se time, and availability. pervasive computing An environment in which computers are taken out of standalone boxes to which people are tied and put into ordinary things, in everyday objects. Also called ubiquitous computing. P-frame (predicted frame) In MPEG terminology, a frame that is only reconstructed from preceding reference frames. It can also be a reference frame because it reconstructs other frames in some instances. PGP (Pretty Good Privacy) PGP (Pretty Good Privacy) A technique for encrypting messages. Using PGP is one of the most common ways to protect messages on the Internet because it is effective, easy to use, and free. PGP is based on the public key method, which uses two keys: a public key disseminated to individ- uals who should receive a message and a private key used to decrypt messages that are received. phantom circuit A third voice circuit that is superimposed on two 2-wire voice circuits. phase The angle of a waveform at a given moment. phase jitter A random distortion of signal lengths caused by the rapid fluctuation of the fre- quency of the transmitted signal. Phase jitter interferes with interpretation of information by changing the timing. phased-array antenna A small, flat antenna that is steered electronically. It is actually a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. It provides great agility and fast tracking, as well as the ability to form multiple antenna beams simultaneously. It allows for very fast and pre- cise steering of the communications beam, which is very important for high-bandwidth commu- nication because the data rate is inversely proportional to the angular offset. PHS (Personal Handyphone System) A Japanese standard for 2G PCS wireless networks. PHY (physical layer) Layer 1 of the OSI model. It defines the electrical, optical, mechanical, and procedural characteri