同版本之间的差异在附录C中作了总结。
应该指出的是，在用于IDT时，本标准与ITU-T X.25的应用范围并不相同。本标准包括了ITU-T X.25加在IDE上的规范。另外，本标准还包含使用DTE间互工作和覆盖DTE到DTE直接操作的附加规范。这种更广泛的适用范围在本标准的应用中必须意识到。

Scope

This standard specifies ITU-T X.25-compliant packet layer procedures, formats and facilities for use by data terminal equipment (DTE) operations. Including virtual call and permanent virtual circuit two operation modes. The cladding protocol specified in this standard can be used in both Open Systems Interconnection (OSI) and non-OSI situations. When used in the OSI context, the packet protocol is included in the network layer of the OSI reference model, namely GB/T 9387.1-1998. This standard covers DTE operation on the packet layer when accessing public or private packet-switched networks conforming to ITU-T X.25 by means of private paths or circuit-switched connections. This standard also covers the additional packet layer procedures necessary when two DTEs conforming to this standard communicate directly (without intervening packet-switched networks) over a dedicated path, a circuit-switched connection, or a local area network (LAN). This standard also includes private networks that use ITU-T X.25 to connect to packet-switched public data networks and can also provide X.25 interfaces to DTEs (see Appendix A). A declaration of those capabilities and options of an implementation is required to assess the conformance of a particular implementation. Such a statement is called "Protocol Implementation Conformity Statement (PICS)", which is defined in GB/T 17178.1-1997. According to the relevant guidelines given by ISO/IEC 9646-7-1997, Appendix B provides the PICS form. The first edition of the corresponding International Standard was based on the 1984 CCITT X.25 Red Book text and also included the necessary provisions for compatibility with the earlier 1980 CCITT X.25 Yellow Book text. The second edition is based on the blue book text of 1988 CCITT X.25, the third edition is based on the 1993 edition of X.25, and the fourth edition is based on the 1996 edition of X.25. This includes the necessary provisions for compatibility with the 1988, 1984, and 1980 X.25 versions. The differences between the different editions of this standard are summarized in Appendix C. It should be pointed out that, when used for DTE, the scope of application of this standard is not the same as that of ITU-T X.25. This standard contains the specification added to DTE by ITU-T X.25. In addition, this standard also contains additional specifications to facilitate interworking between DTEs and to cover DTE-to-DTE direct operations. This wider applicability must be recognized in the application of this standard.