n system which uses an inter- of frequencies or data rates. mediate point for the storage and forwarding of messages Near end crosstalk. Crosstalk which occurs on the transmission between two communicating systems. line at the end where the signal source is located. The energy Microcells. Small cells used within cellular communication sys- usually propagates in the opposite direction to the main signal. items, e.g. PCN, so that r.f. propagation is confined to a small (See also far end crosstalk.) local area. Negative acknowledgement. Error control procedure where the Microcode. Usually refers to software code which is designed to recipient raises a flag when an error is detected in received data. be unalterable. See also positive acknowledgement. Microprocessor. A powerful and flexible integrated circuit which Negative temperature coefficient. Aparameter which decreases as can be changed by software for different functions. the temperature is increased. Mid span meet. Term usually applied to the capability to inter- Network layer. The layer in the ISO OSI model which is respon- operate equipment (sourced from different vendors) at the sible for managing data on a network wide basis. transmission cable interconnection level. This is also referred Network topology. The physical or logical arrangement of nodes to as mid fibre meet when the transmission media is optical located on a network. Examples of topologies are star, tree, fibre cable. ring, bus, etc. Modulation. The processing of a signal to make it suitable for N-key lockout. A feature of keyboards which prevents an output sending over a transmission line. signal when two or more keys are depressed simultaneously. Modulo-N counter. A counter which counts up to N before reset- See also N-key rollover. ting to zero. N-key rollover. A feature of keyboards which enables an operator Monochromatic. A beam of light which has a single frequency or, to depress keys in rapid sequency. See also N-key lockout. in practice, a very narrow band of frequencies. Node. The co