P p See pico. P-ROM Partial ROM. An optical disc medium made by Verbatim that has magneto-optical and write-once sectors on the same 3.5-inch disk. P6 The code name from Intel for the successor to the Pentium. P7 Code name from Intel for the sucessor to the P6. Chip is expected to have as many a 25 million transistors. PA Public address. PABX See private automatic branch exchange. pacing The process of sending a predefined quantity of information before receiving a response acknowledging receipt of the previously transmitted data. packet An arbitrary collection of data grouped and transmitted with its user identification over a shared facility. packet assembler/dissembler (PAD) Equipment in a packet network used to convert the format of data from asynchronous or synchronous terminals for transmission over a packet network. packet radio Digital, connectionless, two-way, wireless data communications services that utilize channels in the 800- to 900 MHz SMR band, operate at between 4800 and 19.2K bps. per channel, and support transparent subscriber roaming and call handoff throughout a national coverage area. packet switched data networks (PSDN) A network designed to carry data in the form of packets. packet switching Process where long messages are broken down into small units called packets that are individually addressed and routed through the network; the channel is occupied only for the duration of the packet and is then available for the transfer of other traffic. packet switching network Network in which data is transmitted in packets. Packets can be routed individually over the best available network connection and reassembled to form a complete message at the destination. packet-switching module interface Provides interface between a PBX and a packet-switching module; data terminals are provided with speed, codes, and protocols to process data calls to packet-switching networks. packing density Amount of storage area per unit length, unit area, or unit volume (e.g., number of bits per inch stored on magnetic tape). pACT Personal air communications technology. PAD See packet assembler/disassembler. page composition The act of laying out a page, placing photos and art in place, determining where text and headlines go. See layout. page decomposition The ability of OCR software to separate text from graphics and maintain column/tabular formatting. page definition A software object containing formatting information for printing raw data in a readable way. page description language A device-independent, high-level language for defining printer output. If an application generates output in a page description language, such as PostScript, the output can be printed on any printer that supports it. page end A non-printing command character that divides pages in an electronic document into pages on a printer. page printer A printer that uses sheets of paper rather than continuous tractor-fed paper. page proof The ways to see a proof of a desktop published document include a desktop black and white laser printer; a desktop color printer; a high-resolution, high-quality color printer from an output service bureau; and contact prints made from the finished output negs. page recognition Software that recognizes the content of a printed page which has been scanned into the computer. pages per minute Measurement of a scanner's or printer's speed. See PPM. paging Direct attendant access and station user dial access to loudspeaker equipment. Special sets with internal speakers provide paging access to speakers, eliminating the need for separate speaker systems. paging by zone By dialing an access code, a telephone can page specific groups of phones or speakers. paging, radio A signal is transmitted via radio to customers carrying pocket, radio receivers (beepers) which emit a buzz or beep tone. The customer then can answer the page by dialing an answering code from a station within the PBX. paging, radio access with answerback Permits access to customer provided paging systems and can connect the paged party, when he or she answers the page, by dialing a special code from the PBX. paint program Graphics software that simulates painting on screen with the use of a graphics tablet or mouse. Paint programs create raster graphics images. PAL 1. Programmed Array Logic. 2. See phase alternate line. 3. Paradox Application Language (Paradox's programming language). palette 1. In computer graphics, the total range of colors that can be used for display. May also refer to the collection of painting tools available to the user. 2. A set of functions or modes. palmtop A computer small enough to be held in one hand and operated with the other. PAM 1. Primary Access Method (Unisys). 2. See pulse-amplitude modulation. pan 1. To move (while viewing) horizontally across a text record. 2. In computer graphics, to move (while viewing) to a different part of an image without changing magnification. paneling When an image is too large for the printer, it can be printed in panels and pasted together. Pantone Matching System A means of describing colors by assigning them numbers. It's a de facto standard color description language adopted by commercial printers. parallel transmission The simultaneous transmission of all bits making up a character or byte, either over separate channels or on different carrier frequencies on the channel. parity The maintenance of sameness where data is checked during transmission to detect whether or not it has been altered. parity bit A binary digit appended to a collection of digits to make the sum of the digits always odd or always even. parity check Addition of non-information bits to data, making the number of ones in a grouping of bits always even or always odd. This permits detection of bit groupings that contain single errors and can be applied to characters, blocks, or any convenient bit grouping. park 1. A feature that enables calls to be transferred to a busy extension; the call can be connected to the extension when it is free. See also camp-on. 2. To retract the read/write hed on a hard disk to its home location before the unit is physically moved in order to prevent damage. part 68 FCC specifications indicating the minimum acceptable protection that communications equipment must provide to the telephone network. party line Telephone line that is shared by multiple subscribers. party line stations Two party telephone service that is expanded to support multi-party service. Pascal A compiler language (named after Blaise Pascal) that is noted for its structured constraints; initially developed as a teaching language password A word or code used to identify an authorized user. password protection The use of passwords as a security measure to protect against unauthorized access. patch Temporary jack and cable connection. Path attenuation The losses undergone by a wave in transit between a transmitter and a receiver. pattern recognition An OCR technique that uses libraries of information about how characters are built - artificial intelligence experts - in fonts and sizes. Also called feature extraction, because the experts examine certain parts, or features, of the characters and cumulatively agree on whether the character meets enough criteria to identify it as one character or another. PATX See private automatic telex exchange. PAX See private automatic exchange. PBX See private branch exchange. PBX attendant position The equipment used by the PBX operator for controlling calls and connecting functions not performed automatically. PBX extension See extension, PBX. PBX main station The designated primary telephone instrument associated with a PBX. PBX station line The permanent electrical circuit and incidental components between the PBX station line circuit appearance and point of connection with the station equipment. PBX station line circuit A network of circuit elements used to connect a PBX to one of its associated lines. PBX tie line See tie trunk. PBX trunk A transmission path between a PBX and a commercial central office. PC 1. Personal Computer. 2. Peripheral Controller. 3. Program Controller. 4. Printed Circuit. PC AT Architecture See Industry Standard Architecture. PC bus The bus architecture used in first-generation IBM PCs. It referes to the original 8-bit bus. PC card 1. A credit-card sized removable module that contains memory, I/O, or a hard disk. It is the PCMCIA's trademark for its PC card standard. See PCMCIA. 2. An expansion board for a PC. PC-DOS Personal Computer Disk Operating System (IBM). PCB Printed circuit board. PCBX See private computerized branch exchange. PCCA Portable Computer and Communications Association. PCI Peripheral Component Interconnect. A local bus for personal computers that provides a high-speed data path between the CPU and peripheral devices. PCL Hewlett-Packard's proprietary page description language (PDL) for its laser printers. Stands for Printer Control Language. PCL Level 5, resident in the HP LaserJet Series III of printers, supports fully scalable typefaces and rotation of text. PCM See pulse-code modulation. PCM-30 An international 2.048M bps T1 and E1 service derived from the fact that 30 channels are available for 64K bps digitized voice each using pulse code modulation (PCM). PCMCIA See Personal Computer Memory Card International Association. PCN See Personal Communications Networks. PCP Primary Control Program. PCS See personal communications service. PCU 1. Program Control Unit. 2. Punched Card Unit. PD-IS pACT data intermediate. PDA See Personal digital assistants. PDL See Page Description Language. PDM See pulse-duration modulation. PDN See public data network. PDP 1. Procedure Definition Processor. 2. Programmable Data Processor (Digital Equipment). PDS Packet Driver Specification; a specification that is similar in purpose to NDIS. Like NDIS, PDS provides a common application programming interface for LAN adapter device drivers and higher level protocols. PDS can support multiple protocols at the same time on a single adapter. PDU Protocol Data Unit. PDX See private digital exchange. peer communications Form of communication in which two parties contact each other as equals rather than in a master/slave relationship. A program and a terminal form a master/slave combination, but two programs form a peer combination. Peer combinations protocols usually provide more powerful interaction facilities. peer-to-peer communication Allows networked users to independently initiate communications sessions at their discretion in order to communicate and share resources with others on the network. Nodes that communicate as peers also share the same protocol layer, or logical unit (LU). peer-to-peer model Enables any workstation on a network to act as a server or client, or both. Networked users can independently initiate communications sessions at their discretion, or work on their own applications while others access their files (or vice-versa). peer-to-peer network 1. A term describing the relationship between a telephone system and the external computer working with it. 2. A network in which every node has equal access to the network and can send and receive data at any time without having to wait for permission from the control node. peer-to-peer resource sharing An architecture that allows any station to contribute resources to the network while still running local application programs. peg count A rough count of some event, usually the number of calls made or received in a given period. PEL Picture Element. PEL Picture element, the smallest graphic unit that can be displayed on a screen. pen plotter See plotter. pen-based computing Entering data into a computer with an electronic stylus or pen, and a pad that accepts the stylus' input. PER Program Event Recording. perfecting Printing both sides of a sheet in the same pass. peripheral Any hardware device attached to a computer, such as a hard drive, printer, scanner, or CD-ROM player. peripheral device or equipment Equipment that works in conjunction with a computer or processor and provides the system with outside communication. permanent virtual circuit A permanent association exists between two units of DTEs, and is a point-to-point switched circuit over which only data, reset, interrupt, and flow control packets can flow. Personal Communications Networks A type of wireless telephone system that communicates via low-power antennas and use light, inexpensive handheld handsets. Personal Communications Service Individualized telecommunications services permitting users to communicate regardless of location. The low power, wireless communications technology utilizes lightweight, inexpensive pocket telephones and personal computers to transmit voice, fax and data from anywhere. Personal Computer Memory Card International Association (PCMCIA) The association that standardizes credit-card size packages for memory and I/O (modems, LAN cards, etc) for computers, laptops, etc. There are three standards for PCMCIA cards, Type one, two, and three. personal digital assistants (PDA) An electronics device that acts like a personal carry along database, calculator, alarm clock, and a personal communicator. It looks like a palmtop computer, and the communications are performed through the telephone or through wireless. PERT Program (or project) Evaluation and Review Technique. PEX (PHIGS Extension to X) The PEX protocol enables an X Window System to render three-dimensional graphics on X terminals. PF Programmable Function. PF/R Power Fail/Restart. PGH Presentation Graphics Facility. PGP Pretty good privacy, a public domain program that provides PC users on the Internet with the ability to encrypt e-mail messages for privacy and security. phantom circuit Circuit configured so that two pairs of wires can provide three speech paths. phase A Phase A is the first part of a fax machine's call process. It is the call establishment. It occurs when transmitting and receiving units connect over the phone line, recognizingone another as fax machines. This is the start of the handshaking procedure. See phase B. phase alternate line (PAL) A color television system developed in West Germany that minimizes phase errors and improves color performance. phase B Phase B is the second part of a fax machine's call process. It is the premessage procedure, where the answering machine identifies itself, describing its capabilities in a burst of digital information packed in frames conforming to the HDLC standard. See phase C. phase C Phase C is the third part of a fax machine's call process. It is the fax transmission portion of the operation. This step consists of two parts C1 and C2 which take place simultaneously. Phase C1 deals with synchronization, line monitoring, and problem detection. Phase C2 includes data transmission. See Phase D. phase change ink jet A form of ink-jet printing in which solid inks are liquefied and jetted onto the page, where the ink immediately resolidifies. phase change recording A rewritable optical recording technique. A laser causes the medium to crystallize in a controlled way, reflecting light either into or away from the reading laser. phase D Phase D is the fourth part of a fax machine's call process. This phase begins once a page has been transmitted. Both the sender and receiver revert to using HDLC packets during Phase B. If the sender has further pages to transmit, it sends an MPS and Phase C recommences for the following page. See Phase E. phase E Phase E is the fifth part of a fax machine's call process. This phase is the call release portion. The side that transmitted last sends a DCN frame and hangs up without awaiting a response. phase jitter Random distortion that results in the intermittent shortening or lengthening of the signals. phase modulation (PM) Modulation in which the angle relative to the unmodulated carrier changes according to the value of the amplitude of the modulating signal. phase-shift keying (PSK) A modulation technique in which the phase of the carrier is modulated by the state of the input signal. PHIGS Programmer's Hierarchical Interactive Graphics Interface; an ANSI and ISO two- and three-dimensional computer graphics development library that became an international standard in 1988. Phong A technique used in 3D illustration rendering that determines the brightest spot and the darkest spot in the image, and spreads out the remaining values. phosphor Substance that emits light when struck by electrons. This is the substance that coats the back of a CRT video display. Photo CD A format for storing and retrieving color images on CD, developed by Kodak and Philips. Can also be used for data. photocomposition The manipulation and transfer of graphic images and text, using photographic means, to a light-sensitive paper or film. photoconductor 1. Any transducer that produces a current which varies in accordance with the incident light energy. A fiber optic communications term. 2. Material, available in many forms (sheets, belts, and drums), which changes in electrical conductivity when acted upon by light. photodiode Device that detects optical power and converts it to electrical current. photooptic memory An optical storage technique that uses a laser to record data onto a photosensitive film. photosensor The light-sensitive reading device used in optical scanners. phototypesetter Device that uses photographic techniques to reproduce machine-readable text on light-sensitive paper and film. physical layer The first (lowest) layer in the seven layer OSI reference model of data communications protocols; it is responsible for the electromechanical interface to communications media. physical medium See link. PIC 1. Priority Interrupt Controller. 2. Processor Interconnection Channel. 3. PICture - a file extension used for graphics formats. PICK Business-oriented operating system (Pick Systems). picker In jukeboxes, the hand on the robotic arm that grasps and moves the disc to or from the storage slot, disc drive, or mailslot. pico (p) A prefix for one trillionth of a unit. PICT A Macintosh bitmapped (raster) image file format. PICT2 An expanded version of PICT. picture element The smallest discrete part of a video image, the size that is controlled by an analog-to-digital conversion sampling process. PILOT Programmed Inquiry, Learning, Or Teaching. pilot model A model of a system used for program testing purposes; the model is simpler than the system it represents. The files used on a pilot model, for example, can contain fewer records than the operational files; there can be fewer lines and fewer terminals per line. pilot tone A tone of a specific frequency that is associated with a certain transmission system. PIN See positive intrinsic negative. ping Packet Internet Groper is a program and UNIX command that is used for testing and debugging network connections. PIO Parallel Input/Output. PIOCS Physical Input/Output Control System. pit Broadly used now for all the data-carrying marks in optical discs. Originally meant the rimless troughs in the photo-resist layer of optical disc masters. Pits and flats (the unmarked areas) represent data. pitch 1. The number of printed characters per inch measured horizontally. 2. The distance between grooves (measured center to center) on an optical disc. pixel See picture element. PKS See public switched network. PL/1 Programming Language/1. planetary camera A microform camera system in which the document is held still on a copyboard while film is being exposed. Once the document is recorded, it is replaced with the next document to be filmed, and the film in the camera is advanced. More accurate, but slower, than a rotary camera. plat When a CAD/CAM plotter prints something, the drawing is called a plat. platemaking The act of exposing a printing plate through a negative or a flat of negatives. platen The cylinder in impact printers and typewriters around which the paper goes and which the printing mechanism strikes to produce an impression. platter 1. Another word for optical disc. 2. The circular recording parts of a hard disk. plot To create an image by drawing a series of lines. In programming, a plot statement creates a single vector (line) or a complete circle or box that is made up of several vectors. plotter A graphics printer that draws images with ink pens. It prints vector graphics, i.e., images created by a series of many straight lines. plotter font A font created in vector graphics, i.e. a series of end prints connected by lines. PLP Presentation Level Protocol. A North American standard protocol for videotex. plug-in-stations A ready-made station that can be moved from one location to another and activated by inserting it into a socket. PM See phase modulation. PMA Priority Memory Access. PMB PROM Memory Board. PMBX See private manual branch exchange. PMF Print Management Facility. How IBM controls page printers from mainframes. PMOS P-channel metal oxide semiconductor. PMS 1. Project Management System (IBM). 2. Public Message Service. 3. Pantone Matching System. point-of-presence (POP) Physical place at which responsibility for managing inter-LATA circuits changes from the local telephone company to the long distance company. point-to-multipoint network A network configuration interconnecting a point to multiple sites. point-to-point A private circuit, conversation or teleconference in which there is one person at each end, usually connected by a dedicated transmission line. point-to-point connection An uninterrupted link between two pieces of equipment. point-to-point network A network configuration that interconnects only two points. The connection can be dedicated or switched. pointer A logical link from a database record to another record, file, or image stored elsewhere. POL Problem Oriented Language (Unisys). polar transmission A method of transmitting teletypewriter signals, whereby the marking signal is represented by direct current flowing in one direction and the spacing signal is represented by an equal current flowing in the opposite direction. In tone signaling, there are three states. Also called polar keying. polarization The direction of the electric field in the lightwave. polling Allows stations on a multipoint circuit to transmit without contending for lines. The communications device or computer polls each terminal by transmitting a message that the terminal acknowledges by indicating its need for service. If a negative response is received by the polling device, polling continues with the next terminal in sequence until all terminals on the polling list are queried. If the terminal has a message to send, the poll causes the terminal to be activated. Polling avoids the problems of contention. polling list The order in which stations are polled -- maintained in a list associated with each channel. A line polling list may also be used to provide priority in line service. PON Passive optical network. POND Print ON Demand. POP See point-of-presence. port 1. Entrance to or exit from a network. 2. In data communications, that part of a data processor dedicated to a single channel for receiving data from or transmitting it to one or more external devices. portability The ease with which an application can be moved from one hardware/software system to another. portal A device that allows unlike systems to communicate over a common network link. portrait Page or monitor orientation in which the page height exceeds the page width. Contrast with landscape. POS 1. Point Of Sale. 2. Primary Operating System. POSI Japan's Promoting Conference for OSI. positive Photographic print that accurately represents the tone values of the original. positive intrinsic negative (PIN) Type of photodetector used to sense lightwave energy and then to convert it into electrical signals. POSIX Portable Operating System Interface for Computer Environments (UNIX-compatible); a standard operating system specified by the IEEE and endorsed by the US Federal Government. Postal Telephone and Telegraph (PTT) Administratio The government agency that acts as a common carrier in most countries outside North America. posterization Video effect which reduces the range of color variation in a video image so that it looks flat or two-dimensional, like a paint-by-number picture. PostScript A page description language from Adobe Systems that translates graphics created in a computer into the printer's machine language by a PostScript interpreter built into the printer. POTS Plain old telephone service. power failure transfer If the PBX is not receiving enough power, this feature provides limited service to and from the exchange network. power level The ratio of the power at a given point to an arbitrary amount of power chosen as a reference. Usually expressed in decibels based on 1 milliwatt (dBm) or 1 watt (dBw). PPB PROM Programmer Board. ppm Pages per minute. The measurement of printing or scanning speed. PPP Point-to-point protocol - a direct-connection protocol standard that lets two computers communicate via a physical link. PPS 1. packets per second. 2. pulses per second. 3. Page Printing System. 4. Page Processing System (Bull HN). PR/SM Processor Resource/Systems Manager (IBM). PRBS A test pattern having the properties of random data, but generated in such a manner that another circuit, operating independently, can synchronize on the pattern, and detect transmission bit errors. preemption The act of interrupting a lower priority call or message to use the same circuit to transmit an urgent message. prefix Any digit dialed before a phone number, usually indicating predetermined instructions to the phone system. prepress The preparation work required, including typesetting and page markup, to turn camera-ready artwork into the printing plates needed for mass production. presentation data value The unit of a file, described in abstract syntax, that is transferred by the Presentation Layer during FTAM activities. presentation layer Sixth layer of the OSI model that provides services to the application layer such as data definition, managing the entry, exchange, display, and control of structured data. preset call forwarding Feature that enables a user to re-route incoming calls to a pre-determined secondary number. pressure-sensitive pad A digitizing tablet that can measure the amount of pressure placed on the stylus, as well as its location. Used in paint packages to great effect. preventive maintenance Maintenance, such as periodic inspection, cleaning, and adjustment intended to prevent system malfunctions. prewrite defect A defect in the groove of a pre-grooved optical medium. Measure in ratio to the available total recording time on the disk. PRI See primary rate interface. primary area A customer's local calling area. primary block See primary group. primary carrier First-choice provider of long-distance service. primary center A Class 3 Central Office or i.e., a control center connecting toll centers. primary group The lowest level of the multiplexing hierarchy in which a group of basic signals are combined by multiplexing. primary rate interface (PRI) An ISDN term that refers to 23B + 1D channels, operating at 1.544M bps in the U.S. or 30B + 1D channels operating at 2.048M bps in Europe. primary routing point The switch designated as the control point for a longhaul telephone call. primary station A station responsible for controlling a data link: controls one or more secondary stations. primitive 1. In computer graphics, a graphics element that is used as a building block for creating images, such as a point, line, arc, cone, or sphere. 2. In programming, a fundamental instruction, statement, or operation. 3. In microprogramming, a microinstruction or elementary machine operation. PRIMIX UNIX-based operating system (Prime Computer). PRIMOS Prime operating system (Prime Computer). print engine The unit within the printer that does the actual printing. print screen The command that instructs a computer to send the image presently on the screen to a printer. See screen dump. print server Program operating on a microcomputer connected to a LAN that permits other network users to obtain shared access to a printer connected to the PC operating the print server program. print spooler A separate cache of memory dedicated to accepting and sending print data to a printer, allowing the user to run another program without interference. printer A device that converts computer output into printed images. printer driver Software routine that converts an application program's printing request into the language the printer understands. printer font A font used for printing. printer server See print server. priority indicators Indicates message priorities (e.g., urgent, rush, routine, or deferred) via a code in the message header in order to determine the sequence of transmission. priority trunk queuing PBX feature that places a caller with a high, self-chosen trunk access level ahead of callers waiting for the same trunk group. PRISM Parallel reduced instruction set multiprocessor (HP/Apollo). privacy and privacy release Prevents anyone from entering a conversation in progress unless the initiating telephone releases the privacy feature. private automatic branch exchange (PABX) Small, local telephone switch for a business in which users must dial 9 to access a local line; virtually synonymous with PBX. private automatic exchange (PAX) Small local telephone switch that serves inside extensions only, with no ability to connect calls to the public telephone system. Also called private exchange. private automatic telex exchange Telephone switch used for a telex network within an organization. private branch exchange (PBX) A private, automatic telephone exchange connected to the public telephone network that manages calls without operator assistance. private central office line See direct department calling. private computerized branch exchange (PCBX) A private branch exchange that is computer controlled, allowing the capabilities and designations of each circuit to be programmed and stored in memory. private digital exchange (PDX) Local telephone switching service utilizing digital technology. private exchange (PX) See private automatic exchange. private line The channel and channel equipment furnished for the exclusive use of a customer organization, without interexchange switching arrangements. private line service An outside exchange that operates independent of the PBX. private manual branch exchange (PMBX) A local telephone system with manual switching under the control of an operator. private network A network established and operated by an individual business or organization. process color A color printed from four separate printing plates. Four-color process printing uses cyan, magenta, yellow, and black (CMYK) inks to produce full color reproduction. processing, batch A method of computer operation in which a number of similar input items are accumulated and grouped for processing. processor A device capable of receiving and manipulating data and supplying results usually of an internally stored program. PROCLIB Procedure library. PROFS Professional Office System (IBM). programmable read-only memory (PROM) Nonvolatile memory that is permanently programmed by the equipment manufacturer into a computer and can be modified only with special equipment that erases the previous program. Unaffected by power loss. progressive conference Parties can be added and sometimes deleted from a conference call while in progress. progressive proofs Also called progs. Color prints of each of the process colors' negatives, layered in register to show what the final printed piece will look like. PROLOG Programming in Logic (programming language). PROM See programmable read-only memory. propagation delay Time it takes for a signal to travel through a conductor from one point to another. proportional spacing Character spacing based on the width of each character. protocol A strict procedure required to initiate and maintain communication. Protocols can exist at many levels in one network, such as link-to-link, end-to-end, and subscriber-to-switch. protocol analyzer A device or software application that allows the user to analyze the performance of network data to ensure that the network is operating within the parameters of the network specifications. protocol conversion Procedure that permits communications between different protocols. prototyping A software engineering process in which a model of a system is quickly created and run or simulated to show users what the final system will do, as well as to determine the viability of a given approach to a system's design. PSC Public Service Commission. See Public Utility Commission. PSDN See packet switched data networks. PSE Packet switching exchange. psec Picosecond (trillionth of a second). pseudorandom noise modulation A spread-spectrum communications technique in which each digital bit is chopped into chips that are phase-shifted in a random pattern by codes to stimulate noise. The chip pattern can be rephased by matched-filter techniques. PSI Peripheral Subsystem Interface. PSK See phase-shift keying. PSL Pocket Select Language (Unisys). PSN See public switched network. PSTN See public switched telephone network. PTAT Private translantic fiber optic cables. PTM/OS Programmable Terminal Multiplexer/Operating System (TI). PTT Public telephone and telecommunications. PU Physical Unit; a term used in the SNA environment to identify a printer, terminal, or PC address. PU 2.1 is an IBM protocol that supports multiple conversations during a session, thereby facilitating cooperative processing. public data network (PDN) A shared-resource data network that offers data communications services to public subscribers. public key system (PKS) An asymmetrical, two-key encryption algorithm that transforms data from plain text to cipher text with one key that is made public, and converts cipher text back to plain text with a different key that remains secret. public switched network (PSN) Any switching system that provides circuit switching to many customers. In the U.S., there are four: Telex, TWX, telephone, and Broadband Exchange. public switched telephone network (PSTN) The complete public telephone system, including trunk lines, exchanges, and telephones. Public Utility Commission (PUC)/Public Service Com State commission regulating intrastate communications. PUC See Public Utility Commission. puck An input device used to create graphics, like a mouse but equipped with a number of special-function pushbuttons. pulse A sharp change in the current or voltage produced in a circuit used to operate an electrical switch. pulse dialing See rotary dial. pulse modulation Transmission of information by modulation of a pulsed or intermittent carrier. Pulse width, count, position, phase, and/or amplitude can be the varied characteristic. pulse stuffing A method for allowing mismatches on timing signals of digital circuits. One flow of data has bits appended so that its final rate is the same as the master clock. pulse width modulation (PWM) Uncommon method of modulating a signal, in which the duration of a pulse width is varied according to some characteristic of the modulating signal. pulse-amplitude modulation (PAM) A form of modulation in which the pulse carrier's modulation varies according to a characteristic of the original analog signal; the pulse width is constant. pulse-code modulation (PCM) Modulation of an analog signal in accordance with a specific code. pulse-duration modulation (PDM) A form of pulse modulation in which the durations of pulses are varied. pulsing Transmission of a telephone number to the telephone company in digital form by sending pulses of current through the circuit. push-down queue A method of maintaining a priority list of items waiting to be processed in a system in which the last item added to the list is the first item processed (LIFO). push-up queue A method of maintaining a priority list of items waiting to be processed in a system in which the first item added to the list is the first item processed (FIFO). pushbutton dialing The use of keys or push buttons instead of a rotary dial to generate a sequence of digits to establish a circuit connection. The signal form is usually multiple tones. Also called tone dialing, touch-call, touch-tone. pushbutton to rotary conversion A feature that allows use of pushbutton telephones even when touch-tone circuits are not provided or are not available from the CO. PVC See Permanent virtual circuit. PVN Private Virtual Network. PWM See pulse width modulation. PX See private exchange.