nnection point of a functional unit to the trans- Monomode fibre. Optical fibre which is designed to carry light of mission line. Often refers to the physical device used for trans- a single wavelength at a time. mission of data within a network. Monopulse. A tracking system which uses multiple feed horns. Noise. Generally refers to any unwanted signal in a transmission Monostable. A component or system which has only one stable system. It is usually random in nature and interferes with the mode, and will return to this mode at the first opportunity. See wanted signal. also bistable. Noise figure. A measure, in units of decibels, of the noise con- u-law. A companding technique used for PCN systems in North tained in a system. Usually stated as the ratio of the system's America. (Also written as mu-law.) input signal to noise ratio to its output signal to noise ratio. 62/28 Glossary Noise equivalent power. A measure of the noise generated in a state lasts and includes the time to detect loss of alignment and device such as a photodiode. It is the amount of light needed to to recover from it. produce a signal equivalent to the noise level. Outsourcing. Term applied to the use of third parties for mainten- Noise suppressor. A filter element which removes or suppresses ance and management of a privately owned network. the unwanted signal from a transmission. Overhead. Information added to a user's transmitted message for Noise temperature. A measure of the noise in a system. It is control purposes, e.g. for error checking, synchronisation, etc. defined as the absolute temperature of a resistive source, in Overlay network. A telecommunication network which is added degrees Kelvin, which delivers an equal noise power. over essentially the same route as an existing network, but does Nonblocking. Usually refers to a switch which has a through not replace it. The overlay network usually carries suplemen- traffic path for every user attached to it, so that it is able to pass tary services. through all calls. Nonpersistent 