GB/T 16974-1997 in English

《GB/T 16974-1997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，TC28SC6（全国信息技术标准化技术委员会数据通信分会）执行，主管部门为国家标准化管理委员会。
本标准规定了数据终端设备(DTE)操作使用的符合ITU-TX.25的包层规格、格式和设施。包括虚呼叫和永久虚电路两种操作方式。

Scope

This standard specifies the ITU-TX.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 layer protocol is included in the network layer of the OSI reference model, that is, in ISO/IEC7498.1. This standard covers DTE operation at the packet level when accessing public or private packet-switched networks conforming to ITU-TX.25 by means of dedicated 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-TX.25 to connect to the packet-switching public data network 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 Consistency Statement (PICS)", defined in ITU-TX.290|ISO/IEC9646-1. According to the relevant guidelines given by ITU-TX.296|ISO/IEC9646-7, Appendix B provides the PICS form table. The first edition of the corresponding international standard is based on the Red Book text of 1984CCITTX.25, and also includes the necessary clauses compatible with the earlier Yellow Book text of 1980CCITTX.25. The second edition is based on the blue book text of 1988CCITTX.25, and this third edition is based on the 1993 edition of X.25. The necessary clauses for compatibility with the 1988, 1984, and 1980 X.25 versions were retained. 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-TX.25. This standard contains the specification added to DTE by ITUTX.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.