OAM cell

  Operation, Administration, and Maintenance cell. ATM Forum specification for cells used to monitor virtual circuits. OAM cells provide a virtual circuit-level loopback in which a router responds to the cells, demonstrating that the circuit is up, and the router is operational.

OARnet

  Ohio Academic Resources Network. Internet service provider that connects a number of U.S. sites, including the Ohio supercomputer center in Columbus, Ohio.

object instance

  Network management term referring to an instance of an object type that has been bound to a value.

OC

  Optical Carrier. Series of physical protocols (OC-1, OC-2, OC-3, and so on), defined for SONET optical signal transmissions. OC signal levels put STS frames onto multimode fiber-optic line at a variety of speeds. The base rate is 51.84 Mbps (OC-1); each signal level thereafter operates at a speed divisible by that number (thus, OC-3 runs at 155.52 Mbps). See also SONET, STS-1, and STS-3c.

ODA

  Open Document Architecture. ISO standard that specifies how documents are represented and transmitted electronically. Formally called Office Document Architecture.

ODI

  Open Data-Link Interface. Novell specification providing a standardized interface for NICs (network interface cards) that allows multiple protocols to use a single NIC. See also NIC (network interface card).

OEMI channel

  See block multiplexer channel.

Office Document Architecture

  See ODA.

Ohio Academic Resources Network

  See OARnet.

OIM

  OSI Internet Management. Group tasked with specifying ways in which OSI network management protocols can be used to manage TCP/IP networks.

OIR

  Online insertion and removal. Feature that permits the addition, replacement, or removal of interface processors in a Cisco router without interrupting the system power, entering console commands, or causing other software or interfaces to shut down. Sometimes called hot swapping. See also power-on servicing.

ONC

  Open Network Computing. Distributed applications architecture designed by Sun Microsystems, currently controlled by a consortium led by Sun. The NFS protocols are part of ONC. See also NFS.

ones density

  Scheme that allows a CSU/DSU to recover the data clock reliably. The CSU/DSU derives the data clock from the data that passes through it. In order to recover the clock, the CSU/DSU hardware must receive at least one 1 bit value for every 8 bits of data that pass through it. Also called pulse density.

online insertion and removal

  See OIR.

on-the-fly packet switching

  See cut-through packet switching.

open architecture

  Architecture with which third-party developers can legally develop products and for which public domain specifications exist.

open circuit

  Broken path along a transmission medium. Open circuits will usually prevent network communication.

Open Data-Link Interface

  See ODI.

Open Document Architecture

  See ODA.

Open Network Computing

  See ONC.

Open Shortest Path First

  See OSPF.

Open System Interconnection

  See OSI.

Open System Interconnection reference model

  See OSI reference model.

oper account

  One of the four default user accounts that are created in the factory on each LightStream 2020 ATM switch. The oper account is for general users. Its default interface is the CLI.

Operation, Administration, and Maintenance cell

  See OAM cell.

Option

  One currently defined: maximum TCP segment size.

Optical Carrier

  See OC.

optical fiber

  See fiber-optic cable.

Organizational Unique Identifier

  See OUI.

oscillation

  Secondary signal on top of the 60-Hz waveform. It has a magnitude that ranges from 15 % to 100 % of the normal voltage carried on the power line. See surge, spike, and sag.

OSI

  Open System Interconnection. International standardization program created by ISO and ITU-T to develop standards for data networking that facilitate multivendor equipment interoperability.

OSI Internet Management

  See OIM.

OSINET

  International association designed to promote OSI in vendor architectures.

OSI reference model

  Open System Interconnection reference model. Network architectural model developed by ISO and ITU-T. The model consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The highest layer (the application layer) is closest to the user; the lowest layer (the physical layer) is closest to the media technology. The next to lowest layer are implemented in hardware and software, while the upper five layers are implemented only in software. The OSI reference model is used universally as a method for teaching and understanding network functionality. Similar in some respects to SNA. See application layer, data link layer, network layer, physical layer, presentation layer, session layer, and transport layer.

OSPF

  Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the ISIS protocol. See also Enhanced IGRP, IGP, IGRP, IS-IS, and RIP.

OUI

> Organizational Unique Identifier. The 3 octets assigned by the IEEE in a block of 48-bit LAN addresses.

outframe

  Maximum number of outstanding frames allowed in an SNA PU 2 server at any time.

out-of-band signaling

  Transmission using frequencies or channels outside the frequencies or channels normally used for information transfer. Out-of-band signaling is often used for error reporting in situations in which in-band signaling can be affected by whatever problems the network might be experiencing. Contrast with in-band signaling.