g, etc. intensity of light emission varies with angular displacement. Protocol stack. Usually refers to the software layers used for com- Polarisation. The rotation of a signal within an electromagnetic munication between systems. Examples are the seven layer OSI wave. protocol stack and the protocol stack used within TCP/IP. Polarised light. Light in which wave motion has been modified. In Proximity switch. A switch which operates when it is within a set linearly polarised light the wave motion is in one direction only, distance of an object, but where there is no mechanical contact in a plane perpendicular to the direction of motion. between the switch and the object being sensed. See also limit Polling. System in which a central master node asks each node in switch. turn to transmit data for a specified time. Pseudorandom noise generator. A noise generator obtained by set- Polling delay. The time interval between two consecutive polls of ting the generator to a very long periodic sequence, SO that the a node by a master node. output stream appears to be random. Port. The physical access point into and out of an electrical equip- Psophometric weighting. A noise weighting filter which is pro- ment or a network. posed within CCITT recommendations. Power spectral density. The case where signal power is distributed Public network. Usually refers to a network owned and operated as a function of frequency. by a licensed telecommunication authority, providing a PSTN Positive acknowledgement. Error control procedure where reci- service to the public. pient acknowledges receipt of correct data. (See also negative acknowledgement.) Positive temperature coefficient. A parameter which increases as Quantisation. Process of sampling an analogue signal to transform the temperature is increased. it to digital form. Post dialling delay. The delay between the completion of dialling Quantisation noise. The difference between a discrete sample by a subscriber and the receipt by him of a signal indicating value represented by a dig