ake place. open system A system that facilitates multivendor, multitechnology integration based on publicly available standards for subsystem interaction. Three characteristics of an open system are portability, scalability, and interoperability. OpenCable A CableLabs initiative to publish specifications that define digital cable net- work interfaces and the nature of next-generation cable set-top boxes. The goal of OpenCable is to help the cable industry deploy interactive services over cable, create a common standard for digital cable TV within the United States, and promote competition among licensed device manufacturers. optical carrier Specifications that define line speeds and transmission encoding and multi- plexing methods for the SDH/SONET fiber-optic backbone network. optical fiber Any filament, or fiber, made of dielectric materials that is used to transmit laser- or LED-generated light signals, usually for digital communications. An optical fiber consists of a core, which carries the signal, and cladding, a substance with a slightly higher refractive index than the core, which surrounds the core and reflects the light signal back into it. Also called lightguide or fiber-optic. optical switch A device that resides at a junction point in an optical backbone and enables signals in optical fibers or integrated optical circuits to be selectively switched from one circuit to another. An optical switch links any of several incoming lines to any of several outgoing lines and automatically reroutes traffic when a network path fails. OPX (off-premises extension) A telephone extension located other than where the main switch is. OSI (Open Systems Interconnection) model A seven-layer logical network architecture used to define network protocol standards to enable an OSI-compatible computer or device to communicate with any other OSI-compliant computer or device for a meaningful exchange of information. Layer 7, the application layer, is responsible for exchanging information between the programs running on a comp