OADM (optical add/drop multiplexer)
An optical multiplexing device that uses special filters to extract the wavelengths that need to be dropped off at a given location. It eliminates the costly electronics that are used to convert between light and electricity in a nonoptical multiplexer.

OC-1 to OC-48 (Optical Carrier-1 to Optical Carrier-48)
OC-1 is the base optical carrier transmission speed of 51.840Mbps. To calculate OC-2 to OC-48 speeds, simply multiply the OC-1 base by the desired magnitude. Common OC levels include OC-1 (51Mbps), OC-3 (155Mbps), OC-12 (622Mbps), OC-48 (2.5Gbps), OC-192 (10Gbps), and OC-768 (40Gbps).

octet
8-bit byte. Used instead of byte when talking about packet services.

OFDM (Orthogonal Frequency Division Multiplexing)
A multicarrier modulation scheme that broadcasts on many frequencies, reducing interference from collisions with walls and objects.

off-hook
A state in which a telephone set is in use (that is, the handset is removed from its cradle).

off-line
(1) Pertaining to equipment or devices not under direct control of the central processing unit. (2) Used to describe terminal equipment that is not connected to a transmission line. (3) Not controlled directly by or communicating with a computer.

offload
To move data or programs out of storage.

OGT (outgoing trunk)
A one-way trunk that carries only outgoing traffic.

OLT (optical line termination)
A switch that sends traffic downstream to subscribers and that also handles the upstream traffic.

one-way trunk
A trunk between a switch (that is, a PBX) and an exchange, or between exchanges, where traffic originates from only one end.

ones density rule
A principle which says that if you transmit more than 15 zeros in a row, the network may lose synchronization, which means transmission errors could occur.

on-hook
A state in which a telephone set is not in use (that is, the handset is resting in the cradle).

online
(1) Being controlled directly by or directly communicating with a computer. (2) Connected to a computer so that data can pass to or from the computer without human intervention. (3) Directly in the line loop.

online services
Computer functions offered to end users not owning a host computer; includes time sharing, archival storage, and prepared software programs.

ONP (Open Network Provision)
A pan-European standard ensuring the provision of the network infrastructure by European telecommunications administrations to users and competitive service providers on terms equal to those for the administrations themselves.

ONU (optical network unit)
A device in which optical-to-electrical conversions takes 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.

optical carrier
Specifications defining line speeds and transmission encoding and multiplexing 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 serves to reflect 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 carriers to string together wavelengths to provide end-to-end connections. It links any of several incoming lines to any of several outgoing lines and automatically reroutes traffic when a network path fails. Sometimes referred to as an optical cross-connect or a wavelength router.

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 that is used for the definition of network protocol standards to enable an OSI compatible computer or device to communicate with any other OSI-complaint computer or device for a meaningful exchange of information. Layer 7, the application layer, is responsible for exchanging information between the programs that are running on a computer and other services on a network. Layer 6, the presentation layer, formats information so that a software application can read it. Layer 5, the session layer, supports connections between sessions and handles administrative tasks and security. Layer 4, the transport layer, corrects transmission errors and ensures that the information is delivered reliably. Layer 3, the network layer, identifies computers on a network and determines how to direct information transfer over that network. Layer 2, the data-link layer, groups data into containers to prepare that data for transfer over a network. Layer 1, the physical layer, defines how a transmission medium connects to a computer, as well as how electrical or optical information is transferred on the transmission medium.

OSP (online service provider)
A provider that organizes online content and provides intuitive user navigation.

OSPF (Open Shortest Path First)
A routing protocol, used on TCP/IP networks, in which routers maintain an internal map of the network and exchange information about the current state of each network link. OSPF’s features include least-cost routing, multipath routing, and load balancing.

OTDR (optical time domain reflectometer)
A device that can be used in fiber networks to detect potential leaks that could be the result of unwanted intrusion.

out-of-band management
A system which management data is communicated through a link, outside the network, typically through a modem or some other serial connection.

out-of-band signaling
Signaling in which the conversation and the signaling take place over different paths. A separate digital channel (called a signaling link) is created, where messages are exchanged between network elements at 56Kbps or 64Kbps. Out-of-band signals run no danger of interference from speech or data, which allows signaling to take place during the conversation. However, the out-of-band signal needs extra bandwidth and extra electronics to handle the signaling band.

output
Data that has been processed.

overflow
Excess traffic, on a particular route, that is offered to another (alternate) route.

overlay network
A high-performance digital network that interconnects with the main public network but which has its own lines, exchanges, and, often, a separate international gateway.

override
To seize a circuit even though the circuit is already occupied.