OAM (Operation and Maintenance) Cell - A cell that is used to monitor virtual circuits.

OCA (Over Current Alarm) - Available as a functional alarm from Marconi Communications battery and converter power plants.

OCC (Other Common Carriers) - Carriers providing long distance communications service other than AT&T. These companies (including AT&T) are called IXCs (IntereXchange Carriers).

OC-n (Optical Carrier level-n) - The optical counterpart of STS-n (the basic rate of 51.84 Mbps on which SONET is based is referred to as OC-1 or STS-1).
OC-48c (Concatenated OC-48) - 2.488 Gigabits per second carried as optical signals
OC-12c (Concatenated OC-12) - 622.08 Megabits per second carried as optical signals
OC-3c (Concatenated OC-3) - 155.52 Megabits per second carried as optical signals

Octet - A grouping of 8 bits; similar, but not identical to, a byte.

OEM (Original Equipment Manufacturer)- The maker of equipment that is marketed by another vendor, usually under the name of the reseller. The OEM may only manufacture certain components, or complete computers, which are then often configured with software and/or other hardware by the reseller.

Office Alarm- Alarm indicating a fault in a central office.

Office Records- A permanent record developed to reflect the actual equipment and arrangements in a central office.

Off-Line System- A standby power system designed to provide power to equipment only when commercial power fails (typically a full inverter system).

Off-Peak- The periods of time after the business day has ended during which carriers offer discounted airtime charges.

Ohm- Measure of resistance. A resistance of one Ohm allows one ampere to flow when a potential difference of one volt is applied to the resistance.

Ohmmeter- Test set which gives resistance readings in Ohms.

Ohm's Law- The law which relates current measured as Amps (I), voltage (E) and resistance measured as Ohms (R). The law is E = I x R. It can also be expressed as I = E/R, or R = E/I.

OID (Object Identifier) - The address of a MIB variable.

ONA (Open Network Architecture) - A concept under which telephone companies are obliged to provide a certain class of service to their own internal value-added divisions and the same class of service to external, nonaffiliated value-added companies.

One’s Density - The requirement for digital transmission lines in the public switched telephone network that eight consecutive "0"s cannot be in a digital data stream; exists because repeaters and clocking devices within the network will lose timing after receiving eight "0"s in a row; a number of techniques are used to insert a "1" after every seventh-consecutive "0" (see Bit Stuffing). 

On-Line Equipment - Equipment which is in direct communication with a switching center or distant terminal. 

On-Line System - A standby power system designed to provide continuous power via the standby power equipment to system equipment independent of the condition of the commercial power (typically a -48VDC system).

OOF (Out-of-Frame) - A signal condition and alarm in which some or all framing bits are lost.

OpenView - Hewlett-Packard's network management software. 

Optical Fiber Cable - Cable made up of glass fibers protected by plastic coverings; sometimes metallic wires are included as strength members. 

Oscilloscope - A device that produces a visual trace of a voltage or current wave shape, generally on a cathode ray tube.

OSI (Open Systems Interconnection) - The 7-layer suite of protocols designed by ISO committees to be the international standard computer network architecture. 

OSP (Outside Plant) - The outside plant includes all cables and wires extending outward from the network protectors on the main distribution frame, supporting structures, and other associated apparatus necessary to connect the terminal equipment to the outside plant.

OSPF (Open Shortest Path First) Protocol - A routing algorithm for IP that incorporates least-cost, equal-cost, and load balancing.

OUI (Organizationally Unique Identifier) - Part of RFC 1483. A three-octet field in the SubNetwork Attachment Point (SNAP) header, identifying an organization which administers the meaning of the following two octet Protocol Identifier (PID) field in the SNAP header. Together they identify a distinct routed or bridged protocol.

Out-of-Band Management - Refers to switch configuration via the serial port or over Ethernet, not ATM. 

Out Of Phase - Waves of the same frequency, but different time relation, that do not pass through their zero and maximum points at the same instant.

Output - (1) Data that flows out of a computer to any device; (2) in specification description language, an action within a transition which generates a signal which, in turn, acts as an input elsewhere; and (3) power, current or voltage at the output of a device.

Output Rating - (1) The power available at the output terminals of a transmitter or power supply when connected to the normal load or its equivalent; and (2) under specified ambient conditions, the power that can be delivered by a device over a long period of time without overheating.

Output Transformer - Transformer which matches power output of an amplifier to impedance of a load.

Outside Link - A link to a lowest level outside node.

Outside Node - A node which is participating in PNNI routing, but which is not a member of a particular peer group. 

Overbooking - A VBR process used to increases total link capacity by a factor.

Overcurrent Protection - Protection of power supplies, conductors, and connected equipment from excessive flow of input or output current, including the short circuited current. 

Overload - (1) A load greater than that for which the power source has been designed; and (2) on a transmission system, a higher level than that for which the system was designed. Overload produces distortion.

Overload Protection - A circuit breaker or mechanical fuse which opens when the current is greater than that which the equipment has been designed to carry.

Overload Rating - Some load in excess of the nominal rating, which a device can carry for a specified length of time without being damaged.

Overtemperature Protection - Protection of the power supply or parts of it against temperatures exceeding specified values.

Overvoltage Protection: - Protection of the power supply and/or connected equipment against output voltage, including the open circuit voltage.

Overvoltage Relay - Alarm relay which operates when voltage across it exceeds a preset level.