lling is responsible for the electrical, mechanical and interface as- interests (usually in a PTT) into the private equity market. pects of transmitted data. Profiles. Within communications usually refers to the selection of Pixel. Represents the smallest unit of a video display which may a limited set of alternatives from base standards, to suit the be coded to an electrical signal for transmission. requirements of a particular application. Also known as func- Plaintext. A message before it is encrypted, after which it is called tional standards. ciphertext. Progress tones. Tones received by a subscriber informing him of Platform. Usually refers to the basic hardware and software ele- the progress of his call. Examples are dialling tone, ringback, ments, (e.g. a computer, operating system, relational database) busy tone, etc. on which an application, such as a network management sys- Propagation delay. The time taken for a signal to travel over a tem, is built. given distance in a medium. Point of presence. In the US refers to the point where the local Protection switching. The facility to recover from a failure by company (LATA) terminates a subscriber's line, for interfacing swtiching to another transmission path. For example 1+1 pro- to the long distance or inter-LATA carrier. tection switching provides a standby path which can be Point to point. A transmission circuit which links two nodes switched to if the main one fails or deteriorates, e.g. due to (users) directly, without intermediate nodes or processors. signal fading or high errors. In N+M protection switching the Polar coding. Line code using two balanced levels, positive and N paths are protected by M alternative circuits. negative, to represent binary data. Also called non return to zero Protocol. Rules, usually defined by a standards making body, for or NRZ coding. carrying out a specific function, such as exchange of informa- Polar diagram. A plot for a light source showing how the relative tion between two systems, synchronisation, error checkin