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.