 the services of the Optical coupler. A component which electrically isolates the input layers below them for doing so. and output signals using light coupling between the two. Peg count. Measure or count of the number of traffic attempts OSI Reference Model. A seven layer reference model defined by made on a circuit during a given period of time. the International Standardisation Organisation (ISO), in which Personal communication networks. High capacity cellular functions are broken into seven layers, each layer providing mobile network for low cost pocket telephones. Radio links in cleary defined services to adjacent layers. the frequency range 1.7-2.3GHz. Out-band signalling. Use of frequencies outside the speech band Phase hit. Unwanted and significant shifts in phase of an analogue (e.g. 3825Hz) for signalling. signal. Out of frame alignment. The state in which frame alignment has Phase jitter. Small, continuous and often random variation in been lost. Out of frame alignment time is the time for which this phase of a transmitted signal. Glossary of terms 62/29 Phase modulation. Encoding system used in transmission in Primitive. Usually refers to the information passed between ad- which the phase shift in the carrier wave is used to represent jacent layers within a layered architecture, and not passed different bit values. outside the architecture. Photometric units. Units concerned with measurement of visible Private line. Another name for a leased line. (See leased line.) light, and with the response of the eye to this radiation. See also Private network. Usually refers to a network which is owned and radiometric units. operated by a private organisation. However the term is also Photopic. Response curve of the human eye at normal illumination used to refer to lines which are leased from a public operator levels. See also scotopic. but run by a private organisation for its own use. Physical layer. The lowest layer within the ISO OSI model, which Privitisation. Refers to the sale by government of its contro