 carry approximately the same density quency range. of traffic. Kelvin. Unit of absolute temperature. Absolute zero is at -273° Loading coil. An inductance added to a transmission line in order Celsius. to reduce amplitude distortion. Ku band. Electromagnetic spectrum in the 10GHz to 12GHz Local area network. A high speed link, usually carrying data, con- range. necting several communicating devices, over a relatively small Keys. Symbols used in the encryption of plaintext to ciphertext. geographical area. Key phone. A telephone which incorporates a switching function, Local exchange. Exchanges which are local to subscribers and obtained by pressing keys. into which their lines terminate. Local loop. The term for the system (cable or radio) used to connect the subscribers' equipment, such as a telephone or PABX, to a local exchange. Labelling algorithm. An algorithm in which labels are used on Logarithmic compression. Compression of a signal based on the individual nodes, the labels being updated as required. The logarithm of its instantaneous amplitude. technique is commonly used for problems such as shortest path Long haul. Refers to telephone circuits operating over long dis- routing. tances, such as between LATAs. Latency. Waiting time between the request for a transmission Longitudinal signal. A signal, usually unwanted, which is im- channel and the acquisition of a channel. pressed across both wires of a two wire transmission line. It Layered architecture. Refers to the structure in which the total may be eliminated by balancing the lines. functionality of the system is broken down into layers of Loopback. A system test method in which a signal is sent to the defined functionality, each layer providing a limited service to device under test, which is then returned to the transmitter for the layers above and below. The most common layered struc- comparison with the original transmission. ture is the ISO OSI seven layer reference model. Loop start. Analogue signalling method in which lifting the tele- L band.