CSMA. A type of CSMA (Carrier Sense Multiple Access) in which if a station, wishing to transmit, senses that the line is in use it backs off for a random time and then Packet switching. Transmission method in which data is formed recommences the sensing operation. into discrete segments, usually with its own control informa- Nonvolatile memory. A storage device which does not lose its tion, and is routed through the network in this envelope, content when the power source is removed. referred to as a packet. Packets occupy a communication chan- Null character. A character which is usually added to a trans- nel for a short duration, SO packets from several users can share mitted signal to fill unused space. It may be added or removed a channel. without affecting the sense of the transmitted data. Pair gain system. Any system in which subscribers can be served Null modem. Name generally applied to the wired RS-232C cable by fewer number of wire pairs than otherwise, e.g. by multi- which allows two DTEs to communicate directly with each plexing, etc. This includes increasing the number of channels other. over the existing pair of wires, as is done in ISDN. It is Nyquist theorem. Theorem postulated by Nyquist which states measured as a ratio of the number of wires needed without the that the original analogue signal can be precisely recreated pair gain system to that required with the pair gain system. provided a specified minimum number of samples are taken of Parallel interface. An interface to equipment where all the bits making up a character are available simultaneously for trans- Nyquist transmission rate. The maximum rate of transmission of mission. See also serial interface. pulses over a transmission channel (equal to twice the band- Parallel processing. A system in which a processor performs two width of the channel) without the risk of intersymbol inter- or more simultaneous processes. ference. Parasitic capacitance. Unwanted capacitance which is unavoid- ably formed when the system is built. This is also k