SNMP4tPC - Network/Computer Glossary of Terms (O to S)

Out-of-Band signaling An extra signal transmitted alongside the information signal to monitor and control the transmission. It uses a separate channel of the LAN and allows network management devices to access LAN devices even when the LAN itself is not functioning, so providing an additional layer of resilience.
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PABX (PBX) A Private Automatic Branch eXchange/telephone exchange linked to the PSTN. It handles calls automatically i.e., unattended. Normally a manual, user-owned exchange.

Packet A collection of bits, including the address, data and control, that are switched and transmitted together. The terms frame and packet are often used synonymously.

Packet Buffer Memory space reserved for storing a packet awaiting transmission or for storing a received packet.

Packet Switching A method of switching data in a network. Individual packets of a set size and format are accepted by the network and delivered to their destination. The sequence of packets is maintained, and destination established, by the exchange of control information (also contained in the packets) between the sending terminal and the network before the transmission starts. The network is open to all users, all the time, with packets from the various nodes being interleaved throughout the network. The packets can be sent in any order, as the control information sent at the beginning of the transmission ensures they are interpreted in the correct order at the receiving end. Because each packet carries its own control instructions, it can use any route to reach its destination. The link lasts only as long as the trarsmission. An ITU-TS standard for packet switched networks information.

Predictive dialing Automated dialing feature in which the systems predicts from pre-set algorithms when an agent/operator will become free and makes the call in advance.

Preview dialing Automated dialing feature in which the system determines the next call to be made but allows the agent/operator as a supervisor to check and activate the call.

Presentation Layer The sixth of seven layers in the OSI model. Its job is to ensure that two computers wanting to communicate establish the ground rules for how they can interoperate.

PRA (Primary Rate Access) An ISDN access method that uses maximum data rates of 2.048 Mbit/s in Europe, or 1.544 Mbit/s in the US and Japan.

PRI (Primary Rate Interface) ISDN's carrier or large user interface comprising 30BD channels in Europe, supporting around 1,920Mbit/s.

Primary domain controller The server at which the master copy of a domain's user accounts database is maintained. The primary domain controller also validates logon requests. A LAN Manager term.

Print Server A computer and/or software to provide users or a network with access to a central printer. The printer server acts as a buffer, holding the information to be printed out in memory until the printer is free. It is possible to program the printer server to print jobs in the order they arrive or to give priority to particular users who, in effect, jump the queue. The advantages of a printer server include maximum exploitation of expensive resources like laser printers and not having to retry if the printer is initially busy.

Printer driver A program that controls printing and sets options such as print quality and paper size for a particular printer. For example, in LAN Manager, each printer queue has a single printer driver associated with it.

Private (dedicated) line Term for a dedicated voice or data circuit, usually leased from a Common Carrier.

Privilege level For user-level security, one of three settings -user, admin or guest is assigned for each user account. The privilege level defines interfaces was drawn up in 1980, known as Recommendation X.25. Hence packet switched networks are often referred to as X.25. In some cases X.25 is being superseded by frame relay and eventually by ATM.

PAD (Packet Assembler/Disassembler) A device used in a packet switched network to convert data to an appropriate format so that a non X.25 (non-packet switching) device can send information over an X.25 (packet switched) network, The pad is responsible for call set up and addressing routines.

Paging A one-way alert or alphanumeric message service. Messages are normally left via a bureau with a human or automatic operator.

Parallel interface An interface that can handle a group of bits transmitted at the same time, by sending each bit over a separate wire. Probably the best known parallel interface of all is the one developed by Centronics, a printer manufacturer, that has 36 pins and can handle a byte of data at a time. The transmission of the data is controlled by a computer generated strobe pulse.

Parity Bit An extra bit added to a group of bits, usually to a seven-bit byte. The parity bit can be of 0 or 1 value so that every byte will then add up to an odd or even number, depending on whether odd or even parity is chosen. When the transmitting device frames a character, it counts the numbers of 0s and 1s in the frame and attaches the appropriate parity bit. The recipient counts all the 0s and 1s and compares the total to the odd or even message contained in the parity bit. If there's a discrepancy the recipient asks for the data to be resent.

PCM (Pulse Code Modulation) One of the most common ways of converting an analog signal to a digital signal. This is done by sampling the signal and coding the sample. It usually samples at 8,000 times per second; each sampling representing 8bits. This produces a transmission rate of 641Kbit/s, with the traffic on the line in interleaved data format. At the receiving end, the interleaved signals are separated and converted back to analog.

Phase Modulation One of the three ways of modulating or altering a signal so that it is able to carry information. The other two are amplitude and frequency modulation. Phase modulation is used in high speed modems.

Polling Method of controlling terminals on a multi-point line by which a computer acting as master calls or polls each of the terminals in turn to find out if they have any data to send. This access method is used in star topology networks but is generally being superseded by the preferred interrupt method.

Port A computer interface capable of attachment to another device, such as a modem for communicating with a remote terminal, or, if the port is within a hub, to a workstation.

PPP (Point to Point Protocol) An addition to the Internet protocol suite to help connect devices where dissimilar transport protocols exist, for example for the transfer of routing the range of actions a user can perform on the network.

Profile A set of parameters defining the way in which a device acts. In the LAN world, a profile is often used by one or more workstations to determine the connection they will have with other devices. Profiles work like batch files, executing a number of commands to save a user time and effort.

Protocol A set of rules governing the information flow within a communications infrastructure, often known as "data link control". Protocols control format, timing, error essential correction and running order. They are essential for a device to be able to interpret incoming information. Suites of protocols are often used in networks, with each protocol responsible for one part of a communications function.

Protocol converter A device that translates between two protocols to facilitate communications between different computers or different systems.

Protocol Independent Routing A proprietary routing method designed for IBM networks developed by General DataComm, improving on Source Routing. The router checks all LAN packets for their source address contained in each frame. Adopting the proprietary DSPF (Discover Shortest Path First) router-to-router protocol, a PIR router finds the best path between pairs of LANs, rather than simply between pairs of routers as OSPF (Open Shortest Path First). Because a router can have several LANs connected to it, OSPF does not necessarily produce the best result, whereas PIR load-balances across each available LAN link for improved throughput and performance. From a Token Ring LAN, PIR looks like a Source Routing bridge; in an Ethernet environment, PIR appears as a Transparent bridge.

Proxy agent A network management agent that sits in front of a device attached to a network that needs to be managed, but has no intrinsic network management capabilities. The proxy agent provides network management "by proxy" for that device.

PSN (Packet Switching Node) The name of an Arpanet packet switch; PSNs were formerly called IMPS.

PDN (Public Data Network) A network established by a PTT, PTO or TA to provide data transmission services for public use.

PUP (Parc Universal Packet) In the internet system developed by Xerox Corporation, a PUP is the fundamental unit of transfer, just as the IP datagram in a TCP/IP internet. The name was derived from the name of the laboratory at which the Xerox internet was developed, the Palo Alto Research Center (PARC).

Punch-down block Telephony term describing the connector arrangements for distributing and connecting UTP/STP wiring inside a building. Typically found in phone wiring closets.

PVC (Permanent Virtual Circuit) A fixed virtual circuit between two users: the public data network equivalent of a leased line. No call setup or clearing procedures are needed.
Q

Q.921/931 ITU-TS (formerly CCITT) Q Series Recommendations describing Lap- D, the Layer Two protocol for an ISDN D-channel.

Qbit (Qualifier bit) In X.25 networks, bit 8 in an octet of packet header. It is used to indicate if a packet contains control information.

QLLC (Qualified Logical Link Control) Routines to provide Synchronous Data Link Control.

Quartet Signaling Part of the 100BaseVG-AnyLAN proposed standard involving transmission over all four wire pairs simultaneously in voice-grade UTP cable, expanding its capacity to 100Mbit/s.
R

RS standards Laid down by the EIA, RS232c approximates to V.24/28; RS422 to V.11; RS423 to V.10 and RS449 to V.36.

RARP (Reverse Address Resolution Protocol) The TCP/IP protocol a diskless machine uses at startup to find its IP address.

Redirect A packet switching function that routes a call to an alternative network address if the link to the original address is not working. It is carried out by end point switches.

Redirector A LAN device driver that translates operating system requests into network events and transmits them to the right protocol stack.

Regeneration A method of boosting a digital signal. Remote bridge LAN-to-LAN bridge that links geographically distant LANs across a wide area link.

RFC (Request For Comment) Document series begun in 1969 describing the Internet suite of protocols and related experiments.

RPC (Remote Procedure Call) A means of communication between two tasks running on separate machines linked by a LAN. One machine can request a service, typically computation, from the other, by executing a high level request known as a language procedure call. RPCs are an underlying architectural element of distributed processing and client/server computing.

RPL (Remote Program Load) Starting a computer and loading the operating system into memory when the operating system software is provided by a server on the network.

Reconfiguration The process of physically altering the location or functionality of network or system elements. Automatic configuration describes the way sophisticated networks can readjust themselves in the event of a link or device failing, enabling the network to continue operation.

Red Book The 1984 compilation of the ITU-TS's standards for telecommunications, including some for ISDN.

Redirect A packet switching function that routes a call to an alternative network address if the link to the original network is not available. It is carried out by the end point switches.

Redundancy Otherwise known as fault-tolerance, in data transmission, this refers to characters and bits that can be removed from a transmission without affecting the message. Ir data processing and data communications, it means providing backup for components: should one of them fail, the system continues to run without operation. Total redundancy is usually impractical, but financial institutions and other organizations with mission-critical applications attempt to install a high level of redundancy on the basis that downtime loses money, lives, depending on the business.

Repeater A device that extends the maximum length of cable that can be used in a single network. In fiber networks, it is an optoelectrical module that receives an optical signal and converts it into an electrical form.

Ring topology A layout scheme in which the network takes the form of a closed loop with the devices attached into the ring. Every workstation is linked to two others, one on each side. All data is passed from node to node in one direction only, each PC acting as a repeater for the next one in the loop. Response time is determined by the number of stations on the ring - the more there are, the slower it works. If one PC fails, the loop is broken, though most rings have self-healing capacity to reconfigure and continue operation. IBM's Token passing ring ensures that the failed station is removed and its neighbors are then directly connected.

Ring in and Ring out Token Ring connectors on the MSAU connecting the unit to trunk cabling. The Wrap feature is implemented at these interfaces.

RMON (Remote Monitoring) SNMP specification for multivendor statistics-gathering by a standards-based (de facto SNMP) management station from de facto standard (RMON)-compliant devices.

Roaming The ability of a mobile communications device to move freely from one part the whole of a network operator's system or another.

RIP (Routing Information Protocol) The protocol used by Berkeley 4BSD Unix systems to exchange routing information among a small group of computers. Implemented by the Unix program "routed", RIP is derived from an earlier protocol of the same name developed at Xerox.

Rlogin (Remote Login) The service offered by Berkeley 4BSD Unix systems that allows users of one machine to connect to other Unix systems across an internet and interact as if their terminals connected to the machines directly. Although Rlogin offers essentially the same service as Telnet, it is superior because the software passes information about the user's environment to the remote machine.

Router Network interconnector device operating at OSI Network Layer (Level Three) that supports a particular Network Layer protocol and related stack, such as TCP/IP, DecNet, XNS, SNA, OSI IP, IPX. These days routers tend to support multiple protocols by a variety of methods, such as Protocol Independent Routing. A router can be used to link LANs together locally or remotely as part of a WAN. A network built using routers is often termed an internetwork.

Routing Process of delivering a message across one or more networks via the most appropriate path.

Routing table Information stored within a router that contains network path and status information. It is used to select the most appropriate route to forward information along.

RS232-C An EIA standard which is the most common way of linking data devices together. An interface for linking DCEs and DTES, it defines the electrical characteristics of the signals from such devices. RS232 is suitable for both synchronous and asynchronous communications and specifies a 25-pin connector, traditionally the DB-25. 20 of the pins carry out routine system operation while the rest are reserved for modem testing or unallocated. For this reason it is never safe to assume that two RS232 devices will work together. Newer, more compact RS232 interfaces have nine pins only and an adapter card is needed. It is functionally similar to the ITU-TS's V.24 and V.28 standards.

RS422 EIA recommended standard to extend the RS232 50ft limit. It is most commonly implemented on 25-pin connectors (DB-25s) and is electrically compatible with the ITU-TS V.11 standard.

RS423 The EIA recommended standard for cable lengths that extend the RS232 50ft limit. It was introduced in tandem with RS422 but is not widely used. Electrically compatible with the ITU-TS's V.10 Recommendation.
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