ital code, during the quantisation success or failure of the operation. process, and the original analogue sample value. Preamble. Usually refers to the pattern of data inserted before the Quantum efficiency. Number of electrons released in a photosen- main message in a transmission frame, and used for control sitive component for each photon of incident radiation of a functions such as synchronisation, error correction, etc. given wavelength. Presentation layer. The layer in the ISO OSI reference model which enables the application layer to interpret the data being exchanged between two systems. Radiometric units. Units used to measure the radiation of all wave- Price cap. Regulation which fixes the maximum price which can lengths within the optical spectrum. It is therefore concerned be charged for certain goods or services. with total radiation detection. See also photometric units. Primary cell. A cell which cannot be recharged and re-used be- Raised cosine channel. A transmission channel having no inter- cause of irreversible chemical reactions which occur in it when symbol interference at the sample times of adjacent signalling it is producing a current. See also secondary cell. intervals. 62/30 Glossary Random noise. A form of noise or interference which is also Secondary cell. A cell which can be recharged and re-used after it known as white noise or Gaussian noise. has become discharged. See also primary cell. Real time. A system in which interchange of data takes place Self test. A facility for testing the equipment without use of any interactively. The response to a message is fast enough to affect external test signals. Usually done by connecting its output to the subsequent message. its input, i.e. loopback. Receive only device. An element which is capable of receiving a Serial interface. An interface to equipment in which the bits mak- transmission but not generating any. Example is a printer con- ing up a character are available serially, i.e. bit by bit. nected to a line. Session layer. The layer 