Overview
ISO/IEC 7776:1995 - "Information technology - Telecommunications and information exchange between systems - High-level data link control procedures - Description of the X.25 LAPB‑compatible DTE data link procedures" - defines the HDLC-based data link behavior for Data Terminal Equipment (DTE) using the X.25 LAPB protocol. This second edition (1995) cancels and replaces the 1986 edition and consolidates later amendments. It specifies frame structures, elements and procedures for DTE↔DCE and DTE↔DTE exchanges over duplex links using synchronous or start/stop transmission.
Key Topics
- Frame structure: Defines basic (modulo‑8) and extended (modulo‑128) frame formats, including address, control, information and FCS fields, with frame delimiting via flag sequences (e.g., 0 1111110).
- Classes of operation: Basic (modulo‑8, BAC, optional functions 2 & 8) and extended (modulo‑128, optional function 10) modes and rules for selecting modes (subscription‑time or bilateral agreement).
- Elements of procedure: Link initialization, flow control, and error control mechanisms (sequence numbers, retransmission, CRC‑based Frame Check Sequence).
- Transparency and encoding:
- Synchronous bit‑stuffing transparency (insert/discard “0” after five contiguous ones).
- Start/stop options: seven‑bit data path transparency (SBDPT) and control‑escape transparency for octet‑based encodings.
- Single Link Procedure (SLP) and Multilink Procedure (MLP): SLP for single links; optional MLP to combine parallel SLPs for resilience and improved throughput.
- Conformance and testing: Static and dynamic conformance requirements and a Protocol Implementation Conformance Statement (PICS) proforma to specify implemented options.
Applications & Who Uses It
ISO/IEC 7776:1995 is used where reliable X.25 LAPB data‑link behavior is required:
- Network equipment vendors implementing legacy X.25 DTE or DCE interfaces.
- Telecommunications engineers maintaining packet‑switching systems or integrating with public data networks that support X.25.
- Protocol developers and testers using the PICS proforma and conformance criteria for interoperability testing.
- System integrators connecting DTEs directly (DTE↔DTE) or via DCEs over duplex synchronous or start/stop links.
Practical benefits include standardized flow control, error detection/recovery, and framing/encoding rules to ensure interoperability across implementations and compatibility with ITU‑T X.25 environments.
Related Standards
ISO/IEC 7776 references and applies several HDLC and conformance standards, including:
Keywords: ISO/IEC 7776:1995, X.25 LAPB, HDLC, DTE, DCE, data link procedures, frame structure, multilink, conformance, PICS, synchronous transmission, start/stop transmission.
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