- 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.
- 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.
- PCS physical coding sublayer
- PDV path delay value
- 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.
- PHY Physical Layer entity sublayer
- PICS protocol implementation conformance statement
- PLS physical signaling sublayer
- PMA physical medium attachment
- PMD physical medium dependent
- PMI physical medium independent
- 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.
- ppd peak-to-peak differential
- PPP (Point to Point Protocol) An acronym for Point-to-Point Protocol, PPP is a communications protocol used to transmit network data over telephone lines. It allows you to connect your computer to the Internet itself, rather than logging on through an Internet service provider's host computer and using UNIX commands through a shell. This type of connection lets you communicate directly with other computers on the network using TCP/IP connections. It is part of the TCP/IP suite of programs necessary to connect to and use the Internet.
- 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.
- 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.
- PVV path variability value