Overview
ISO/IEC 9040:1997 - Information technology - Open Systems Interconnection - Virtual Terminal Basic Class Service defines, at an abstract level, the externally visible service that a Virtual Terminal (VT) Basic Class provides in the OSI Application Layer. The standard specifies the VT service model: the primitives (actions/events), associated parameters, valid sequences and interaction patterns between VT users. It is an abstract service definition (not an implementation specification) and is intended to be used together with the VT Basic Class Protocol (ISO/IEC 9041), ACSE and the Presentation Service.
Key topics and technical requirements
- Service model and primitives: Defines VT primitives for establishment, termination, negotiation and data transfer (examples: VT-ASSOCIATE, VT-RELEASE, VT-DATA).
- VTE (Virtual Terminal Environment): Describes VTE components, VTE-parameters, consistency rules and directed graphs for environment configuration.
- Display, control and device objects: Formalizes display objects, control objects, reference information objects (RIOs) and device objects used to model terminal state and behaviour.
- Modes and access rules: Specifies S-mode and A-mode operation, access-rules, ownership and token/dialogue control (VT-GIVE-TOKENS, VT-REQUEST-TOKENS).
- Delivery control and synchronization: Defines delivery-control options (no delivery-control, simple, quarantine, implicit delivery), update queues and priority handling.
- Negotiation and profiles: Defines negotiation facilities for VTE-profiles, and includes default VTE-profiles for common modes.
- Exception, interrupt and dialogue facilities: Covers interrupt semantics (VT-BREAK), exception reporting (VT-P-EXCEPTION) and dialogue management.
- Registration and identifiers: Includes ASN.1 Object Identifier values for registered VT objects (Annex C).
- Informative annexes: Explanatory notes, default profiles and sizing guidance (Annexes A–D).
Applications and who uses it
- Systems integrators and architects designing terminal-oriented interactive applications, remote management consoles, and terminal emulators.
- Software developers implementing VT protocol stacks or application-layer interfaces that require consistent terminal semantics.
- Vendors building networked terminal services, legacy host access solutions and transaction systems that model terminal screens and fields.
- Standards engineers and interoperability test teams validating conformance between VT service implementations and related OSI Application Layer components.
Practical uses include remote terminal emulation, modelling multi-device terminal environments, defining screen/field semantics for interactive applications, and supporting negotiated terminal capabilities.
Related standards
Keywords: ISO/IEC 9040:1997, Virtual Terminal Basic Class Service, VTE, virtual terminal, VT protocol, ACSE, ASN.1, VTE-profile, display objects, delivery control.