GB/T 7421-2008Information technology -- Telecommunications and information exchange between systems -- High-level data link control (HDLC) procedures (English PDF)

Foreword

This standard is identical with ISO/IEC 13239.:2002 "Information Technology Systems for Remote Communication and Information Exchange Advanced Data Link Control (HDLC) Regulations (English version).

This standard replaces GB/T 7421-1987 "Information Processing Systems Data Communication Advanced Data Link Control Regulations" GB/T 7496-1987 "Advanced data link control protocol frame structure of information processing system data communication", GB/T 7575-1987 "Data Communication Advanced Data Link Control Procedures Rules Elements Compilation", GB/T 14400-1993 "Information Processing Systems Data Communications Advanced data link control balance class procedures for the exchange of data link layer address decision/negotiation "and GB/T 15698-1995 "Information Technology Inter-system Telecommunications and Information Exchange Advanced Data Link Control Procedures General XID Frame Information Field Contents and format".

This standard and GB/T 7421-1987, GB/T 7496-1987, GB/T 7575-1987, GB/T 14400-1993 and GB/T 15698-1995 The difference is that.

--- This standard is GB/T 7421-1987, GB/T 7496-1987, GB/T 7575-1987, GB/T 14400-1993 and GB/T 15698-1995 integration;

--- This standard is named "Advanced Interlinkage Control (HDLC) for Telecommunications and Information Exchange between Information Systems Systems Process ";

--- Added HDLC frame non-basic format;

--- Added connectionless category (unbalanced connectionless category and balanced connectionless category) related content;

--- Increased start/stop transmission of the relevant content;

--- Added modal 32768 and modal 2147483648 control field formats;

--- Added some options of optional functions and the use of optional functions;

--- Increased frame inspection sequence negotiation rules.

This standard Appendix A ~ Appendix H are informative appendix.

This standard by the National Information Technology Standardization Technical Committee and focal point.

This standard was drafted. China Electronics Standardization Institute.

Introduction

Advanced Data Link Control (HDLC) procedures are designed for synchronous or on/off, transparent data transmission. Two data stations the normal period of code transparent data communication consists of the transmission of the data source to the data sink information frame and the transmission of the reverse acknowledgment frame. Usually at Before the data station containing the data source receives the acknowledgment, it should save the original information in memory for use when it needs to be retransmitted.

In those cases where it is required, the data sequence integrity between the data source and the data sink is implemented numerically. The number in this standard the provisions of the module cycle, number units to frame. The combination of each data source/data sink on the data link uses a separate numbering method.

The data sink implements the acknowledgment function by notifying the data source of the next expected sequence number. This confirmation function is available one Individual, non-informative frames or within the control fields of the informative frame.

HDLC procedures apply to unbalanced and balanced data links.

Unbalanced data link an unbalanced data link contains two or more data stations. For control purposes, there is a data station on the data link responsible for it Organize data flows and handle unrecoverable data link layer errors. The station with this responsibility is unbalanced in connection with the data the link is called the master and is called the control station in the unbalanced connectionless data link. The frame sent by the master/control station is a command frame. data the other data stations on the link are called secondary stations in the data link of the unbalanced connection mode, and are referred to as secondary stations in the data link of the unbalanced connectionless mode Help station The frame sent by the secondary station/secondary station is the response frame.

For data transmission between the master/control station and the secondary/secondary stations, two data link control scenarios are considered (see Figure A and

Figure B).

Case 1. The data station containing the data source executes the master/control station data link control function and controls the package via the select type command Data Stations with Data Stations with Secondary/Secondary Data Link Control.

Case 2. A data station containing a data sink performs master/control station data link control functions and controls packets via interrogation commands Data station with data source with secondary/secondary data link control functions.

Information flows from the data source to the data sink, but acknowledgments are always sent in the opposite direction.

The two control situations can be combined so that two-way alternating or two-way simultaneous communications can be made on the data link.

Figure A unbalanced data link function (case 1)

Figure B Unbalanced data link function (case 2)

Balanced data links

A balanced data link contains only two data stations. For control purposes, each data station is responsible for organizing the data flow and negative Responsible for handling the irreversible data link layer error status generated in the respective initiated transmission. Each data station balances the number of connections GB/T 7421-2008/ISO/IEC 13239.:2002 According to the link are called combined stations, both in the balanced connectionless data link are called peer stations. They both send and receive command frames Response frame.

For data transfer between compound stations/peers, use the data link control function shown in Figure C. Each combination station /the data source in the peer controls the data sink in the other combining station/peer by means of a select command. Information flows from the data source to the data sink, Confirmation is always sent in the opposite direction. Each combination station/peer can make use of the inquiry order request from another combination station/peer Station confirmation and status response.

Figure C balances data link functions

Data link configuration the categories of advanced data link control (HDLC) procedures are some of the ways to describe data link operations. These methods allow for various logics Synchronous or start/stop, transparent data transmission between data stations of a configuration and physical configuration. These categories of procedures are throughout HDLC architecture, defined in a consistent manner. One of the purposes of this standard is unbalanced procedures, balanced procedures and no connection procedures Maintaining maximum compatibility between the basic categories is particularly needed for data stations with configurable capabilities that operate on a specific communication basis the example requirements may have the characteristics of a master station, a secondary station, a compound station, a control station, a secondary station or a peer station.

This standard defines five basic types of procedures (two unbalanced, one balanced and two unconnected), unbalanced categories apply Point-to-point and multi-point configurations on private or switched data transmission facilities (see Figure D, using Master/Slave terminology). Unbalanced category the characteristic is that the data link has only one master at one end and one or more secondary stations at the other end. The management of the data link is performed by the master alone Therefore, it is called the "unbalanced" procedure category.

Figure D Unbalanced data link configuration

Unbalanced Unconnected categories are suitable for point-to-point configuration on private or switched data transmission facilities or on private data transmission facilities Point-to-multipoint configuration (see Figure D, using control station/secondary station terminology). The feature of the unbalanced connectionless class is that there is only one end of the data link Control stations, and one or more secondary stations on the other end. The control station is responsible for determining when the secondary station is allowed to send. Control station and auxiliary station Neither support any form of connection establishment/disconnection procedures, flow control procedures, data transfer confirmation procedures or error recovery procedures, so the designation "No connection" procedure category.

The balance category applies to point-to-point configurations on private or switched data transmission facilities (see Figure E, using combined station terminology). Balance class Another feature is that on a logical data link, there are two data stations called combinational data stations, which have the same responsibility for the management of the data link.

It is therefore referred to as the "Balance" protocol category.

Figure E Balanced Data Link Configuration

GB/T 7421-2008/ISO/IEC 13239.:2002 Balancing The connectionless category applies to point-to-point configurations on private or switched data transmission facilities (see Figure E, Using Peer Terminology).

The feature of the balanced connectionless category is that on a data link, there are two data stations, called peer stations, which control their when give away. Each peer does not support any form of connection establishment/disconnection procedures, flow control procedures, data transfer confirmation procedures or error recovery Duplicate procedures, so specify the "no connection" procedure category.

For each type of procedure, a method of operation may be provided in accordance with the order and responsiveness available on the basic type of watch in that class. At the same time, also Lists a variety of different optional features. And defines a description of the procedures for using these optional features.

It should be recognized that the unbalanced protocol class defined by this standard can be configured symmetrically on a single data circuit working. For example, combining two unbalanced procedures in the opposite direction (I frames are used only as commands) will result in a symmetrical point Point-to-point configuration (see Figure F).

Figure F Symmetric data link configuration

These HDLC procedures define the exchange identifier (XID) command/response frame as an optional function of exchanging data link information Knowledge, parameters, functional abilities, etc.). Defines the content and format of the generic XID frame information field.

These HDLC procedures also specify that two data stations be used to mutually determine the data link layer address before the logical data link is established the parameters and procedures.

GB/T 7421-2008/ISO/IEC 13239.:2002 Information technology - Telecommunications and information exchange between systems Advanced Data Link Control (HDLC) procedures

1 Scope

This standard specifies the frame structure for data communication systems that use bit-oriented advanced data link control (HDLC) procedures, which the content and format of the prime, procedure category, common exchange identifier (XID) frame, and the data link layer address in the switching environment are determined/negotiated means.

Note. The phrase "bit-oriented" use of HDLC control procedures and the non-integer number of bits of various subfields used for HDLC control purposes Send about. However, frames as a whole may be made of octet-oriented cells (eg, on/off mode) for transmission purposes.

The frame structure section defines the relative positions of the various components of the basic frame format and the non-basic frame format. Also defined to get in the mechanism by which the intra-bit pattern is independent (transparent) anywhere at any time. In addition, three frame check sequences are specified (FCS), which defines the extension rules for address fields and describes the addressing conventions available.

The Procedural Elements section specifies the data link for synchronous or start/stop transparent data transmission using independent frame numbers in both directions Control procedures elements.

These HDLC protocol elements are specifically defined according to what happens when the secondary station, secondary station, peer station or composite station receives the order.

This standard applies to a wide range of applications, for example, used in the usually buffer capacity of data stations between the unidirectional, bidirectional alternating or double to the simultaneous, including in different types (such as multi-point/point to point, full/half duplex, exchange/non exchange, synchronization/start/stop, etc.) Work on data communication.

The defined HDLC protocol elements can serve as a common basis for establishing different types of control procedures. This standard does not define any single System, nor should it be seen as a data communication system specification. For a specific system implementation, not all commands or responses are need.

The Classification of Procedures section describes the HDLC unbalanced procedures category for synchronization or start/stop data transmission, the HDLC Balanced Procedures category and HDLC no connection rules category.

For unbalanced categories, the data link consists of a master plus one or more secondary stations and operates normally in point-to-point or multi-point configurations Response mode or asynchronous response mode of operation. For the balance category, the data link consists of two composite stations and is configured asynchronously in a point-to-point configuration Balance mode of operation. For the unbalanced connectionless category, the data link consists of a control station plus one or more secondary stations, Point or multi-point configuration with unbalanced connectionless operation. For the balanced connectionless category, the data link is made up of two peers and is at the point Point-to-point configuration to balance the connection-free operation. The basic tables of commands and responses are defined in each case. However, the data link Ability to be modified with optional features.

Balanced operations are expected to require equally controlled environments for either end of the data link. Follow the entire HDLC architecture Include these operational requirements.

The content and format of the General Exchange Identifier (XID) frame part The main purpose of establishing an XID frame is to transmit and receive data in two or more HDLC stations in fact, exchange data link information. For this standard, the data link information should include any or all of the basic operating characteristics, such as such as identifying, and/or selecting optional features and facilities for each station. When one or more stations are able to offer multiple options, Ben the standard defines a single exchange negotiation procedure for establishing operational characteristics.

This standard is the means by which at least one data link connection between two parties in need of communication is provided to provide the information required for the exchange. This standard is this purpose describes the content and format of the general XID frame information field.

This standard only defines the basic HDLC standards and information coding. Provided at the same time the definition of the basic parameters of the consultation A mechanism for using generic XID frame information fields to negotiate private parameters in a single XID exchange.

GB/T 7421-2008/ISO/IEC 13239.:2002