信息技术.系统间远程通信和信息交换.高级数据链路控制(HDLC)规程

[中文适用范围]： 本国际标准规定了帧结构、过程元素、过程类别、通用交换标识（XID）帧的内容和格式，以及交换环境中数据链路层地址的解析/协商方法使用面向位的高级数据链路控制（HDLC）程序的数据通信系统。 注：在涉及 HDLC 控制过程时，短语“面向比特”的使用涉及将非整数比特分配给用于 HDLC 控制目的的各种子字段。 然而，出于传输目的，帧作为整体可以由面向八位位组的单元构造（例如，开始-停止模式）。 帧结构部分定义基本帧格式和非基本帧格式的各个组成部分的相对位置。 还定义了用于在帧内需要时实现位模式独立性（透明度）的机制。 此外，还规定了三种帧检查序列（FCS）；定义了地址字段扩展的规则；并描述了可用的寻址约定。 过程部分的元素指定用于同步或开始/停止、在两个方向上使用独立帧编号的代码透明数据传输的数据链路控制过程的元素。 这些过程的HDLC元素是根据在次站、支路站、对等站或组合站处接收到命令时发生的动作来具体定义的。 本国际标准旨在涵盖广泛的应用；例如，通常被缓冲的数据站之间的单向、双向交替或双向同时数据通信，包括对不同类型的数据电路的操作；例如多点/点对点、双工/半双工、交换/非交换、同步/启停等。 程序的定义元素应被视为建立不同类型数据链路的共同基础控制程序。 本国际标准没有定义任何单一系统，不应被视为数据通信系统的规范。 并非所有命令或响应都是任何特定系统实现所必需的。 过程类别部分描述了用于同步或启动/停止数据传输的HDLC不平衡过程类别、HDLC平衡过程类别以及HDLC无连接过程类别。 对于非平衡类，数据链路由一个主站加上一个或多个从站组成，并在点对点或多点配置中以正常响应模式或异步响应模式运行。 对于平衡类，数据链路由两个组合站组成，并在点对点配置中以异步平衡模式运行。 对于不平衡无连接类别，数据链路由一个控制站加上一个或多个支路站组成，并在点对点或多点配置中以不平衡无连接模式运行。 对于平衡无连接类别，数据链路由两个对等站组成，并在点对点配置中以平衡无连接模式运行。 在每个类中，定义了命令和响应的基本指令集，但可以通过使用可选功能来修改数据链路的能力。 平衡操作适用于数据链路两端需要同等控制的情况。 根据整体 HDLC 架构涵盖了操作要求。 交换标识(XID)帧部分的内容和格式建立在以下事实之上：XID帧的主要用途是在两个或多个HDLC站之间交换数据链路信息。 就本国际标准而言，数据链接信息应包括任何和所有数据链接信息。 [外文原描述]： This International Standard specifies the frame structures, the elements of procedures, the classes of procedures, the content and format of the general purpose Exchange Identification (XID) frame, and a means for resolution/negotiation of a data link layer address in switched environments for data communication systems using bit-oriented high-level data link control (HDLC) procedures. NOTE The use of the phrase "bit-oriented", referring to the HDLC control procedures, pertains to the allocation of a non-integral number of bits to various subfields used for HDLC control purposes. However, the frame as an entirety may be constructed from octet-oriented units (e.g., start-stop mode) for transmission purposes. The frame structure portion defines the relative positions of the various components of the basic frame format and the nonbasic frame format. The mechanisms used to achieve bit pattern independence (transparency), where and when required, within the frame are also defined. In addition, three frame checking sequences (FCS) are specified; the rules for address field extension are defined; and the addressing conventions available are described. The elements of procedures portion specifies elements of data link control procedures for synchronous or start/stop, codetransparent data transmission using independent frame numbering in both directions. These HDLC elements of procedures are defined specifically in terms of the actions that occur on receipt of commands at a secondary station, a tributary station, a peer station, or a combined station. This International Standard is intended to cover a wide range of applications; for example one-way, two-way alternate or twoway simultaneous data communication between data stations which are usually buffered, including operations on different types of data circuits; for example multipoint/point-to-point, duplex/half-duplex, switched/non-switched, synchronous/startstop, etc. The defined elements of procedures are to be considered as a common basis for establishing different types of data link control procedures. This International Standard does not define any single system and should not be regarded as a specification for a data communication system. Not all of the commands or responses are required for any particular system implementation. The classes of procedures portion describes the HDLC unbalanced classes of procedures, the HDLC balanced class of procedures, and the HDLC connectionless classes of procedures for synchronous or start/stop data transmission. For the unbalanced classes, the data link consists of a primary station plus one or more secondary stations and operates in either the normal response mode or the asynchronous response mode in a point-to-point or multipoint configuration. For the balanced class, the data link consists of two combined stations and operates in the asynchronous balanced mode in a point-topoint configuration. For the unbalanced connectionless class, the data link consists of a control station plus one or more tributary stations and operates in the unbalanced connectionless-mode in a point-to-point or multipoint configuration. For the balanced connectionless class, the data link consists of two peer stations and operates in the balanced connectionless-mode in a point-to-point configuration. In each class, a basic repertoire of commands and responses is defined, but the capability of the data link may be modified by the use of optional functions. Balanced operation is intended for use in circumstances which require equal control at either end of the data link. Operational requirements are covered in accordance with the overall HDLC architecture. The content and format of the Exchange Identification (XID) frame portion builds on the fact that the principal use of the XID frame is to exchange data link information between two or more HDLC stations. For the purpose of this International Standard, ISO/IEC 13239:2002(E) 2 © ISO/IEC 2002 ? All rights reserved data link information shall include any and all essential operational characteristics such as identification, authentication and/or selection of optional functions and facilities concerning each station. This International Standard defines a single-exchange negotiation procedure for establishing operational characteristics when either one or more stations are capable of providing multiple selections. This International Standard provides a means for exchanging the necessary information to establish, at a minimum, a data link connection between two correspondents wishing to communicate. It describes a general purpose XID frame information field content and format for that purpose. It defines encoding for information related to the basic HDLC standards only. Mechanisms are provided to permit the general purpose XID frame information field to be used to negotiate private parameters in a single XID exchange simultaneously with negotiation of the defined basic parameters. This International Standard does not limit or restrict the use of the XID frame information field from defining other standard formats for use in specific applications. The following are examples of potential uses of the XID command/response frame interchange: a) Identification of the calling and called stations when using circuit switched networks (including switched network backup applications). b) Identification of stations operating on non-switched networks requiring identification at start-up. c) The XID command frame with an individual, group or all-station address may be used to solicit XID response frame(s) from other station(s) on the data link, prior to or following data link establishment. d) Negotiation of the Frame Check Sequence (FCS) to be used for subsequent information interchange, by stations that support both 16-bit FCS and 32-bit FCS capabilities. e) Convey higher layer information that may be required prior to data link establishment. f) Transmission of an XID response frame at any respond opportunity to request an XID exchange to modify some of the operational parameters (for example, window size) following data link establishment. g) Negotiation of the number of protected bits in the frame when an Unnumbered Information with Header check (UIH) frame is used. The means for resolution/negotiation of a data link layer address in switched environments portion is applicable to data stations employing HDLC balanced classes of procedures which provide the XID command/response capability with the two specific parameter fields, identified below. It is used to select a pair of operational link addresses when preassigned, system designated addresses are not known on an a priori basis; e.g., switched circuited data links. Additional XID frame functions (including the exchange of operational parameters, command/response support, higher layer information, etc.) may be accomplished in conjunction with data link layer address determination or following address determination, with additional XID frame exchanges. NOTE Address resolution procedures for situations where the remote DTE does not support XID frames, the "all-station" address, or complete address support capabilities as defined in clause 8 below are not within the scope of this International Standard.