Glossary P

O

OC

Optical Carrier: A series of physical protocols, designated as OC-1, OC-2, OC-3, and so on, for SONET optical signal transmissions. OC signal levels place STS frames on a multi-mode fiber-optic line at various speeds, of which 51.84Mbps is the lowest (OC-1). Each subsequent protocol runs at a speed divisible by 51.84. See also: SONET.

octet

Base-8 numbering system used to identify a section of a dotted decimal IP address. Also referred to as a byte.

ones density

Also known as 'pulse density,' this is a method of signal clocking. The CSU/ DSU retrieves the clocking information from data that passes through it. For this scheme to work, the data needs to be encoded to contain at least one binary 1 for each 8 bits transmitted. See also: CSU and DSU.

one-time challenge tokens

Used to provide a single use password. This prevents replay attacks and snooping; however, it also requires the user to have a device that provides the token. This physical component of the security model works to prevent hackers from guessing or obtaining the user's password.

OSI

Open Systems Interconnection: International standardization program designed by ISO and ITU-T for the development of data networking standards that make multivendor equipment interoperability a reality.

OSI reference model

Open Systems Interconnection reference model: A conceptual model defined by the International Organization for Standardization (ISO), describing how any combination of devices can be connected for the purpose of communication. The OSI model divides the task into seven functional layers, forming a hierarchy with the applications at the top and the physical medium at the bottom, and it defines the functions each layer must provide. See also: Application layer, Data Link layer, Network layer, Physical layer, Presentation layer, Session layer, and Transport layer.

OSPF

Open Shortest Path First: A link-state, hierarchical IGP routing algorithm derived from an earlier version of the IS-IS protocol, whose features include multipath routing, load balancing, and least-cost routing. OSPF is the suggested successor to RIP in the Internet environment. See also: Enhanced IGRP, IGP, and IP.

OUI

Organizationally Unique Identifier: An identifier assigned by the IEEE to an organization that makes network interface cards. The organization then puts this OUI on each and every card they manufacture. The OUI is 3 bytes (24 bits) long. The manufacturer then adds a 3-byte identifier to uniquely identify the host on an internetwork. The total length of the address is 48 bits (6 bytes) and is called a hardware address or MAC address.

out-of-band management

Management 'outside' of the network's physical channels-for example, using a console connection not directly interfaced through the local LAN or WAN or a dial-in modem. Compare to: in-band management.

out-of-band signaling

Within a network, any transmission that uses physical channels or frequencies separate from those ordinarily used for data transfer. For example, the initial configuration of a Cisco Catalyst switch requires an out-of-band connection via a console port. Compare to: in-band signaling.

output buffer priorities

Refers to the use of queues applied to an interface buffer.