scope:

This International Standard 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.

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 procedures use the HDLc frame structure defined in ISO/IEC 3309 and the HDLC elements of procedures described in ISO/IEC 4335.

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-to-point 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.