This International Standard specifies general principles for the mappings between the OSI Data Link service (ISO/IEC 8886), both connection-mode (CO-DLS) and connectionless-mode (CL-DLS), and standard Data Link protocols, as follows: M1: CO-DLS — HDLC X.25 LAPB-compatible DTE procedures, single link procedures (ISO/IEC 7776) M2: CO-DLS — HDLC Unbalanced operation Normal response mode Class, UNC (ISO/IEC 3309, ISO/IEC 4335 and ISO/IEC 7809) M3: CO-DLS — Logical link control (LLC) Type 2 (ISO/IEC 8802-2) M4: CL-DLS — LLC Type 1 (ISO/IEC 8802-2) M5: CL-DLS — HDLC Balanced operation Connectionless-mode Class, BCC (ISO/IEC 3309, ISO/IEC 4335 and ISO/IEC 7809) M6: CL-DLS — HDLC Unbalanced operation Connectionless-mode Class, UCC (ISO/IEC 3309, ISO/IEC 4335 and ISO/IEC 7809) This International Standard specifies the detailed mappings Ml, M2, M5 and M6; it also specifies the main features of the mappings M3 and M4. This International Standard does not specify individual implementations or products, nor does it constrain the implementation of Data Link entities and interfaces within an information processing system. NOTES 1 The above designations M1 to M6 for the mappings are used elsewhere in this International Standard. 2 The possibility of adding further mappings to the above list in the future is not precluded (for example, to cover Frame Relay protocols).