nique.) passed between layers in the OSI Seven Layer Programmable Read Only Memory. (Memory tech- model.) nology, usually semiconductor, where the data is Peak Envelope Power. written, or programmed, once by the user and there- Presentation Function. after can only be read and not changed. See also Power Flux Density. (Measure of spectral emission ROM.) strength.) Packet Switched Data Network. (Or Public Pulse Frequency Modulation. (An analogue modu- Switched Data Network. X.25 network, which may lation technique.) be private or public.) Personal Identification Number. (Security number PSPDN Packet Switched Public Data Network. used for items such as remote database entry.) Packet Switching Exchange. Positive Intrinsic Negative. (Semiconductor struc- Phase Shift Keying. (Analogue phase modulation ture used for some types of diodes.) technique.) Plant Level Controller. Packet Switched Network. Phase Locked Loop. (Technique for recovering the Public Switched Network. clock in transmitted data. Often performed by an Packet Switched Service. (Data service offered by integrated circuit.) Public Land Mobile Network. Public Switched Telephone Network. (Term used to Packet Level Protocol. describe the public dial up voice telephone network, Payload Manager. (Within an SDH.) operated by a PTT.) Phase Modulation. (Analogue signal modulation Pacific Telecommunications Council. technique.) Public Telecommunications Network. Private Manual Branch Exchange. (PBX with con- Public Telecommunication Operator. (A licensed nections between extensions done by an operator.) telecommunication operator. Usually used to refer to Public Mobile Radio. a PTT.) POCSAG Post Office Code Standardisation Advisory Group. Postal, Telegaph and Telephone. (Usually refers to (Name given by study group to U.K. digital paging the telephone authority within a country, often a system. Adopted by CCIR as Radio Paging Code publicly owned body. The term is also loosely used RPC No. 1.) to describe any large telecomunications carrier.) Priority-Oriented Dem