r the propagation of signals, usually in the form of modulated radio, light, or acoustic waves, from one point to another (e.g., optical fiber, cable, wire, water, air, free space). medium frequency Frequencies in the range between 300KHz and 3MHz. mega 1 million. Megaco (Media Gateway Control) An ITU standard that describes how the media gate- way should behave and function. It is standardized under ITU-T H.248. memory An area of a computer system that accepts, holds, and provides access to information. MEMS (microelectromechanical system) switch An optical switch that uses an array of microscopic mirrors to reflect light from an input port to an output port. menu A displayed list of items from which a user can make a selection. menu-driven Programs that are run by instructions presented as a list of commands and available options. Unlike in a command-driven program, the user only has to select the desired option. MEO (middle-earth-orbit) satellite A satellite that orbits at an elevation of about 6,200 to 9,400 miles (9,900 to 15,000 km) above the earth. mesh A topology in which nodes are connected in an unconstrained way and may contain loops. A fully connected mesh has every pair of nodes directly connected. message An arbitrary amount of information whose beginning and end are defined. In data communications, a message consists of a header, a body, and a trailer. message authentication Authentication that verifies the integrity of an electronic message and also verifies that an electronic message was sent by a particular entity. message format Rules for the placement of such portions of a message as message heading, address, text, end-of-message indication, and error-detecting bits. message numbering Identification of each message within a communications system by the assignment of sequential numbers. message switching message switching A technique that transfers messages between points that are not directly connected. The switching facility receives messages, stores them in queues for each destination point,