1、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
2、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 fra
3、me 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 non-basic frame format. The mechanisms used to achieve bit pattern i
4、ndependence (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 specif
5、ies elements of data link control procedures for synchronous or start/stop, code-transparent 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 stat
6、ion, 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 two-way simultaneous data communication between data stations which are usually buffered, including operations on d
7、ifferent types of data circuits; for example multipoint/point-to-point, duplex/half-duplex, switched/non-switched, synchronous/start-stop, etc.The defined elements of procedures are to be considered as a common basis for establishing different types of data link control procedures. This Internationa
8、l 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
9、, 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 th
10、e 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-to-point configuration. For the unbalanced connectionless class, the data link consists of a
11、 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-p
12、oint 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. Operati
13、onal 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,data link information shall include any and all esse
