 requested resource either at the application layer or at the session or transport layer. PSK (phase-shift keying) A modulation technique for transforming digital information to analog whereby that information is conveyed as varying phases of a carrier signal. PSTN (public switched telephone network) The complete traditional public telephone system, including telephones, local and interexchange trunks, transport equipment, and exchanges. PTO (public telecommunications operator) An incumbent carrier. This term is used in countries other than the United States. PTT (post, telegraph, and telephone) organization Usually a governmental depart- ment that acts as its nation's common carrier. Also called PT&T. public key A device used by algorithms that encrypt and decrypt using asymmetric yet math- ematically linked keys. Each security module is assigned a pair of keys: The encryption key is public and does not require distribution by secure means. The decryption, or private, key cannot be discovered through knowledge of the public key or its underlying algorithm. Public key algo- rithms can apply to one or more of the following: key distribution, encryption, authentication, or digital signature. pulse A momentary, sharp alteration in the current or voltage produced in a circuit to operate a switch or relay that can be detected by a logic circuit. A pulse is a sharp rise and fall of finite duration. punchdown block A common termination point in the wiring closet for wires going out to the individual offices and wall sockets. push technology A program that updates news, weather, or other selected information on a computer user's desktop interface through periodic and generally unobtrusive transmission over the World Wide Web. push-to-talk A mobile application that turns a mobile phone into a walkie-talkie by giving it trunked radio functionality. Sometimes abbreviated PTT. push-to-x A mobile application that is based on push-to-talk and combined with other appli- cations, including presence, picture sharing, and video cl